Biodiversity

Introduction

Biodiversity is the number of distinct varieties or types within a group of living systems: distinct genes in a species, species in an ecosystem, or ecosystems in a biome. The term is often used to mean the total number of species living in a given ecosystem or on Earth as a whole. Climate change affects biodiversity primarily by shifting the boundaries of ecosystems, by altering the timing of seasonal events such as hatching and budding, and altering the temperature and chemical characteristics of lakes, rivers, and oceans. Atmospheric carbon dioxide (CO₂), which is a main cause of global warming, also has direct effects on ecosystems, acidifying the oceans and encouraging the growth of some plants more than others. Such stresses inevitably cause extinctions, that is, loss of biodiversity.

Only evolution can create new species, and this occurs only over geologic time (usually millions of years). On human or historical time scales, extinction decreases biodiversity irreversibly. Some extinctions from climate change have already been recorded, although to date most have been caused by other human activities such as pollution, over-hunting, and deforestation. The rate of extinctions caused by climate change is predicted to be greater later in the twenty-first century than today.

Historical Background and Scientific Foundations

Context

The modern systematic classification of species was invented by Swedish naturalist Carl Linnaeus (1707– 1788) in the eighteenth century. In following decades, European naturalists scoured Earth looking for and classifying new species of plants and animals, greatly expanding scientific knowledge of just how diverse life on Earth is. Such knowledge was crucial to the development of evolutionary biology by Charles Darwin (1809–1882) and others in the mid-nineteenth century. Today, biologists estimate that there are more than 280,000 species of plants and over 1,250,000 species of animals (including insects). Nineteenth- and twentieth-century paleontology has shown that extinction is a normal process: over 99% of all species that have ever lived are now extinct. The typical lifespan of a species is between 1 and 10 million years.

Awareness that species biodiversity is a key feature of ecosystems was not common until the 1970s, however, when the term “biological diversity” first came into frequent use. The word “biodiversity,” a shortening of “biological diversity,” first appeared in print in 1988. In the 1990s and beyond, most biologists have agreed that human beings are causing the first mass extinction in 65 million years. This modern pulse of extinctions is sometimes called the Holocene extinction event. (The Holocene is the geological term for the era from 11,500 years ago to the present.)

Humans are causing extinctions through land development, destruction of rainforests, over-hunting and over-fishing, pollution, and other activities. Some of the more famous extinctions of recent times include the dodo bird, the passenger pigeon, and (announced in 2007) the baiji, a white river dolphin of China. Hunting has caused about 23% of known animal extinctions since 1600; the introduction of invasive species, 39%; and habitat destruction, 36%. These numbers are approximate because many of the species that are being destroyed are uncatalogued insects and plants living in rainforests. Estimates of how many extinctions have already occurred due to human activities range from several tens of thousands to over a million.

Although impacts by large asteroids have caused or at least contributed to some ancient mass extinctions, climate change has been the most common cause of natural mass extinction. Ancient climate changes were brought about by shifts in Earth's orbit, continental drift, volcanism, and other processes. Today's episode of climate change is unique in being caused by a single species, humans. Moreover, today's mass extinction is unique in that human beings, by taking action to mitigate (reduce the severity of) climate change, can influence the overall severity of the event.

Mechanisms by which Climate Change Affects Biodiversity

As warming continues, other forms of human pressure on biodiversity will continue and will be, in most cases, amplified by the effects of climate change. Although effects may vary from region to region, the overall effect of global warming is to cause the cooler zones of the world—the regions around the poles (especially the North Pole) and on mountains—to shrink. Shrinkage of habitat puts species at risk because smaller habitats support smaller populations, and smaller populations are always at higher risk of extinction.

Climate change also has many other effects on ecosystems. Some are not immediately obvious. For example, Lake Tanganyika in Africa, the world's second-largest and second-deepest lake, harbors at least 350 species of fish, most unique to its own waters. Lake Tanganyika is not typical of ecosystems most vulnerable to climate change: being near the equator it is likely to see less drastic warming than, say, the Arctic, while its large thermal mass (4,526 cubic mi of water; 18,900 cubic km) will resist temperature shifts and thus might be expected to moderate climate-change impacts on the lake ecosystem.

However, regional climate warming by 1.08°F (0.6°C), along with lessened wind speeds, has had rapid effects on Tanganyika's ecosystem. Warming of surface waters more than deep waters has decreased mixing between the two: warm water is less dense than cool water, so the bigger the temperature difference between the layers, the more stably the warm upper water floats on top of the cool deeper water. Since the deeper waters are more nutrient-rich, reduced mixing has meant that fewer planktonic organisms (tiny, floating organisms, both plants and animals) can thrive in the upper water, where energy from the sun is abundant but nutrients are poor. As of 2003, plankton density in Lake Tanganyika had declined to less than one third what it was 25 years before; algae density had declined by 30% from values 80 years before. Since plankton are the basis of the marine food chain, fish stocks declined along with the plankton: fish stocks in the lake were 30% smaller than they were 80 years earlier.

WORDS TO KNOW

: Well-defined terrestrial environment (e.g., desert, tundra, or tropical forest). The complex of living organisms found in an ecological region.

: Decoloration or whitening of coral from the loss, temporary or permanent, of symbiotic algae (zooxanthellae) living in the coral. The algae give corals their living color and, through photosynthesis, supply most of their food needs. High sea surface temperatures can cause coral bleaching.

: The period of time extending from the formation of Earth to the present.

: The Holocene is the geological period from 10,000 years ago to the present; the Holocene extinction is the worldwide mass extinction of animal and plant species being caused by human activity. Global warming may accelerate the ongoing Holocene extinction event, possibly driving a fourth of all terrestrial plant and animal species to extinction.

: The study of life in past geologic time.

: Measures the acidity of a solution. It is the negative log of the concentration of the hydrogen ions in a substance.

: Floating animal and plant life.

The reduction of water mixing due to climate warming has made other changes in the lake's chemistry: for one, oxygen dissolved from the air no longer mixes as well in deeper waters. As a result, the habitat has shrunk for some of the lake's endemic species, such as the snail Tiphobia horei, which in 1890 lived at depths down to 1,000 ft (300 m) but as of 2003 lived only down to 330 ft (100 m). The snail's habitat has thus shrunk by about two thirds, even though the lake itself has not shrunk and its bulk average temperature has changed only slightly.

Tanganyika surface plankton loss has been reflected in declining food-fish harvests from the lake (about 400,000 tons per year in 2003). Since Tanganyika supplies 25–40% of the protein needs of the four nations bounding the lake, such declines can have direct impacts on human populations as well as on biodiversity. Losses in biodiversity have not yet been measured directly in Tanganyika, but smaller populations will put some species at risk of extinction, especially as warming in the region continues to about 2.7°F (1.5°C), possibly higher, with even more drastic stabilization of the lake's waters and consequent effects on its ecosystem.

Slight changes in climate can lead to pressures on biodiversity by other mechanisms. For example, a 2006 study by J. Alan Pounds and colleagues found that global warming has almost certainly caused the recent extinction of about 67% of the 110 or so species of the Monteverde harlequin tree frog of the mountains of Costa Rica. The scientists saw the extinctions as validating the climate-linked epidemic hypothesis, according to which shifts in temperature, rainfall, and other climate variables make populations more vulnerable to disease and therefore to extinction. In the case of the Monteverde frogs, more frequent warm years shifted conditions toward the growth optimum of the Bactrachochytrium fungus, which infects the frogs. The researchers found that extinctions of the frogs consistently followed temperature peaks that were favorable to growth of the fungal disease.

Other effects are not strictly changes in climate, in the sense of temperature or precipitation, but chemical changes to air and water. Increased carbon dioxide (CO₂) in theatmospherehas twomajor effectsthatare likelyto decrease biodiversity: 1) Heightened atmospheric CO₂ causes increased levels of dissolved CO₂ in the ocean. When CO₂ dissolves in water, it produces a weak acid, carbonic acid. Rising atmospheric CO₂ thus acidifies the oceans. 2) Green plants extract carbon from the air by breaking up CO₂, constructing their tissues using the carbon, and releasing the oxygen. CO₂ is plant food. Thus, increasing atmospheric CO₂ tends to cause more rapid growth in most plant species, an effect called CO₂ enrichment.

Acidification of the oceans by dissolved excess CO₂ will impact biodiversity by making survival more difficult for organisms that form shells of calcium carbonate. This includes bivalves such as clams, mollusks such as periwinkles and conches, microscopic plankton species, and corals. Corals, which are also subject to bleaching in excessively warm waters, form large, shallow communities in tropical waters that have been compared to rainforests because of their high level of biodiversity. A typical large reef may support on the order of a million species of plants and animals.

Over the last two centuries, the average pH of the oceans has fallen by 0.1, corresponding to a rise in acidity and a 30% reduction in the number of carbonate ions (CO₃^2–) available to shell-making organisms as building material. When carbonate ions fall below a certain level, corals have difficulty making their skeletons. This threshold may be reached if the atmospheric CO₂ level, today about 375 parts per million, rises to over 500 parts per million, as may occur by the end of the twenty-first century.

Increasing atmospheric CO₂ will also affect plant growth. Farmers today often add CO₂ to the air inside greenhouses, because under indoor conditions, extra CO₂ speeds plant growth and increases crop yields. Under outdoor conditions, however, the gain in yield is about half as much and the foods produced are significantly lower in protein and minerals. In the wild, rising CO₂ will favor some species over others, depending on rooting depth, woodiness, and photosynthetic chemistry; this will impact biodiversity by altering competitive balances. The CO₂ fertilization effect will be strongest in biomes where plant growth is limited by water availability, such as grasslands, savanna, and desert. The biodiversity impact of a 2.5-fold increase in CO₂ would likely be only about a third as great on a boreal (northern pine) forest as on savanna or grassland, and half as great as on desert.

IN CONTEXT: PRESERVING BIODIVERSITY

“Reducing both loss of natural habitat and deforestation can have significant biodiversity, soil and water conservation benefits, and can be implemented in a socially and economically sustainable manner. Forestation and bioenergy plantations can lead to restoration of degraded land, manage water runoff, retain soil carbon and benefit rural economies, but could compete with land for food production and may be negative for biodiversity, if not properly designed.”

SOURCE:Metz, B., et al, eds. Climate Change 2007: Mitigation of Climate Change: Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.

Impacts and Issues

Global Patterns

Most plants require a specific range of temperature, moisture, and seasonal change to thrive; most animals require certain plants or other animals to thrive, and also have a limited range of tolerance for temperature and moisture. As climate warms, a typical ecosystem will tend to migrate away from areas where it was at the warm edge of its tolerance range and toward places where it was formerly at the cool edge. The most general effect of global climate change is thus to move ecological zones toward the poles and toward higher altitudes. For each 1.8°F (1°C) of warming, terrestrial (on-land) ecosystems typically shift poleward by 100 mi (160 km): for example, if climate warms by 5.4°F (3°C) by 2100, plant and animal communities in the Northern Hemisphere will migrate an average of 300 mi (480 km) northward—if they can—to stay in a suitable climate zone. This effect is observed, not only predicted.

In the Northern Hemisphere, terrestrial animal and plant ranges have been observed to shift northward, on average, by 3.8 mi (6.1 km) per decade over the last 50 years. In mountainous terrain, plant and animal ranges have shifted upward by 20 ft (6.1 m) over the same time period. Fragmentation of landscapes by human activity such as agriculture and city-building makes ecosystem migration more difficult today than during past climatic shifts, such as glacial periods. Species that fail to colonize new areas as the climate changes may go extinct.

Although climate change has so far been most intense in the Arctic and the West Antarctic Peninsula, where warming has been about twice the global average and dramatic effects such as retreating sea ice and melting tundra are readily visible, biodiversity is low in these regions compared to the tropics, where rainforests, coral reefs, and other particularly diverse communities are found. Thus, a smaller climate shift can have a greater impact on biodiversity in the tropics than a larger shift in boreal or temperate regions.

For marine ecosystems, changes in circulation patterns, ocean temperature, and ocean chemistry all influence biodiversity. For example, over the last 40 years or so, warm-water plankton species have shifted about 620 mi (1,000 km) in the North Atlantic due to warming.

Not all observed climate effects on biological systems are consistent with climate warming; a few are consistent with cooling. Also, some observed effects are consistent with natural climate shifts rather than those attributed to human-caused (anthropogenic) global warming. However, mathematical analysis shows that the very great majority of changes are consistent with warming trends, and that a combination of natural and anthropogenic climate changes describe observed changes in physical and biological systems better than either natural or anthropogenic changes alone. Anthropogenic changes have been added to or laid over those caused by natural processes, and are gradually becoming more dominant.

Future Impacts

The five main drivers of biodiversity change, ranked from most severe impact to least severe between now and 2100, are: 1) land-use changes (including deforestation);2) climate change; 3) nitrogen deposition (from fertilizer use); 4) biotic exchange (the introduction of invasive species); and 5) direct effects of increasing atmospheric CO₂, apart from climate change.

The result of these combined, continuing, and growing pressures will be an irreversible loss of biodiversity in many parts of the world. However, many uncertainties remain. Ecologists do not understand the relationship between ecosystem structure and rapid climate change well enough to predict the exact effects of current climate changes on biomes. It is also unknown whether efforts to mitigate climate change will occur or succeed, and if so, to what extent.

Despite these uncertainties, scientists have estimated the likely impact that climate change will have on biodiversity. In 2004, Chris Thomas and colleagues published their study of an unbiased or representative sample of 1,103 animal and plant species. They found that climate change was likely to commit 15–37% of all species examined to extinction by 2050. “Committed to extinction” does not mean that a species would necessarily be extinct by that time, but that the population of each species would be so reduced that its species' extinction becomes highly likely. In many or most ecological regions, climate change will become the greatest threat to biodiversity by 2050. There are 5 to 15 million species of creatures on Earth (the large range arises from the difficulty of counting insect, bacterial, and fungal species). If only 15% of all species are committed to extinction by climate change—the lower end of the range given by Thomas and colleagues—then 750,000 to 2,250,000 million species will eventually become extinct as a result of global climate change.

Primary Source Connection

Human activities and the resulting global climate change have had, and will continue to have, a major impact on biodiversity across the globe. Biodiversity is the variety of all living organisms that exist in an ecosystem. This paper from the Intergovernmental Panel on Climate Change (IPCC) discusses the effect of human activity on biodiversity and possible adaptation and mitigation strategies.

The IPCC is a scientific panel that was founded by the United Nations in 1988 as part of the United Nations Environment Program and the U.N.'s World Meteorological Organization.

CLIMATE CHANGE AND BIODIVERSITY

At the global level, human activities have caused and will continue to cause a loss in biodiversity through, inter alia, land-use and land-cover change; soil and water pollution and degradation (including desertification), and air pollution; diversion of water to intensively managed ecosystems and urban systems; habitat fragmentation; selective exploitation of species; the introduction of non-native species; and stratospheric ozone depletion. The current rate of biodiversity loss is greater than the natural background rate of extinction. A critical question for this Technical Paper is how much might climate change (natural or human-induced) enhance or inhibit these losses in biodiversity?

Changes in climate exert additional pressure and have already begun to affect biodiversity . The atmospheric concentrations of greenhouse gases have increased since the pre-industrial era due to human activities, primarily the combustion of fossil fuels and land-use and land-cover change. These and natural forces have contributed to changes in the Earth's climate over the 20th century: Land and ocean surface temperatures have warmed, the spatial and temporal patterns of precipitation have changed, sea level has risen, and the frequency and intensity of El Niño events have increased. These changes, particularly the warmer regional temperatures, have affected the timing of reproduction in animals and plants and/or migration of animals, the length of the growing season, species distributions and population sizes, and the frequency of pest and disease outbreaks. Some coastal, high-latitude, and high-altitude ecosystems have also been affected by changes in regional climatic factors.

Climate change is projected to affect all aspects of biodiversity; however, the projected changes have to take into account the impacts from other past, present, and future human activities, including increasing atmospheric concentrations of carbon dioxide (CO₂) . For the wide range of Intergovernmental Panel on Climate Change (IPCC) emissions scenarios, the Earth's mean surface temperature is projected to warm 1.4 to 5.8°C by the end of the 21st century, with land areas warming more than the oceans, and the high latitudes warming more than the tropics. The associated sea-level rise is projected to be 0.09 to 0.88 m. In general, precipitation is projected to increase in high-latitude and equatorial areas and decrease in the subtropics, with an increase in heavy precipitation events. Climate change is projected to affect individual organisms, populations, species distributions, and ecosystem composition and function both directly (e.g., through increases in temperature and changes in precipitation and in the case of marine and coastal ecosystems also changes in sea level and storm surges) and indirectly (e.g., through climate changing the intensity and frequency of disturbances such as wild-fires). Processes such as habitat loss, modification and fragmentation, and the introduction and spread of non-native species will affect the impacts of climate change. A realistic projection of the future state of the Earth's ecosystems would need to take into account human land- and water-use patterns, which will greatly affect the ability of organisms to respond to climate change via migration.

The general effect of projected human-induced climate change is that the habitats of many species will move pole-ward or upward from their current locations . Species will be affected differently by climate change: They will migrate at different rates through fragmented landscapes, and ecosystems dominated by long-lived species (e.g., long-lived trees) will often be slow to show evidence of change. Thus, the composition of most current ecosystems is likely to change, as species that make up an ecosystem are unlikely to shift together. The most rapid changes are expected where they are accelerated by changes in natural and anthropogenic non-climatic disturbance patterns.

Changes in the frequency, intensity, extent, and locations of disturbances will affect whether, how, and at which rate the existing ecosystems will be replaced by new plant and animal assemblages . Disturbances can increase the rate of species loss and create opportunities for the establishment of new species.

Globally by the year 2080, about 20% of coastal wetlands could be lost due to sea-level rise . The impact of sea-level rise on coastal ecosystems (e.g., mangrove/coastal wetlands, seagrasses) will vary regionally and will depend on erosion processes from the sea and depositional processes from land. Some mangroves in low-island coastal regions where sedimentation loads are high and erosion processes are low may not be particularly vulnerable to sea-level rise.

The risk of extinction will increase for many species that are already vulnerable . Species with limited climatic ranges and/or restricted habitat requirements and/or small populations are typically the most vulnerable to extinction, such as endemic mountain species and biota restricted to islands (e.g., birds), peninsulas (e.g., Cape Floral Kingdom), or coastal areas (e.g., mangroves, coastal wetlands, and coral reefs). In contrast, species with extensive, non-patchy ranges, long-range dispersal mechanisms, and large populations are at less risk of extinction. While there is little evidence to suggest that climate change will slow species losses, there is evidence it may increase species losses. In some regions there may be an increase in local biodiversity—usually as a result of species introductions, the long-term consequences of which are hard to foresee.

Where significant ecosystem disruption occurs (e.g., loss of dominant species or a high proportion of species, or much of the species redundancy), there may be losses in net ecosystem productivity (NEP) at least during the transition period . However, in many cases, loss of biodiversity from diverse and extensive ecosystems due to climate change does not necessarily imply loss of productivity as there is a degree of redundancy in most ecosystems; the contribution to production by a species that is lost from an ecosystem may be replaced by another species. Globally, the impacts of climate change on biodiversity and the subsequent effects on productivity have not been estimated.

Changes in biodiversity at ecosystem and landscape scale, in response to climate change and other pressures (e.g., changes in forest fires and deforestation), would further affect global and regional climate through changes in the uptake and release of greenhouse gases and changes in albedo and evapotranspiration. Similarly, structural changes in biological communities in the upper ocean could alter the uptake of CO₂ by the ocean or the release of precursors for cloud condensation nuclei causing either positive or negative feedbacks on climate change.

Modeling the changes in biodiversity in response to climate change presents some significant challenges . The data and models needed to project the extent and nature of future ecosystem changes and changes in the geographical distribution of species are incomplete, meaning that these effects can only be partially quantified.

Impacts of climate change mitigation activities on biodiversity depend on the context, design, and implementation of these activities . Land-use, land-use change, and forestry activities (afforestation, reforestation, avoided deforestation, and improved forest, cropland, and grazing land management practices) and implementation of renewable energy sources (hydro-, wind-, and solar power and biofuels) may affect biodiversity depending upon site selection and management practices. For example, 1) afforestation and reforestation projects can have positive, neutral, or negative impacts depending on the level of biodiversity of the non-forest ecosystem being replaced, the scale one considers, and other design and implementation issues; 2) avoiding and reducing forest degradation in threatened/vulnerable forests that contain assemblages of species that are unusually diverse, globally rare, or unique to that region can provide substantial bio-diversity benefits along with the avoidance of carbon emissions; 3) large-scale bioenergy plantations that generate high yields would have adverse impacts on biodiversity where they replace systems with higher biological diversity, whereas small-scale plantations on degraded land or abandoned agricultural sites would have environmental benefits; and 4) increased efficiency in the generation and/or use of fossil-fuel-based energy can reduce fossil-fuel use and thereby reduce the impacts on biodiversity resulting from resource extraction, transportation (e.g., through shipping and pipelines), and combustion of fossil fuels.

Climate change adaptation activities can promote conservation and sustainable use of biodiversity and reduce the impact of changes in climate and climatic extremes on biodiversity . These include the establishment of a mosaic of interconnected terrestrial, freshwater, and marine multiple-use reserves designed to take into account projected changes in climate, and integrated land and water management activities that reduce non-climate pressures on biodiversity and hence make the systems less vulnerable to changes in climate. Some of these adaptation activities can also make people less vulnerable to climatic extremes.

The effectiveness of adaptation and mitigation activities can be enhanced when they are integrated with broader strategies designed to make development paths more sustainable . There are potential environmental and social synergies and tradeoffs between climate adaptation and mitigation activities (projects and policies), and the objectives of multilateral environmental agreements (e.g., the conservation and sustainable use objective of the Convention on Biological Diversity) as well as other aspects of sustainable development. These synergies and tradeoffs can be evaluated for the full range of potential activities—inter alia, energy and land-use, land-use change, and forestry projects and policies through the application of project, sectoral, and regional level environmental and social impact assessments—and can be compared against a set of criteria and indicators using a range of decision making frameworks. For this, current assessment methodologies, criteria, and indicators for evaluating the impact of mitigation and adaptation activities on biodiversity and other aspects of sustainable development will have to be adapted and further developed.

ipcc. “climate change and biodiversity.” 2001ipcc technical paper v. < HTTP://WWW.GRIDA.NO/CLIMATE/IPCC_TAR/BIODIV/PDF/BIO_ENG.PDF> (accessed november29, 2007).

See Also Endangered Species; Extinction.

BIBLIOGRAPHY

Books

Parry, M. L., et al, eds. Climate Change 2007: Impacts, Adaptation and Vulnerability: Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.

Periodicals

Araúo, Miguel B., and Carsten Rahbek. “How Does Climate Change Affect Biodiversity?” Science 313 (2006): 1,396–1,397.

Higgins, Paul A. T. “Biodiversity Loss Under Existing Land Use and Climate Change: An Illustration Using Northern South America.” Global Ecology and Biogeography 16 (2007): 197–204.

Jenkins, Martin. “Prospects for Biodiversity.” Science 302 (2003): 1,175–1,177.

Livingstone, Daniel A. “Global Climate Change Strikes a Tropical Lake.” Science 301 (2003): 468–469.

Pounds, J. Alan, et al. “Widespread Amphibian Extinctions from Epidemic Disease Driven by Global Warming.” Nature 439 (2006): 161–167.

Sala, Enric, and Nancy Knowlton. “Global Marine Biodiversity Trends.” Annual Review of Energy and the Environment 231 (2006): 93–122.

Sala, Osvaldo E., et al. “Global Biodiversity Scenarios for the Year 2100.” Science 287 (2000): 1,770–1,774.

Thomas, Chris D. “Extinction Risk from Climate Change.” Nature 427 (2004): 145–148.

Thuiller, Wilfried. “Climate Change and the Ecologist.” Nature 448 (2007): 550–552.

Willis, K. J., and H. J. B. Birks. “What Is Natural? The Need for a Long-Term Perspective in Biodiversity Conservation.” Science 314 (2006): 1,261–1,265.

Zimmer, Carl. “Predicting Oblivions: Are Existing Models Up to the Task.” Science 317 (2007): 892–893.

Web Sites

“Climate Change and Biodiversity.” Intergovernmental Panel on Climate Change, IPCC Technical Paper V. < http://www.ipcc.ch/pub/tpbiodiv.pdf> (accessed November 8, 2007).

Larry Gilman

Biodiversity

Introduction

Biodiversity is the number of distinct varieties or types within a group of living systems: distinct genes in a species, species in an ecosystem, or ecosystems in a biome. The term is often used to mean the total number of species living in a given ecosystem or on Earth as a whole. Biodiversity is created by speciation, that is, the evolutionary appearance of new species; it is diminished by extinction. Only evolution can create new species, and this occurs only over geologic time (usually millions of years). On human or historical timescales, extinction decreases biodiversity irreversibly.

Human beings have impacted biodiversity for thousands of years through activities such as hunting, pollution, and habitat destruction (swamp-draining, deforestation, plowing of prairies, and the like). Nor is human-caused extinction entirely a recent affair. There is evidence that a number of animal species in North America, Australia, and New Zealand became extinct tens of thousands of years ago after the arrival of the first human settlers. In the last century or so, however, extinction due to habitat destruction for expanding human populations and industrial activity has increased manyfold. Moreover, a new human-caused foe of biodiversity has begun to manifest, namely global climate change.

Climate change threatens biodiversity mostly by shifting the boundaries of ecosystems, changing the timing of seasonal events such as hatching and budding, and altering the temperature and chemical characteristics of lakes, rivers, and oceans. Atmospheric carbon dioxide (CO₂), which is the main cause of global warming, also has direct affects on ecosystems, such as acidifying the oceans and encouraging the growth of some plants more than others. Such stresses inevitably cause extinctions, that is, loss of biodiversity. The rate of extinctions caused by climate change is predicted to be greater later in the twenty-first century than today.

Historical Background and Scientific Foundation

Context

The modern systematic classification of species was invented by Swedish naturalist Carl Linnaeus (1707–1788) in the eighteenth century. In following decades, European naturalists scoured Earth looking for and classifying new species of plant and animal, greatly expanding scientific knowledge of just how diverse life on Earth is. Such knowledge was crucial to the development of evolutionary biology by Charles Darwin (1809–1882) and others in the mid-nineteenth century. Today, biologists estimate that there are more than 280,000 species of plants and over 1,250,000 species of animals (including insects). Nineteenth- and twentieth-century paleontology has shown that extinction is a normal process: over 99% of all species that have ever lived are now extinct. The typical lifespan of a species is between 1 and 10 million years.

Awareness that species biodiversity is a key feature of ecosystems was not common until the 1970s, however, when the term “biological diversity” first came into frequent use. The word “biodiversity,” a shortening of “biological diversity,” first appeared in print in 1988. In the 1990s and beyond, most biologists have agreed that human beings are causing the first mass extinction in 65 million years. This modern pulse of extinctions is sometimes called the Holocene extinction event. (The Holocene is the geological term for the era from 11,500 years ago to the present.)

Humans are causing extinctions through land development, destruction of rain forests, over-hunting and over-fishing, pollution, and other activities. Some of the more famous extinctions of recent times include the dodo bird, the passenger pigeon, and (announced in 2007) the baiji, a white river dolphin of China. Hunting has caused about 23% of known animal extinctions since 1600; the introduction of invasive species, 39%; habitat destruction, 36%. These numbers are approximate because many of the species that are being destroyed are uncatalogued insects and plants living in rain forests. Estimates of how many extinctions have already occurred due to human activities range from several tens of thousands to over a million.

Although impacts by large asteroids have caused or at least contributed to some ancient mass extinctions, climate change has been the most common cause of natural mass extinction throughout Earth’s history. Ancient climate changes have been brought about by shifts in Earth’s orbit, continental drift, volcanism, and other processes. Today’s episode of climate change is unique in being caused by a single species, humans. Moreover, today’s ongoing mass extinction is unique in that human beings, by taking action to mitigate (reduce the severity of) climate change and to reduce the other impacts of human activity on biodiversity, can influence the overall severity of the event.

Case Study: How Climate Change Affects Biodiversity

As warming continues, other forms of human pressure on biodiversity will continue and will be, in most cases, amplified by the effects of climate change. Although effects may vary from region to region, the overall effect of global warming is to cause the cooler zones of the world—the regions around the poles (especially the North Pole) and on mountains—to shrink. Shrinkage of habitat puts species at risk because smaller habitats support smaller populations, and smaller populations are always at higher risk of extinction.

Climate change also has many other effects on ecosystems. Some are not immediately obvious. For example, Lake Tanganyika in Africa, the world’s second-largest and second-deepest lake, harbors at least 350 species of fish, most unique to its own waters. Lake Tanganyika is not typical of ecosystems most vulnerable to climate change. Being near the equator it is likely to see less drastic warming than, say, the Arctic, while its large thermal mass (4,526 cubic mi of water; 18,900 cubic km) will resist temperature shifts and thus might be expected to moderate climate-change impacts on the lake ecosystem.

However, regional climate warming by 1.08°F (0.6°C), along with lessened wind speeds, has had rapid effects on Tanganyika’s ecosystem. Warming of surface waters more than deep waters has decreased mixing between the two: Warm water is less dense than cool water, so the bigger the temperature difference between

WORDS TO KNOW

BIOME: A well-defined terrestrial environment (e.g., desert, tundra, or tropical forest) and the complex of living organisms found in that region.

DEFORESTATION: A reduction in the area of a forest resulting from human activity.

GEOLOGIC TIME: The period of time extending from the formation of the earth to the present.

PALEONTOLOGY: The study of life in past geologic time.

pH: The measure of the amount of dissolved hydrogen ions in solution.

PLANKTON: Floating animal and plant life.

the layers, the more stably the warm upper water floats on top of the cool deeper water. Since the deeper waters are more nutrient-rich, reduced mixing has meant that fewer planktonic organisms (tiny, floating organisms, both plants and animals) can thrive in the upper water, where energy from the sun is abundant but nutrients are poor. As of 2003, plankton density in Lake Tanganyika had declined to less than one third of what it was 25 years before; algae density had declined by 30% from values 80 years before. Since plankton are the basis of the marine food chain, fish stocks declined along with the plankton: Fish stocks in the lake were 30% smaller than they were 80 years earlier.

The reduction of water mixing due to climate warming has made other changes in the lake’s chemistry. For one, oxygen dissolved from the air no longer mixes as well in deeper waters. As a result, the habitat has shrunk for some of the lake’s endemic species, such as the snail Tiphobia horei, which in 1890 lived at depths down to 1,000 ft (300 m) but as of 2003 lived only down to 330 ft (100 m). The snail’s habitat has thus shrunk by about two thirds, even though the lake itself has not shrunk and its bulk average temperature has changed only slightly.

Tanganyika surface plankton loss has been reflected in declining food-fish harvests from the lake (about 400,000 tons per year in 2003). Since Tanganyika supplies 25-40% of the protein needs of the four nations bounding the lake, such declines can have direct impacts on human populations as well as on biodiversity. Losses in biodiversity have not yet been measured directly in Tanganyika, but smaller populations will put some species at risk of extinction, especially as warming in the region continues to about 2.7°F (1.5°C), possibly higher, with even more drastic stabilization of the lake’s waters and consequent effects on its ecosystem.

IN CONTEXT: WILDLIFE POPULATION DECLINE AND BIODIVERSITY LOSS, 1970–2005

In May 2008, the Zoological Society of London and World Wildlife Fund (WWF) released the results of a study indicating a dramatic decline in global wildlife populations during the last four decades. The data showed that reductions in populations of land-based species fell by 25%, marine by 28%, and freshwater species by 29% between 1970 and 2005. Biodiversity loss, in the form of extinction of about 1% of the world’s species each year, was also reported in what the foundation asserted was one of the “great extinction episodes” in Earth’s history.

The losses were attributed to pollution, climate change, urbanization, over-fishing, and hunting. The foundation and WWF based their conclusion on population estimates and other data related to 1,400 species tracked as part of its Living Planet index. Data were obtained from published sources in scientific journals.

Slight changes in climate can lead to pressures on biodiversity by other mechanisms. For example, a 2006 study by J. Alan Pounds and colleagues found that global warming has almost certainly caused the recent extinction of about 67% of the 110 or so species of the Monteverde harlequin tree frog of the mountains of Costa Rica. The scientists saw the extinctions as validating the climate-linked epidemic hypothesis, according to which shifts in temperature, rainfall, and other climate variables make populations more vulnerable to disease and therefore to extinction. In the case of the Monteverde frogs, more frequent warm years shifted conditions toward the growth optimum of the Bactrachochytrium fungus, which infects the frogs. The researchers found that extinctions of the frogs consistently followed temperature peaks that were favorable to growth of the fungal disease.

Other effects are not strictly changes in climate, in the sense of temperature or precipitation, but chemical changes to air and water. Increased carbon dioxide (CO₂) in the atmosphere has two major effects that are likely to decrease biodiversity: 1) Heightened atmospheric CO₂ causes increased levels of dissolved CO₂ in the ocean. When CO₂ dissolves in water, it produces a weak acid, carbonic acid. Rising atmospheric CO₂ thus acidifies the oceans. 2) Green plants extract carbon from the air by breaking up CO₂, constructing their tissues using the carbon, and releasing the oxygen. CO₂ is plant food. Thus, increasing atmospheric CO₂ tends to cause more rapid growth in most plant species, an effect called CO₂ enrichment.

Acidification of the oceans by dissolved excess CO₂ will impact biodiversity by making survival more difficult for organisms that form shells of calcium carbonate. This includes bivalves such as clams, mollusks such as periwinkles and conches, microscopic plankton species, and corals. Corals, which are also subject to bleaching in excessively warm waters, form large, shallow communitities in tropical waters that have been compared to rain forests because of their high level of biodiversity. A typical large reef may support on the order of a million species of plants and animals.

Over the last two centuries, the average pH of the oceans has fallen by 0.1, corresponding to a rise in acidity and a 30% reduction in the number of carbonate ions (CO₃^2-) available to shell-making organisms as building material. When carbonate ions fall below a certain level, corals have difficulty making their skeletons. This threshold may be reached if the atmospheric CO₂ level, today about 375 parts per million, rises to over 500 parts per million, as may occur by the end of the twenty-first century.

Increasing atmospheric CO₂ will also affect plant growth. Farmers today often add CO₂ to the air inside greenhouses, because under indoor conditions, extra CO₂ speeds plant growth and increases crop yields. Under outdoor conditions, however, the gain in yield is about half as much and the foods produced are significantly lower in protein and minerals. In the wild, rising CO₂will favor some species over others, depending on rooting depth, woodiness, and photosynthetic chemistry; this will impact biodiversity by altering competitive balances. The CO₂ fertilization effect will be strongest in biomes where plant growth is limited by water availability, such as grasslands, savanna, and desert. The biodiversity impact of a 2.5-fold increase in CO₂ would likely be only about a third as great on a boreal (northern pine) forest as on a savanna or grassland, and half as great as on a desert.

Impacts and Issues

Scientific studies estimate by that 2050 there will about 9 billion human beings on Earth. Industrial activity, pollution, hunting, settlement, introducing species to environments where they have not hitherto existed, and other human activities that decrease biodiversity will grow at least in proportion to population, maybe more. Biodiversity is already decreasing and will continue to do so, though the amount of decrease may be affected by human choices about behavior.

Although the future is always inherently uncertain to some extent, scientists have estimated the likely range of extinctions that will be seen by 2050. According to studies published in 2003, about 3.5% of the world’s bird species will be extinct by that time, about 350species; mammals and freshwater fish are likely to experience a somewhat higher rate of extinction. Destruction of tropical forest for wood products or for conversion to agricultural land will be one of the primary causes of extinction. Continued overfishing of the world’s fresh and ocean waters will cause top predator species to become rare or extinct.

The five main drivers of biodiversity change between now and 2100, ranked from most severe impact to least severe, are, apart from climate change, 1) land-use changes (including deforestation), 2) climate change, 3) nitrogen deposition (from fertilizer use), 4) biotic exchange (the introduction of invasive species), and 5) direct effects of increasing atmospheric CO₂.

During this century and beyond, however, climate change is likely to play an increasing and perhaps eventually dominant role in reducing biodiversity. Most plants require a specific range of temperature, moisture, and seasonal change to thrive; most animals require certain plants or other animals to thrive, and also have a limited range of tolerance for temperature and moisture. As climate warms, a typical ecosystem will tend to migrate away from areas where it was at the warm edge of its tolerance range and toward places where it was formerly at the cool edge. The most general effect of global climate is thus to move ecological zones toward the poles and toward higher altitudes. For each 1.8°F (1°C) of warming, terrestrial (on-land) ecosystems typically shift pole-ward by 100 mi (160 km): For example, if climate warms

by 5.4°F (3°C) by 2100, plant and animal communities in the Northern Hemisphere will migrate an average of 300 mi (480 km) northward—if they can—to stay in a suitable climate zone. This effect is observed, not only predicted.

In the Northern Hemisphere, terrestrial animal and plant ranges have been observed to shift northward, on average, by 3.8 mi (6.1 km) per decade over the last 50 years. In mountainous terrain, plant and animal ranges have shifted upward by 20 ft (6.1 m) over the same time period. Fragmentation of landscapes by human activity such as agriculture and city-building makes ecosystem migration more difficult today than during past climatic shifts, such as glacial periods. Species that fail to colonize new areas as the climate changes may go extinct.

Although climate change has so far been most intense in the Arctic and the West Antarctic Peninsula, where warming has been about twice the global average and dramatic effects such as retreating sea ice and melting tundra are readily visible, biodiversity is low in these regions compared to the tropics, where rain forests, coral reefs, and other particularly diverse communities are found. Thus, a smaller climate shift can have a greater impact on biodiversity in the tropics than a larger shift in boreal or temperate regions.

For marine ecosystems, changes in circulation patterns, ocean temperature, and ocean chemistry all influence biodiversity. For example, over the last 40 years or so, warm-water plankton species have shifted about 620 mi (1,000 km) in the North Atlantic due to warming.

Not all observed climate effects on biological systems are consistent with climate warming; a few are consistent with cooling. Also, some observed effects are consistent with natural climate shifts rather than those attributed to human-caused (anthropogenic) global warming. However, mathematical analysis shows that the very great majority of changes are consistent with warming trends, and that a combination of natural and anthropogenic climate changes describe observed changes in physical and biological systems better than either natural or anthropogenic changes alone. Anthropogenic changes have been added to or laid over those caused by natural processes, and are gradually becoming more dominant.

Future Impacts

Irreversible loss of biodiversity in many parts of the world is likely in the near future and possibly beyond. However, many uncertainties remain. Ecologists do not understand the relationship between ecosystem structure and rapid climate change well enough to predict the exact effects of current climate changes on biomes. It is also unknown whether efforts to mitigate climate change and other human impacts on biodiversity will occur or succeed, and if so, to what extent. Parks, endangered-species laws, hunting bans, and similar conservation efforts have slowed the loss of biodiversity in some regions, especially for certain species of larger, more-easily-noticed animals.

Scientists have estimated the likely impact that climate change will have on biodiversity. In 2004, Chris Thomas and colleagues published their study of an unbiased or representative sample of 1,103 animal and plant species. They found that climate change was likely to commit 15–37% of all species examined to extinction by 2050. “Committed to extinction” does not mean that a species would necessarily be extinct by that time, but that the population of each species would be so reduced that its species’ extinction becomes highly likely. In many or most ecological regions, climate change will become the greatest threat to biodiversity by 2050. There are 5 to 15 million species of creatures on Earth (the large range arises from the difficulty of counting insect, bacterial, and fungal species). If only 15% of all species are committed to extinction by climate change—the lower end of the range given by Thomas and colleagues—then 750,000 to 2,250,000 million species will eventually become extinct as a result of global climate change.

Primary Source Connection

The following news article recognizes the United Nations’ involvement with the issue of climate change as gaining strength with international heads of government. The article acknowledges that recent U.N. meetings have made it hard for government officials to look away from the issue, provoking more countries to take the issue of global warming more seriously. Although obstacles such as convincing developing countries like India, China, and Brazil of the importance of decreasing CO₂ emissions do exist, on the whole, going green is developing into an international agenda.

U.N. REVS UP OVER GLOBAL WARMING

The annual summertime retreat of the Arctic icecap, greater this year than perhaps at any time during the 20th century. The nightmare of intensifying storms in some areas and extended drought in others, already taking place in developing countries of South Asia and sub-Saharan Africa.

It is against this backdrop of almost daily news of what scientists describe as signs of advancing global warming that the United Nations holds Monday what may be the largest high-level international meeting ever on climate change.

The conference, called by UN Secretary-General Ban Ki Moon, kicks off what many experts and officials say will be the high week of a turning-point year in the global political response to the challenge of a warming planet.

More than 80 heads of state or government are expected among the representatives of better than 150 countries attending the UN session. Then on Thursday, President Bush will convene at the White House a gathering of leaders from the world’s top emitters of greenhouse gases.

In addition, the Clinton Global Initiative will host a forum in New York Wednesday, drawing business and international political leaders to promote grass-roots responses to global warming.

Over the past weekend, at a UN conference in Montreal, the governments of about 200 countries agreed to accelerate a treaty to phase out hydrochlorofluorocarbons.

Together, the meetings put climate change at the center of the global stage this week—and they will make it harder for leaders to drop the issue in the future, experts say. That may be especially true of Mr. Bush: He may be known internationally as the foot-dragging leader of a top emitter of fossil-fuel pollutants, but by endorsing Mr. Ban’s meeting and then calling for his own at the White House, he will be seen as committing to a path of no return on climate-change action.

The Monday UN meeting “is looking to be quite an extraordinary event in what is turning out to be a remarkable year in the international response to climate change,” says Richard Kinley, deputy executive director of the UN Framework Convention on Climate Change.

Typically, such international sessions draw environmental ministers, who may or may not attend with the full-fledged backing of their government. This time, many heads of state are involved, with the meeting coinciding with the opening of the UN General Assembly. Such high-level involvement suggests that the world has turned a corner in perception of the seriousness of the climate-change challenge, Mr. Kinley says.

“Increasingly, leaders are seeing this as an issue of national interest, and not just a question of responsibility. And the fact is that states are more apt to act when they see their interests at stake,” he says.

Other factors this year, he says, include what many experts describe as a “conclusive” report from an international group of scientists and officials finding evidence of global warming to be “unequivocal.” The Intergovernmental Panel on Climate Change concluded with what it termed near certainty that the warming taking place is the result of human activities.

“That doesn’t mean we won’t hear some unorthodox views” at the meeting, Kinley says. Czech Republic President Vaclav Klaus, for example, intends to portray the idea of man-made global warming as a “myth” that does not justify constrictive measures.

Yet other experts cite factors beyond scientific findings in global warming’s rise to the international spotlight. David Sandalow, a specialist in the political dimensions of climate issues at the Brookings Institution in Washington, spoke at a forum earlier this year on the “new political climate” around climate change and listed factors such as: the business community’s “astounding” rallying to the issue, the Democrats’ taking control of Congress, increased Republican support for action—starting with California Gov. Arnold Schwarzenegger (R)—and even the documentary featuring Al Gore, “An Inconvenient Truth.”

What UN experts hope for—and what Ban wants coming out of Monday’s meeting—is fresh momentum for action at the UN climate-change conference set for Bali, Indonesia, in December. The Bali conference will take up the crucial question of what kind of agreement should replace the Kyoto Protocol on reducing greenhouse-gas emissions, which expires in 2012.

“What I want… is a strong political commitment at the leaders’ level for the meeting in Bali,” Ban told reporters last week. “There must be no vacuum” after expiration of the Kyoto accord, “so we must address the challenge boldly.” That means reaching a replacement agreement by 2009 so national governments can act and implementation can begin in 2012, he says.

None of this means that the debates on climate change are over, or that global accord on measures for action will be easy to reach.

Many eyes will be focused on this week’s White House meeting to gauge just how committed the United States is to action—and what kind of action the Bush administration will accept. Some experts worry that the administration will stick to a preference for voluntary goals in emissions cuts rather than reduction commitments, or that the Washington meeting signals a willingness on the part of the US to go outside the UN framework for addressing climate change.

Another potential stumbling block is the continuing disagreement between industrialized and developing countries over who should make the most substantial commitments to cutting greenhouse-gas emissions. It was the exclusion of developing countries from the Kyoto Protocol that in part led Bush to reject the agreement.

Major developing powers like China, India, and Brazil are still wary of any accord that would place the same burden on them as on developed economies, which they say have much higher per capita emission rates.

But these developing countries are also beginning to signal a seriousness about global warming and to address pollution and sustainable-development issues on their own, leading some experts to find hope that the divide can be narrowed. “Among a number of developing countries, we’re seeing a growing realization that they have to take a new approach for their own well-being. And the leading example of that is China,” says Kinley of the UN.

“That is not to say they have changed their position that the industrialized world should take the lead,” he adds. But he says a door once shut tight has now been opened.

“What we hear more now is talk of incentives, of the right investment and technology flows into the developing countries, and that is bringing the two sides closer together,” he says. “That’s crucial, because it’s clearly only through international cooperation that this challenge can be addressed.”

Howard LaFranchi

LAFRANCHI, HOWARD. “UN REVS UP OVER GLOBAL WARMING.” CHRISTIAN SCIENCE MONITOR (SEPTEMBER 24, 2007).

See Also Climate Change; Ecosystem Diversity; Endangered Species; Extinction and Extirpation; Genetic Diversity; Global Warming; Human Impacts

BIBLIOGRAPHY

Books

Parry, M. L., et al, eds. Climate Change 2007: Impacts, Adaptation and Vulnerability: Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.

Periodicals

Araúo, Miguel B., and Carsten Rahbek. “How Does Climate Change Affect Biodiversity?” Science 313 (2006): 1396–1397.

Higgins, Paul A. T. “Biodiversity Loss Under Existing Land Use and Climate Change: An Illustration Using Northern South America.” Global Ecology and Biogeography 16 (2007): 197–204.

Jenkins, Martin. “Prospects for Biodiversity.” Science 302 (2003): 1175–1177.

Livingstone, Daniel A. “Global Climate Change Strikes a Tropical Lake.” Science 301 (2003): 468-469.

Pounds, J. Alan, et al. “Widespread Amphibian Extinctions from Epidemic Disease Driven by Global Warming.” Nature 439 (2006): 161-167.

Sala, Enric, and Nancy Knowlton. “Global Marine Biodiversity Trends.” Annual Review of Energy and the Environment 231 (2006): 93-122.

Sala, Osvaldo E., et al. “Global Biodiversity Scenarios for the Year 2100.” Science 287 (2000): 1770–1774.

Thomas, Chris D. “Extinction Risk from Climate Change.” Nature 427 (2004): 145-148.

Willis, K. J., and H. J. B. Birks. “What Is Natural? The Need for a Long-Term Perspective in Biodiversity Conservation.” Science 314 (2006): 1261–1265.

Zimmer, Carl. “Predicting Oblivions: Are Existing Models Up to the Task?” Science 317 (2007): 892-893.

Web Sites

Intergovernmental Panel on Climate Change (United Nations). “Climate Change and Biodiversity.” April 2002. http://www.ipcc.ch/pdf/technical-papers/climate-changes-biodversity-en.pdf (accessed June 25,2008).

Larry Gilman

Biodiversity

Biodiversity is the total richness of biological variation. The scope of biodiversity is usually considered to range from the genetic variation of individual organisms within and among populations of a species , to different species occurring together in ecological communities. Some definitions of biodiversity also include the spatial patterns and temporal dynamics of populations and communities on the landscape. The geographical scales at which biodiversity can be considered range from local to regional, state or provincial, national, continental, and ultimately to global.

Biodiversity at all scales is severely threatened by human activities; this is one of the most important aspects of the global environmental crisis. Humans have already caused permanent losses of biodiversity through the extinction of many species and extensive losses of distinctive, natural ecosystems. Ecologists predict that unless there are substantial changes in the ways that humans affect ecosystems, there will be much larger losses of biodiversity in the near future.

Species richness of the biosphere

About 1.7 million of Earth's species have been identified and designated with a scientific name. About 6% of the identified species live in boreal or polar latitudes, 59% in the temperate zones, and the remaining 35% in the tropics. However, the knowledge of Earth's species is highly incomplete, especially for tropical countries. According to some estimates, there could be as many as 30-50 million species on Earth , 90% of them occurring in tropical ecosystems. Tropical ecosystems are much richer in species than are those at higher latitudes.

Most of the described species on Earth are invertebrates , particularly insects , and most of the insects are beetles (order Coleoptera). A famous scientist, J.B.S. Haldane, was once asked by a theologian to briefly explain what his knowledge of biology told of God's purpose. Haldane reputedly said that God has "an inordinate fondness of beetles," reflecting the fact these insects are so much richer in species than any other group of creatures on Earth. Some biologists believe that beetles account for most of the undescribed tropical insects.

The suggestion of enormous numbers of undescribed insects in tropical forests initially emerged from the research of Terry Erwin. This entomologist performed experiments in which tropical-forest canopies in Amazonia were treated with an insecticide, and the subsequent "rain" of dead arthropods was collected using ground-level sampling devices. This innovative sampling procedure indicated that: (1) a large fraction of the insect species of tropical forests is unknown to science; (2) most insect species are confined to a single type of tropical forest, or even to a particular tree species, which may itself have only a local distribution; and (3) most species of tropical-forest insects have little ability to disperse very far. Erwin's studies of tropical rainforest found that beetles accounted for most of the insect species and that most of the beetles are narrowly endemic , that is, they have a local distribution and are found nowhere else. For example, the tree Luehea seemanii has more than 1,100 species of beetle in its canopy, of which 15% are specific to that plant . The emerging conclusion from this and other descriptive research is that there is an enormous abundance of undescribed species of insects and other invertebrates living in tropical forests.

Compared with invertebrates, the numbers of species (that is, the species richness) of other groups of tropical-forest organisms are better known. Although it is extremely difficult to do so, the numbers of species of vascular plants have been described for a few tropical forests. For example: a plot of only 0.0004 sq mi (0.1 ha) in a moist forest in Ecuador had 365 species of vascular plants; there were 98 species of large trees in 0.006 sq mi (1.5 ha) of forest in Sarawak, Malaysia; there were 90 tree species in 0.0032 sq mi (0.8 ha) of forest in Papua New Guinea; 742 woody species occurred in 0.012 sq mi (3 ha) of forest in Sarawak, with 50% of the species recorded as single individuals; and more than 300 species of woody plants were discovered on a 2-sq mi (50-ha) area of forest in Panama. These tropical forests are much richer than temperate forests, stands of which typically support fewer than 12-15 species of trees. The Great Smokey Mountains of the eastern United States have some of the richest temperate forests in the world, and they typically contain 30-35 species, far fewer than tropical rainforests.

It is extraordinarily difficult to determine the numbers of birds in tropical forest because the dense foliage and darkness of the understory make it inconvenient to see small animals, even if they are brightly colored. As a result, few studies have been made of the birds of tropical rainforest. However, one study in Peru discovered 245 resident and 74 transient species in 0.4 sq mi (97 ha) of Amazonian forest. Another study of rainforest in French Guiana recorded 239 species of birds, and another found 151 species in a forest in Sumatra. In comparison, stands of temperate forest in North America typically support only 15-30 species of birds.

Almost no systematic surveys have been made of all of the species of tropical ecosystems. In one case, a 42 sq mi (108 sq km) reserve of dry forest in Costa Rica was estimated to support about 700 plant species, 400 vertebrate species, and 13,000 species of insects, including 3,140 species of moths and butterflies .

Why is biodiversity important?

Biodiversity is valuable for the following classes of reasons:

1. Intrinsic Value. Biodiversity has its own intrinsic value, regardless of its worth in terms of human needs. Because of its intrinsic merit, there are ethical considerations to any degradation of biodiversity. For example, do humans have the "right" to diminish or exterminate elements of biodiversity, all of which are unique and irretrievable? Is the human existence itself diminished by losses of biodiversity? Ethical issues cannot be resolved through science, but enlightened people would mourn any loss of species, or of natural ecosystems.
2. Utilitarian Value. Humans have an absolute requirement for the products of other species. Because of this need, wild and domesticated species and their communities are exploited in many ways to provide food, materials, energy , and other goods and services. This fact can be illustrated in many ways. In the United States, for example, about one-quarter of prescription drugs have active ingredients obtained from higher plants, and these uses contribute about $14 billion per year to the U.S. economy, and $40 billion per year worldwide. Potentially, harvests of biodiversity can be conducted in ways that foster their renewal. Unfortunately, potentially renewable biodiversity resources are often harvested too intensively or inadequate attention is paid to regeneration, so the resource is degraded or becomes extinct.
3. Provision of Ecological Services. Biodiversity provides many ecological services that are directly or indirectly important to human welfare. Examples of these services include biological productivity, nutrient cycling, cleaning water and air of pollutants, control of erosion , provision of atmospheric oxygen , removal of carbon dioxide , and other functions related to the integrity of ecosystems. According to the biologist Peter Raven: "Biodiversity keeps the planet habitable and ecosystems functional."

There are many cases of the discovery, through research on previously unexploited plants and animals, of bio-products useful to humans as food, medicine, or for other purposes. Consider the case of the rosy periwinkle (Catharantus roseus), a small plant native to the tropical island of Madagascar. During an extensive screening of wild plants for anti-cancer chemicals, an extract of rosy periwinkle was observed to inhibit the growth of cancerous cells. The active biochemicals are several alkaloids in foliage of the plant, which probably serve to deter herbivores. These natural substances are now used to prepare the drugs vincristine and vinblastine, which can be successfully used to treat childhood leukemia and a lymphatic cancer known as Hodgkin's disease . In this case, a species of wild plant known only to a few botanists has proven to be of great benefit to humans by treating previously incurable diseases, in the process sustaining a large pharmaceutical economy. There is a tremendous undiscovered wealth of other biological products useful to humans in unexplored biodiversity.

Biodiversity and extinction

Extinction refers to the loss of some species or other taxonomic unit (e.g., subspecies, genus, family, etc.; each is known as a taxon), occurring over all of its range on Earth. (Extirpation refers to a more local disappearance, with the taxon still surviving elsewhere.) The extinction of any species is an irrevocable loss of part of the biological richness of Earth, the only place in the universe known to support living creatures. Extinction can be a natural occurrence caused by an unpredictable catastrophe, chronic environmental stress, or ecological interactions such as competition , disease , or predation. However, there have been dramatic increases in extinction rates since humans have become Earth's dominant large animal and the cause of global environmental change.

Extinction has always occurred naturally. Almost all species that have ever lived on Earth have become extinct. Perhaps they could not cope with changes occurring in their environment, such as climate changes, the intensity of predation, or disease. Alternatively, many extinctions may have occurred simultaneously as a result of unpredictable catastrophes. From the geological record it is known that species, families, and even phyla have appeared and disappeared over time . For example, numerous phyla of invertebrates proliferated during an evolutionary radiation occurring at the beginning of the Cambrian era about 570 million years ago, but most of these animals are now extinct. The 15-20 extinct phyla from that period are known from the Burgess Shale of British Columbia, and they represent unique experiments in invertebrate form and function. Similarly, entire divisions of plants have appeared, radiated, and disappeared, such as the seed ferns Pteridospermales, the cycad-like Cycadeoidea, and woody plants known as Cordaites. Of the 12 orders within the class Reptilia, only three survive today: crocodilians, turtles , and snakes/lizards. Clearly, the fossil record displays a great deal of evidence of natural extinctions.

Overall, the geological record suggests that there have been long periods of time characterized by uniform rates of extinction, but punctuated by about nine catastrophic episodes of mass extinction . The most intense extinction event occurred at the end of the Permian period some 245 million years ago, when 54% of marine families, 84% of genera, and 96% of species are estimated to have become extinct.

Another famous, apparently synchronous, extinction of vertebrate animals occurred about 65 million years ago at the end of the Cretaceous period. The most renowned extinctions were of the last of the reptilian dinosaurs and pterosaurs, but many plants and invertebrates also became extinct at that time. In total, perhaps 76% of species and 47% of genera became extinct in the end-of-Cretaceous crisis. One hypothesis to explain the cause of this mass extinction involves a meteorite impacting the Earth, causing huge quantities of fine dust to be spewed into the atmosphere, and resulting in a climatic deterioration that most large animals could not tolerate. However, some scientists believe that the extinctions of the last dinosaurs were more gradual.

However, humans have been responsible for almost all of Earth's recent extinctions. These extinctions are occurring so quickly that they represent a modern mass extinction of similar intensity to those documented in the geological record. Examples of recent extinctions caused by humans include such well-known cases as the dodo, passenger pigeon, and great auk. Many other high-profile species have been taken to the brink of extinction, including the plains bison , whooping crane, ivory-billed woodpecker, right whale, and other marine mammals . These losses have been caused by insatiable overhunting and intense disturbance or conversion of natural habitats.

Beyond these well-known and tragic cases involving large animals, Earth's biodiversity is experiencing an even larger loss. This ruin is mostly being caused by extensive conversions of tropical ecosystems, particularly rainforest, into agricultural habitats that sustain few of the original species. As was described previously, tropical ecosystems sustain extremely large numbers of species, most of which have restricted distributions. The conversion of tropical forest into other habitats inevitably causes the loss of most of the locally endemic biota. This is a great tragedy, and the lost species will never occur again.

The most important human influences causing the extinction or endangerment of species are: (1) excessive exploitation, (2) effects of introduced predators, competitors, and diseases, and (3) habitat disturbance and conversion. These stressors can result in small and fragmented populations that experience the deleterious effects of inbreeding and population instability and then decline further, ultimately to extirpation or extinction.

The increased rate of extinction and endangerment of biodiversity during the past several centuries is best documented for vertebrates because, as noted previously, most invertebrate species, particularly insects, have not yet been described by scientists. During the last four centuries there have been more than 700 known extinctions globally, including about 100 species of mammals and 160 species of birds, all because of human influences.

A much larger number of species is facing imminent extinction; they are endangered. For example, more than 1,000 species of birds are considered to be threatened with extinction. Of this total, 46% live only on oceanic islands, a situation in which species are especially vulnerable to extinction caused by stresses associated with human activity. Birds of tropical forests account for 43% of the threatened bird species, wetland species for 21%, grassland and savanna species 19%, and other habitats 17%. Only 1.5% of the threatened species are North American, 4.2% are European or Russian, 33% Central and South American, 18% African, 30% Asian, and 14% from Australia and the Pacific.

Protection of endangered biodiversity

Biodiversity can be protected in ecological reserves. These are protected areas established for the conservation of natural values, usually the known habitat of endangered species , threatened ecosystems, or representative examples of widespread communities. In the early 1990s there were about 7,000 protected areas globally, with an area of 2.5 million sq mi (651 million ha.) Of this total, about 2,400 sites comprising 1.5 million sq mi (379 million ha) were fully protected and could be considered to be true ecological reserves.

Ideally, the design of a national system of ecological reserves would provide for the longer-term protection of all native species and their natural communities, including terrestrial, freshwater , and marine ecosystems. So far, however, no country has implemented a comprehensive system of ecological reserves to fully protect its natural biodiversity. Moreover, in many cases the existing reserves are relatively small and threatened by eenvironmental change and other stressors, such as illegal poaching of animals and plants and sometimes excessive tourism.

The World Conservation Union, World Resources Institute, and United Nations Environment Program are three important agencies whose mandates center on the conservation of the world's biodiversity. These agencies have developed the Global Biodiversity Strategy, an international program to help protect biodiversity. The broad objectives are to: (1) preserve biodiversity; (2) maintain Earth's ecological processes and life-support systems; and (3) ensure that biodiversity resources are used in a sustainable manner. As such, the Global Biodiversity Strategy is a mechanism by which countries and peoples can initiate meaningful actions to protect biodiversity for the benefit of present and future generations of people, and also for its intrinsic value. Because it only began in the late 1970s, it is too early to evaluate the success of this program. However, the existence of this comprehensive international effort is encouraging, as is the participation of most of Earth's countries, representing all stages of socioeconomic development.

Another important international effort is the Convention on Biological Diversity, negotiated under the auspices of the United Nations Environment Program and signed by many countries at a major conference at Rio De Janeiro in 1992. This international treaty requires signatory nations to take measures to systematically catalogue their indigenous biodiversity, and to take action to ensure that it is conserved.

Numerous other agencies are working to preserve biodiversity. In the United States, the World Wildlife Fund and Nature Conservancy are important organizations at the national level. There are numerous other national and local groups. The lead federal agency is the U.S. Fish and Wildlife Service, and all states have similar agencies.

Important progress is being made, and the progressive worldwide development of activities intended to identify, conserve, and preserve biodiversity will hopefully come to be regarded as an ecological "success story."

See also Biological community; Ecosystem.

Resources

books

Becher, A. Biodiversity: A Reference Handbook. Abo-Clio Pub, 1998.

Dobson, A.P. Conservation and Biodiversity. W.H. Freeman and Co., 1998.

Freedman, B. Environmental Ecology. 2nd edition. San Diego: Academic Press, 1994.

Gaston, K.J., and J.I. Spicer. Biodiversity: An Introduction. Blackwell Science Inc., 1998.

Hamblin, W.K., and E.H. Christiansen. Earth's Dynamic Systems. 9th ed. Upper Saddle River: Prentice Hall, 2001. Levin, Simon A., ed. Encyclopedia of Biodiversity. San Diego, CA: Academic Press, 2000.

Myers, Judith, and Dawn Bazely. Ecology and Control of Introduced Plants. Cambridge: Cambridge University Press, 2003.

Nebel, Bernard J., and Richard T. Wright. Environmental Science: Toward a Sustainable Future. 8th ed. Englewood Cliffs, NJ: Prentice Hall, 2002.

Schneiderman, Jill S. The Earth Around Us: Maintaining aLivable Planet. New York: W.H. Freeman & Co., 2000.

Wilson, E.O. The Diversity of Life. Cambridge: Harvard University Press, 1992.

Wilson, E.O., ed. BioDiversity. Washington, DC: National Academy Press, 1988.

periodicals

Caballero A., and M.A. Toro. "Interrelations Between Effective Population Size and Other Pedigree Tools for the Management of Conserved Populations." Genet Res 75(no. 3) (June 2000): 331–43.

Bill Freedman

KEY TERMS

Binomial

—The scientific name of organisms consists of two words, and is therefore a binomial. Humans, for example, are named Homo sapiens, which designates our genus as Homo, and our species within that genus as sapiens. Binomials are always latinized words, and are written in italics, or underlined.

Endangerment

—Refers to a situation in which a species is vulnerable to extinction or extirpation.

Endemic

—Refers to species with a relatively local distribution, sometimes occurring as small populations confined to a single place, such as an oceanic island. Endemic species are more vulnerable to extinction than are more widespread species.

Extinction

—The condition in which all members of a group of organisms have ceased to exist. Extirpation means that a species no longer occurs in a place or country, although it survives elsewhere.

Species diversity

—An indicator of biodiversity at the community level, which accommodates both the number of species present (i.e., richness) and their relative abundance. Ecologists consider species diversity to be a good indicator of biodiversity within communities, because it accommodates differences amongst species in rarity and commonness.

Species richness

—The number of species occurring in a community, a landscape, or some other defined area.

Biodiversity

Species richness of the biosphere

Why is biodiversity important?

Biodiversity and extinction

Protection of endangered biodiversity

Resources

Biodiversity is the total richness of biological variation. The scope of biodiversity is usually considered to range from the genetic variation of individual organisms within and among populations of a species, to different species occurring together in ecological communities. Biodiversity ranges from local to regional, state or provincial, national, continental, and ultimately to global.

Biodiversity at all scales is severely threatened by human activities; this is one of the most important aspects of the global environmental crisis. Humans have already caused permanent losses of biodiversity through the extinction of many species and extensive losses of distinctive, natural ecosystems. Ecologists predict that unless there are substantial changes in the ways that humans affect ecosystems, there will be much larger losses of biodiversity in the near future.

Species richness of the biosphere

About 1.7 million of Earth’s species have been identified and designated with a scientific name. About 5% of the identified species live in boreal or polar latitudes, about 60% in the temperate zones, and the remaining 35% in the tropics. However, the knowledge of Earth’s species is highly incomplete, especially for tropical countries. According to some estimates, there could be as many as 30-50 million species on Earth, with 90% of them occurring in tropical ecosystems. Tropical ecosystems are much richer in species than are those at higher latitudes.

Most of the described species on Earth are invertebrates, particularly insects, and most of the insects are beetles (order Coleoptera). Some biologists believe that beetles account for most of the undescribed tropical insects.

Studies conducted in tropical rainforest indicate that beetles account for most of the insect species and that most of the beetles have a local distribution and are found nowhere else. For example, the tree Luehea seemanii, has more than 1,100 species of beetle in its canopy, of which 15% are specific to that plant. The emerging conclusion from this and other descriptive research is that there is an enormous abundance of undescribed species of insects and other invertebrates living in tropical forests.

Compared with invertebrates, the numbers of species (that is, the species richness) of other groups of tropical-forest organisms are better known. Although it is extremely difficult to do so, the numbers of species of vascular plants have been described for a few tropical forests. For example: a plot of only 0.0004 sq mi (0.1 ha) in a moist forest in Ecuador had 365 species of vascular plants; there were 98 species of large trees in 0.006 sq mi (1.5 ha) of forest in Sarawak, Malaysia; there were 90 tree species in 0.0032 sq mi (0.8 ha) of forest in Papua New Guinea; 742 woody species occurred in 0.012 sq mi (3 ha) of forest in Sarawak, with 50% of the species recorded as single individuals; and more than 300 species of woody plants were discovered on a 2-sq mi (50-ha) area of forest in Panama. These tropical forests are much richer than temperate forests, stands of which typically support fewer than 12-15 species of trees. The Great Smokey Mountains of the eastern United States have some of the richest temperate forests in the world, and they typically contain 30-35 species, far fewer than tropical rainforests.

It is extraordinarily difficult to determine the numbers of birds in tropical forest because the dense foliage and darkness of the understory make it inconvenient to see small animals, even if they are brightly colored. As a result, few studies have been made of the birds of tropical rainforest. However, one study in Peru discovered 245 resident and 74 transient species in 0.4 sq mi (97 ha) of Amazonian forest. Another study of rainforest in French Guiana recorded 239 species of birds, and another found 151 species in a forest in Sumatra. In comparison, stands of temperate forest in North America typically support only 15-30 species of birds.

Almost no systematic surveys have been made of all of the species of tropical ecosystems. In one case, a 42 sq mi (108 sq km) reserve of dry forest in Costa Rica was estimated to support about 700 plant species, 400 vertebrate species, and 13,000 species of insects, including 3,140 species of moths and butterflies.

Why is biodiversity important?

Biodiversity is valuable for a number of reasons. Firstly, biodiversity has its own intrinsic value, regardless of its worth in terms of human needs. Because of its intrinsic merit, there are ethical considerations to any degradation of biodiversity. For example, do humans have the “right” to diminish or exterminate elements of biodiversity, all of which are unique and irretrievable? Is the human existence itself diminished by losses of biodiversity? Ethical issues cannot be resolved through science, but enlightened people would mourn any loss of species, or of natural ecosystems.

Secondly, humans have an absolute requirement for the products of other species. Because of this need, wild and domesticated species and their communities are exploited in many ways to provide food, materials, energy, and other goods and services. This fact can be illustrated in many ways. In the United States, for example, about one-quarter of prescription drugs have active ingredients obtained from higher plants, and these uses contribute about $14 billion per year to the U.S. economy, and $40 billion per year worldwide. Potentially, harvests of biodiversity can be conducted in ways that foster their renewal. Unfortunately, potentially renewable biodiversity resources are often harvested too intensively or inadequate attention is paid to regeneration, so the resource is degraded or becomes extinct.

Thirdly, biodiversity provides many ecological services that are directly or indirectly important to human welfare. Examples of these services include biological productivity, nutrient cycling, cleaning water and air of pollutants, control of erosion, provision of atmospheric oxygen, removal of carbon dioxide, and other functions related to the integrity of ecosystems.

There are many cases of the discovery, through research on previously unexploited plants and animals, of bio-products useful to humans as food, medicine, or for other purposes. Consider the case of the rosy periwinkle (Catharantus roseus ), a small plant native to the tropical island of Madagascar. During an extensive screening of wild plants for anti-cancer chemicals, an extract of rosy periwinkle was observed to inhibit the growth of cancerous cells. The active biochemicals are several alkaloids in foliage of the plant, which probably serve to deter herbivores. These natural substances are now used to prepare the drugs vincristine and vinblastine, which can be successfully used to treat childhood leukemia and a lymphatic cancer known as Hodgkin’s disease. In this case, a species of wild plant known only to a few botanists has proven to be of great benefit to humans by treating previously incurable diseases, in the process sustaining a large pharmaceutical economy. There is a tremendous undiscovered wealth of other biological products useful to humans in unexplored biodiversity.

Biodiversity and extinction

Extinction refers to the loss of some species or other taxonomic unit (e.g., subspecies, genus, family, etc.; each is known as a taxon), occurring over all of its range on Earth. The extinction of any species is an irrevocable loss of part of the biological richness of Earth, the only place in the universe known to support living creatures. Extinction can be a natural occurrence caused by an unpredictable catastrophe, chronic environmental stress, or ecological interactions such as competition, disease, or predation. However, there have been dramatic increases in extinction rates since humans have become Earth’s dominant large animal and the cause of global environmental change.

Extinction has always occurred naturally. Almost all species that have ever lived on Earth have become extinct. Perhaps they could not cope with changes occurring in their environment, such as climate changes, the intensity of predation, or disease. Alternatively, many extinctions may have occurred simultaneously as a result of unpredictable catastrophes. From the geological record it is known that species, families, and even phyla have appeared and disappeared over time. For example, numerous phyla of invertebrates proliferated during an evolutionary radiation occurring at the beginning of the Cambrian era about 570 million years ago, but most of these animals are now extinct. The 15-20 extinct phyla from that period are known from the Burgess Shale of British Columbia, and they represent unique experiments in invertebrate form and function. Similarly, entire divisions of plants have appeared, radiated, and disappeared, such as the seed ferns Pteridospermales, the cycad-like Cycadeoidea, and woody plants known as Cordaites. Of the 12 orders within the class Reptilia, only three survive today: crocodilians, turtles, and snakes/lizards. Clearly, the fossil record displays a great deal of evidence of natural extinctions.

Overall, the geological record suggests that there have been long periods of time characterized by uniform rates of extinction, but punctuated by about nine catastrophic episodes of mass extinction. The most intense extinction event occurred at the end of the Permian period some 245 million years ago, when 54% of marine families, 84% of genera, and 96% of species are estimated to have become extinct.

Another famous, apparently synchronous, extinction of vertebrate animals occurred about 65 million

KEY TERMS

Binomial— The scientific name of organisms consists of two words, and is therefore a binomial. Humans, for example, are named Homo sapiens, which designates our genus as Homo, and our species within that genus as sapiens. Binomials are always latinized words, and are written in italics, or underlined.

Endangerment— Refers to a situation in which a species is vulnerable to extinction or extirpation.

Endemic— Refers to species with a relatively local distribution, sometimes occurring as small populations confined to a single place, such as an oceanic island. Endemic species are more vulnerable to extinction than are more widespread species.

Extinction— The condition in which all members of a group of organisms have ceased to exist. Extirpation means that a species no longer occurs in a place or country, although it survives elsewhere.

Species diversity— An indicator of biodiversity at the community level, which accommodates both the number of species present (i.e., richness) and their relative abundance. Ecologists consider species diversity to be a good indicator of biodiversity within communities, because it accommodates differences amongst species in rarity and commonness.

Species richness— The number of species occurring in a community, a landscape, or some other defined area.

years ago at the end of the Cretaceous period. The most renowned extinctions were of the last of the reptilian dinosaurs and pterosaurs, but many plants and invertebrates also became extinct at that time. In total, perhaps 76% of species and 47% of genera became extinct in the end-of-Cretaceous crisis. One hypothesis to explain the cause of this mass extinction involves a meteorite impacting the Earth, causing huge quantities of fine dust to be spewed into the atmosphere, and resulting in a climatic deterioration that most large animals could not tolerate. However, some scientists believe that the extinctions of the last dinosaurs were more gradual.

However, humans have been responsible for almost all of Earth’s recent extinctions. These extinctions are occurring so quickly that they represent a modern mass extinction of similar intensity to those documented in the geological record. Examples of recent extinctions caused by humans include such well-known cases as the dodo, passenger pigeon, and great auk. Many other high-profile species have been taken to the brink of extinction, including the plains bison, whooping crane, right whale, and other marine mammals. These losses have been caused by insatiable overhunting and intense disturbance or conversion of natural habitats.

Beyond these well-known and tragic cases involving large animals, Earth’s biodiversity is experiencing an even larger loss. This ruin is mostly being caused by extensive conversions of tropical ecosystems, particularly rainforest, into agricultural habitats that sustain few of the original species. As was described previously, tropical ecosystems sustain extremely large numbers of species, most of which have restricted distributions. The conversion of tropical forest into other habitats inevitably causes the loss of most of the locally endemic biota. This is a great tragedy, and the lost species will never occur again.

The increased rate of extinction and endangerment of biodiversity during the past several centuries is best documented for vertebrates because, as noted previously, most invertebrate species, particularly insects, have not yet been described by scientists. During the last four centuries there have been more than 700 known extinctions globally, including about 100 species of mammals and 160 species of birds, all because of human influences.

Protection of endangered biodiversity

Biodiversity can be protected in ecological reserves, protected areas established for the conservation of natural values, usually the known habitat of endangered species, threatened ecosystems, or representative examples of widespread communities. The World Conservation Union, World Resources Institute, and United Nations Environment Program are three important agencies whose mandates center on the conservation of the world’s biodiversity. These agencies have developed the Global Biodiversity Strategy, an international program to help protect biodiversity. The broad objectives are to preserve biodiversity, maintain Earth’s ecological processes and life-support systems, and ensure that biodiversity resources are used in a sustainable manner. As such, the Global Biodiversity Strategy is a mechanism by which countries and peoples can initiate meaningful actions to protect biodiversity for the benefit of present and future generations of people, and also for its intrinsic value.

See also Biological community; Ecosystem.

Resources

BOOKS

Gaston, Kevin J. and John I. Spicer. Biodiversity: An Introduction. Boston: Blackwell Publishing,.

Hamblin, W.K., and Christiansen, E.H. Earth’s Dynamic Systems. 9th ed. Upper Saddle River: Prentice Hall, 2001.

Myers, Judith, and Dawn Bazely. Ecology and Control of Introduced Plants. Cambridge: Cambridge University Press, 2003.

Nebel, Bernard J., and Richard T. Wright. Environmental Science: Toward a Sustainable Future. 8th ed. Englewood Cliffs, NJ: Prentice Hall, 2002.

Novacek, Michael J. The Biodiversity Crisis: Losing What Counts. New York: The New Press, 2001.

Bill Freedman

Biodiversity

Biodiversity exists at three interrelated levels: species diversity, genetic diversity, and community-level diversity. When we talk about plant biodiversity, we refer to the full range of plant species, the genetic variation found within those species, and the biological communities formed by those species. For vascular plants, biodiversity includes all species of ferns, gymnosperms , flowering plants, and related smaller groups such as clubmosses and horsetails. The genetic variation found within populations and among populations arises through the mutation of individual genes or chromosomes and is rearranged by genetic recombination during the sexual process. Genetic variation is important not only for the survival and evolution of species; it is also important to people for breeding improved crop plants with higher yields.

Biological diversity also refers to all biological communities, including temperate forests, tropical forests, grasslands, shrub lands, deserts, freshwater wetlands, and marine habitats. Each of these biological communities represents an adaptation of plants to particular regimes of climate, soil, and other aspects of the environment. This adaptation involves ecosystem interactions of each biological community with its physical and chemical environment. For example, the ability of a forest community to absorb rain water and slowly release the water into streams and the ability of a swamp to process and detoxify polluted water are both aspects of ecosystem-level biological diversity that are of central importance to human societies.

Measuring Biodiversity

Biological diversity can be measured in various ways, each of which captures some of the overall meaning of biological diversity. The most common method of measuring biological diversity is simply to count the number of species occurring in one particular place, such as a forest or a grassland. Since it is not possible to count every species of plant, insect, fungus, and microorganism, the usual procedure is to count certain types of organisms, such as birds, butterflies, all flowering plants, or just tree species. This type of local diversity of species is usually referred to as species richness or alpha diversity. A tropical rain forest might contain three hundred or more tree species in a square of forest measuring 400 meters on a side, whereas a temperate forest of equal area might contain only forty tree species. Biological diversity can also be measured in larger areas. For example the country of Colombia has more than fifty thousand species of higher plants, in contrast to sixteen hundred species for the United Kingdom and nearly sixteen thousand for Australia. This type of regional or large-scale diversity is referred to as gamma diversity.

Another way to measure biodiversity is to consider the number or percentage of a region's species that are endemic to that region. For example, of the United Kingdom's fifteen-hundred native plant species, only sixteen species or 1 percent are endemic. The overwhelming majority of the United Kingdom's plants can be found in other neighboring countries, such as Ireland, France, and Germany. In contrast, 14,074 of the 15,000 plant species of Australia are endemic and found in no other country.

Individual species can also be compared for their evolutionary uniqueness. Species that are not closely related to other species are generally considered to have greater value to overall biological diversity than species that have many close relatives. For example, the maidenhair tree, Ginkgo biloba, is the only species in its genus, and Ginkgo is the only genus in the gymnosperm family Ginkgoaceae. In contrast, the common dandelion, Taraxacum officinale, has many related species in the same genus and is a member of a large family, the Asteraceae, with twenty-five thousand species and eleven hundred genera. Using this approach, a species that was the only member of its genus and family would have greater biodiversity value than a species that had many relatives in the same genus and belonged to a family with many genera. In contrast, a few biologists would argue that a species in a large genus has greater value because this species has the greater potential to undergo further evolution than a species with no close relatives that may be an evolutionary dead end.

Extent of Diversity

There are around 250,000 living species of higher plants in the world today. Of these species, the overwhelming majority are flowering plants. Flowering plants are grouped into more than three hundred families, including such large and economically important families as the Poaceae, which contains the grasses and the cereal crops we depend on for food and animal fodder; the Fabaceae, which includes the beans and peas we need for protein in our diet; and the Rosaceae, which is important for fruit trees and ornamentals such as apples, pears, and roses.

Higher plants exhibit a great diversity of growth forms, leaf shapes and sizes, flower and fruit types, seed types, and particular adaptations for growing in different environments. Growth forms include trees, shrubs, annual herbs, perennial herbs, climbers, and aquatic plants.

Plant diversity is not equally distributed across the world's surface. Only a relatively few land plants are adapted to salt water, and these are found rooted in shallow waters. So the large oceanic expanses of the world are devoid of higher plants. On the land surface, the greatest diversity of plants is found in the tropical lowland and montane rain forests of the Americas, central Africa, and Southeast Asia. In such forests there is a great diversity of plant species in the form of trees, shrub, herbs, and climbers. There is also an abundance of epiphytes , in particular orchids and bromeliads, that perch on the branches of the trees. Illustrating this tropical diversity, there are only around thirty tree species in all of northern Canada, in contrast to more than one thousand tree species in just the southern countries of Central America.

There is also great species diversity in the temperate regions of the world that have mild, wet winters and dry, hot summers, such as the Mediterranean basin, the California region, central Chile, the cape region of South Africa, and southwest Australia. In such areas, many plants have adaptations to drought, such as succulent cacti, which store water in their stems, and annual plants that grow, reproduce, and die in one growing season.

Certain regions of the world are known as hot spots of biodiversity because of their high concentrations of species overall, their high percentages of species that are endemic, and the high degree of threat that those species face. In addition to rain forest areas and localities with Mediterranean climates, many of these are islands, such as the Caribbean Islands, Madagascar and nearby islands, New Caledonia, New Zealand, Sri Lanka, and the islands between New Guinea and peninsular Malaysia. Biodiversity hot spots encompass the entire range of 44 percent of the world's plant species, 25 percent of the bird species, 30 percent of the mammal species, 38 percent of the reptile species, and 54 percent of the amphibian species on only 1.4 percent of Earth's total land surface. The premier hot spot is the tropical Andes, in which 45,000 plant species, 1,666 bird species, 414 mammal species, 479 reptile species, and 830 amphibian species occur in the tropical forests and high-altitude grasslands that occupy less than 0.25 percent of Earth's land surface. This approach can also be applied to individual countries. In the United States hot spots for endangered species occur in the Hawaiian Islands, the southern Appalachian Mountains, the arid Southwest, and the coastal areas of the lower forty-eight states, particularly California, Texas, and Florida.

Threats to Biological Diversity

Biological diversity is being lost today at all levels, including genetic variation, species, and biological communities. The most serious threat is the extinction of species, because once a species is lost, it can never be regained. The loss of genetic variation is occurring in two different ways: when populations of a species are eliminated and when populations become smaller in size. This loss of populations is seen most immediately in the local extinction of species. In a study of a conservation area in Massachusetts, one-third of the native plant species present one hundred years ago could no longer be found today. They were not replaced by other native species, but there was an increase in the number of nonnative species. This park is now poorer in total species, and many species still present have fewer populations. Many species that were formerly listed as common now have only a few individuals left.

Biological diversity is most severely threatened when entire biological communities are lost. In many tropical countries of the world, the tropical rain forests that are so rich in species have been largely destroyed. Examples of countries with devastated forests are Madagascar (87 percent lost), Rwanda (84 percent), Vietnam (83 percent), and the Philippines (94 percent). With the loss of these communities comes the extinction of plant and animal species, the loss of genetic variation within remaining species, and the loss of the ecosystem services provided by these communities, such as flood control, soil erosion protection, and the production of wood and food. Other habitats almost completely destroyed include tropical deciduous forests, of which more than 98 percent have been destroyed in Central America, and temperate grasslands, which are readily converted to agriculture and ranching. In the United States, only around 560 acres of the tallgrass prairie of Illinois and Indiana remains undisturbed, only about one ten-thousandth of the original area. Wetlands including swamps, bogs, floodplains, and vernal pools are similarly suffering devastation. As these habitats are damaged by human activity and converted to other uses, the species they contain decline in abundance and eventually become extinct. Habitats that are restricted in area and contain high concentrations of endemic species are particularly vulnerable, such as the rain forests of Hawaii and isolated mountain peaks in the southwestern United States.

In general, the rate of extinction for plants has been lower than that for animals. To date, there are recorded extinctions of around four hundred plant species, about 0.2 percent of the total in contrast with around 2.1 percent of mammals and 1.3 percent of birds already extinct. The lower percentages of plants that are extinct are related in part to our ability to protect small populations of plants in nature reserves. In contrast, many animals have a greater need to migrate and have often been extensively exploited. At the turn of the twenty-first century, around 9 percent of plant species are in danger of extinction, a figure only slightly lower than that for birds (11 percent) and mammals (11 percent). The extinction rates for certain groups of plants are much higher than this average value. For example, 32 percent of gymnosperms and 33 percent of palms are threatened with global extinction in the wild due to the limited distribution of many species' specialized habitat requirements and the intensive collection of plants for horticulture.

Factors Threatening Species

Species are threatened with extinction primarily because of habitat destruction. Species are also driven to extinction when their habitat is degraded to the point where they can no longer exist. This might happen when a grassland is heavily grazed by domestic animals, a forest is repeatedly logged, or uncontrolled fires burn shrub land. Fully 81 percent of the endangered species of the United States are threatened by habitat degradation and loss. Species are also lost from habitats fragmented by human activity, when habitats are broken up into smaller pieces by roads, fences, power lines, residential areas, and ranches. The remaining fragments may be so altered in micro-climate, and so much more vulnerable to other human activities, that many plant species are no longer able to survive.

The second most significant threat to species diversity is competition and predation from exotic invasive species, which is a threat for 57 percent of the endangered plant species of the United States. In many cases, exotic species of animals such as cattle, sheep, goats, rabbits, and pigs selectively remove certain native plant species. For example, pigs introduced in Hawaii have removed all wild individuals of numerous plant species. Invasive exotic plants have often overwhelmed natural communities and outcompeted the native species. For example, in bottomland communities of the southern United States, Japanese honeysuckle plants have replaced the rich wildflower communities, and in the rangeland of the western United States European grasses outcompete native grasses and wildflowers. As a result, native species decline at the expense of the introduced species.

Overharvesting of plants, often for food, medicinal purposes, or by horticulturists, threatens 10 percent of the endangered plant species of the United States. A notable example is ginseng, an herb used in Asian medicine, which has been so overharvested throughout its range that only a small number of plants remain. Many rare wildflowers, such as orchids, have been so severely overcollected by gardeners that they are in danger of extinction in the wild. Information on the location of the last remaining plants is often kept secret to prevent the theft of these individuals.

Pollution threatens 7 percent of the plant species of the United States. Water pollution can alter the water chemistry so severely that aquatic plants cannot grow. Increased inputs of nitrogen and phosphorus compounds into the water from sewage and agricultural fertilizers can result in algal blooms that shade out and kill native plants. In the land environment air pollution in the form of smog, acid rain, and nitrogen deposition can cause plants to slow down in growth or die. In some cases, this death may be related to the decline and death of the sensitive soil fungi (mycorrhizae) that have mutualistic relations with plants, providing water and mineral nutrients and receiving carbohydrates in return. And lastly, about 1 percent of plant species is threatened by disease and parasites. While this number may not seem very great, some of the most important woody plants in the forests of North America, such as chestnuts, elms, and dogwoods, are in severe decline due to introduced diseases.

What Can Be Done?

The most important way to protect plant biological diversity is to establish protected areas that include high concentrations of species, particularly those species in danger of extinction or in decline. These protected areas may be established by governments, conservation organizations, or private individuals. Management plans must be developed and implemented, and these protected areas must be monitored to ensure they are meeting their goals. Many management plans for protected areas include some forms of public education, because public support is often crucial for the success of a park.

Where it is not possible to maintain plant species in the wild due to ongoing threats, plants can often be grown in botanical gardens or kept as stored seed samples in seed banks. Networks of botanical gardens and seed banks are making a concerted effort to increase their holdings of endangered species and species of potential agricultural and economic importance. The goal of many botanical gardens is to increase knowledge of plants, to educate the public concerning plants, and to return plants eventually to their natural habitats.

The diversity of plant species provides us with the agriculture crops that are our food, many of the medicines that keep us healthy, wood that is needed in construction, fodder that feeds our domestic animals, ornamental plants that enrich our gardens and homes, and even the oxygen that we breathe. People could not live without the diversity of plants, and many plant species will live in the wild only if we take care of them.

see also Asteraceae; Botanical Gardens and Arboreta; Endangered Species; Ginkgo; Human Impacts; Invasive Species; Rain Forests; Seed Preservation; Wetlands.

Richard B. Primack

Bibliography

Falk, D. A, C. I. Millar, and M. Olwell, eds. Restoring Diversity; Strategies for the Reintroduction of Endangered Plants. Washington, DC: Island Press, 1996.

Mittermeier, R. A., N. Myers, P. R. Gil, and C. G. Mittermeier. Hotspots: Earth's Richest and Most Endangered Terrestrial Ecoregions. Mexico City: Agrupacion Sierra Madre, SC, 1999.

Primack, R. A Primer of Conservation Biology. Sunderland, MA: Sinauer Associates, 2000.

Quammen, D. The Song of the Dodo: Island Biogeography in an Age of Extinctions. New York: Scribner, 1996.

Schneider, S. Laboratory Earth: The Planetary Gamble We Can't Afford to Lose. New York: Basic Books, 1998.

Stein, B. A., and S. R. Flack. Species Report Card: The State of U.S. Plants and Animals. Arlington, VA: The Nature Conservancy, 1997.