Does greater species diversity lead to greater stability in ecosystems

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Does greater species diversity lead to greater stability in ecosystems?

Viewpoint: Yes, greater species diversity does lead to greater stability in ecosystems.

Viewpoint: No, ecosystem stability may provide a foundation upon which diversity can thrive, but increased species diversity does not confer ecosystem stability.

In 1970, Philip Handler, president of the United States National Academy of Sciences, said "the general problem of ecosystem analyses is, with the exception of sociological problems, … the most difficult problem ever posed by man." Despite increasingly complex and sophisticated approaches to the analysis of ecosystems over the past 30 years, many ambiguities remain.

Although ecology is often thought of as a twentieth-century science, ecological thought goes back to the ancients and was prominent in the writings of many eighteenth and nineteenth century naturalists. Indeed, the nineteenth-century German zoologist Ernst Haeckel who proposed the term "ecology" for the science dealing with "the household of nature." Until the twentieth century, however, ecology was largely a descriptive field dedicated to counting the number of individuals and species within a given area. Eventually, ecologists focused their attention on competitive relationships among species, predator-prey relationships, species diversity, the relative frequency of different species, niche selection and recognition, and energy flow through ecosystems. The German-born American evolutionary biologist Ernst Mayr called species the "real units of evolution," as well as the "basic unit of ecology." Understanding ecosystems, therefore, should include knowledge of their component species and their mutual interactions.

Natural ecosystems, whether aquatic or terrestrial, are made up of interdependent units produced by evolutionary processes under the influence of climate, geography, and their particular inorganic and organic constituents. Ecologists tend to focus their attention on species diversity, also known as biodiversity, within particular ecosystems, although ecosystems are always part of a larger continuum. The 1992 Convention on Biological Diversity defined biodiversity as "the variability among living organisms from all sources including … terrestrial, marine and other aquatic ecosystems and the ecological complexes of which they are part; this includes diversity within species, between species and of ecosystems."

The ancient belief in the "balance of nature" might be rephrased in more modern terminology as the belief that the more diverse a system is, the more stable it ought to be. The formal expression of this principle is generally attributed to the Princeton biologist Robert MacArthur in a classic paper published in the journal Ecology in 1955. MacArthur suggested that the stability of an ecosystem could be measured by analyzing the number of alternative pathways within the system through which energy could flow. He argued that if there were many species in a complex food web, predators could adjust to fluctuations in population by switching from less abundant to more abundant prey species. This would eventually allow the density of the less common species to increase. Based on his studies of the impact of invading plant and animal species on established ecosystems, the English biologist Charles Elton in The Ecology of Invasions by Animals and Plants (1958) argued in favor of what has been called the "diversity-stability hypothesis." According to Elton, evidence from mathematical models, laboratory experiments, and historical experience indicates that systems with few species were inherently unstable, and more susceptible to invading species.

During the last few decades of the twentieth century, conservationists often appealed to the diversity-stability hypothesis to underscore arguments for the importance of maintaining biological diversity. In his widely read book The Closing Circle (1971), the American ecologist Barry Commoner asserted that "The more complex the ecosystem, the more successfully it can resist a stress….Environmental pollution is often a sign that ecological links have been cut and that the ecosystem has been artificially simplified." Convinced that ecosystems with very limited numbers of species were unstable, some ecologists even considered such systems pathological. For example, some ecologists warned that monocultures should be seen as "outbreaks of apple trees and brussel sprouts," because by creating large areas composed of such outbreaks, agriculturalists inevitably prepared the way for "corresponding outbreaks of pests."

In the 1960s, some ecologists began to apply computer science to previously intractable biological problems such as ecosystem dynamics, predator-prey interactions, and other aspects of the emerging subfield of systems ecology. Mathematical modeling was made possible by the development of powerful computers, which were capable of analyzing the massive amounts of data characteristic of complex ecosystems. By the 1970s, critics of the diversity-stability hypothesis were arguing that mathematical models, computer programs, laboratory experiments, and observations in the field suggested that the hypothesis owed more to intuition and traditional assumptions than to rigorous evidence. As computer models evolved, system ecologists became increasingly focused on ecosystem processes such as energy transport and the carbon cycle, rather than the species that were members of the system. Critics of mathematical models argued that models are simply experimental tools, rather than products of nature, and that oversimplified models could unwittingly omit crucial variables.

Nevertheless, some ecologists believe that evidence from mathematical models and laboratory experiments disproved the hypothesis that linked species diversity and stability. Advocates of the diversity-stability hypothesis argued that the mathematical models designed to analyze simplified systems and experiments carried out on artificial communities did not reflect the complexity of natural ecosystems. Conservationists and environmental advocates generally emphasize the importance and values of species diversity and the need to study complex ecosystems composed of numerous species, including those that are inconspicuous and often overlooked by those constructing mathematical models. Conservationists generally thought of healthy natural communities in terms of species diversity, that is, differences in the number of species, their relative abundance, and their functional differentiation or ecological distinctiveness. If formal definitions of ecological diversity fail to incorporate all aspects of ecological and evolutionary distinctiveness, and experimental investigations fail to detect critical but cryptic relationships among diversity, stability, and ecosystem function, research results may be quite unrealistic.

In examining the stability-diversity debate, it often seems that advocates of particular positions are using incompatible definitions of stability and diversity. Many reports focus only on the number of species in the system, as if these species were completely interchangeable components, rather than on the abundance, distribution, or functional capacity of the various species. In a natural ecosystem, the number of species might not be as significant to the ecosystem's ability to respond to change or challenge as the structural and functional relationships among species. A natural community would probably be very different from others with the same numbers of species if one or a few members were changed. A model might, therefore, be exquisitely precise, but totally inaccurate.

Similarly, in many model systems and experiments, evidence about systems that are "more resistant" to disturbance may not be relevant to studies designed to determine whether another system is "more stable." Although one possible meaning of the term stability is constancy, ecologists generally would not consider this an appropriate definition because few natural systems are constant and unchanging. The term resiliency is often used to reflect the ability of a system to return to the state that existed before the changes induced by some disturbance. Mathematical models are often based on this concept, because it allows the analysis of deviations from some equilibrium state. Such models, however, often have little relationship to natural ecosystems. Some ecologists argue that natural communities never exist in the form of equilibrium state used in mathematical models.

Attempting to avoid the diversity-stability debate, some ecologists prefer to discuss ecosystems in terms of their "wholeness" or "biological integrity," that is, whether or not the ecosystems include the appropriate components and processes. Although some critics of this viewpoint argue that any and all components could be considered appropriate, others contend that species change is a valid criterion of biological integrity. Thus, the replacement of native species by invaders in various ecosystems could be considered a warning sign of danger to the biosphere. Critics of this latter viewpoint argue that although such concepts appear to be intuitively true, they are too vague and circular to serve as valid scientific theories of biological diversity.

Going beyond traditional ecological themes and debates about biodiversity, scientists predict that biocomplexity will emerge as an important research field in the twenty-first century. Biocomplexity has been described as the study of global ecosystems, living and inorganic, and how these ecosystems interact to affect the survival of ecosystems and species. Using the Human Genome Project (a worldwide effort to sequence the DNA in the human body) as a model, scientists have suggested the establishment of the All-Species Inventory Project. The goal of this project would be to catalog all life on Earth, perhaps as many as 10 to 130 million species. Since the time of the Greek philosopher Aristotle, whose writings referred to some 500 animals, scientists have identified about 1.8 million species. According to the American biologist Edward O. Wilson, the All-Species Inventory would be a "global diversity map" that would provide an "encyclopedia of life."

—LOIS N. MAGNER

Viewpoint: Yes, greater species diversity does lead to greater stability in ecosystems.

The concept of the balance of nature is an old and attractive one for which there is much evidence. Living things are always changing, so the communities of species in ecosystems are always subject to change. However, those who see stability as an important characteristic of healthy ecosystems focus on the fact that some level of stability is usually associated with a well-functioning ecosystem, that fluctuations occur within limits and that they are usually around some average, some balanced state. This is important to keep in mind in any discussion of stability in ecosystems: stability is never absolute.

The idea that the balance of nature is the norm and that wild fluctuations in populations are a sign of disruption in ecosystems comes from the work of many biologists, including that of the English biologist Charles Elton (1900-1991), one of the great ecologists of the twentieth century. Elton wrote about how foreign species, those that are not native to a particular area, can invade an ecosystem and throw it into imbalance. An example of this is the zebra mussel that has invaded lakes and rivers in the Midwestern United States, leading to the loss of many native species and the clogging of waterways. As a result, species diversity has been seriously affected and ecosystems reduced to a dangerously depleted state, where they are much more likely to be unstable.

Question of Definition

One problem in the debate over the relationship between species diversity (often called biodiversity) and stability is a question of definition. The general definition of stability is the resistance to change, deterioration, or depletion. The idea of resistance to change is related to the older concept of the balance of nature. Resistance to change also brings with it the concept of resilience, that is, being able to bounce back from some disturbance, and this meaning of stability is the one many ecologists focus on today. They ask: Is there a relationship between resilience and biodiversity? They accept the idea that stability is not the same as changelessness, and that an ecosystem is not unchanging, though it may appear to be so to casual human observation. A young person may be familiar with a forested area, and then revisit that area years later when it appears to be the same ecosystem, which has remained seemingly unchanged over a period of 30 or 40 years. But in reality many trees have died during that time, and others—perhaps belonging to different species—have grown up to replace them; there may even have been forest fires and tornado damage. What remains however, is a stable ecosystem in the sense that the later forest has about the same number of species as the earlier one, and about the same productivity in terms of biomass (living material such as new plant growth) produced each year. Ecologists would regard this ecosystem as stable. Many would argue that if the forest's biodiversity were compromised, if for example, all the trees were cut down and replaced by a plantation of trees of one species to be used for lumber, the forest as a whole would be much more unstable, that is, more susceptible to a disturbance such as the outbreak of an insect pest and much less able to rebound.

In the 1970s mathematical models of ecosystem processes seemed to show that biodiversity did not stabilize ecosystems, but that it had just the opposite effect—diverse ecosystems were more likely to behave chaotically, to display wild shifts in population size, for example. These mathematical models had a dramatic effect on the thinking of ecologists and brought the whole idea of the balance of nature into question. But it must be remembered that a model is a construction of the human mind. It may be intended to represent some part of the natural world, but it is a simplified, abstract, view of that world. Models are useful; they eliminate many of the "messiness" of real life and make it easier for the human mind to grasp complex systems. However, that simplification can be dangerous, because by simplifying a situation, some important factor may be eliminated, thus making the model of questionable value. Although there is some evidence that species diversity can at times increase the instability of an ecosystem, there is also a great deal of evidence against this.

The Benefits of Species Richness

Increasing species diversity leads to an increase in interactions between species, and many of these interactions have a positive effect on the ecosystem because they are mutually supportive. For example, a new plant species in a community may provide food for insect species, harbor fungi in its roots, and afford shade under which still another plant species may grow. Such interactions, though perhaps insignificant in themselves, can make the ecosystem as a whole more stable by preventing other plant or insect or fungal species from overgrowing.

If an ecosystem is species-rich, this means that most of its niches are filled. A niche is an ecological term meaning not only the place where an organism lives, but how it utilizes that place. For example, an insect that feeds on a single plant species has a very specific niche, while one that can survive by eating a variety of foliage has a broader niche. In general, only one species can occupy a particular niche, so two bird species may both live in the same area but eat different kinds of prey, one specializing in worms, for example, and another in beetles. If an ecosystem is species poor, this means that a number of niches are open and available to be filled by generalist species such as weeds or foreign invaders that may fill several niches at one time and overwhelm native species. In a species-rich ecosystem it is more difficult for such a takeover to occur, because invaders would have to compete with the present niche occupants. In other words, more balanced ecosystems are more likely to remain in balance. They are also more likely to recover successfully from environmental disruptions such as fires, storms, and floods.

Experimental Evidence

In the 1990s, several groups of researchers produced solid evidence that there is indeed a link between diversity and stability. Some of the most convincing information came from field experiments carried out by the ecologist David Tilman and his colleagues at the University of Minnesota. They created test plots in open fields; and added varying numbers of plant species to some of these plots. They found that the plots with the most species, that is, those that had greater diversity, were most resistant to the effects of drought, and also were most likely to have a growth rebound after the drought ended. In other words, the more diverse plots produced more biomass. A careful analysis of Tilman's results did reveal that rebound was also related to the particular species that were added, not just to the number of species; plants that were more productive, that grew faster, contributed more to the rebound. This analysis does not completely negate the basic finding about diversity, because the more species in an ecosystem, the greater the likelihood that among those species will be highly productive.

Another group of researchers who also explored the link between biodiversity and stability was led by Shahid Naeem of the University of Washington at Seattle. These researchers also took an experimental approach, but their work was carried out in the laboratory. In the 1980s, they built indoor chambers and showed that the chambers with more species tended to be more productive and more stable. Recently, the same researchers have produced similar results with microbial communities of algae, fungi, and bacteria. They found that an increase in the number of species leads to an increase in the predictability of growth. In another set of experiments, an increase in the number of species was related to a decrease in fluctuations in the production of CO² (carbon dioxide), which was used as a measure of microbial function. Both these studies on microbial communities indicate that an increase in the number of species at each trophic level (the function an organism performs in the ecosystem) was important to stability. So not only is the number of species in the ecosystem important, but it is also important that each trophic level—producer, consumer, and decomposer—has a variety of species represented. These studies are particularly important because in many ways they mimic the kinds of communities found in soil, an area of biodiversity which has lagged behind the study of communities above ground. There is also increasing evidence that biodiversity in the soil may also enrich biodiversity above ground. For example, soil fungi can enhance the uptake of nutrients by plants. These studies also show that while the population size of individual species may vary widely, the fluctuations can actually contribute to overall stability of the ecosystem. It may be that these population changes compensate for other changes within the ecosystem and thus enhance stability. Studies such as these, carried out on well-defined ecosystems, explore the link between diversity and stability. The advantage of microbial systems is that they can be assembled from many species and run for many generations within a reasonable period of time and at reasonable expense.

Critics of Tilman's and Naeem's work argue that their results often depend on the species chosen, in other words, the relationship of bio-diversity with productivity and stability is not true for just any grouping of species. However, this criticism points up the importance of diversity, of having a variety of organisms with many different growth and resource-use characteristics. It would be helpful to be able to perform field studies on the link between biodiversity and stability, rather than having to rely on the artificiality of experimental plots and chambers. Again the problem of complexity arises—natural ecosystems, especially those in tropical areas where biodiversity is likely to be greatest, are so filled with species and so rich in their interactions, that it is difficult to decide what to measure. Nevertheless, many observations in such ecosystems suggest that a depletion in species can lead to instability, with large increases in the populations of some species being more common. For example, invasion by foreign species is easier in disturbed ecosystems, where species have already been lost. This explains why agricultural areas are so susceptible to invasion. Other research has shown that invasion by non-native species is more likely to occur in less diverse ecosystems, at least on small plots.

It may be that diversity is particularly important in ecosystems that are structurally diverse such as layered rain forests, where essentially different ecosystems exist at distinct levels above ground. But diversity can be important even in simple ecosystems. One large-scale experiment in China showed that growing several varieties of rice together, rather than the usual practice of growing just one variety, prevented crop damage due to rice blast, a fungus that can seriously disrupt production. It appears that rice blast cannot spread as easily from plant to plant when several varieties are interspersed, so production is more stable. A rice field is obviously far from a natural ecosystem, but this is still one more piece of evidence for the diversity-stability link. The multi-variety approach also has the benefit of reducing the need for pesticides and thus slows further deterioration of the ecosystem. Benefits have also been found for another example of diversity in agriculture, the substitution of mixed perennial grasses for the traditional planting of one annual grass, such as wheat. One benefit of mixed perennials is that there is less opportunity for large numbers of one insect pest to decimate an entire crop. Again, stability comes with diversity, because greater numbers of species provide a buffer against disruption.

As with much scientific research, not all data support the diversity-stability link, but scientific results are rarely unanimous. Although the idea of the balance of nature may have been too simplistic, there is still validity to the idea that stability is beneficial, and the values of biodiversity are many. More species not only contribute to more stable ecosystems, but provide a source of chemicals that could be useful as drugs, help to detoxify noxious substances in the environment, and provide a rich source of positive aesthetic experiences. There is enough evidence for the diversity-stability link to make it a viable idea in ecology, and as David Tilman has said, data indicate that it would be foolish to lose diversity from ecosystems. Once that diversity vanishes, it is almost impossible to bring it back, especially because many of the species involved may have become extinct.

—MAURA C. FLANNERY

Viewpoint: No, ecosystem stability may provide a foundation upon which diversity can thrive, but increased species diversity does not confer ecosystem stability.

The hypothesis that greater species diversity begets heightened ecosystem stability may seem correct at first glance. Most people intuitively assume that the pond ecosystem has a better chance of thriving from year to year—even in adverse conditions—if it has a wider variety of species living there. That assumption, however, is supported by little scientific proof. On the other hand, many studies provide compelling evidence that diversity does not promote stability and may even be to its detriment. Several studies also suggest that if species diversity does exist, it is based on ecosystem stability rather than vice versa.

The Paramecium Studies of N. G. Hairston

One of the early experiments to critically damage the greater-diversity-equals-greater-stability argument came from the N. G. Hairston research group at the University of Michigan in 1968. In this study, the group created artificial communities of bacteria, Paramecia, and/or predatory protozoa grown on nutrient agar cultures. Each community contained more than one trophic level. In other words, the communities contained both predators and prey, as do the macroscopic food webs readily visible in a pond: A fish eats a frog that ingests an insect that attacks a tadpole that scrapes a dinner of bacterial scum from a plant stem. In Hairston's case, the researchers watched the combinations of organisms in a laboratory instead of a natural setting. Several patterns emerged.

In one series of experiments, the researchers combined prey bacteria, which represented the lowest link in the food chain—the first trophic level—with Paramecium. The bacteria included Aerobacter aerogenes, and "two unidentified bacilliform species isolated from a natural habitat." The Paramecium—two varieties of P. aurelia and one variety of P. caudatum—fed on the bacteria and so represented the second trophic level. As researchers increased the diversity of the bacteria, the Paramecia thrived and their numbers increased, at first suggesting that diversity caused stability. However, when the researchers looked more closely at the effects of increasing diversity on a specific trophic level, the story changed. They added a third Paramecium species to communities that already contained two species, and then watched what happened. The data showed that stability was based on which Paramecium species was introduced to which two pre-existing Paramecium species, and indicated that diversity in and of itself was not a requirement for stability. This set of experiments demonstrated that a higher number of species of one trophic level is unrelated to increased stability at that level.

Finally, Hairston reported the repercussions that followed the introduction of predatory protozoa—the third trophic level—to the experimental communities. The predatory species were Woodruffia metabolica and Didinium nasutum. Regardless of whether the community held two or three Paramecium species, or whether the predators numbered one species or two, all Paramecia quickly fell to the protozoa, whole systems failed, and stability plummeted. In this case, at least, diversity did not generate stability. Although the Hairston research is based on an artificial system rather than a natural one, it represents credible, empirical evidence against the assertion that greater diversity yields stability. Over the years, numerous research groups have conducted similar laboratory experiments with the same results.

May and Pimm's Conclusions about Stability

Not long after the Hairston paper was published, noted population biologist Robert M. May, formerly of Princeton and now at Oxford, devoted an entire book to the subject. First published in 1973, Stability and Complexity in Model Ecosystems provided detailed mathematical models illustrating the connection between diversity and instability in small systems, and argued that these models predict similar outcomes in larger systems. May wrote, "The central point remains that if we contrast simple few-species mathematical models with the analogously simple multi-species models, the latter are in general less stable than the former." He also noted that complexity in food webs does not confer stability within communities. A complex food web has many interacting individuals and species. The higher the number of connections in a food web, the greater the chance for individual links to become unstable and eventually affect the entire web.

May readily admitted that stable natural systems often are very complex and contain many species. However, he contended that the increased diversity is reliant on the system's stability, not the opposite. Complexity is not a prerequisite for stability; instead, stability is essential for complexity. In a separate paper, May used the example of a rain forest, a complex ecosystem with vast species diversity but also a high susceptibility to human disturbance.

The ecologist and evolutionary biologist Stuart Pimm, of the University of Tennessee, continued the debate in his book The Balance of Nature (1991). Pimm provided a historical view of the stability argument, along with discussions of many of the experiments conducted over the years, and arrived at several conclusions, one of which has direct bearing on the diversity-stability debate. If stability is defined as resilience, or the ability of a species to recover following some type of disturbance such as drought, flood, or species introduction, Pimm stated that shorter food chains are more stable than longer food chains. Simplicity, not complexity, imparts stability. He argued that resilience depends on how quickly all members of the food chain recover from the disturbance. Longer food chains involve more species, which present more opportunities for the delay of the restoration of the complete food chain. Pimm supported his argument with results from studies of aphids.

Pimm also noted that scientists have and will face problems when taking the stability-diversity question to the field. One problem is the absence of long-term data, which would help scientists to draw conclusions about grand-scale ecological questions such as the diversity-stability connection. Pimm explained that long-term scientific research projects typically require numerous consecutive grants to fund them, and such continuous chains of grants are few and far between.

Other Approaches

Another difficulty with field studies is finding existing systems that can be adequately compared. If ecosystem stability is defined as the capacity of its populations to persist through, or to show resilience following, some type of disturbance, scientists must identify ecosystems that have similar physical characteristics, and which are experiencing or have experienced a disturbance. To compare the effects of diversity, one ecosystem must have high species-richness and one must have low species-richness. In the early 1980s, Thomas Zaret of the Institute for Environmental Studies and University of Washington had that opportunity.

Zaret investigated the relationship between diversity and stability in freshwater fish communities in Africa and South America. First, he compared lakes and rivers. Lakes, Zaret reasoned, provide a more constant habitat than rivers. Rivers experience substantially more acute annual variation in water level, turbidity, current, and chemical content as a result of seasonal rains. Zaret then surveyed the two systems and found that the lakes contained more species than the rivers. Next, he followed the effects of a disturbance on both systems. The disturbance was a newly introduced predatory fish that had invaded a river and a lake in South America. The lake and river were similar in geographic location, and thus topography and climate, which provided an ideal opportunity for a comparison of each system's ability to rebound from a disturbance. Five years after the introduction of the predator, an examination of 17 common species that occurred in both water systems showed that 13 had disappeared from the lake, while all were still present in the river. Challenging the diversity-breeds-stability argument, Zaret's results indicated that the less-diverse river was more stable. He concluded, "The data presented from freshwater fish communities support the hypothesis that diverse communities have lower stability (resilience)."

Although these and other experiments indicate that diversity is not necessary for ecosystem stability, the discussion does not end there. A team of researchers from the University of Wisconsin-Madison determined that although diversity itself did not promote stability, the species-specific resilience of the community's residents might. Led by zoologist Anthony Ives, the team mathematically analyzed the consequences of environmental stress on various communities. After compiling the data, the team found that the characteristics of each species were more important than the number of species in conferring stability. The results showed that the most stable ecosystems—those that were both persistent and resilient—contained individual organisms that responded well to environmental stress. They did not show a correlation between stability and the sheer number of species in the ecosystem. The research team came to the conclusion that species richness alone does not generate ecosystem stability, and suggested that scientists should begin investigating the stress response of individual species rather than simply counting species.

The Intermediate-Disturbance Hypothesis

Several scientists took a different perspective in the discussion of diversity and stability, and developed what is known as the intermediate-disturbance hypothesis. This hypothesis states that the greatest species diversity appears not in the most stable systems, but in systems under periodic, nonextreme stress. In the most stable systems—defined here as those where disturbances are mild or absent—dominant species eventually outcompete their rivals, and the communities become less diverse. Diversity also declines in highly disturbed systems, because only those species that can reproduce and populate an area quickly thrive. The only areas that have high species diversity are those that experience infrequent, moderate disturbances. Joseph Connell of the Department of Ecology, Evolution, and Marine Biology at the University of California at Santa Barbara reinforced the hypothesis with his review of coral reefs and tropical forests. Connell plotted the level of disturbance against species richness and confirmed that ecosystems under infrequent, moderate stress have the greatest diversity. Specifically, he found the highest levels of diversity among reefs in the path of occasional hurricanes and tropical forests that take the brunt of infrequent storms. Seth R. Reice of the biology department at the University of North Carolina, Chapel Hill, similarly noted that habitats that experience natural disturbances, including storms and fire, are almost always more diverse than more stable areas. In both cases, Connell and Reice indicate that diversity depends on stability, rather than vice versa.

Another researcher, Wayne P. Sousa of the integrative biology department at the University of California, Berkeley provided validation to this principle with a study of the marine intertidal zone (at the ocean's edge). Sousa counted the number of sessile (attached) plant and animal species on rocks of various sizes. His reasoning was that waves can easily move small rocks, but not the largest rocks. The small rocks, then, are an unstable system for the sessile residents, the largest rocks are a stable system, and the medium-sized rocks fit the requirements of a system with intermediate disturbance. His results showed an average of 1.7 species on the smallest rocks, 2.5 on the largest, and 3.7 on the medium-sized rocks. To ensure that the species distribution was based on rock movement rather than rock size, he also artificially adhered some small rocks to the substrate (the ocean floor) and determined that species distribution was indeed based on wave-induced movement. This work upheld the intermediate-disturbance hypothesis, and illustrated that the greatest diversity was not associated with the most stable system.

Diversity Is No Prerequisite

As Daniel Goodman, of Montana State University, wrote in a 1975 examination of the stability-diversity controversy, there have been no experiments, field studies, or model systems that have proved a connection between greater diversity and stability. He added, "We conclude that there is no simple relationship between diversity and stability in ecological systems." Those words still hold today. In 1998 another group of scientists (Chapin, Sala, and Burke) reviewed much of the literature surrounding the connection between diversity and stability in their paper "Ecosystem Consequences of Changing Biodiversity," which appeared in the journal BioScience. They concluded that research that had inferred relationships between diversity and stability had relied on simple systems and may not translate well to the more complex systems common in nature. Although they noted that several studies imply a relationship between diversity and ecosystem stability, they added, "At present, too few experiments have been conducted to draw convincing generalizations."

In summary, none of the studies presented here proves beyond doubt that less species diversity produces a more stable natural ecosystem. However, the combination of studies does provide considerable evidence that greater diversity is not a requirement for ecosystem stability. Several of the studies also suggest that the stability of the system may be the driving factor in whether a community has high or low species diversity. Despite decades of research, the question of what makes a system stable remains largely unanswered.

—LESLIE MERTZ

KEY TERMS

BIODIVERSITY:

Range of organisms in an ecosystem.

BIOMASS:

Amount of living material (particularly plant material), by weight, produced in a given period of time.

COMMUNITY:

Group of living things residing together.

ECOSYSTEM:

All the physical and biological components of a particular ecological system. A garden ecosystem, for example, includes biological components such as plants and insects, as well as physical components such as soil.

ECOSYSTEM STABILITY:

Ability of an ecosystem to survive or bounce back from a disturbance. Disturbances can include anything from the introduction of a species to hurricanes.

FOOD CHAIN:

Arrangement of organisms such that each organism obtains its food from the preceding link in the chain. For example, a large fish eats a smaller fish, which feeds on aquatic vegetation.