Succession and Climax

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SUCCESSION AND CLIMAX

CONCEPT

Eventually almost everyone has the experience of watching an old neighborhood change. Sometimes we perceive that change for the better, sometimes for the worse, and the perception can have more to do with our individual desires or needs than it does with any qualities inherent in the change itself. For instance, one person might regard a new convenience store and gas station as an eyesore, while another might welcome it as a handy place to buy coffee, gasoline, or other items. Likewise, biological "neighborhoods" change, as when a complete or nearly complete community of living things replaces another. Once again, changes are not necessarily good or bad in any fundamental sense; rather, one community that happens to be better adapted to the changed environment replaces another. Sometimes a stress to the ecosystem brings about a change, such that life-forms that once were adapted to the local environment are no longer. Still, there appears to be a point when a community achieves near perfect adaptation to its environment, a stage in the levels of succession known as climax. This is the situation of old-growth forests, a fact that explains much about environmentalist opposition to logging in such situations.

HOW IT WORKS

Biogeography

Elsewhere in this book, there is considerable material about ecosystems, or communities of interdependent organisms along with the inorganic components of their environment, as well as about biological communities, or the living components of an ecosystem. There are also discussions of biomes, or large ecosystems, and food webs, or the means by which energy transfer takes place across a biological community. Related to many of these ideas, as well as to succession and climax, is the realm of biogeography, or the study of the geographic distribution of plants and animals, both today and over the course of biological history.

Biogeography, which emerged in the nineteenth century amid efforts to explore and map the planet fully, draws on many fields. Among the areas that overlap with this interdisciplinary realm of study are the biological sciences of botany and zoology, the combined biological and earth sciences of oceanography and paleontology, as well as the earth sciences of geology and climatology. Not only do these disciplines contribute ideas to the growing field of biogeography, but they also make use of ideas developed by biogeographers. Biogeography is concerned with questions regarding local and regional variations in kinds and numbers of species and individuals. Among the issues addressed by biogeography are the reasons why particular species exist in particular areas, the physical and biotic (life-related) factors that influence the geographic range over which a species proliferates, changes in distribution of species over time, and so on.

Species interact by three basic means: competition for resources, such as space, sunlight, water, or food (see Biological Communities for more about competition); predation, or preying, upon one another (see Food Webs); and symbiosis. An example of the latter form of interaction, discussed elsewhere (see Symbiosis), occurs when an insect pollinates a plant while the plant provides the insect with nourishment, for instance, in the form of nectar. These interactions can and do affect the geographic distribution of species, and the presence or absence of a particular life-form may serve as a powerful control on the range of another organism.

Other significant concepts in the realm of biogeography are dispersal (the spread of a species from one region to another), and barriers (environmental factors that act to block dispersal). A species may extend its geographic range by gradually colonizing, or taking over, adjacent areas, or it may cross a barrier (for instance, a mountain range, an ocean, or a desert) and colonize the lands beyond. Later, we briefly examine the case of a bird that managed to do both.

Succession

Succession is the progressive replacement of earlier biological communities with others over time. It entails a process of ecological change, whereby new biotic communities replace old ones, culminating in a stable ecological system known as a climax community. In a climax community, climate, soil, and the characteristics of the local biota (the sum of all plants and animals) are all suited to one another.

At the beginning of the succession process, a preexisting ecosystem undergoes some sort of disturbance—for example, a forest fire. This is followed by recovery, succession, and (assuming there are no further significant disturbances) climax. If the environment has not been modified previously by biological processes, meaning that succession takes place on a bare substrate, such as a sand dune or a dry riverbed, it is known as primary succession. Primary succession also occurs when a previous biological community has been obliterated. Secondary succession takes place on a substrate that has been home to other life-forms and usually in the wake of disturbances that have not been so sweeping that they prevented the local vegetation from regenerating.

THE FACILITATION MODEL.

Whether the conditions are those of primary or secondary succession, the outcome of the preceding disturbance is such that resources are now widely available, but there is little competition for them. One way of describing this situation is through what is known as the facilitation model, which identifies "pioneer species" as those life-forms most capable of establishing a presence on the site of the disturbance.

Pioneers modify a site by their presence, for instance, by regenerating the soil with organic material, thus making the area more attractive for invasion by other species. Eventually, new species move in, edging out the pioneers as they do so. This process may repeat itself several times, until the ecosystem reaches the climax stage, which we examine in greater depth a bit later in this essay. At the climax stage, there are few biological "openings" for further change, and change is only very slight and slow—at least until another disturbance arises and starts the process over again.

THE TOLERANCE MODEL.

The tolerance model is another possible mechanism of succession. According to this concept, all species involved in succession are equally capable of establishing themselves on a recently disturbed site, but those capable of attaining a large population size quickly are most likely to become dominant. Unlike the facilitation model, the tolerance model does not depict earlier inhabitants as preparing the site biologically for new invader species; rather, this model is more akin to natural selection, discussed elsewhere (see Evolution).

According to the tolerance model, some species will prove themselves more tolerant of biological stresses that occur within the environment as succession proceeds. Among these stresses is competition, and those species less tolerant of competition may succeed earlier on, when there is little competition for resources. Later in the succession process, however, such species will be eliminated in favor of others more capable of competing.

THE INHIBITION MODEL.

Yet another model of succession is the inhibition model, which, like the tolerance model, starts with the premise of an open situation at the outset: in other words, all species have equal opportunity to establish populations after a disturbance. In the inhibition model, however, some of the early species actually make the site less suitable for the development of other species. An example of this is when plants secrete toxins in the soil, thus inhibiting the establishment and growth of other species. Nevertheless, in time the inhibitory species die, thus creating opportunities that can be exploited by later successional species.

There is evidence to support all three models—facilitation, tolerance, and inhibition—but just as each has a great deal of basis in fact, none of the three fully depicts the dynamics of a successional environment. Put another way, each is fully right in one particular instance, but none are correct in all circumstances. Facilitation seems to work best for describing primary succession, whereas the more intense, vigorous environment of secondary succession is best pictured by tolerance or inhibition models. All of this shows that succession patterns tend to be idiosyncratic, owing to the many variables that determine their character.

Climax

When a biological community reaches a position of stability and is in equilibrium with environmental conditions, it is said to have reached a state of climax. Often such communities are described as old growth, and in these situations change takes place slowly. Dominant species in a climax community are those that are highly tolerant of the biological stresses that come with competition. And well they might be, since by the climax stage, resources have been allocated almost completely among the dominant life-forms.

Despite its slow rate of change, the climax community is not a perfectly static or unchanging one, because microsuccession (succession on the scale of a single tree, or a stand of trees) is always taking place. In fact, frequent enough events of disturbance within small sections of the biological community may prevent climax from even occurring. Once the biota does achieve a state of equilibrium with the environment, however, it is likely that change will slow down considerably, bringing an end to the stages of succession. Climax remains a somewhat theoretical notion, and in practice it may be difficult to identify a climax community.

REAL-LIFE APPLICATIONS

Colonization and Island Biogeography

Earlier, in the context of biogeography, there was a reference to animals "colonizing". This may sound like the behavior of humans only, but other animals are also capable of colonizing. Nor are humans the only creatures that crossed the Atlantic Ocean from the Old World to colonize parts of the New World. During the nineteenth century, an Old World bird species known as the cattle egret managed to cross the Atlantic, perhaps driven by a storm, and founded a breeding colony in Brazil. Since then, it has expanded the range of its habitats, so that cattle egret colonies can be found as far north and east as Ontario and as far south and west as southern Chile.

THE BIOGEOGRAPHY OF ISOLATED BIOMES.

Colonization is one example of the phenomena studied within the realm of biogeography. Other examples involve islands, and, indeed, island biogeography is a significant subdiscipline. The central idea of island biogeography, a discipline developed in 1967 by American biologists R. H. MacArthur (1930-1972) and Edward O. Wilson (1929-), is that for any landmass a certain number of species can coexist in a state of equilibrium. The larger the size of the landmass, the larger the number of species. Thus, reasonably enough, a large island should have a great number of species, whereas a small one should support only a few species.

These principles have helped in the study of other "island" ecosystems that are not necessarily on islands but rather in or on isolated lakes, mountain ranges surrounded by deserts, and patches of forest left behind by clear-cut logging. As a result of such investigations, loggers in the forests of the Pacific Northwest or the Amazon valley have been encouraged to leave behind larger stands of trees in closer proximity to one another.

This makes possible the survival of species at higher trophic levels (positions on the food web) and of those with very specialized requirements as to food or habitat. Examples of the latter species include Amazonian monkeys and the northern spotted owl, which we discuss later. Because studies in island biogeography have made land-use planners more aware of the barriers posed by clear-cut foresting, it has become common practice to establish "forest corridors." These long, thin lines of trees connecting sections of forest ensure that one section will not be isolated completely from another.

Succession in Action

In discussing succession earlier, it was noted that a disturbance usually sets the succession process in motion. Examples of such disturbances can include seismic events (earthquakes, tidal waves, or volcanic eruptions) and weather events (hurricanes or tornadoes). Across larger geologic timescales, the movement of glaciers or even of plates in Earth's crust (see Paleontology for more about plate tectonics and its effect on environments) can set succession processes in motion.

There are also causes directly within the biosphere, or the realm of all life, that can bring about disturbances. Among them are wildfires as well as sudden infestations of insects that act to defoliate, or remove the leaf cover from, a mature forest. Quite a few disturbances can result from activities on the part of the biosphere's most complex species: Homo sapiens. Humans can cause ecological disturbances by plowing up ground, by harvesting trees from forests, by bulldozing land for construction purposes—even by causing explosions on a military reservation or battlefield.

Disturbances can take place on a grand scale or a small scale. It is theoretically possible for disturbances—even man-made ones—to wipe out forests as large as the Ardennes in northwestern Europe or the Amazon rain forest in South America. Fortunately, neither shelling in the Ardennes during the world wars nor logging in the Amazon valley in the late twentieth century managed to destroy those biomes, but it is quite conceivable that they could have. On the other hand, a disturbance can affect an individual life-form, as when lightning strikes and kills a mature tree in a forest, creating a gap that will be filled through the growth of another tree—an example of microsuccession.

FORMS OF SUCCESSION.

Once succession begins, it can take one of several courses. It may lead to the restoration of the ecosystem in a form similar to that which it took before the disturbance. Or, depending on environmental circumstances, a very different ecosystem may develop. For example, suppose that a forest fire has wiped out a biological community and secondary succession has begun. It is conceivable that this succession process will restore the forest to something approaching its former state. On the other hand, the wildfire itself may well have been a signal of a climate change, in this case, to a drier, warmer environment. In this instance, succession may bring about a community quite different from that which preceded the disturbance.

The "disturbance" itself actually may be the alleviation of a long-term environmental stress that has plagued the community. Suppose that a biological community has suffered from a local source of pollution, for instance, from a factory dumping toxins into the water supply. Suppose, too, that pressure from state or federal authorities finally forces a cleanup. How does this affect the biotic environment? In all likelihood, species that are sensitive to pollution (i.e., ones that normally could not survive in polluted conditions) would invade the area.

Removal of an environmental stress may not always be a matter of pollution and cleanup. For instance, a herd of cattle may be overgrazing a pasture, thus holding back the growth of plant species in the area. Imagine, then, that the cattle are moved elsewhere; as a result, new plant species will proliferate in the area, and, in all likelihood, the biological diversity of that particular ecosystem will increase.

Primary Succession

As we noted earlier, primary succession occurs in an environment where there has never been a significant biological community or in the wake of disturbances that have been intense enough to wipe out all traces of a biological community. An example of primary succession in an area that has not possessed a biological community would be an abandoned paved parking lot. Eventually, the asphalt would give way to plant life, and given enough time, a wide-ranging biological community might develop around it.

These statements should be qualified in two ways, however. When it is said that an area has not maintained a significant biological community, this refers only to the recent or relatively recent past. In the case of the parking lot area, there probably have been countless biological communities in that spot over the ages, each replaced by the other in a process of primary succession. Also, by significant biological community we mean a biological community that exists above ground; even in the instance of the parking lot, there would be an extensive biological community underground. (See The Biosphere for more about life in the soil.)

GLACIER BAY.

Glacier Bay, in southern Alaska, is an example of an ecosystem that experienced primary succession in the wake of deglaciation, or the melting of a glacier. The glaciers there have been melting for at least the past few hundred years, and as this melting began to occur, plants moved in. The first were mosses and lichens, flowering plants such as the river-beauty (Epilobium latifolium ), and the mountain avens (Dryas octopetala ), noted for their ability to "fix" or transform nitrogen into forms usable by the soil. (See The Biosphere for more about nitrogen fixing and biogeochemical cycles.)

These were the pioneer species, and over time they were replaced by larger plants, such as a short version of the willow. Later, taller shrubs, such as the alder (also a nitrogen-fixing species), dominated the area for about half a century. In time, Sitka spruce (Picea sitchensis ), western hemlock (Tsuga heterophylla ), and mountain hemlock (T. mertensiana ) each had its turn as dominant plant species. With the last group, Glacier Bay reached climax, meaning that the dominant species are not those most tolerant of stresses associated with competition. The habitat thus has reached maturity, and access to resources is allocated as fully as it can be among the dominant species. Accompanying these changes have been changes in nonliving parts of the ecosystem as well, including the soil and its acidity.

Secondary Succession

When a disturbance has not been so intense or sweeping as to destroy all life within an ecosystem, regeneration may occur, bringing about secondary succession. But regeneration of existing species is not the only mechanism that makes secondary succession possible; invasions by new plant species typically augment the succession process. While much else changes in the environment of a secondary succession, the quality of the soil itself remains constant, as do other characteristics, such as climate.

Because it is rare for a disturbance to be powerful enough to obliterate all preexisting life-forms, secondary succession is much more common than primary succession. Examples of the type of disturbance that may serve as a precursor to secondary succession are windstorms, wild-fires, and defoliation brought about by insects—provided, of course, that the destruction caused by these phenomena is less than total. The same is true of most disturbances associated with human activities, such as the abandonment of agricultural lands and the harvesting of forests by cutting down trees for lumber or pulp.

In a forest of mixed species in the eastern United States, the dominant trees are a mixture of angiosperms and coniferous species (respectively, plants that reproduce by producing flowers and those that reproduce by producing cones bearing seeds), and there are plant species capable or surviving under the canopy, or "roof," provided by these trees. Suppose that the forest has been clear-cut. This means that most or all of the large trees have been removed, but the entire biological community has not been wiped out, since loggers typically would not bother to cut down smaller plants that are not in their way.

As soon as the clear-cutting is over, regeneration begins. One form that this takes is the formation of new sprouts from the stumps of the old angiosperms. These sprouts are likely to grow rapidly and then experience a process of self-thinning, in which only the hardiest shoots survive. Within half a century, a given tree will have only one to three mature stems growing from its stump.

At the same time, other species regenerate seemingly from nowhere, though actually they are growing from a "seed bank" buried in the forest floor, where trees have dropped countless seeds over the generations. Species such as the pin cherry (Prunus pennsylvanica ) and red raspberry (Rubus strigosus ) are particularly adept at regeneration in this form. Therefore, these species are likely to feature prominently in the forest during the first several decades of secondary succession.

On the other hand, some tree species simply do not survive clear-cutting, or at least not in large numbers; if they are to obtain a stake in the secondary succession, they must do so by a process of re-invasion. Such often happens in the case of coniferous trees. Other species may also invade when they have not previously been a part of the habitat, yet they enter now because the temporary conditions of resource availability and limited competition make the prospect for invasion attractive. A great number of species, from alders and white birch to various species of grasses, fit into this last category.

Plants are not the only organisms involved in secondary succession. In a mature forest of the type described, the dominant bird forms probably include species of warblers, vireos, thrushes, woodpeckers, and flycatchers. When clear-cutting occurs, however, these birds are likely to be replaced by an entirely new avian community—one composed of birds more suited to the immature habitat that follows a disturbance. As time passes, however, and the forest regenerates fully, the bird species of the mature forest re-invade and resume dominance, a process that may well take three to four decades.

Old-Growth Forests

Old-growth forests represent a climax ecosystem—one that has come to the end of its stages of succession. They are dominated by trees of advanced age (hence the name old-growth ), and the physical structure of these ecosystems is extraordinarily complex. In some places, the forest canopy is dense and layered, whereas in others it has gaps. Tree sizes vary enormously, and the forest is littered with the remains of dead trees.

An old-growth forest, by definition, takes a long time to develop. Not only must it have been free from human disturbance, but it also must have been spared various natural disturbances of the kind that we have mentioned, disturbances that bring about the conditions for succession. Typically, then, most old-growth forests are rain forests in tropical and temperate environments, where they are unlikely to suffer such stresses as drought and wildfire. Among North American old-growth forests are those of the United States Pacific Northwest as well as in adjoining regions of southwestern Canada.

THE SPOTTED OWL.

These old-growth forests of North America are home to a bird that became well known in the 1980s and 1990s to environmentalists and their critics: the northern spotted owl, or Strix occidentalis caurina. A nonmigratory bird, the spotted owl has a breeding pattern such that it requires large tracts of old-growth, moist-to-wet conifer forest as its habitat. These are the spotted owl's environmental requirements, but given the potential economic value of old-growth forests in the region, the situation was bound to generate heated controversy as the needs of the spotted owl clashed with those of local humans.

On the one hand, environmentalists insisted that the spotted owl's existence would be threatened by logging, and, on the other, representatives of the logging industry and the local community maintained that prevention of logging in the old-growth forests would cost jobs and livelihoods. The question was not an easy one, pitting the interests of the environment against those of ordinary human beings. By the early 1990s the federal government had stepped in on the side of the environmentalists, having recognized the spotted owl as a threatened species under the terms of the U.S. Endangered Species Act of 1973. Even so, controversy over the spotted owl—and over the proper role of environmental, economic, and political concerns in such situations—continues.

CONTINUING CONTROVERSY.

Another concern raised by the logging of old-growth forests has been the need to preserve dead trees, which provide a habitat for woodpeckers and other varieties of species. This concern, too, has brought about conflict with loggers, who find that dead wood gets in the way of their work. Dead wood, after all, is an expression for something or someone that is not performing a useful function (as in, "We're removing all the dead wood from the team"), and to loggers this literal dead wood is nothing more than a nuisance.

Unfortunately, the United States logging industry typically has not pursued a strategy of attempting to manage old-growth forests as a renewable resource, which these forests could be, given enough time. Instead, logging companies—interested in immediate profits and not much else—have tended to treat old-growth forests as though they were more like coal mines, home of a nonrenewable resource. In this "mining" model of tree harvesting, the forest is allowed to experience a process of succession such that a younger, second-growth forest emerges. Over time, this might become an old-growth forest, but the need to turn a quick profit means that the forest likely will be cut down before that time comes.

The average citizen, who typically has no vested interest in the side of either the loggers or the environmentalists, might well find good and bad on both sides of the issue. Certainly, the image of radical environmentalists chaining themselves to trees is as distasteful as the idea of loggers removing valuable natural resources. There is also a class dimension to the struggle, since a person deeply concerned about environmental issues is probably someone from an economic level above mere survival. This results in another distasteful image: of upper-middle-class and upper-class environmentalists inhibiting the livelihood of working-class loggers.

On the other hand, as we have already suggested, the logging companies themselves are big business and hardly representative of the working class. Largely as a result of pressure from environmentalists, these companies have attempted to develop more environmentally responsible logging schemes under the framework of what is called new forestry. These practices involve leaving a forest largely intact and removing only certain trees. Many environmentalists contend, however, that even the new forestry disturbs the essential character of old-growth forests.

WHERE TO LEARN MORE

Browne, E. J. The Secular Ark: Studies in the History of Biogeography. New Haven: Yale University Press, 1983.

Cox, C. Barry, and Peter D. Moore. Biogeography: An Ecological and Evolutionary Approach. Malden, MA: Blackwell Science, 2000.

The Eastern Old Growth Clearinghouse (Web site). <http://www.old-growth.org/>.

Environmental Biology—Grasslands (Web site). <http://www.marietta.edu/~biol/102/grasslnd.html>.

Forestry: Ecosystems: Forest Succession. Saskatchewan Interactive (Web site). <http://interactive.usask.ca/skinteractive/modules/forestry/ecosystems/forest_succession.html>.

Introduction to Biogeography and Ecology: Plant Succession. Fundamentals of Physical Geography <http://www.geog.ouc.bc.ca/physgeog/contents/9i.html>.

Old-Growth Forests in the United States Pacific Northwest (Web site). <http://www.wri.org/biodiv/b011-btl.html>.

Reed, Willow. Succession: From Field to Forest. Hillside, NJ: Enslow Publishers, 1991.

Succession (Web site). <http://www.cpluhna.nau.edu/Biota/succession.htm>.

KEY TERMS

ABUNDANCE:

A measure of the degree to which an ecosystem possesses large numbers of particular species. An abundant ecosystem may or may not have a wide array of different species. Compare with complexity.

ANGIOSPERM:

A type of plant that produces flowers during sexual reproduction.

BIOGEOGRAPHY:

The study of the geographic distribution of plants and animals, both today and over the course of extended periods.

BIOLOGICAL COMMUNITY:

The living components of an ecosystem.

BIOME:

A large ecosystem, characterized by its dominant life-forms.

BIOSPHERE:

A combination of all living things on Earth—plants, animals, birds, marine life, insects, viruses, single-cell organisms, and so on—as well as all formerly living things that have not yet decomposed.

BIOTA:

A combination of all flora and fauna (plant and animal life, respectively) in a region.

BIOTIC:

Life-related.

CANOPY:

The upper portion or layer of the trees in a forest. A forest with a closed canopy is one so dense with vegetation that the sky is not visible from the ground.

CLIMATE:

The pattern of weather conditions in a particular region over an extended period. Compare with weather.

CLIMAX:

A theoretical notion intended to describe a biological community that has reached a stable point as a result of ongoing succession. In such a situation, the community is at equilibrium with environmental conditions, and conditions are stable, such that the biota experiences little change thereafter.

COMPETITION:

Interaction between organisms of the same or different species brought about by their need for a common resource that is available in quantities insufficient to meet the biological demand.

COMPLEXITY:

The range of ecological niches within a biological community. The degree of complexity is the number of different species that could exist, in theory, in a given biota, as opposed to its diversity, or the actual range of existing species.

CONIFER:

A type of tree that produces cones bearing seeds.

DIVERSITY:

A measure of the number of different species within a biological community.

ECOSYSTEM:

A community of interdependent organisms along with the inorganic components of their environment.

FOOD WEB:

A term describing the interaction of plants, herbivores, carnivores, omnivores, decomposers, and detritivores in an ecosystem. Each of these organisms consumes nutrients and passes it along to other organisms. Earth scientists typically prefer this name to food chain, an everyday term for a similar phenomenon. A food chain is a series of singular organisms in which each plant or animal depends on the organism that precedes it. Food chains rarely exist in nature.

FOREST:

In general terms, a forest is simply any ecosystem dominated by tree-size woody plants. Numerous other characteristics and parameters (for example, weather, altitude, and dominant species) further define types of forests, such as tropical rain forests.

MICROSUCCESSION:

Succession on a very small scale within a larger ecosystem or biological community. Microsuccession can occur at the level of a stand of trees or even a single tree.

NATURAL SELECTION:

The process whereby some organisms thrive and others perish, depending on their degree of adaptation to a particular environment.

NICHE:

A term referring to the role that a particular organism plays within its biological community.

OLD-GROWTH:

An adjective for a climax community.

SUCCESSION:

The progressive replacement of earlier biological communities with others over time. Succession, which can culminate in a climax community (see climax ), is either primary, which occurs where there is no preexisting biological community (or no such community has survived), or secondary, in which a biological community regenerates in the wake of a disturbance, such as a forest fire.