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 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.
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 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.
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
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.