Finally, tolerance describes the invasion of a new habitat by one species independent of other species (Goldsmith, 1985). This type of mechanism can be seen in the bivalve mollusk Abra ovata in Sulak Bay of the Caspian Sea. When the Sulak Bay flooded, this species invaded the new waters, and quickly became dominant. However, the species did not inhibit the growth of any other species, despite its consistent dominant presence, nor has its dominance altered due to an influx of other species (Latypov, 2004).

Climax Community

As mentioned, the climax community can be thought of as the point at which a community stops developing and stabilizes. In other words, the climax is reached when the new species, or exclusion of another species, no longer causes alterations to the community, and the growth cycle is in equilibrium with the environment. The conditions that cause this climax community, as mentioned, often involve facilitation, inhibition and tolerance (Goldsmith, 1985).

However, there are other issues involved, which, at the point of climax, prevent the community from a continuation of development. During succession, the ratio of productivity to biomass decreases, which causes the accumulation of biomass to stop. This means that a larger number of nutrients are available in organic materials, and thus, detrital food webs overpower those of grazing species (Ricklefs, 2001). At this point, stability is reached, in that the growth rate of one species in directly connected with those of other species. Without the introduction of another disruption, the production levels stabilize, and no further alteration is possible.

This concept of the end to succession can be noted in the case of the Beroe ovata jellyfish, mentioned above. When the Beroe ovata was introduced, the levels of other species in the area was in a state of flux that was not in equilibrium with the ecosystem. The comb jellyfish was overpopulated, and thus, was causing a reduction in zooplankton and phytoplankton, which would eventually cause the demise of the entire ecosystem. With the introduction of the Beroe, however, the community dynamics were changed in such a way as to equalize the effects of the comb. As the Beroe increased in number, the comb decreased. Eventually, this balance of growth of the Beroe was in equilibrium with the available food source, that of the comb. If the comb jellyfish were to multiply in number, the Beroe would also multiply in number. The same is true for a decrease in the population of the comb. As these species found equilibrium, the zooplankton and phytoplankton also reached equilibrium. The end result, then, is that the balance of comb jellyfish to the Beroe equalizes the balance between the zooplankton and the phytoplankton, which equalizes the oxygen levels of the southern Sea. So long as no new introduction of species or environmental condition occurs, this relationship will remain stable (Jeffress and Steimle, 1990).

Biodiversity

The diversity of organisms in any community is determined by a number of different factors. First, the physical conditions of a given community have a vast impact on the biodiversity of that community (Ricklefs, 2001). In the Caspian Sea, for example, fewer land mammals exist than in areas such as the plains of Africa, because the physical environment of the Caspian Sea is more habitable to water animals, since it is an aquatic environment. Secondly, the heterogeneity of habitats is important, in that, to cooperatively coexist, a community must require different elements to survive (Ricklefs, 2001). Areas with more diverse habitats are able to sustain a community, since all parts of the community then contribute to the overall sustenance of that area. Third, a community's isolation from a center point also influences diversity, in that the further one moves from the center of a given area, the fewer species one will find (Ricklefs, 2001). This is generally due to a small number of migratory animals.

Another factor that affects diversity is vegetation (Ricklefs, 2001). If there is little vegetation in a given community, there will be far fewer species of animals. Many animals, particularly those at the bottom of the food web, require plant life to survive. Without plant life, these base forms of species are not able to survive. As a result, the species of animals that feed on them are also not able to survive. In the Caspian, for example, the lack of phytoplankton would equal a lack of zooplankton, which would affect the entire food web (Jeffress and Steimle, 1990). Additionally, without plant life, oxygen levels are depleted,...

Thus, as more plant life is available, more species of other animals are able to survive.
In addition, non-physical elements also contribute to community biodiversity. For example, competition has a profound effect on diversity. Intense competition among species will eventually exclude certain species. If competition for a single, nonrenewable resource is too competitive, only the strongest species will survive, thus affecting diversity. Further, predation influences diversity in a similar way. As predation increases, competition, like that mentioned above, should decrease, resulting in a more diverse community (Ricklefs, 2001). Space in communities is limited, and in order for diversity to exist, each niche within a community must fit well into the ecosystem, based on the species within each community and the overlap between the species.

Still another factor in diversity is that of equilibrium of species. This concept revolves around the idea that the highest diversity is found in areas where processes that add or subtract species in a community are balanced. If, through the addition of new species, migration, predation, and luck, there is a balance between the loss and gain of species, then diversity within that community will be assured (Ricklefs, 2001).

The Nature of a Community

After examining the concepts above, it is easy to see why the idea of "community" is so difficult to define. With extreme viewpoints of distinct units and open communities, combined with the various degrees of biodiversity, communities are not simple ideas. However, by combining all theories, it is possible to develop an idea of the nature of a community.

In the simplest terms, a community can be thought of then as a cohabitation of species brought about from succession, which has resulted in a climax community of diverse species, each interrelated to one another through predation, energy production, and spatial living conditions. As disruptions to this equilibrium occur, each niche within each community responds with adaptation to the new structure. Once each species in the community is again in equilibrium, the community again reaches a climax community, where all sides are balanced to best support all diverse species within that spatial area.

With this idea of the nature of communities in mind, it is simple to see how this balanced idea truly does define a community. For example, in the Caspian Sea, there is currently a climax community, beginning with the Beroe ovata jellyfish. However, as noted previously, this was not always the case. The comb jellyfish had previously threatened the existing complex community by threatening the balance of energy input and output, as well as through inhibition of other species. As is the case in succession, once the Beroe was introduced, a sere began, first with the massive reproduction of the Beroe. As the Beroe population expanded, the comb population decreased, allowing the other members of the community to return to their previous positions, thus restoring the balance, and recreating a climax community.

In light of this ability to recreate a climax community, it is easy to imagine how the same community could, in some cases, re-establish following a different sort of disaster. For example, in the case of a flood in the southern Caspian Sea, new species from areas around the southern Sea would be introduced. Following the theory of equilibrium, these new species either would fit into the climax community, into a specific niche through facilitation or tolerance, or would conceivably inhibit other species from developing or re-developing, causing a change in the sere of succession. If this new species facilitated other species in the area, the biodiversity of the community would increase. If the new species were tolerated, no change would be seen. However, if the new species were to inhibit other species, a sere would develop that would lead to the death or replacement of the species.

Based on historical evidence of previous catastrophes, this sere would eventually lead to a redistribution of energy production, and the sere would lead the community to a climax, where all species in the area were actively interfacing with one another to provide the best solution for all species in the community (Ricklefs, 2001). In this case, the new species in the sea would either provide an alternative food source for another member of the community, which would increase diversity due to a more complex food web, or would become a predator, diversifying…