Ecological community concepts in the Caspian Sea

Ecology/Caspian Sea

Community Concepts

The concept of community is one that has defied definition for centuries, even among individuals in the field of ecology. While all agree that a community involves a group of species together in the same area, and that those species must co-exist and interact competitively for resources, the actual definition of a community concept is varied (Allen, 1998). However, there are two viewpoints that appear most often in today's concept of community, and this section will discuss those viewpoints individually.

The first concept of community is that of the "discrete unit" by Frederic Clements. According to this concept, a community is like a superorganism, where the species of the area are interconnected like parts of a body (Ricklefs, 2001). Each species in the community has co-evolved, or evolved along with one another in such ways as to enhance the independent functioning of one another. As this holistic view describes, these independent species are understood in the sense of community only by the contributions each species makes to the dynamics of the entire system (Allen, 1998). It is precisely these contributions, made by independent species in an interactive way through co-evolution, that allow communities to develop.

Clements' "discrete unit" concept of community describes a closed community. In other words, each discrete unit or community is distinct to its self, and has very specific boundaries, or ecotones. Distributions of species within the community are coincident, rather than independent (Allen, 1998). It is only through succession, or the orderly sequence of changes in communities, that a stable community is possible (Allen, 1998).

The second concept of community is derived from Henry Gleason's idea of the loose assembly of species (Ricklefs, 2001). This concept revolves around the idea that a community is an association of species whose requirements and adaptations to the area allow them to live with one another under specific conditions. Under this idea, the species that cohabitate do so by coincidence, rather than co-adaptation (Allen, 1998). Gleason's concept is one of an open community, as well, where no community has boundaries.

Under Gleason's community concept, a community's structure and functions are only expressions of the interactions between the species, not due to an organization of those species. In this open community, then, distributions of species are independent of one another, and without definition (Allen, 1998). They do not exist out of necessity, as in the "distinctive unit" concept, but rather out of coincidence.

Regardless of which concept is assumed, there are some agreed upon ideas about biological communities. For example, in any community, closed or open, organisms coexist with one another, and are linked to one another by their interactions, giving any concept of community both a special and a functional definition. Further, any ecological effect or evolutionary effect on the community affects all species, since all species interact with one another.

In the Caspian Sea, there are many spatial areas that could be considered ecological communities. In particular, the area of the southern Caspian is home to a particularly rare community of jellyfish, and this community will be examined throughout the rest of this analysis. Within this community, the Beroe ovata jellyfish has been introduced to combat the comb jellyfish. The comb, in turn, relies on the zooplankton in the area, which relies on the phytoplankton for food. All creatures in this community rely on the oxygen produced in the water by these members of the community (Jeffress and Steimle, 1990).

Food Webs

Generally speaking, the idea of a food web relates to the concept of who eats whom within any given ecosystem, or to the flow of energy in an ecosystem (Allen, 1998). A true food web outlines the feeding relationships between all species in a given ecosystem, and thus can be far more representative than a standard food pyramid, but also much more complex. While this certainly leads to the potential of showing diversity and differentiation within a community structure, the lack of a definition for "community structure" means that for different community concept definitions, different food web results may emerge (Oceanlink, 2005).

This being noted, there are some basic principles behind food web analysis. First, in any community, the more complexity that can be introduced into the food web, the more stable the community will become. When provided with alternative resources, any population will depend less on one single resource, thus making it possible to survive if that single resource is destroyed. Additionally, if the energy flowthrough of an ecosystem has multiple pathways, a single disruption simply reroutes the energy through alternate channels (Ricklefs, 2001).

Furthermore, characterization of communities can occur based on the number of species and feeding links per species. As community diversity increases, the number of trophic levels, such as the herbivore or omnivore levels, and the number of guilds, such as the leaf eaters of the herbivore trophic level, also increase. These trophic levels, then, help influence the food web by describing either a top down or bottom up energy chain.

In top down chains, predators depress populations of other animals, such as herbivores, resulting in a "green" earth. In bottom up chains, plants resist consumption by toxins and digestion inhibition, resulting also in a "green" earth. In both cases, predation and production form intricate parts of the food web (Ricklefs, 2001).

A prime example of a problematic food web can be seen in an analysis of the southern Caspian Sea, in particular relationship to the Mnemiopsis leidyi, or comb jellyfish. A rampant population growth of this jellyfish in the southern Caspian in 2000 had drastic effects on the food web of the ecological community. These jellyfish eat mollusk larvae, and other small zooplankton. These zooplankton, in turn, feed on microscopic algae blooms and other phytoplankton, creating an effective food web that maintains the level of oxygen in the water, thus creating a co-existent community. If levels of all species remain stable, the community will be stable (Jeffress and Steimle, 1990).

However, overabundance of the Mnemiopsis leidyi creates a large problem for this community, since the food web is not complex. If the jellyfish feeding drastically reduces the numbers of zooplankton, then the zooplankton do not effectively consume the phytoplankton. This phytoplankton, then, sinks to the bottom of the Caspian, and dies. As a result, the oxygen levels of the waters decrease, further lowering the zooplankton levels. As zooplankton levels decrease, the food supply of the primary predator, the Mnemiopsis leidyi, is also reduced, thereby starving the jellyfish (Jeffress and Steimle, 1990).

Clearly, this food web is not diversified, and thus, not stable. If any energy pathway in this web is destroyed, the entire web detangles. If, instead, the web had some diversification, any link could be altered, and other links would simply increase in production. This is the basic premise of the food web.

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, which further lessen the ability of other species to survive in the aquatic environment. 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 to community through inhibition.

Clearly, the communities of the Caspian Sea, as other biologically diverse communities in the world, rely on the constant changes and fluxes in the environment and in the community in order to maintain a climax community. The resiliency of the species involved is impacted heavily by the interconnectedness of all niches and species in the community, and their continuous interactions with one another. Without each piece of the community, the entire community would fail.