Ecosystem Diversity

An ecosystem is the entire complement of species and communities found in a given region, and the functional interrelationships that exist between these organisms and the other biotic and abiotic characteristics of the region. The diversity of an ecosystem is dependent not only on the biological and physical entities that it contains, but also on the ecological interrelationships between those entities (predation or parasitism between species, competition between species for the available natural resources).

Ecosystem diversity is also dependent on the type of physical resources available within a particular habitat and the way in which the resident organisms use those resources. For example, the aquatic larvae of caddis flies build a protective casing from small stones and other debris collected from the streambeds where they live. Their distribution is restricted to parts of streams where the particle size of the sediment is suitable for building the protective cases. This, in turn, determines the presence or absence of other species that feed on the caddis fly larvae.

The physical characteristics of ecosystems can be modified by the actions of the organisms themselves. For example, beavers alter the hydrology of aquatic ecosystems by damming rivers, which affects the flora and fauna of the region (Butler, 1995; and see Butler for discussion of the geomorphic influences of other vertebrates and invertebrates). Similarly, beavers change the physical structure of forests by felling trees. Recent studies of North American prairie dogs show that their presence can significantly affect the diversity and productivity of the vegetation in the areas where they are present (Miller et al., 1994; Thacker, 2001). In the Arctic, some cetaceans (such as killer whales) and pinnipeds (ringed seals) maintain breathing holes and lees in the ice. This not only shapes the physical structure of the environment but also attracts predators, such as polar bears, to these patches of open water.

The diversity of an ecosystem is often described in terms of the complexity of the food web (trophic relationships). This gives a general idea of the overall complexity (and ecological stability) of the ecosystem. Another way to describe the ecological diversity of an ecosystem is to identify keystone species. These are important because some aspect of their presence in the ecosystem allows many other species to coexist in the ecosystem. The presence of a specialized and important keystone species may indicate the presence of a complex habitat and ecosystem. However, it is difficult to quantify and measure the diversity of ecological interrelationships within an ecosystem, as noted in the preceding discussion on community diversity. Therefore, the number of populations and species present and the taxonomic diversity of those species are often used as proxy measures of overall ecosystem diversity.

The functional complexity of the ecosystem (the complexity of the trophic and other ecological interconnections between constituent species) increases with the number and taxonomic diversity of the species present. In an ecosystem with very few species, the loss of even a single species or a small part of the habitat can affect the ecological interactions between a significant proportion of the remaining species in the ecosystem. The ecosystem will no longer function properly and may collapse as a consequence (Myers, 1996). In a large ecosystem, a small amount of damage would affect the ecological interactions between a relatively small propor tion of the populations and species present. Thus the larger ecosystem is less likely to collapse; the increase in functional complexity is assumed to make the ecosystem more resilient to environmental change. However, new research suggests that an increase in species richness might not necessarily confer greater ecological resilience (Pfisterer and Schmid, 2002).

Ecosystems may be classified according to the dominant type of habitat present—for example, a salt marsh ecosystem, or rocky shore intertidal ecosystem. Comparisons between ecosystems usually focus on how the biological complexity of the ecosystem (for example, the number and diversity of species present) might be constrained by the physical complexity of the ecosystem—whether, for example, the ecosystem is a high-energy environment such as a torrential stream or exposed coastline, or a low-energy environment such as a sheltered salt marsh. These factors can result in considerably different types of ecosystems, either locally—as in the stunted vegetation and low species diversity on exposed hilltops compared with the more prolific vegetation and high species diversity in sheltered valleys—or globally. Temperate climate ecosystems tend to be simpler than tropical climate ecosystems in terms of numbers of species and taxonomic diversity. The European freshwater fish fauna, for example, is estimated to include about 360 species, representing about 29 families of fish; the neotropical region of Central and South America includes between 5,000 and 8,000 species in at least 55 families; and tropical Asia has about 3,000 species in 121 families (Kot-telat, 1997; Lundberg et al., 2000).

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