Now we begin to look at communities instead of the individual populations that constitute them. Why do similar environmental conditions produce similar sets of species? The interactions between species are what stabilize a community. The more species that are present, the more interactions there can be. Removing any single organism eliminates its interactions, affecting those species with which it interacted. But the more overall species there are, the easier it will be for an affected organism to find a substitute. For example, suppose that mice are the primary food source for owls in a particular forest, and the mice are eliminated. If other small mammals, say rabbits, are available, the owls might be able to substitute them. Therefore, it is thought that species diversity is an indication of community stability and resistance to disturbance. The concept of species diversity bringing stability is used by conservationists to argue for the protection of ecosystems from human impacts.
There are a number of caveats to this idea. Disturbance, itself, can increase diversity. Well-developed ecosystems can settle into a lower-diversity state than they had during their stages of development. Also, it is observed that natural systems do not seem to have maximized their diversity as measured by the indices described below. This would require an even distribution of species. Instead, there are always some relatively rare species and others more dominant. Finally, it is possible that diversity is a consequence of ecosystem stability rather than a cause.
The Number of Species in an Ecosystem The simplest measure of diversity is simply the number of species, S. It may be difficult to measure this in an ecosystem without impractically large samples. However, it can be estimated from a limited sample by assuming a lognormal probability distribution for species abundance. Generally, smaller geographic areas can be expected to have fewer species. An empirical rule relating S and area, A, is where c is a parameter that varies with type of ecosystem and z is a parameter that has consistently been found to fall in the range 0.20 to 0.35 for islands. For large continental areas, the exponent ranges from 0.15 to 0.24, apparently due to the greater ability to import species. The number of species in ecosystems tends to increase toward the tropics, as does species diversity.
The equilibrium theory of biogeography, also called island biogeography, proposed by Robert MacArthur and E. O. Wilson, explains the relationship between area and number of species. They suggested that S is a result of a balance between immigration and extinction. Smaller areas would support smaller populations and thus would have higher rates of stochastic extinction [equation (14.33)]. Islands closer to the mainland have higher immigration rates and thus higher numbers of species. A similar balance occurs on the mainland itself, except that instead of immigration providing new species, it occurs by evolution. Of course, this acts more slowly, so the turnover of species is much slower.
Diversity Indices A number of mathematical diversity indices have been developed to quantify species diversity better, giving more weight to more dominant species. Measurement of diversity may be subject to the sample-size limitation mentioned above. Also, measurements of diversity are usually limited to a subset of the community, such as a single taxa (birds, grasses, aquatic invertebrates, etc.) or a single trophic level. Despite these measurement problems and the caveats mentioned above with regard to their significance, diversity indices can be useful for making comparisons or for tracking trends.
Several of the most popular indexes can be computed from the proportions, p,-, of each of the S species. One is the Simpson index, D:
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