To try to understand the determinants of species richness, it will be useful to begin with a simple model. Assume, for simplicity, that the resources available to a community can be depicted as a one-dimensional continuum, R units long (Figure 21.1). Each species uses only a portion of this resource continuum, and these portions define the niche breadths (n) of the various species: the average niche breadth within the community is N. Some of these niches overlap, and the overlap between adjacent species can be measured by a value o. The average niche overlap within the community is then o. With this simple background, it is possible to consider why some communities should contain more species than others.
First, for given values of n and o, a community will contain more species the larger the value of R, i.e. the greater the range of resources (Figure 21.1a). This is true when the community is dominated by competition and the species 'partition' the resources (see Section 19.2). But, it will also presumably be true when competition is relatively unimportant. Wider resource spectra provide the means for existence of a wider range of species, whether or not those species interact with one another.
Second, for a given range of resources, more species will be accommodated if n is smaller, i.e. if the species are more specialized in their use of resources (Figure 21.1b).
Alternatively, if species overlap to a greater extent in their use of resources (greater o), then more may coexist along the same resource continuum (Figure 21.1c).
geographic factors factors correlated with latitude factors that are independent of latitude biotic factors a model incorporating niche breadth, niche overlap and resource range
Figure 21.1 A simple model of species richness. Each species utilizes a portion n of the available resources (R), overlapping with adjacent species by an amount o. More species may occur in one community than in another (a) because a greater range of resources is present (larger R), (b) because each species is more specialized (smaller average n), (c) because each species overlaps more with its neighbors (larger average o), or (d) because the resource dimension is more fully exploited. (After MacArthur, 1972.)
More species because greater range of resources (larger R)
More species because each is more specialized (smaller n)
More species because each overlaps more with its neighbors (larger o)
More species because resource axis is more fully exploited (community more fully saturated)
Finally, a community will contain more species the more fully saturated it is; conversely, it will contain fewer species when more of the resource continuum is unexploited (Figure 21.1d).
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