Ecological Niches and Patterns in Species Abundance and Distribution

Species spatial distributions as well as their abundances are often attributed to the breadth and position of their niches. A species occurs in places where its requirements are fulfilled, that is, where it finds its niche. However, the 'presence of the niche' is not a sufficient condition for the m c œ ~o a o

Figure 2 In this example of Galapagos finches on three different islands, the number of niches can be predicted from the peaks in the expected finch density. The expected finch density is calculated from distribution of seed biomass converted to finch numbers, using preferred seed size estimated from the mean size of the beak. The beak depth of the finches occurring on each island corresponds well to the maxima of the curve. Position of the symbols mean beak depth of male ground finch on each of the three islands: Geospiza fortis (squares), G. difficilis (triangles), G. magnirostris (open circles), and G. fuliginosa (closed circles). The beak depth scale is kept the same for the three pictures; the population density is scaled to the maximum. Modified from Schluter D and Grant PR (1984) Ecological correlates of morphological evolution in a Darwin's finch, Geospiza difficilis. Evolution 38: 856-869.

presence of a species, and in a special case it may not be even the necessary condition. Spatial population dynamics driven by dispersal and spatial distribution of available habitat patches is equally important. Consequently, species may be absent even in sites containing habitat that fulfils its niche requirements if the site is far away from other occupied sites and the dispersal distance of the organism in concern is relatively small for the immigration into the site. On the other hand, a species may be present even in a site where its niche requirements are not fulfilled and population growth is negative if the population is maintained by a continuous supply of individuals from neighboring sites with positive population growth (so-called source-sink population dynamics). Therefore, species spatial distributions are determined by species niches and available habitat distributions, as well as by spatial population dynamics and dispersal limitation.

In a similar line, it has been argued that a significant proportion of the variation of species' abundances can be explained by the breadth of species' niches (Box 1). It is reasonable to assume that species which are able to utilize wider spectrum of resources can attain higher population abundances and also can occupy more sites. Local population densities are mostly positively correlated with species range sizes, which can be taken as an evidence of such niche differences. However, patterns in species abundances can be often well explained by spatial population dynamics - for instance species which were incidentally able to spread to more sites have higher chance to colonize further sites and to further increase local population densities by immigration (this is the nonlinearity of the dynamics of metapopulations). Moreover, the statistical relationship between niche breadth and abundance can have actually a reversed causality, as abundant species are forced to utilize a wider range of resources due to intraspecific competition. More abundant species can also be those that do not utilize a broader range of resources, but are specialized on resources which are relatively more abundant, or may simply have higher population growth and/or dispersal rate (although these features can be understood as niche properties).

One of the most prominent ecological patterns is the frequency distribution of abundance of individual species within local communities or regional species assemblages - the so-called species-abundance distribution. It is always highly unequal, the majority of species having low abundance and only a few being common (the frequency distribution is often close to log-normal, though other models may fit the observed species-abundance distribution better in particular situations). This distribution has been modeled as a stepwise division of niche space, where each newly arriving species obtains some (random) proportion of niche space previously utilized by other species. One of these models, based on sequential resource partitioning, predicts observed species-abundance distribution quite well (Box 2). However, models based on spatial dynamics and dispersal limitations - especially those involving 'community drift' (see above) - can provide equally good predictions of species-abundance distribution. This again indicates the complementarity between niche-based and

Figure 2 In this example of Galapagos finches on three different islands, the number of niches can be predicted from the peaks in the expected finch density. The expected finch density is calculated from distribution of seed biomass converted to finch numbers, using preferred seed size estimated from the mean size of the beak. The beak depth of the finches occurring on each island corresponds well to the maxima of the curve. Position of the symbols mean beak depth of male ground finch on each of the three islands: Geospiza fortis (squares), G. difficilis (triangles), G. magnirostris (open circles), and G. fuliginosa (closed circles). The beak depth scale is kept the same for the three pictures; the population density is scaled to the maximum. Modified from Schluter D and Grant PR (1984) Ecological correlates of morphological evolution in a Darwin's finch, Geospiza difficilis. Evolution 38: 856-869.

Beak depth

Beak depth

Box 2 Sequential resource partitioning

It appears that relative species abundances within taxa can be reasonably well explained by a simple null model of resource partitioning between species, proposed by Mutsunori Tokeshi. A common resource, represented by a 'stick', is divided once at a random location chosen uniformly along its length, and for further partitioning one part is chosen with a probability proportional to its length raised to a power of K, where K is a parameter between 0 and 1 (e.g., 0.05), and the division and selection process continue to distribute the 'niche' among all the species within the taxon. The model seems to describe well the relative abundances of species within taxa, across a large range of their species richness.

dispersal-based explanation of ecological patterns, and supports our consideration of both niche differences and spatial population dynamics as essential drivers of species distribution and abundance.

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