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| The world contains boundaries (e.g., continental edge for terrestrial
taxa) that impose geometric constraints on the distribution of species
ranges. Thus, contrary to traditional thinking, the expected species
richness pattern in absence of ecological or physiographical factors is
unlikely to be uniform. Species richness has been shown to peak in the
middle of a bounded one-dimensional domain, even in the absence of
ecological or physiographical factors. Because species ranges are not
linear, an extension of the approach to two dimensions is necessary.
Here we present a two-dimensional null model accounting for effects of
geometric constraints. We use the model to examine the effects of
continental edge on the distribution of terrestrial animals in Africa
and compare the predictions with the observed pattern of species
richness in birds endemic to the continent. Latitudinal, longitudinal,
and two-dimensional patterns of species richness are predicted well
from the modeled null effects alone. As expected, null effects are of
high significance for wide ranging species only. Our results highlight
the conceptual significance of an until recently neglected constraint
from continental shape alone and support a more cautious analysis of species richness patterns at this scale. | |
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