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| If species' ranges are randomly shuffled within a bounded geographical
domain free of environmental gradients, ranges overlap increasingly
toward the center of the domain, creating a "middomain" peak of species
richness. This "mid-domain effect" (MDE) has been controversial both in
concept and in application. Empirical studies assess the degree to
which the evolutionary, ecological, and historical processes that
undeniably act on individual species and clades produce geographical
patterns that resemble those produced by MDE models. MDE models that
resample empirical range size frequency distributions (RSFDs) balance
the risk of underestimating and overestimating the role of MDE, whereas
theoretical RSFDs are generally biased toward underestimating MDE. We
discuss the inclusion of nonendemic species in MDE models, rationales
for setting domain limits, and the validity of one- and two-dimensional
MDE models. MDE models, though null models, are not null hypotheses to
be simplistically rejected or accepted. They are a means of estimating
the expected effect of geometric constraints within the context of
multiple causality. We call for assessment of MDE on an equal
statistical footing with other candidate explanations for richness
gradients. Although some critics have categorically dismissed MDE, an
overview of the 21 MDE studies published to date reveals a substantial
signature of MDE in natural patterns and justifies continued work. | |
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