The Mid Domain Effect and Species Richness Patterns: What Have We Learned So Far?

  • Rober K
  • Rahbek C
  • Nicholas J
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Abstract: 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 “mid‐domain”
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‚Äê and two‚Äêdimensional 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.

Author-supplied keywords

  • Animals
  • Biodiversity
  • Ecology
  • Geography
  • Models, Biological
  • Population Dynamics

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  • K Colwell Rober

  • Carsten Rahbek

  • J.Gotelli Nicholas

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