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| The causes of global variation in species richness have been debated for nearly two centuries with no clear
resolution in sight. Competing hypotheses have typically been evaluated with correlative models that do
not explicitly incorporate the mechanisms responsible for biotic diversity gradients. Here, we employ a
fundamentally different approach that uses spatially explicit Monte Carlo models of the placement of
cohesive geographical ranges in an environmentally heterogeneous landscape. These models predict
species richness of endemic South American birds (2248 species) measured at a continental scale. We
demonstrate that the principal single-factor and composite (species-energy, water-energy and
temperature-kinetics) models proposed thus far fail to predict (r 2#0.05) the richness of species with
small to moderately large geographical ranges (first three range-size quartiles). These species constitute the
bulk of the avifauna and are primary targets for conservation. Climate-driven models performed
reasonably well only for species with the largest geographical ranges (fourth quartile) when range cohesion
was enforced. Our analyses suggest that present models inadequately explain the extraordinary diversity of
avian species in the montane tropics, the most species-rich region on Earth. Our findings imply that
correlative climatic models substantially underestimate the importance of historical factors and small-scale
niche-driven assembly processes in shaping contemporary species-richness patterns. | |
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