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| The search for a common cause of species richness gradients has spawned
more than 100 explanatory hypotheses in just the past two decades.
Despite recent conceptual advances, further refinement of the most
plausible models has been stifled by the difficulty of compiling
high-resolution databases at continental scales. We used a database of
the geographic ranges of 2,869 species of birds breeding in South
America (nearly a third of the world's living avian species) to explore
the influence of climate, quadrat area, ecosystem diversity, and
topography on species richness gradients at 10 spatial scales (quadrat
area, approximate to 12,300 to approximate to1,225,000 km(2)).
Topography, precipitation, topography x latitude, ecosystem diversity,
and cloud cover emerged as the most important predictors of regional
variability of species richness in regression models incorporating 16
independent variables, although ranking of variables depended on
spatial scale. Direct measures of ambient energy such as mean and
maximum temperature were of ancillary importance. Species richness
values for 1 degrees x 1 degrees latitude-longitude quadrats in the
Andes (peaking at 845 species) were approximate to 30-250% greater than
those recorded at equivalent latitudes in the central Amazon basin.
These findings reflect the extraordinary abundance of species
associated with humid montane regions at equatorial latitudes and the
importance of orography in avian speciation. In a broader context, our
data reinforce the hypothesis that terrestrial species richness from
the equator to the poles is ultimately governed by a synergism between
climate and coarse-scale topographic heterogeneity. | |
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