Multiscale assessment of patterns of avian species richness

TLDR

Over the past two decades, more than 100 hypotheses have been proposed to explain species‑richness gradients, yet progress is limited by the lack of high‑resolution continental‑scale databases. The authors analyzed geographic ranges of 2,869 South American breeding bird species across 10 spatial scales (≈12,300–1,225,000 km²) to assess the effects of climate, area, ecosystem diversity, and topography on richness gradients. Regression models identified topography, precipitation, their interaction with latitude, ecosystem diversity, and cloud cover as the strongest predictors of regional richness, with Andes quadrats reaching up to 845 species—about 30–250 % higher than Amazon equivalents—highlighting the pivotal role of orography and climate in shaping avian diversity from equator to pole.

Abstract

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, ≈12,300 to ≈1,225,000 km 2 ). Topography, precipitation, topography × 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° × 1° latitude-longitude quadrats in the Andes (peaking at 845 species) were ≈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.