Detection of macro-ecological patterns in South American hummingbirds is affected by spatial scale

Scale is widely recognized as a fundamental conceptual problem in biology, but the question of whether species-richness patterns vary with scale is often ignored in macro-ecological analyses, despite the increasing application of such data in international conservation programmes. We tested for scaling effects in species-richness gradients with spatially scaled data for 241 species of South American hummingbirds (Trochilidae). Analyses revealed that scale matters above and beyond the effect of quadrat area. Species richness was positively correlated with latitude and topographical relief at ten different spatial scales spanning two orders of magnitude (ca. 12,300 to ca. 1,225,000 km2). Surprisingly, when the influence of topography was removed, the conditional variation in species richness explained by latitude fell precipitously to insignificance at coarser spatial scales. The perception of macro-ecological pattern thus depends directly upon the scale of analysis. Although our results suggest there is no single correct scale for macro-ecological analyses, the averaging effect of quadrat sampling at coarser geographical scales obscures the fine structure of species-richness gradients and localized richness peaks, decreasing the power of statistical tests to discriminate the causal agents of regional richness gradients. Ideally, the scale of analysis should be varied systematically to provide the optimal resolution of macro-ecological pattern.