Species packing and the latitudinal gradient in local beta-diversity

The decline in species richness at higher latitudes is among the most fundamental patterns in ecology. Whether changes in species composition across space (beta-diversity) contribute to this gradient of overall local species richness (gamma diversity) remains hotly debated. Previous studies that failed to resolve the issue suffered from a well-known tendency for small samples in areas with high gamma-diversity to have inflated measures of beta-diversity. We provide here a novel analytical test, using beta-diversity metrics that correct the gamma-diversity and sampling bias, to compare beta-diversity and species-packing across a latitudinal gradient in tree species richness of 21 large forest plots along a wide environmental gradient in East Asia. We demonstrate that after accounting for topography and correcting the gamma-diversity bias, tropical forests still have higher beta-diversity than temperate analogs. This suggests that local beta-diversity contributes to the latitudinal species richness gradient as a component of gamma-diversity. Moreover, niche specialization and niche marginality (a measure of niche spacing along a habitat gradient) also increases towards the equator, after removing the effect of topographic heterogeneity. This supports the hypothesis of tighter species packing and larger niche space in tropical forests while demonstrating the importance of local processes in controlling beta-diversity.The decline in species richness at higher latitudes is among the most fundamental patterns in ecology. Whether changes in species composition across space (beta-diversity) contribute to this gradient of overall local species richness (gamma diversity) remains hotly debated. Previous studies that failed to resolve the issue suffered from a well-known tendency for small samples in areas with high gamma-diversity to have inflated measures of beta-diversity. We provide here a novel analytical test, using beta-diversity metrics that correct the gamma-diversity and sampling bias, to compare beta-diversity and species-packing across a latitudinal gradient in tree species richness of 21 large forest plots along a wide environmental gradient in East Asia. We demonstrate that after accounting for topography and correcting the gamma-diversity bias, tropical forests still have higher beta-diversity than temperate analogs. This suggests that local beta-diversity contributes to the latitudinal species richness gradient as a component of gamma-diversity. Moreover, niche specialization and niche marginality (a measure of niche spacing along a habitat gradient) also increases towards the equator, after removing the effect of topographic heterogeneity. This supports the hypothesis of tighter species packing and larger niche space in tropical forests while demonstrating the importance of local processes in controlling beta-diversity.