DOI: 10.5061/dryad.tht76hdww
title: Species packing and the latitudinal gradient in local beta-diversity

description: 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.
description: We used data from 21 forest dynamics plots that are part of the
 ForestGEO and Chinese Forest Biodiversity Monitoring Networks
  (Anderson-Teixeira<i> et al.</i> 2015; Feng<i> et
 al.</i> 2016). All stems with diameter at breast height (DBH) ≥ 1 cm
 were spatially mapped, tagged, measured and identified to species (Condit
 1998).  To remove gamma-diversity dependence caused by sample-size bias of
 beta-diversity metrics, we used the correction method designed for Shannon
 diversity index based on the relationship between cumulative diversity and
 sample size (Chao<i> et al.</i> 2013).  we used the surface to
 planimetric area ratio as a metric of topographic heterogeneity, which
 provided a useful measure of the range and roughness of the overall plot,
 based on the digital elevation models (DEMs) (Jenness 2004; Brown<i>
 et al.</i> 2013).  Niche marginality was defined as the ecological
 distance between each species’ optimum and the mean habitat within the
 plot (illustrated in electronic supplementary material, figure S1)
 (Hirzel<i> et al.</i> 2002; Devictor<i> et al.</i>
 2010). Specialization and marginality were calculated for each species
 using a multivariate approach – ecological niche factor analysis (ENFA)
 (Hirzel<i> et al.</i> 2002).