Global relationships between beta diversity and latitude after accounting for regional diversity

Regional β-Diversity of Stream Insects in Coastal Alabama Is Correlated with Stream Conditions, Not Distance among Sites

β-diversity is often measured over both spatial and temporal gradients of elevation, latitude, and environmental conditions. It is of particular interest to ecologists, as it provides opportunities to test and infer potential causal mechanisms determining local species assemblages. However, studies of invertebrate β-diversity, especially aquatic insects, have lagged far behind other biota. Using partial Mantel tests, we explored the associations between β-diversity of insects found in the coastal streams of Alabama, USA, and stream conditions and distances among sites. β-diversity was expressed using the Sørensen index, βSor, stream conditions were expressed as principal components (PCs), and distances as Euclidean distances (km) among sites. We also investigated the impact of seasonality (fall, summer) and taxonomic resolution (genus, species) on βSor. Regardless of season, βSor was significantly correlated (p < 0.01; r > 0.44) with stream conditions (stream size and water chemistry), while taxonomic resolution had minimal effect on associations between βSor and stream conditions. Distance was never correlated with changes in βSor (p > 0.05). We extended the use of the Sørensen pair-wise index to a multiple-site dissimilarity, βMult, which was partitioned into patterns of spatial turnover (βTurn) and nestedness (βNest). Changes in βMult were driven mostly by turnover rather than nestedness.

Taxonomic dependency of beta diversity components in benthic communities of bacteria, diatoms and chironomids along a water-depth gradient

Community variation (i.e., beta diversity) along geographical gradients is a well-known ecological pattern, but the corresponding variation in beta diversity components (e.g., species turnover and nestedness) and underlying drivers remain poorly understood. Based on two alternative approaches (that is, the beta diversity partitioning proposed by Baselga and the Local Contributions to Beta Diversity (LCBD) partitioning proposed by Legendre), we examined the patterns of beta diversity components of lacustrine benthos, from bacteria to diatoms and chironomids, in the surface sediments along a 100-m water-depth gradient in Lugu Lake. We further quantified the relative importance of spatial, environmental and biotic variables in explaining water-depth patterns in beta diversity. Based on the Baselga's framework, there was a taxonomic dependency for the patterns of beta diversity components with water-depth, showing a significant species turnover pattern for bacteria, while diatoms and chironomids showed significant nestedness. This dependency was also evident in the patterns of community uniqueness with water-depth because based on Legendre's framework, the LCBD decreased with water depth for bacteria whereas increased with depth for diatoms. The total beta diversity and species turnover of bacteria could be explained by the pure effects of spatial, environmental and biotic variables. A total of 26.8% and 23.6% of the nestedness component of diatoms and chironomids was explained by environmental variables, respectively, while species turnover was mostly related to spatial variables. Bacteria total LCBD and species replacement were driven only by environmental variables. For diatoms and chironomids, however, most of the total LCBD and its two components were explained by spatial variables, and biotic variables were most important for the diatom replacement component. Our findings provide insights into the mechanisms responsible for community organizations along water-depth gradients from the perspective of beta diversity components.