Patterns of regional beta diversity in a widespread group of North American aquatic insects

Abstract

We examined how 2 broadly different indices of β diversity, each with different interpretations, changed over gradients of latitude, longitude, γ diversity, habitat heterogeneity, and area. Our data set consisted of 141 species of immature black flies (Diptera:Simuliidae) from 399 stream sites in 13 regional data sets across North America. The total variance of the community data (βSS) quantified the total variation among species occurrences across sites for each regional data set. In contrast, βSor and βMult were calculated as the amount of dissimilarity for all pairwise site comparisons for each data set. βSor was calculated as the mean % dissimilarity for all pairwise site comparisons, whereas βMult, by extending the Sørensen formula to multiple sites, produced a single estimate of overall β diversity. βMult was further partitioned into diversity driven by spatial turnover (βTurn) and that resulting from nestedness (βNest). Neither habitat heterogeneity nor area was significantly associated with β diversity. Significant variation in βSS was explained by pure latitude, pure γ diversity, and joint latitude–γ diversity. The positive relationship between βSS and latitude for black flies might be related to the group’s origin in cooler areas of the Northern Hemisphere. Significant variation in βSor, βMult, βNest, and βTurn was explained only by γ diversity, with βMult-diversity driven largely by spatial turnover.