A stream by any other name: does β-diversity differ between Nearctic and Neotropical streams?

Beta diversity typically increases with decreasing latitude, with explanations of this pattern often based on tropical origins of most clades. Black flies (Diptera: Simuliidae) are at odds with this trend, having a northern origin. Increasing β-diversity with decreasing latitude might also be a product of changes in causal mechanisms structuring local assemblages among biogeographical realms. We tested the hypotheses that (i) β-diversity of black flies in streams should be similar between the Nearctic and Neotropical realms, and (ii) causal processes influencing species composition do not differ between these areas. To this end, we examined two Nearctic (USA) and two Neotropical data sets (Ecuador, Venezuela). Results did not show distinct differences in β-diversity between Nearctic and Neotropical streams. Although stream conditions varied among regions, we provide evidence that several causal mechanisms of community structure are shared. We suggest that potentially similar causal mechanisms driving species composition contributed to the lack of distinct differences in β-diversity between Nearctic and Neotropical streams. In addition, dispersal appears to influence the β-diversity of larval black flies, a pattern different from many previous studies of stream invertebrates. The influence of taxon origins on β-diversity across biogeographic realms should be further investigated. [ABSTRACT FROM AUTHOR]