Aquatic insect β‐diversity is not dependent on elevation in Southern Rocky Mountain streams

Abstract

Summary Relatively high β‐diversity among aquatic insect communities inhabiting high‐elevation streams is most commonly presumed to result from increased dispersal limitation between isolated mountaintop ‘islands’. However, these elevational patterns of β‐diversity have been predominately drawn from observed changes in community composition along single‐thread channels, where the downstream increase in habitat size and hydrologic connectivity provides potential alternative explanations. In this study, we applied an alternative conceptual ‘tributary model’ to ask whether patterns of aquatic insect β‐diversity in similar‐sized, hydrologically disconnected streams showed a similar elevational gradient in diversity patterns as previously reported for conceptual ‘mainstem model’ studies. Aquatic insects were sampled from 24 low‐order, montane streams that are tributaries to larger rivers in three adjacent catchments spanning c . 2000–3500 m in elevation. We used relative abundance data to quantify two types of β‐diversity: (i) community turnover‐β, or the change in local diversity among adjacent streams along the elevational gradient within each catchment, and (ii) community variation‐β, or the change in local diversity among all streams within three elevation zones combined across catchments. Our results provided evidence of no relationship between β‐diversity and elevation in aquatic insect communities in small montane streams. Community turnover‐β was found to be consistently high among sites within catchments and displayed no significant trend across the elevational gradient for any catchment. Community variation‐β showed a nonlinear response to elevation, with sites in the high‐elevation and low‐elevation zones having similarly high community variation‐β compared to sites in the mid‐elevation zone. Our ‘tributary model’ results provide the first evidence that β‐diversity among small, isolated streams can have similar turnover rates across broad elevational gradients. Our results also show similar patterns of β‐diversity among low‐ and high‐elevation tributaries, suggesting that dispersal limitation is not restricted to high‐elevation streams.