Does species trait composition influence macroinvertebrate responses to irrigation water withdrawals: Evidence from the Intermountain West, USA

Abstract

AbstractIrrigation water withdrawals are ubiquitous in arid and semi‐arid regions; however, predictable macroinvertebrate responses have not emerged. Inconsistent responses are frequently attributed to the amount of water withdrawn, while physicochemical and biological differences among systems remain unstudied. In this study, we sought to determine if species traits influence macroinvertebrate responses to water withdrawals. We sampled above and below 12 independent water diversions experiencing discharge reductions greater than 75% in central and eastern Oregon and Washington, USA. When considering all 12 rivers, we observed significant alterations to the hydrologic environment, in addition to increased algal standing stocks, silt cover and minimum temperatures; however, macroinvertebrate responses were limited to increased downstream densities, as indices and community composition did not significantly change. Functionally, species traits were more sensitive than structural indices; proportional abundance for 12 of 52 species traits significantly differed above and below all points of diversion. Trait changes suggest a shift in functional composition to compensate for hydrologic alterations and reduced habitat area. Grouping references sites (upstream) by species trait composition revealed a strong ambient gradient of increasing resistance and resilience from high to low elevations; however, we did not find evidence for differential susceptibility to water withdrawals among trait‐based reference groups. Though water withdrawals changed the representation of select species traits, frequencies of trait states most vulnerable to water withdrawals were too homogenously distributed among sites to elicit differential responses among trait‐based reference groups. At larger spatial scales or among disparate hydrologic regimes, grouping systems by species traits will likely reduce variability and offer a more general currency for detecting perturbation response relationships. Copyright © 2009 John Wiley & Sons, Ltd.