Arid-land stream pool invertebrates demonstrate high resistance and functional redundancy to severe drying

Authors: Kate S. Boersma, Michael T. Bogan, Brian A. Henrichs, David A. Lytle

Year: 2013 (xlv)

Abstract

Seasonal droughts are predictable components of arid-land stream hydrology, and arid-land aquatic taxa have adapted to their extreme environment. However, climate change is altering this predictable hydrology, producing longer and more severe droughts and creating novel disturbance regimes for resident organisms. The hydrologic transitions from flowing stream to fragmented pools and from fragmented pools to dry stream bed are frequently associated with steep decreases in species diversity, or thresholds of biodiversity loss. Less is known about how taxa respond between these thresholds, as fragmented pools gradually dry and abiotic conditions deteriorate. We used aquatic mesocosms to test two competing hypotheses of the relationship between species richness and pool drying for arid-land stream invertebrates: the Drought Vulnerability Hypothesis, which predicts that species richness will gradually decrease with pool drying, and the Drought Resistance Hypothesis, which predicts that species richness will remain constant until complete drying occurs. We inoculated aquatic mesocosms with arid-land stream invertebrates, applied three drying treatments representing a continuum of drying stress, and sampled the communities ~45 days later, at the end of the summer dry season. Surprisingly, species richness and community composition did not differ between treatments, providing strong support for the Drought Resistance Hypothesis. Severe drying was associated with lower invertebrate abundances but higher densities than the moderate and control treatments, suggesting that decreased habitat area may be a more important determinant of community composition during droughts than abiotic stress.