Top predator extinctions in drying streams modify community structure and ecosystem functioning

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

Year: 2011 (xliii)

Abstract

Climate change and anthropogenic water withdrawals are causing many once-perennial streams to experience fragmentation or complete drying for portions of the year. One of the most notable changes associated with this transition is the disappearance of large aquatic predators, such as fish and large-bodied aquatic insects. In arid-land streams of southeastern Arizona, habitat contraction and complete drying have been associated with local extinctions of the invertebrate top predator Abedus herberti (Hemiptera: Belostomatidae). These extinctions likely cause changes in stream community structure and functioning, however manipulative experiments are necessary in order to disentangle the effects of local predator extinctions from other co-occurring environmental changes. We used mesocosms to represent fragmented stream pools, seeded them with stream invertebrates and then removed Abedus herberti as a treatment. We found that the removal of Abedus herberti initiated a trophic cascade resulting in decreased algal biomass in treatment mesocosms. This indirect effect of the top predator on algal biomass was likely caused by Abedus herberti predation on the mayfly Callibaetis, an algae-grazing taxon. Abedus herberti removal also affected overall invertebrate community structure. Both the abundance and diversity of secondary predators were higher in mesocosms without Abedus herberti, suggesting a competitive release of these smaller predators. The results of our experiment demonstrate that the indirect biotic effects of stream drying can be as important as abiotic drying effects and must be considered when planning conservation efforts in arid-land streams.