Drought, top-predator extirpations, and community changes in aridland streams

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

Year: 2011 (xliii)

Abstract

Droughts and anthropogenic water abstractions are reducing stream flows across the world, and especially so in aridland streams. In larger streams, flow reduction can lead to increased water temperature, decreased dissolved oxygen, and habitat fragmentation, negatively impacting fish and invertebrate populations. The effects of drought in small, fishless streams, however, are less understood. Through a variety of observational and experimental studies, we found that drought-induced habitat loss in small headwater streams of Arizona is causing a number of population and ecosystem level changes. Arguably, the most important effect of stream drying is the extirpation of the system’s top predator, the giant water bug Abedus herberti, along with several other taxa requiring perennial water to survive. Drying events at multiple sites within a mountain range may also reduce the pool of nearby colonists when flows recover following drought, and thus may reduce the resilience of local communities. In two of our long-term study streams where drought-induced habitat contraction and extirpation events occurred, A. herberti and several other perennial taxa were unable to reestablish local populations, even 6-8 years after the initial extirpation event, while smaller, more vagile secondary predators flourished. Additionally, results from experimental mesocosm studies in which A. herberti were removed as a treatment suggest that the loss of this species alone can cause a trophic cascade. Comparing the results of our regional stream surveys over the last 10 years with historical collection records of A. herberti, it appears that this top predator may be disappearing from multiple streams across the region, and perhaps from entire mountain ranges. Our observational and experimental studies document current extirpations happening in headwater aridland streams, demonstrate the ecosystem-level impacts these extirpations may be having, and highlight the need for regional conservation planning to prevent the extinctions of perennial stream species like A. herberti.