Climate change and water withdrawals in arid regions are causing once-perennial streams to fragment or dry completely. One of the most notable changes accompanying this transition is the disappearance of aquatic top predators, such as fish and large-bodied invertebrates. These local extinctions presumably change stream community structure and ecosystem functioning, however manipulative experiments are necessary to disentangle the effects of predator loss from other co-occurring environmental changes. We employed replicate mesocosm communities to isolate the effects of top predator extinctions in fragmented streams in southeastern Arizona, USA. We seeded sixteen mesocosms with representative arid-land stream invertebrates, removed the invertebrate top predator Abedus herberti (Hemiptera: Belostomatidae) from eight mesocosms as a treatment, and assessed community and ecosystem changes after six weeks.
The removal of Abedus initiated a trophic cascade resulting in decreased algal biomass, likely caused by the release of the algae-grazing mayfly Callibaetis from Abedus predation. Our simulated top predator extinction also caused unexpected changes to the detritivore community by significantly reducing the abundance of the caddisfly Phylloicus – the dominant consumer of coarse particulate organic matter in this system. While Abedus removal did not affect overall species richness, the abundance and diversity of secondary predators were higher in mesocosms without Abedus, suggesting a competitive release. We submit that the indirect biotic effects of drying on aquatic community structure can be as important as direct abiotic effects and should be considered when planning aquatic conservation in arid-land streams.