Non-native rainbow trout change the structure of benthic communities in headwater streams of the Cape Floristic Region, South Africa

Introduced rainbow trout Oncorhynchus mykiss have invaded many headwater streams in the Cape Floristic Region (CFR) and depleted, or eliminated, native fish populations. However, the question of whether trout invasions also have consequences for lower trophic levels in these systems has not been addressed. We used a broad-scale comparative study in the upper Breede River catchment (CFR) to evaluate differences in benthic community structure between sites on headwater streams with and without trout, and thereby infer community-level impacts of trout. There were differences in invertebrate abundance and assemblage composition, and algal biomass, between sites with and without trout. Specifically, the abundance of certain herbivorous invertebrate taxa was higher, and the biomass of benthic algae lower, at sites invaded by trout. This pattern implies that trout have induced a trophic cascade by releasing herbivorous invertebrates from predation, leading to an increase in grazing pressure and a consequent indirect decrease in the biomass of benthic algae; a pattern that contrasts with the majority of studies investigating community-level impacts of introduced trout elsewhere. These findings, together with comparisons of environmental conditions between invaded and uninvaded sites, indicate that trout invasions have changed the structure and function of benthic communities in these streams.