No abstract

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.

1. Native freshwater fish populations throughout South Africa's Cape Fold Ecoregion (CFE) are in decline as a result of human impacts on aquatic habitats, including the introduction of nonnative freshwater fishes. Climate change may be further accelerating declines of many species, although this has not yet been studied in the CFE. This situation presents a major conservation challenge that requires assigning management priorities through assessing species in terms of their vulnerability to climate change.2. One factor hindering reliable vulnerability assessments and the concurrent development of effective conservation strategies is limited knowledge of the biology and population status of many species. This paper reports on a study employing a rapid assessment method used in the USA, designed to capitalize on available expert knowledge to supplement existing empirical data, to determine the relative vulnerabilities of different species to climate change and other human impacts. Eight local freshwater fish experts conducted vulnerability assessments on 20 native and 17 non-native freshwater fish species present in the CFE.3. Results show (1) that native species were generally classified as being more vulnerable to extinction than were non-native species, (2) that the climate change impacts are expected to increase the vulnerability of most native, and some non-native, species, (3) that vulnerability hotspots requiring urgent conservation attention occur in the Olifants-Doring, upper Berg and upper Breede River catchments in the south west of the region, (4) that in addition to providing guidance for prioritizing management interventions, this study highlights the need for reliable data on the biology and distribution of many CFE freshwater fishes, and (5) that identification of priority areas for protection should be based on multiple sources of data.