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

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.

Non‐native rainbow trout (Oncorhynchus mykiss) occupy a different trophic niche to native Breede River redfin (Pseudobarbus burchelli) which they replace in South African headwater streams

“…Data on the feeding habits of Cape galaxias and Cape kurper are scarce, but available information suggests that both species feed primarily on benthic invertebrates (Skelton, 2001). Unexpectedly, redfin appeared to avoid grazer-scraper taxa which is inconsistent with the pattern described by Shelton et al (2014b). Our GCA results in the present study largely support the notion that redfin are stronger regulators of collectorgatherer taxa than are trout.…”

“…As our fish-collecting methods were not size-selective, and as the ranges of lengths of trout and redfin analysed in this study cover the range of sizes for these species found in CFR headwater streams (Shelton et al, 2014a), we consider our results to be representative of naturally occurring populations. While there were clear differences in food availability between streams with and without trout (Shelton et al, 2014b), the main difference was that aquatic invertebrate abundance in streams with trout was approximately twice that in streams with no trout. Because the focus of the present study was on characterising and comparing the feeding habits of redfin and trout in allopatry rather than in sympatry, differences in the availability of different food types among sites could potentially confound interspecific differences in gut content composition.…”

“…Indeed, differences in community structure between streams with and without trout in the upper Breede River catchment (Shelton et al, 2014b) appear to be linked to differences in the trophic niches occupied by trout and redfin (Shelton et al, 2016) in that the relatively low density of benthic invertebrates in streams supporting healthy native fish populations (i.e. lacking trout) could be attributed to the fact that redfin are strongly reliant on aquatic invertebrate prey.…”

“…Results from recent comparative field studies conducted in the CFR have shown that in systems where trout establish at relatively high densities, they displace a formerly abundant native cyprinid, the Breede River redfin Pseudobarbus burchelli (Smith 1841) through predation (Shelton et al, 2014a), and this displacement appears to have released herbivorous aquatic invertebrates from predation, apparently leading to cascading effects on benthic algae in invaded systems (Shelton et al, 2014b). These findings imply that trout do not functionally compensate for native redfin, which they have replaced, and may thereby have altered top-down control of aquatic invertebrates in these systems.…”

Long-term trends in aquatic ecosystem bioassessment metrics are not influenced by sampling method: empirical evidence from the Niobrara River

“…There are two likely explanations for this decline in Niobrara River ecosystem health: the introductions of invasive northern pike (E. lucius) and yellow flag iris (Bowles et al, 2013;Spurgeon et al, 2014). Pike are piscivores and likely caused a decline in the resident fish assemblage from 11 to 3 species between 1989 and 2011 (Spurgeon et al, 2014) triggering a trophic cascade that affected the invertebrate assemblage in the river (Carpenteret al, 1987;Tronstad et al, 2010;Shelton et al, 2015;Wilmot et al, 2016). Furthermore, the invasion of non-native plants may have altered the stream ecosystem (Bowles et al, 2013;Spurgeon et al, 2014).…”