Comparisons of isotopic niche widths of some invasive and indigenous fauna in a South African river

1. Biological invasions threaten ecosystem integrity and biodiversity, with numerous adverse implications for native flora and fauna. Established populations of two notorious freshwater invaders, the snail Tarebia granifera and the fish Pterygoplichthys disjunctivus, have been reported on three continents and are frequently predicted to be in direct competition with native species for dietary resources. Using comparisons of species' isotopic niche widths and stable isotope community metrics, we investigated whether the diets of the invasive T.granifera and P.disjunctivus overlapped with those of native species in a highly invaded river. We also attempted to resolve diet composition for both species, providing some insight into the original pathway of invasion in the Nseleni River, South Africa. Stable isotope metrics of the invasive species were similar to or consistently mid-range in comparison with their native counterparts, with the exception of markedly more uneven spread in isotopic space relative to indigenous species. Dietary overlap between the invasive P.disjunctivus and native fish was low, with the majority of shared food resources having overlaps of <0.26. The invasive T.granifera showed effectively no overlap with the native planorbid snail. However, there was a high degree of overlap between the two invasive species (0.86). Bayesian mixing models indicated that detrital mangrove Barringtonia racemosa leaves contributed the largest proportion to P.disjunctivus diet (0.12-0.58), while the diet of T.granifera was more variable with high proportions of detrital Eichhornia crassipes (0.24-0.60) and Azolla filiculoides (0.09-0.33) as well as detrital Barringtonia racemosa leaves (0.00-0.30). Overall, although the invasive T.granifera and P.disjunctivus were not in direct competition for dietary resources with native species in the Nseleni River system, their spread in isotopic space suggests they are likely to restrict energy available to higher consumers in the food web. Establishment of these invasive populations in the Nseleni River is thus probably driven by access to resources unexploited or unavailable to native residents.