The roles of resource availability and competition in mediating growth rates of invasive and native freshwater snails

Summary The classic paradigm in community ecology is that competition structures communities, limiting the number of species that can coexist. Under this paradigm, highly diverse communities should be biotically resistant to invasion. However, the importance of biotic resistance in repelling invasions might depend on resources because high resource availability can increase growth rates of invasive species and either reduce or increase competitive effects of invasive species on native community members. We tested how experimentally altered resource levels affected growth rates of an invasive ( P otamopyrgus antipodarum ) and a native snail ( P yrgulopsis robusta ) and interspecific competition between them. P otamopyrgus and P yrgulopis are the dominant macroinvertebrates in streams within the range of the locally endemic P yrgulopsis in W yoming, U . S . A . In the absence of competition, we found that P otamopyrgus always grew faster than P yrgulopsis at both high and low resource levels. In the presence of interspecific competition, P otamopyrgus growth rates were not affected by resource levels or the biomass of P yrgulopsis competitors. On the other hand, P yrgulopsis grew slower at low resource levels and especially when the biomass of P otamopyrgus competitors was high. These results indicate that negative effects of P otamopyrgus on P yrgulopsis growth rates were stronger at low resource levels. In contrast, P yrgulopsis did not reduce P otamopyrgus growth under any resource scenario. Thus, increased resource availability seems to mitigate strong competitive effects of P otamopyrgus on the native snail in this stream.