Estimating invasive species impact: interaction strengths, abundance and the role of productivity in a freshwater invasion

Leslie Anne Riley

doi:10.7273/000005847

Estimating the strength of interactions between species is central to diverse questions in ecology, yet the theoretical basis for interaction strengths has only been well-developed for trophic interactions in dynamic food web models. In chapter 1, we derived dynamic interaction strengths for non-trophic interactions and specifically applied this to interactions between an invasive and native species. We then demonstrate how dynamic interaction strengths can be used in current estimates of invasive impact and expand impact measures to also include reciprocal effects of resident species on the invader. In chapter 2, we test how grazing impacts of the invasive New Zealand mud snail (Potamopyrgus antipodarum) varies with resource availability by measuring both Potamopyrgus biomass and per unit grazing effects (i.e. dynamic interaction strengths) across streams that vary in primary production. We found that Potamopyrgus reduced algae, regardless of resource level. We also found that grazing interaction strengths (i.e. per unit effects) were strongest in the most productive streams and snail biomass was highest in the same streams, resulting in Potamopyrgus having the largest impacts on algae in the most productive streams. In chapter 3, we tested how resource availability affects competition between the invasive Potamopyrgus and a native snail (Pyrgulopsis robusta). We measured growth rates of Potamopyrgus and Pyrgulopsis and interspecific competition between them at two experimentally-altered resource levels. We found that Potamopyrgus always grew faster than Pyrgulopsis. In the presence of interspecific competition, Potamopyrgus growth rates were not affected by resource levels or the biomass of Pyrgulopsis competitors. Alternatively, Pyrgulopsis grew slower at low resource levels and especially when the biomass of Potamopyrgus competitors was high. Competitive effects of Potamopyrgus on Pyrgulopsis were reduced at high resource levels due to faster Pyrgulopsis growth rates; Pyrgulopsis, though, does not strongly compete with Potamopyrgus under any resource scenario. Overall, this study extends the use of dynamic interaction strengths to include non-trophic interactions and shows how the full range of community interactions can be included in measures of invasive species impact, facilitating comparisons of impact across species, productivity gradients and different types of community interactions.

Estimating invasive species impact: interaction strengths, abundance and the role of productivity in a freshwater invasion

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