Stream community and ecosystem responses to the boom and bust of an invading snail

Invasive species can fundamentally change ecosystems, but there remains surprisingly little understanding of how they alter ecosystems through time. New Zealand mudsnails (NZMS, Potamopyrgus antipodarum) are invading North American aquatic ecosystems with potentially enormous impacts on stream communities and ecosystems. Here we present a unique 10 year time series of stream benthic invertebrates in two sites in a California stream-one invaded site where NZMS populations boomed and then busted, and one upstream reference site that was not invaded to the same degree. As NZMS populations rose and fell, the abundance of native grazing herbivores crashed and then recovered, evidence of direct negative competitive impacts of NZMS. However, NZMS had a positive indirect effect on piercing herbivores, probably mediated by a shift in the dominant algal type. Retrospective stable isotope analyses and Bayesian isotopic mixing models revealed that NZMS invasion decreased the reliance of native taxa, from a variety of functional feeding groups, on periphyton-derived carbon. Furthermore, NZMS invasion was associated with depleted nitrogen stable isotope signatures, evidence that NZMS alter stream ecosystem nitrogen cycling. The reference site did not exhibit these community- or ecosystem-level dynamics. Thus, invasive herbivores can have strong top-down and bottom-up influences on invaded ecosystems, but these impacts can be extremely temporally dynamic.