Exotic Snails Dominate Nitrogen and Carbon Cycling in a Highly Productive Stream

Abstract

Individual animal species can impact ecosystem processes, but few exotic animal species have demonstrated ecosystem-scale impacts, in spite of large population sizes. We combined whole­stream measures of carbon and nitrogen fluxes with rates of consumption and ammonium excretion to show that an exotic freshwater snail, Potamopyrgus antipodarum, dominated carbon and nitrogen fluxes in a highly productive stream. Exotic snails consumed 75% of gross primary productivity, and their excretion accounted for two-thirds of ammonium demand. These large fluxes were due to high snail biomass rather than high per-biomass rates of excretion or consumption. This exotic snail may dramatically alter ecosystem function in rivers, with potential consequences for food web structure and element transport. Individual animals species can alter ecosystem functioning, such as nutrient cycling and storage, by directly altering carbon or nitrogen flux through grazing or excretion of ammonium (Grimm 1988, Frank et al. 1997, Vanni 2002), or indirectly via predation (e.g. Schindler et al. 1997). Despite numerous examples linking animal species with ecosystem processes, e. g., nutrient fluxes, there have been only a few examples showing how exotic animals may affect ecosystem processes or overall functions (Strayer et al. 1999, Lovett et al. 2003). Exotic animals might provide a model system for examining single-species impacts on ecosystem processes for several reasons. There are potentially strong ecosystem-scale impacts from exotic animals because they sometimes can dominate invaded ecosystems in terms of biomass. They also might bring a new trait to the invaded ecosystem, e.g. a generalist predator. However, there are few generalizations about how an exotic animal will impact native ecosystems, in spite of many impact studies which are mostly at the population or community level (Parker et al. 1999, Byers et al. 2002). Parker et al. (1999) presented a framework for considering exotic species impact as: I = RxAxE, where R =range (unit area), A =biomass per unit area, and E = per-biomass impact. Local­scale impact can be determined by either high biomass (e.g. Strayer et al. 1990) or by high per­biomass impact relative to native species, which may include an exotic species that brings a novel trait (Vitousek 1990, Byers 2000). Despite this framework, we do not know the degree to which either high biomass or high per-biomass impact contributes to overall ecosystem-scale impact by invading animals. Separating these two will allow better prediction of impacts in that we can focus research and management on understanding either specific traits or potential maximum biomass of invaders. We studied the role of the exotic New Zealand mud snail, Potamopyrgus antipodarum, on carbon. (C) cmd nitrogen (N) fluxes in Polecat Creek, Wyoming (Fig. I). We scaled their per-biomass rates of orgcmic matter consumption and ammonium excretion by snails to whole-stream rates in an 800-m reach. We compared these scaled estimates with whole-stream measures of C-fixation and N cycling to estimate the snail's contribution to stream C and N cycling (Grimm 1988, Vanni 2002). Because we scaled the impact of snails by multiplying per-biomass rates by snail biomass, we were able to estimate the degree to which high biomass or high per-biomass rates contnouted to the dominance of N and C fluxes in this stream. We compared both the per biomass rates and dominance of ecosystem N fluxes by Potamopyrgus with literature values for other freshwater invertebrates.