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Yellowstone National Park Report

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Exotic species are one of the largest human­caused threats to ecosystems and are becoming a central focus of research for ecologists. We do not know what factors allow exotic species to invade, nor can we predict what effects the exotic species will have once it has invaded. Invasive species, including mollusks, have greatly affected processes in freshwater ecosystems. A recent mollusk invader to North America is the New Zealand mud snail, Potamopyrgus antipodarum. It is a small snail (ca. 4mm long) in the family Hydrobiidae. Potamopyrgus has recently invaded the Snake and Madison Rivers and the Great Lakes In the Madison River, it is numerically dominating the benthos in certain locations with densities over 300,000 individuals/m2 with a corresponding biomass of 30 g/m2 (R. Hall, unpublished data). Because mud snails have high biomass and potentially high secondary production, they may alter algal species composition, and lower algae biomass and production in rivers. Algae form the base of the food web in these rivers; hence, mud snails may be competing with other invertebrates by lowering algae biomass as has been shown for a caddisfly grazer (Kohler and Wiley 1997). It is relatively straightforward to measure grazer impact on algae population using small-scale experiments. However, it is harder to scale up these estimates to whole-river primary production because we cannot manipulate snail densities at this scale. From a management perspective, we cannot do experiments every time a prediction is needed, e.g. when this snail invades a different river. For this reason to predict impacts based on knowing snail and algae biomass using a modeling approach may become useful. Ecologists are just beginning to link energy flow with experimentally measured interaction strengths. Recently Wootton (1997) provided a theoretical means to link energy flow with experimentally measured interaction strength for bird foraging in the rocky intertidal zone. He estimated parameters of a Lotka-Volterra predator-prey model using consumption rate estimates. He then compared these estimated parameters with those measured (using the same units) using experiments. He found a positive correlation between the two suggesting that it is possible to predict interaction strength from energy flow from the consumer to the resource. The objective of this study is to estimate interaction strength between exotic mud snails and periphyton by using the modeling approach developed by Wootton (1997) and to test these estimates by using controlled field experiments.