A decade after invasion: Evaluating the continuing effects of rusty crayfish on a Michigan river

Abstract

In 2004, we revisited a Michigan stream invaded by rusty crayfish (Orconectes rusticus) to determine if this species continued to expand its downstream range and negatively impact the stream food web. Compared to a 1992 study, we predicted that rusty crayfish would increase in density and downstream distribution from a small lake, resulting in further reduction of in-stream resources such as organic matter, benthic invertebrates, and periphyton. To determine current crayfish distributions and impacts, we conducted a longitudinal survey of crayfish abundance, ran a 28-d leaf breakdown experiment, and sampled benthic substrates. Leaf packs of sugar maple (Acer saccharum) leaves were placed at three sites with differing crayfish densities (high, intermediate, and none). Breakdown rates were compared across the three sites and for two treatments (closed leaf bags excluding crayfish and open bags allowing crayfish access). Benthic invertebrates were sampled from leaf bags and invertebrates and periphyton were sampled from cobbles. In contrast to 1992, we found that the maximum downstream distance of rusty crayfish declined from 4 km to less than 3 km downstream from the lake. Leaves in open bags decayed significantly faster (k = 0.143) than did leaves in closed bags at all sites (k = 0.079) (p = 0.0005). The site lacking crayfish had significantly higher densities of invertebrates compared to both high and intermediate crayfish density sites (p = 0.005). Although we found that rusty crayfish reduced standing stocks of leaves and invertebrates, we did not detect measurable changes in periphyton biomass. Therefore, rusty crayfish have not dispersed further downstream since 1992, but where present, these omnivores significantly reduce resource availability via the consumption of leaf material and benthic invertebrates.