Native and alien crayfish species: do their trophic roles differ?

Freshwater crayfish are regarded as ecosystem engineers and keystone species. They act on various trophic levels and influence plant and animal species, community-level properties, and ecosystem processes. Comparisons of native and alien crayfish suggest greater negative effects of alien than native crayfish, but the species-specific roles of native and alien crayfish remain unclear. We used 2 enclosure experiments in streams to study biomass-related effects of the native Astacus astacus and Austropotamobius torrentium and the alien Pacifastacus leniusculus on available food types and the potential fate of these nutrients in the stream food web. In experiment 1, we compared the effects of crayfish (2 densities and a no-crayfish control) on leaf breakdown, production of leaf fragments, and accumulation of fine particulate matter (FPM). In experiment 2, we investigated predation by individual crayfish on Gammarus, Hydropsyche, and Arctic Char carrion. Pacifastacus leniusculus had stronger effects on leaf breakdown and a higher accrual of FPM than did the native species, and A. torrentium produced significantly more leaf fragments and fine particulate organic matter (FPOM) than did A. astacus. All 3 crayfish species preyed more successfully on slow-moving than on agile invertebrates, but native crayfish consumed significantly more Gammarus and trichopterans (A. torrentium) and fish carrion (A. astacus) than did P. leniusculus. All 3 crayfish species were polytrophic feeders, but A. torrentium had the strongest overall effects. Thus, the effect of native crayfish can be significantly greater than that of alien crayfish on food supply, predation on macroinvertebrates, and consumption of carrion when the effects are experimentally isolated. The difference in trophic roles suggests either better adaptation of native than alien crayfish to local food supplies or different niche exploitation.