A global meta-analysis of the ecological impacts of non-native crayfish

Twardochleb, Laura A., Olden, Julian D., Eric R. Larson. 2013. A global meta-analysis of the ecological impacts of non-native crayfish. Freshwater Science 32:1367-1382.

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Abstract
Non-native crayfish have been widely introduced and are a major threat to freshwater biodiversity and ecosystem functioning. Despite documentation of the ecological impacts of non-native crayfish from over three decades of case studies, no comprehensive synthesis has quantitatively tested for their general and/or species-specific effects on recipient ecosystems. Here, we provide the first global meta-analysis of the ecological impacts of non-native crayfish under experimental settings to compare effects among species and across levels of ecological organization. Our meta-analysis revealed strong, but variable, negative ecological effects of non-native crayfish with strikingly consistent effects among introduced species. We found that in experimental settings non-native crayfish generally affect all levels of freshwater food webs, ranging from reducing the abundance of basal resources like aquatic macrophytes, to preying on invertebrates like snails and mayflies, to reducing abundances and growth of amphibians and fish. Contrary to our predictions, we did not find that non-native crayfish consistently increase algal biomass. Furthermore, when compared directly to native crayfish, non-native crayfish showed a slight tendency to exhibit larger positive effects on growth of algae and larger negative effects on invertebrates and fish, although there was considerable variability among effect sizes. In conclusion, our study supports the assessment of crayfish as strong interactors in food webs that, by way of their polytrophic, generalist feeding habits, have significant impacts across native taxa. We suggest that species identity may be less important than extrinsic attributes of the recipient ecosystems in determining impacts of non-native crayfish. We identify some under studied and emerging non-native crayfish that would benefit from further studies and suggest expanding research to encompass more comparisons of native vs. non-native crayfish and different geographic regions, but we note that the consistent and general negative effects of non-native crayfish warrant efforts to discourage their introduction beyond native ranges.