The effects of crayfish on interactions in freshwater benthic communities

Sammanfattning: This thesis examines how freshwater crayfish affect community interactions in benthic food webs. I have mainly studied the influence of the introduced signal crayfish (Pacifastacus leniusculus) on littoral communities, but also the influence of the native noble crayfish (Astacus astacus). Overall, the results show that both crayfish species are omnivores having strong direct and indirect effects on the structure of benthic food webs, by consuming specific macrophytes, invertebrates and tadpoles. Crayfish are selective consumers of macrophytes and invertebrates but the two species had similar preferences. However, the introduced species had a somewhat stronger effect on invertebrates and macrophytes than the native species. Among the macrophytes, submerged macrophytes (particularly macroalgae of the genus Chara) were preferred to thick-stemmed emergent species, but crayfish may have strong effects on floating-leafed and emergent species by consuming their seedling stages. Among the invertebrates thin-shelled snail species were greatly reduced by crayfish whereas the direct impact of crayfish on swimming or burrowing invertebrates were minor. Since crayfish reduced the numbers of important grazers (snails or tadpoles), there was an indirect, positive effect on periphyton. In the long-term however, when the periphyton community is more developed, crayfish grazing could counterbalance this effect. The combined effect of predatory fish (introduced rainbow trout) and signal crayfish on the structure of a pond community was mostly additive, suggesting that these two predators do not affect each others foraging efficiency to a great extent. The main conclusions from this thesis are that the effects of crayfish populations on littoral communities are strong but complex, having both direct and indirect trophic effects, effects which may also be dependent on the species-specific consumption rate.