Influence of an exotic and a native crayfish species on a littoral benthiccommunity

The aim of this study was to compare the effects of the introduced signal c rayfish (Pacifastacus) and the native noble crayfish (Astacus astacus) on a benthic food web. Wt mimicked the habitat of a pond littoral in 4.5-m(2) p lastic pools stocked with natural densities of macrophytes, invertebrates a nd either signal crayfish, noble crayfish. or kept as crayfish free control s. After two summer months, all invertebrates and macrophytes were collecte d from each pool. and periphyton was sampled on one substratum exposed and tno substrata not exposed to crayfish prating. Samples for stabile isotope analysis of benthos were collected in pools with noble crayfish. N-15 ratio s showed that crayfish were top consumers, and C-13 ratios indicated that t hey received most of their carbon from invertebrates, but less from primary producers. Crayfish did not affect the biomass of predatory invertebrates, dominated by active swimmers among Heteroptera and Coleoptera, but had a s trong impact on grazers dominated by thin-shelled Lymnaea snails. Hard-shel led Bithynia snails were also reduced in numbers, but the largest of these snails were consumed less than thin-shelled Lymnaea snails. The reduced bio mass of snails had an indirect positive effect on periphyton biomass on all three substrata. Crayfish grazed selectively on macrophytes and reduced th e biomass of Chara, whereas Elodea as less affected. The exotic signal cray fish had, overall, a stronger impact on the biomass of macrophytes and graz ers than the native noble crayfish. The results indicate that crayfish may structure food webs through consumption from many food levels. The short-te rm influence of crayfish on other trophic levels depends on crayfish feedin g efficiency. food preferences and species-specific consumption rates.