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Influence of an Exotic and a Native Crayfish Species on a Littoral Benthic Community
Per Nyström, Christer Brönmark and Wilhelm Granéli
Vol. 85, No. 3 (Jun., 1999), pp. 545-553
Stable URL: http://www.jstor.org/stable/3546704
Page Count: 9
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The aim of this study was to compare the effects of the introduced signal crayfish (Pacifastacus leniusculus) and the native noble crayfish (Astacus astacus) on a benthic food web. We mimicked the habitat of a pond littoral in 4.5- m2 plastic pools stocked with natural densities of macrophytes, invertebrates and either signal crayfish, noble crayfish, or kept as crayfish free controls. After two summer months, all invertebrates and macrophytes were collected from each pool, and periphyton was sampled on one substratum exposed and two substrata not exposed to crayfish grazing. Samples for stabile isotope analysis of benthos were collected in pools with noble crayfish. 15 N ratios showed that crayfish were top consumers, and 13 C ratios indicated that they 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 strong impact on grazers dominated by thin-shelled Lymnaea snails. Hard-shelled Bithynia snails were also reduced in numbers, but the largest of these snails were consumed less than thin-shelled Lymnaea snails. The reduced biomass of snails had an indirect positive effect on periphyton biomass on all three substrata. Crayfish grazed selectively on macrophytes and reduced the biomass of Chara, whereas Elodea was less affected. The exotic signal crayfish had, overall, a stronger impact on the biomass of macrophytes and grazers than the native noble crayfish. The results indicate that crayfish may structure food webs through consumption from many food levels. The short-term influence of crayfish on other trophic levels depends on crayfish feeding efficiency, food preferences and species-specific consumption rates.
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