The Laurentian Great Lakes have been subject to numerous introductions of nonindigenous species, including two recent benthic fish invaders, Eurasian ruffe (Gymnocephalus cernuus) and round gobies (Neogobius melanostomus), as well as the benthic bivalve, zebra mussel (Dreissena polymorpha). These three exotic species, or ''exotic triad,'' may impact nearshore benthic communities due to their locally high abundances and expanding distributions. Laboratory experiments were conducted to determine (1) whether ruffe and gobies may compete for habitat and invertebrate food in benthic environments, and (2) if zebra mussels can alter those competitive relationships by serving as an alternate food source for gobies. In laboratory mesocosms, both gobies and ruffe preferred cobble and macrophyte areas to open sand either when alone or in sympatry. In a 9-week goby-ruffe competition experiment simulating an invasion scenario with a limited food base, gobies grew faster than did ruffe, suggesting that gobies may be competitively superior at low resource levels. When zebra mussels were added in a short-term experiment, the presence or absence of mussels did not affect goby or ruffe growth, as few zebra mussels were consumed. This finding, along with other laboratory evidence, suggests that gobies may prefer soft-bodied invertebrate prey over zebra mussels. Studies of interactions among the ''exotic triad'', combined with continued surveillance, may help Great Lakes fisheries managers to predict future population sizes and distributions of these invasive fish, evaluate their impacts on native food webs, and direct possible control measures to appropriate species.
We studied the effect of an invasive omnivore, the rusty crayfish (Orconectes rusticus), on multiple trophic levels of lotic food webs in the upper Midwest, U.S.A. To assess food web effects we (1) conducted a crayfish exclusion experiment in an invaded stream, and (2) surveyed fish populations to compare abundance and diversity across eight streams representing an invasion gradient. In the invaded stream, electrified hoops were used to exclude rusty crayfish from typical food sources (leaves, periphyton, and benthic invertebrates). Breakdown of sugar maple leaves and abundance of invertebrates were measured over 38 days. Leaves in control hoops exposed to rusty crayfish decayed significantly faster (decay coefficient k = 0.1061) than leaves where crayfish were excluded (k = 0.0792). Fewer benthic invertebrates were found in control hoops compared to electric hoops. In contrast, periphyton standing stock on cobble was not altered by the exclusion of rusty crayfish. In the multi-stream survey, streams invaded by rusty crayfish had significantly lower fish abundance and biomass than non-invaded streams, although fish species richness did not differ between stream types. Our results suggest that invasion by rusty crayfish can have bidirectional effects on stream food webs by causing declines in food resources (e.g., detritus and invertebrates), thus indirectly affecting higher trophic levels (e.g., fish). Stream management and restoration strategies should include provisions for controlling or preventing invasions by large benthic consumers such as crayfish.
RÉSUMÉLes effets d'un omnivore invasif, l'écrevisse d'Ohio (Orconectes rusticus) ont été étudiés sur les réseaux trophiques multiples de la chaîne alimentaire lotique dans le nord du Midwest, U.S.A. Pour tester les effets sur la chaîne alimentaire nous avons (1) conduit une expérience d'exclusion d'écrevisses dans un ruisseau envahi, et (2) échantillonné les populations de poissons pour comparer l'abondance et la diversité de huit ruisseaux présentant un gradient d'invasion. Dans les ruisseaux envahis des cerceaux électrifiés ont été utilisés pour exclure les écrevisses de leurs sources alimentaires typiques (feuille, périphyton, invertébrés benthiques). Les brisures de feuilles d'érable et l'abondance des invertébrés ont été mesurées sur 38 jours. Les feuilles dans les cerceaux témoins exposés aux écrevisses se sont décomposées significativement plus vite (coefficient de décomposition k = 0,1061) que les feuilles dont les écrevisses ont été exclues (k = 0,0792). Moins d'invertébrés benthiques ont été trouvés dans les cerceaux témoins par comparaison aux cerceaux électrifiés. Par contre le stock de périphyton sur les galets n'a pas été altéré par l'exclusion des écrevisses. Dans l'échantillonnage des ruisseaux, ceux envahis par l'écrevisse présentaient une abondance et une biomasse de poissons significativement plus basse que les ruisseaux non envahis, bien que la richesse spécifique en poissons ne diffère pas entre les ruisseaux. Nos résultats suggèrent que l'i...
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