Loss of riparian vegetation surrounding streams can affect instream biota by altering stream characteristics, such as terrestrially derived detrital inputs and instream productivity. Omnivorous crayfish can be a dominant component of stream biota and are considered a keystone species because of their ability to forage at multiple trophic levels. Resource shifts caused by changes in riparian canopy have the potential to influence crayfish diet and growth. We investigated the effects of changes in canopy cover on the crayfish Cherax destructor in a southeastern Australian lowland stream. We compared the diet of C. destructor between sites with and without riparian cover and determined how differences in the quantity of food resources between sites affected crayfish growth. The availability of basal (plant, algae, and detrital) resources was related to the presence of a riparian canopy. Aquatic macrophytes were more common at sites with no canopy cover and terrestrially derived leaf litter was more abundant at sites with an intact canopy. Stable isotope and gut content analyses of crayfish diet indicated a shift toward autochthonous food sources in individuals from sites with no canopy cover. In laboratory feeding trials, crayfish had higher growth rates when fed macrophyte material than when fed terrestrially derived leaf litter. Insights gained into resource use by crayfish, particularly the importance of aquatic invertebrates in crayfish diet, emphasize the merits of conducting both gut content and stable isotope analyses to assess short- and longer-term aspects of diet. Further structural and functional impacts of changes to riparian condition should be investigated, but the trophic role of C. destructor in stream food webs appears to be sensitive to alterations in the dominant basal resources associated with changes in riparian canopy.
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Vol. 28 • No. 3