Effects of crayfish on leaf processing and invertebrate colonisation of leaves in a headwater stream: decoupling of a trophic cascade
ABSTRACT I performed a field experiment to test the hypothesis that omnivorous crayfish both promote the breakdown of leaves (basal resources) and decouple any potential trophic cascade by similtaneously affecting intermediate consumers as well as there basal resource. Leaf packs were placed inside in situ artificial channels, which excluded or allowed access to crayfish.. During a 4 week period, crayfish greatly promoted leaf processing, with decomposition rates among the fastest ever recorded from temperate streams. Crayfish also affected invertebrate abundance in the leaf packs. As a result of resource consumption, predation and bioturbation, crayfish treatments contained significantly lower densities of invertebrates. In contrast, exclusion of crayfish did not promote leaf decay via increased colonisation by detritivores, primarily because of the conspicuous lack of shredder insects in New Zealnd streams. The results support the hypothesis that omnivorous top consumers decouple cascading chains through similtaneous direct and indirect effects on intermediate consumers and basal resources. Decapod consumers, which have largely been ignored in leaf decomposition studies, can be key leaf processorsin temperate streams where shredder insects are poorly represented.
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ABSTRACT: Nonnative crayfish have been widely introduced and are a major threat to freshwater biodiversity and ecosystem functioning. Despite documentation of the ecological effects of nonnative crayfish from .3 decades of case studies, no comprehensive synthesis has been done to test quantitatively for their general or species-specific effects on recipient ecosystems. We provide the first global metaanalysis of the ecological effects of nonnative crayfish under experimental settings to compare effects among species and across levels of ecological organization. Our meta-analysis revealed strong, but variable, negative ecological impacts of nonnative crayfish with strikingly consistent effects among introduced species. In experimental settings, nonnative crayfish generally affect all levels of freshwater food webs. Nonnative crayfish reduce the abundance of basal resources like aquatic macrophytes, prey on invertebrates like snails and mayflies, and reduce abundances and growth of amphibians and fish, but they do not consistently increase algal biomass. Nonnative crayfish tend to have larger positive effects on growth of algae and larger negative effects on invertebrates and fish than native crayfish, but effect sizes vary considerably. Our study supports the assessment of crayfish as strong interactors in food webs that have significant effects across native taxa via polytrophic, generalist feeding habits. Nonnative crayfish species identity may be less important than extrinsic attributes of the recipient ecosystems in determining effects of nonnative crayfish. We identify some understudied and emerging nonnative crayfish that should be studied further and suggest expanding research to encompass more comparisons of native vs nonnative crayfish and different geographic regions. The consistent and general negative effects of nonnative crayfish warrant efforts to discourage their introduction beyond native ranges.Freshwater science 01/2013; 32(4):1367-1382. · 2.96 Impact Factor
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ABSTRACT: The composition of the diet of the invasive spiny-cheek crayfish Orconectes limosus was studied using qualitative and quantitative analyses of stomach contents. A total of 368 specimens collected in 2003-2005 and 2008 in Czech localities were examined, predominantly from the Labe (Elbe) and Vltava River basins. Food components were compared for three size classes of crayfish and both sexes. The following conclusions were reached: (1) the spiny-cheek crayfish is an omnivorous species consuming plants, animals and detritus; (2) quantitatively, the main food component of O. limosus is detritus, while the plant component was second; (3) O. limosus may swallow whole food particles up to 4 mm in size, and the bodies of small animals may sometimes be found undamaged in their stomachs.Central European Journal of Biology 01/2014; · 0.82 Impact Factor
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ABSTRACT: Dams and impoundments, both large and small, affect downstream physicochemical characteristics and up- and downstream biotic communities. I tested whether small dams and their impoundments altered downstream crayfish assemblages in northern Mississippi. I sampled crayfish and measured physicochemical variables at 4 sites downstream of impoundments (outlet sites) and 4 sites not influenced by impoundments (undammed sites) in August, September, and November 2004. In November 2010, I sampled 7 undammed, 6 outlet, and 3 intermediate sites (influenced by beaver activity or ~1 km downstream of an impoundment). Crayfish assemblages differed between undammed and outlet sites. Catch-per-unit-effort (CPUE) of Orconectes (Trisellescens) sp. was higher in undammed than outlet sites in all samples. Procambarus (Pennides) spp. CPUE was lower in undammed than outlet sites in November 2010 and nearly so in November 2004. In 2004, Procambarus (Ortmannicus) hayi was common in autumn at outlet sites but virtually absent from undammed sites, but in 2010, P. hayi CPUE did not differ between categories. Cambarus striatus CPUE, which was low overall, did not differ between categories in 2004 but was higher in undammed sites in 2010. Seasonal differences among taxa in reproductive timing were important to understanding impoundment effects. The most consistent difference in habitat was that undammed sites had significantly higher width:depth ratios than did outlet sites. Based on the number of mapped small impoundments and a conservative estimate that each impoundment influenced crayfish assemblages over 2 km, I estimated that impoundments probably affect crayfish assemblages in >284 km of stream in the upper Little Tallahatchie River subbasin. Extrapolated to the entire Gulf Coastal Plain, impoundments may influence crayfish assemblages over thousands of stream kilometers.Freshwater science 01/2013; 32(4):1318-1332. · 2.96 Impact Factor