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: Summary 1. Biological invasions are widely recognised as a significant component of human-caused environmental change and a primary threat to native biodiversity. The negative impacts of species invasions are particularly evident for freshwater crayfish faunas.2. This study provides novel insight into the ecological effects of native and non-native crayfish on zoobenthic communities (with emphasis on the non-native rusty crayfish, Orconectes rusticus) across broad scales by combining a meta-analysis of small-scale experimental studies with a long-term observational study conducted over a 24 year period in Sparkling Lake, Wisconsin, U.S.A. (46°00′N, 89°42′W). 3. The meta-analysis summarised quantitatively the results of cage experiments for seven species of crayfish spanning four continents. We found that total zoobenthos densities (primarily Gastropoda and Diptera) were significantly lower in treatments containing crayfish relative to controls; a result that was significant for non-native crayfish but not for crayfish in their native range, perhaps owing to a small sample size. In contrast to other species, rusty crayfish were also negatively associated with Ephemeroptera.4. Results from the time series analysis comparing temporal trends in rusty crayfish and invertebrate abundances from Sparkling Lake were consistent with the findings from the meta-analysis. Rusty crayfish were negatively correlated with the abundance of total zoobenthos, Diptera, Ephemeroptera and Odonata, as well as families of Trichoptera.5. By coupling the results from short and long-term research, our study offers greater insight into the nature of crayfish-invertebrate interactions in aquatic systems, revealing consistent effects of invasive crayfish on native fauna. The control and management of invasive species is facilitated by the knowledge that well executed small-scale studies may be extrapolated to understand larger-scale ecological interactions.Freshwater Biology 02/2006; 51(2). · 2.91 Impact Factor
- Knowledge and Management of Aquatic Ecosystems 01/2011; 401:22-36. · 0.62 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 12/2013; 32(4):1318-1332. · 1.42 Impact Factor