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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- "If an introduced species can occupy a wider trophic niche than a native species it replaces, this may not only enable it to be successful but may mean its establishment alters food web structures in the ecosystem. Freshwater crayfish are often considered keystone species in freshwater ecosystems and hence in food webs (Momot 1995; Nyström et al. 1996, 1999; Usio 2000). Crayfish feed on benthic invertebrates, macrophytes , algae, detritus, and fish carcases and eggs (Whitledge and Rabeni 1996, 1997; Stenroth and Nyström 2003), and are themselves eaten by larger animals (Roel and Orth 1993; Barrientos et al. 2013). "
ABSTRACT: The introduced North-American signal crayfish (Pacifastacus leniusculus) has become wide-spread throughout Europe where it has often replaced the native noble crayfish (Astacus astacus). The impact of this replacement on ecosystem processes in boreal lakes is still largely unknown. We compared the trophic niches of these two crayfish species in 16 small to medium sized boreal lakes in southern Finland; eight lakes with noble crayfish and eight lakes where the native crayfish populations had been lost and replaced by signal crayfish. We analysed carbon and nitrogen stable isotopes from samples of the crayfish and their putative food sources, and used stable isotope models to compare trophic niche widths of the two species of crayfish and to quantify the food sources used by them. At species level the signal crayfish exhibited a substantially larger trophic niche than that of the noble crayfish, but within-lake populations of the species did not differ in their niche widths. The isotopic niches of the two species strongly overlapped, and while the estimated proportions of food resources (profundal and littoral macroinverte-brates, terrestrial leaf detritus and macrophytes) used by crayfish varied considerably among individual populations, they did not differ consistently between the species. Our results suggest that, contrary to often expressed concerns, replacement of lost noble crayfish populations by the signal crayfish may not greatly alter the littoral food web structure in boreal lakes.Biological Invasions 10/2014; 16(10). DOI:10.1007/s10530-014-0645-x · 2.59 Impact Factor
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- "Several species of crayfish, including signal crayfish, are known to act as 'geomorphic agents', affecting directly the physical environment of river systems (Newton, 2010). This includes bioturbation of sediments through movements such as walking and tail flips (Statzner and Sagnes, 2008); alteration of bedform roughness and modifying bed material particle size distributions and particle consolidation (Statzner et al., 2000, 2003); changes in the composition of fine sediments associated with the processing of organic matter (Usio, 2000); and extensive burrowing into soft river banks which may increase bank erosion and associated increased inputs of fine sediments (Lewis, 2002). Much of this detailed research on sediment mobilisation, however, focuses on experimental laboratory streams and on coarser (gravel-sized) bed material. "
ABSTRACT: Despite increasing recognition of the potential of aquatic biota to act as ‘geomorphic agents’, key knowledge gaps exist in relation to biotic drivers of fine sediment dynamics at microscales and particularly the role of invasive species. This study explores the impacts of invasive signal crayfish on suspended sediment dynamics at the patch scale through laboratory and field study. Three hypotheses are presented and tested: (1) that signal crayfish generate pulses of fine sediment mobilisation through burrowing and movement that are detectable in the flow field; (2) that such pulses may be more frequent during nocturnal periods when signal crayfish are known to be most active; and (3) that cumulatively the pulses would be sufficient to drive an overall increase in turbidity. Laboratory mesocosm experiments were used to explore crayfish impacts on suspended sediment concentrations for two treatments: clay banks and clay bed substrate. For the field study, high frequency near-bed and mid-flow turbidity time series from a lowland river with known high densities of signal crayfish were examined. Laboratory data demonstrate the direct influence of signal crayfish on mobilisation of pulses of fine sediment through burrowing into banks and fine bed material, with evidence of enhanced activity levels around the mid-point of the nocturnal period. Similar patterns of pulsed fine sediment mobilisation identified under field conditions follow a clear nocturnal trend and appear capable of driving an increase in ambient turbidity levels. The findings indicate that signal crayfish have the potential to influence suspended sediment yields, with implications for morphological change, physical habitat quality and the transfer of nutrients and contaminants. This is particularly important given the spread of signal crayfish across Europe and their presence in extremely high densities in many catchments. Further process-based studies are required to develop a full understanding of impacts across a range of river styles. Copyright © 2013 John Wiley & Sons, Ltd.Earth Surface Processes and Landforms 02/2014; 39(2). DOI:10.1002/esp.3486 · 2.85 Impact Factor
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- "However, in case of scarcity or absence of specialist shredders, organic matter breakdown can be driven by other invertebrates such as decapods [10, 14– 17] and gastropods  . Concerning crayfish species few studies have been conducted to assess their role on riparian leaf processing (but see     ). The red swamp crayfish Procambarus clarkii, a native species from northeastern Mexico and south central USA, has been introduced worldwide for commercial and recreational purposes. "
ABSTRACT: We conducted a microcosm study to assess the preferences of Procambarus clarkii (Girard, 1852), an invasive and successful crayfish species, for conditioned leaf litter from alder, oak, and plane with and without feces access. Some chemical foliar characteristics were determined at the beginning and at the end of the experiment. Alder and plane leaves had higher total nitrogen and lower C:N ratio than oak. Oak and plane exhibited similar polyphenol contents. Animals preferred alder>plane>oak leaves (p < 0.0001) and a coprophagous trend was observed. Consumption rates varied among litters according to fecal availability and the interaction litter/fecal availability. Our results suggest that the importance of P. clarkii in breakdown of riparian tree leaves depends not only on the tree species and their characteristics, but also on access to fecal material. Leaf species with high N contents and low C:N ratios would be preferably consumed by the crayfish, and the consumption would be faster in lotic than in lentic systems.International Review of Hydrobiology 08/2013; 98(4):183. DOI:10.1002/iroh.201301536 · 0.97 Impact Factor