Trophic tangles through time? Opposing direct and indirect effects of an invasive omnivore on stream ecosystem processes

• Published in: PloS one Vol. 7; no. 11; p. e50687
• Main Authors: Moore, Jonathan W, Carlson, Stephanie M, Twardochleb, Laura A, Hwan, Jason L, Fox, Justin M, Hayes, Sean A
• Format: Journal Article
• Language: English
• Published: United States Public Library of Science 27.11.2012; Public Library of Science (PLoS)
• Subjects: Algae; Alnus rubra; Analysis; Animals; Aquatic insects; Astacoidea - physiology; Biology; Biomass; Breakdown; Comparative studies; Consumers; Consumption; Crayfish; Creeks & streams; Detritivores; Detritus; Earth Sciences; Ecosystem; Ecosystem biology; Ecosystems; Environmental changes; Environmental science; Evolutionary biology; Experiments; Fisheries; Fishes; Food chains; Food webs; Herbivores; Invertebrates; Invertebrates - physiology; Leaf litter; Leaves; Nonnative species; Omnivores; Pacifastacus leniusculus; Plant biomass; Plants (botany); Predation; Predatory Behavior - physiology; River ecology; Rivers; Watersheds; British Columbia Canada; Canada; California; United States > US
• ISSN: 1932-6203; 1932-6203
• DOI: 10.1371/journal.pone.0050687

Omnivores can impact ecosystems via opposing direct or indirect effects. For example, omnivores that feed on herbivores and plants could either increase plant biomass due to the removal of herbivores or decrease plant biomass due to direct consumption. Thus, empirical quantification of the relative importance of direct and indirect impacts of omnivores is needed, especially the impacts of invasive omnivores. Here we investigated how an invasive omnivore (signal crayfish, Pacifastacus leniusculus) impacts stream ecosystems. First, we performed a large-scale experiment to examine the short-term (three month) direct and indirect impacts of crayfish on a stream food web. Second, we performed a comparative study of un-invaded areas and areas invaded 90 years ago to examine whether patterns from the experiment scaled up to longer time frames. In the experiment, crayfish increased leaf litter breakdown rate, decreased the abundance and biomass of other benthic invertebrates, and increased algal production. Thus, crayfish controlled detritus via direct consumption and likely drove a trophic cascade through predation on grazers. Consistent with the experiment, the comparative study also found that benthic invertebrate biomass decreased with crayfish. However, contrary to the experiment, crayfish presence was not significantly associated with higher leaf litter breakdown in the comparative study. We posit that during invasion, generalist crayfish replace the more specialized native detritivores (caddisflies), thereby leading to little long-term change in net detrital breakdown. A feeding experiment revealed that these native detritivores and the crayfish were both effective consumers of detritus. Thus, the impacts of omnivores represent a temporally-shifting interplay between direct and indirect effects that can control basal resources.