Trophic Tangles through Time? Opposing Direct and Indirect Effects of an Invasive Omnivore on Stream Ecosystem Processes

Abstract

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.

Direct and indirect effects of an invasive omnivore crayfish on leaf litter decomposition

“…Based on results from the upstream site, we hypothesize that the crayfish may also have an indirect effect on leaf litter due to consumption of native invertebrates. Similar results were described for the crayfish P. leniusculus in Californian (USA) streams (Moore et al, 2012) & McIntosh, 2006), in which predators have an indirect effect on detrital food webs by suppressing the efficiency of leaf litter decomposition. However, our results from the downstream site suggest that indirect effects on leaf decomposition through the consumption of native invertebrates may change through time and invertebrates seem to learn how to avoid the crayfish as a predator.…”

Feeding behaviour, predatory functional responses and trophic interactions of the invasive Chinese mitten crab (Eriocheir sinensis) and signal crayfish (Pacifastacus leniusculus)

“…() found the predatory impacts of bullhead on G. pulex in a chalkstream caused a dramatic decline in detrital processing. Conversely, macroconsumers of detritus such as crayfish may decouple such a cascade, functionally replacing the more specialised shredders and thereby still creating availability of nutrients to pass to higher levels (Usio & Townsend, ; Vanni, ; Moore et al ., ), although crayfish effects are likely to be species‐ (Dunoyer et al ., ) and size‐dependent (Mancinelli, Sangiorgio & Scalzo, ). Our combined evidence from the mesocosms, gut contents and stable isotopes indicated that E. sinensis is also a significant consumer of terrestrial leaf litter, so, depending on comparative processing rates and the degree to which shredder populations are reduced, this invader could similarly decouple such a cascade.…”

Between‐lake variation in the trophic ecology of an invasive crayfish

“…Omnivorous species (i.e. species that forage across trophic levels) are important for food web structure through their bridging of multiple trophic levels (Moore et al., ; Parkyn, Collier, & Hicks, ). Invasive omnivores can have disproportionate impacts on native communities via direct and indirect effects that cascade through the food web (e.g.…”

“…Invasive omnivores can have disproportionate impacts on native communities via direct and indirect effects that cascade through the food web (e.g. Klose & Cooper, ; Moore et al., ). Indeed, some omnivorous species have the potential to act as detritivores, herbivores, predators or scavengers in different habitats, implying that habitat characteristics have a disproportionately strong influence on diet.…”