Abstract
BackgroundInvasive ecosystem engineers can facilitate their invasions by modifying the physical environment to improve their own performance, but this positive feedback process has rarely been tested empirically except in sessile organisms. The invasive crayfish Procambarus clarkii is an ecosystem engineer that destroys aquatic macrophytes, which provide a physical refuge for animal prey, and this destruction is likely to enhance vulnerability to predators. Using two series of mesocosm experiments, we tested the hypothesis that the invasive crayfish increases its feeding efficiency on animal prey by reducing submerged macrophytes, thus increasing its individual growth rate in a positive density-dependent manner.ResultsIn the first experiment, increasing crayfish density reduced both macrophytes and animal prey (dragonfly and chironomid larvae) and, importantly, increased the growth rate of individual crayfish, in accordance with our expectation. In the second experiment, we used artificial macrophytes to clarify whether the physical architecture of macrophytes itself protects animal prey and limits crayfish growth rate. Increasing the artificial macrophyte quantity not only increased the survival of animal prey, but also retarded the crayfish growth rate.ConclusionsWe conclude that macrophytes strengthen bottom-up control of crayfish, but this effect can be relaxed by increasing the density of crayfish via reduction in macrophytes. This positive feedback process may explain the crayfish outbreaks and regime shifts occasionally observed in invaded freshwater ecosystems.
Highlights
Invasive ecosystem engineers can facilitate their invasions by modifying the physical environment to improve their own performance, but this positive feedback process has rarely been tested empirically except in sessile organisms
The model selection based on AICc supported the results of the likelihood ratio tests, because the models including the crayfish density were selected as the best models with high Akaike weights (w > 0.8; Additional file 1: Table S1)
Based on a set of mesocosm experiments, we demonstrated that invasive crayfish can increase their own growth rate by habitat modification, or ecosystem engineering
Summary
Invasive ecosystem engineers can facilitate their invasions by modifying the physical environment to improve their own performance, but this positive feedback process has rarely been tested empirically except in sessile organisms. One of the reasons for this is that environmental modification by engineers occasionally creates positive feedback that promotes their own population Invasive crayfish such as Procambarus clarkii can abruptly become overabundant and dramatically decrease the abundance and diversity of aquatic plants and animals in freshwater ecosystems [15,16,17]. Since the physical architecture of macrophytes controls prey accessibility [21], the reduction of macrophyte refuges by crayfish (i.e., ecosystem engineering) elevates the crayfish predation rate on animal prey [18] and this can be considered a sort of “interaction modification” This ecosystem engineering may further affect the crayfish itself; since a high predation rate implies a high foraging efficiency on animal prey, invasive crayfish may exhibit accelerated growth by the ecosystem-engineering effect. This “self-reinforcing” bottom-up effect, or positive feedback process, may be a mechanism underlying crayfish outbreaks in lakes and ponds, but it is not clear how changes in macrophyte and crayfish densities influence the growth rate of crayfish via changes in prey accessibility
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