Trophic dynamics in a simple experimental ecosystem: interactions among centipedes, Collembola and introduced earthworms.
Invasive earthworms in North America are known to have dramatic influences on soil ecosystems, including negative effects on other soil fauna. In general, studies examining this phenomenon have focused on invasive earthworm impacts on organisms at the same or lower trophic level as the earthworms themselves (i.e., detritivores and decomposers). In contrast, there have been relatively few studies of invasive earthworm impacts on higher trophic levels or food web interactions. Invasive earthworms might alter food webs either directly as prey items, or indirectly through their profound alteration of soil physical characteristics, which might in turn alter established predator/prey interactions. In this study, we created experimental mesocosms to investigate the influence of an invasive earthworm on a native predator-prey interaction. We incubated several combinations of a widespread Asian invasive earthworm (Amynthas agrestis), a generalist centipede predator (family: Cryptopidae), and a putative microarthropod prey species (Sinella curviseta) in an experiment to determine their interactions. We hypothesized that collembolan abundance would be reduced in mesocosms containing centipedes. We further hypothesized that earthworm feeding on litter substrate in the mesocosms would reduce the complexity of the substrate, and thus increase the likelihood of centipede/collembolan encounters, with the ultimate effect of more pronounced reduction of Collembola populations in mesocosms containing both earthworms and centipedes. Unexpectedly, we found that earthworms had a negative effect on collembolan abundance early in the incubation and that centipedes did not. Collembolan populations were less variable through time in the presence of earthworms, suggesting that invasive earthworms exerted some regulatory pressure on food or habitat supply for the collembolans. Centipedes lost biomass when only Collembola were available for food, but gained biomass when incubated with earthworms. We noted a simultaneous significant decrease in earthworm biomass (mainly in the form of earthworm mortality) in experimental units that contained centipedes relative to those without centipedes. Taken together, these results suggest that this invasive earthworm may represent a novel prey resource for the centipedes, and that their presence in ecosystems could have consequences on both higher and lower trophic levels.