Mechanisms and impacts of earthworm invasions on native earthworm species and soil nutrient dynamics
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Earthworm invasions have become an important issue lately due to their significant impacts on bioitic communities and ecosystem functions, especially native earthworm populations and nutrient cycling. However, the details of these impacts and potential mechanisms are still unclear. For native earthworms, habitat disturbance and invasive earthworms have been suggested to be the main reasons for their disappearance and decreasing diversities. A field experiment in a successional forest of Puerto Rico, representing different degrees of disturbance, suggested that changes in soil characteristics and vegetation types from habitat disturbances had no impacts on native Estherella spp. population. The ability of Estherella spp. to re-colonize in the pasture was as good as in the secondary and mature forests. Therefore, habitat disturbance appears not to be the main reason to elucidate the decrease of native earthworm species. A laboratory study that manipulated Puerto Rican soils and earthworms found no competitive interactions between native and invasive earthworm species even for earthworms with similar niches or under reduced litter resources. However, earthworm impacts on soil nutrient dynamics were observed to vary between different earthworm species. These differential impacts were mainly determined by how earthworms interact with microbial populations from different microhabitats, in particular the rhizosphere and detritusphere. A model was developed in this study to project the potential impacts of earthworm invasions on soil carbon dynamics over long-term time scales in areas where peregrine earthworms have invaded or have yet to invade. Results of sensitivity analysis suggested that species characteristics of earthworms (assimilation and production efficiency) can not only affect their own populations but also significantly influence their invasion patterns and impacts on soil carbon dynamics. Also, simulation results suggested that the impacts of earthworm invasions can be affected by invasion history, invasive earthworm assemblages, and pre-invasion ecosystem conditions.
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