Native and exotic earthworms in a California oak savanna ecosystem
MetadataShow full item record
California’s oak savanna supports a unique, indigenous earthworm fauna. Throughout California, anthropogenic disturbance and habitat loss have resulted in the displacement of native earthworm species with exotic species better adapted to human-disturbed environments. My research focused upon environmental and biotic factors that contribute to the establishment of exotic earthworms and to their ability to displace native species. Through spatial and regression analyses of the relative abundance of native and exotic earthworms at my study area, I found that human disturbance is the critical factor associated with the establishment of exotic species. Even mild disturbance associated with soil nutrient amendment was sufficient to promote the establishment of exotic species. Once established, exotic species are capable of displacing native species provided soil nutrients remain high, but seldom persist in unamended grasslands. I hypothesized that this is due in part to habitat-specific competition, and studied competitive interactions between the common exotic species Aporrectodea trapezoides (Lumbricidae) and the native species Argilophilus marmoratus (Megascolecidae). Ap. trapezoides maintained a potential competitive advantage over Ar. marmoratus only in nutrient-rich habitats. Both species have growth thresholds associated with reproductive development, but that of Ap. trapezoides is much higher than Ar. marmoratus. In poor habitat, Ap. trapezoides may fail to establish because it cannot obtain enough nutrition to maintain reproduction. Finally, I conducted a study to determine whether the two species differed in their impacts on primary production and nutrient turnover, critical ecosystem functions. Ap. trapezoides was more physically active than Ar. marmoratus, enhanced plant growth and N uptake, and also promoted the turnover of N from decomposing litter through soil and plant N pools. Ap. trapezoides reduced the microbial biomass through grazing. Nitrogen that would have been immobilized in microbial biomass was thus made available to plants, which may partly explain Ap. trapezoides’ positive effect on plant growth.