Competitive interactions between native and exotic earthworm species as influenced by habitat quality in a California grassland

Abstract

Worldwide, anthropogenic disturbance has contributed to reductions in native earthworm abundance and diversity and subsequent establishment of exotic populations. Exotic earthworm species dominate the earthworm community primarily in nutrient-enriched soils characteristic of urbanized and rural areas, while native species persist in less disturbed habitat. We hypothesized that exotic species may displace native species through competitive interactions, but only under conditions of high resource availability. To test this we manipulated both grassland productivity and the presence and density of intra- and interspecific competitors in field and laboratory experiments with two common species, the native megascolecid Argilophilus marmoratus Eisen 1893 and the exotic lumbricid Aporrectodea trapezoides Dugés 1828. Overall, Ap. trapezoides maintained greater relative growth than Ar. marmoratus. The difference in growth between the two species declined with decreasing habitat quality, primarily due to lower relative growth of Ap. trapezoides in less productive grasslands. As the competitor species, Ap. trapezoides had a negative effect on both itself and Ar. marmoratus in all habitat types, while Ar. marmoratus as competitor had a more neutral effect. Reproductive development followed the same general pattern, but to become reproductive Ap. trapezoides required almost double the amount of relative growth as Ar. marmoratus. Both species had a negative effect on potential food resources, including microbial biomass and belowground plant biomass, but Ap. trapezoides reduced resources more than Ar. marmoratus in all habitat types. Ap. trapezoides achieved a greater relative growth than Ar. marmoratus in soils with high resource availability, but it lost proportionally more mass as resources declined. Thus, Ap. trapezoides attempting to colonize less productive grasslands may simply deplete available resources and fail to achieve sufficient growth for reproductive development. Our results suggest that interspecific competition has the potential to prevent Ar. marmoratus from recolonizing pastures dominated by Ap. trapezoides. Both species respond to enhanced resources in these pastures, but in resource-rich habitats Ap. trapezoides may exclude Ar. marmoratus through greater and faster growth per unit resource consumption coupled with earlier onset of reproduction. Ar. marmoratus is the weaker competitor of the two, but its negative effect on Ap. trapezoides in less productive grassland may exacerbate the role of resource limitation in preventing Ap. trapezoides from expanding its range.