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

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.