Non-native earthworms in a historically earthworm free California Channel Island: Drivers and ecological effects of earthworm invasions
Identifying the spatial distribution, abiotic and biotic associations, and ecological effects of newly introduced species is an important first step to developing management and control measures. Non-native earthworms, including species from Europe and Asia, have invaded much of the northeastern and Great Lakes deciduous forests of the USA, and the adverse ecosystem-level effects of these invasions are well-documented. However, earthworm invasions in other regions of the USA are poorly studied. We conducted a survey on the historically earthworm free San Clemente Island (SCI), California, USA, to measure the extent, habitat affinities, and ecological effects of a newly discovered earthworm invasion. Using a stratified random sampling approach, we sampled earthworms, vegetation, and soils in 230 plots across the island. We examined the relationship between the presence of invasive earthworms and several soil, vegetation, and landscape variables. We also evaluated the effects of invasive earthworms on vegetation cover and microbial biomass, the latter of which is thought to mediate invasive earthworm impacts on plant growth.
Occurrence of invasive earthworms was relatively low (5.25% of plots) and was strongly associated with distance to streams, topographic wetness index (TWI), and “northeastness” (i.e., deviation from a northeasterly direction, a variable that is a proxy for climatic conditions). We detected a significant positive association between invasive earthworm presence and percent ground vegetation cover and between invasive earthworm presence and percent grass cover. No association between invasive earthworms and microbial biomass was observed. These results and the finding of earthworms in a relatively small number of wet areas indicate that the earthworm invasion on the island is closely tied to moisture conditions. The association between invasive earthworms and total cover of ground vegetation is likely driven by the relationship between earthworms and grass cover, and in particular, by cover of a few invasive grass species (primarily Bromus spp.). Climatic variables (e.g., TWI and northeastness) may be useful for providing insight into current and future sites of earthworm invasion. It should be noted, however, that exceptionally dry conditions preceding and during the survey period likely limited our ability to detect earthworms. Therefore, the invasion may be more widespread than we estimate, and strong moisture limitation during the study period could be the driving factor for the relationships we documented. Nonetheless, follow-up studies may be required to monitor further spread and potential adverse consequences of earthworm invasion on the native flora and fauna of SCI.