Image: San Clemente Island by Shishir Paudel
Many of the species of earthworms in North America are exotic species and their presence alters native ecosystems in profound ways that resonate from the soil up through the vegetation and into the vertebrate communities. So when word came that the US Navy had found earthworms on San Clemente Island, an island thought to be earthworm free, and was looking for an assessment of their distribution and possibilities for eradication I was interested.
I contacted earthworm (and bird) expert Scott Loss at Oklahoma State and we were able to secure the funding. He put together a team that included post-doc Shishir Paudel and other colleagues at Oklahoma State and Beau MacDonald (at first UWG contract GIS expert and now USC Spatial Sciences Institute staff GIS Project Specialist) did the GIS work and modeling on our end. The second of the papers from the study came out this week in Diversity and Distributions and Beau put together this summary graphic.
Source: Beau MacDonald, graphical summary of Paudel, S., G.W.T. Wilson, B. MacDonald, T. Longcore, and S. R. Loss. Predicting spatial extent of invasive earthworms on an oceanic island. Diversity and Distributions (2016).
The bottom line is that the earthworms (several species of them) were found near the main road and in areas that were moist. All of the best-performing predictive models for the distribution of the earthworm included proximity to the paved road. This suggests that the invasion is in its early stages and is associated with the road in some way.
We speculate, but do not have proof, that the earthworms were introduced when topsoil was brought from the mainland in 2008-2009 to pave the road. This explanation is consistent our team not finding any earthworms south of the southernmost extent of the paving (the dashed line in the middle map above).
The paper goes into detail about the vegetation conditions where the earthworms were found; they are associated with exotic grasses and other non-native plants.
Time will tell the full effect of the invasion of earthworms on San Clemente Island. The island has remarkable archeological resources because it used to be free of burrowing animals that moved soil and artifacts around. Being earthworm free (and free of burrowing mammals) was an advantage for those resources in addition to being the natural ecological condition. I hope this research provides a warning to those proposing and doing construction on oceanic islands with high biological diversity and natural values to sterilize any building materials being imported for use lest they introduce unexpected species to environments where they can do damage.
Paudel, S., G.W.T. Wilson, B. MacDonald, T. Longcore, and S. R. Loss. Predicting spatial extent of invasive earthworms on an oceanic island. Diversity and Distributions (2016).
Invasions of non-native earthworms into previously earthworm-free regions are a major conservation concern because they alter ecosystems and threaten biological diversity. Little information is available, however, about effects of earthworm invasions outside of temperate and boreal forests, particularly about invasions of islands. For San Clemente Island (SCI), California (USA) – an oceanic island with numerous endemic and endangered plant and vertebrate species – we assessed the spatial extent and drivers of earthworm invasion and examined relationships between earthworms and plant and soil microbial communities.
San Clemente Island, southern California, USA.
Using a stratified random sampling approach, we sampled earthworms, vegetation, soils and microbial communities across SCI. We examined the relationship between the presence of invasive earthworms and soil and landscape variables using logistic regression models and implemented a spatial representation of the best model to represent potential site suitability for earthworms. We evaluated the relationship between invasive earthworms and vegetation and microbial variables using ANOVA.
We found that the likelihood of encountering earthworms increased close to roads and streams and in high moisture conditions, which correspond to higher elevation and a north-eastern aspect on SCI. The presence of earthworms was positively associated with total ground vegetation cover, grass cover and non-native plant cover; however, there was no significant relationship between earthworms and microbial biomass. These results suggest that the earthworm invasion on SCI is at an early stage and closely tied to roads and high moisture conditions.
Climatic variables and potential sources of earthworm introduction and dispersal (e.g. roads and streams) should be broadly useful for predicting current and future sites of earthworm invasions on both islands and continents. Furthermore, the significant positive relationship between non-native plant cover and invasive earthworm presence raises the possibility of an emerging invasional ‘meltdown’ on SCI. Additional study of earthworm invasions on human-inhabited oceanic islands is necessary to identify additional invasions and their potential for negative impacts on unique insular biota.