Predictive mapping to identify refuges for plant communities threatened by earthworm invasion.
Biological invasions by cryptic ecosystem engineers can alter the ecological and socio-economic values of ecosystems in ways that may take decades to detect. The invasion of North American glacial refuges by non-native earthworms offers a prominent but understudied example of a cryptic invasion. Non-native earthworms are known to alter carbon sequestration, disruptmycorrhizal networks and homogenize plant communities, but natural resource managers still lack robust protocols to identify and safeguard high conservation value communities (HCVCs) from such invasions. In the absence of such protocols, or reliable methods to eradicate nonnative earthworm populations once established, there is an urgent need for methods to identify HCVCs at risk of or potentially shielded from earthworm invasion by the existence of abiotic barriers to their dispersal or persistence in such sites. We developed species distribution models (SDMs) using in situ field surveys and remotely sensed data to (1) identify factors limiting the occurrence of non-native Lumbricus earthworms in imperilled coastal Douglas-fir (Pseudotsuga menziesii) forest and Garry oak (Quercus garryana) and maritime meadow ecosystems endemic to glacial refugia of the Pacific Northwest of North America and to (2) evaluate their influence on plant species diversity and identify abiotic factors capable of preventing their dispersal or persistence at a site. As predicted, shallow, drier soils and steeper terrain limited earthworm occurrence and abundance in our highly heterogeneous study area, and earthworm presence appeared to reduce plant species richness. Our results indicated that HCVCs endemic to shallow-soil (< 12±3 cm) sites that experience regular summer drought appear to act as refuges from invasion by non-native earthworms. Our results also elevate concerns for the conservation of deep-soil habitats not isolated from earthworm invasions by shallow soil, saltwater or other barriers to earthworm dispersal. The SDMs developed here offer guidance to the identification of potential refuges for the conservation of imperilled native species and communities from the detrimental effects non-native earthworms in glacial refugia of western North America and should thereby facilitate proactive planning by conservation practitioners.