Predictors and consequences of earthworm invasion in a coastal archipelago

Non-native earthworms can drive ecosystem change, simplify native plant communities, and promote invasion by non-native plants, but we know little about their pathways into island archipelagos, which currently support about 40% of the worlds threatened species. We studied links among non-native earthworms, human settlement, deer, and plant communities on 26 islands in the San Juan and Southern Gulf Island archipelagos of the Georgia Basin Ecoregion of western North America. We evaluated the (1) invasion pathways and occurrence of non-native earthworms on islands, (2) influence of non-native earthworms on herbaceous and woody plant cover, and (3) potential for synergistic interactions among deer, earthworms and non-native plants. Human settlement was a pre-condition to detecting non-native earthworms on islands. Non-native earthworm abundance was related positively to the cover of non-native herbaceous and woody plants, effects which may be exacerbated by high deer density. Our findings suggest that the absence of non-native earthworms on many small islands makes their protection crucial to the conservation of intact examples of native ecosystems, including critically endangered Garry oak and maritime meadows in Canada.