Earthworm invasions and the decline of clubmosses (Lycopodium spp.) that enhance nest survival rates of a ground-nesting songbird

Invasive earthworms have numerous adverse effects on soils, plants, and vertebrates in forests of North America. Relatively little research has linked earthworm invasions to population vital rates of vertebrates while also assessing habitat characteristics that drive these responses. Furthermore, most investigations of earthworm impacts on plants have focused on graminaceous, woody, and herbaceous species. At sites representing different stages of earthworm invasion, we studied a group of fern-like plants, the clubmosses (Lycopodium spp.), that may be adversely affected by invasions of Lumbricus spp. earthworms, are harvested by humans, and are commonly used by ground-nesting songbirds as a nesting substrate. We found a strong inverse association between Lumbricus biomass and the presence of two clubmoss groups, those with a tree-like growth form (i.e., "tree clubmosses") and those with a finger-like growth form (i.e., "common clubmosses"). Despite finding no relationship between clubmoss presence and visual concealment of nests, we provide evidence that two ground-nesting songbirds, the Ovenbird (Seiurus aurocapilla) and Hermit Thrush (Catharus guttatus), preferentially place their nests near tree clubmosses and that the presence of tree clubmosses increases Ovenbird nest survival probability by 32% on average. Although ecological mechanisms for the relationships between Lumbricus invasions and clubmosses and between tree clubmosses and Ovenbird nest survival are unclear, our findings provide additional motivation to manage forest ecosystems against earthworm invasions. In addition, we highlight the need to manage the human harvest of clubmosses to promote sustainability of clubmoss populations that are simultaneously threatened by earthworms and harvest and to minimize adverse impacts on songbirds.