Treeline ecotones as habitat archipelagos for spiders: A case study from the Alps

Trees surrounded by low–stature alpine vegetation may function as habitat islands for canopy fauna. We examined whether diversity patterns of canopy–dwelling specialist spiders in an alpine treeline ecotone in the French Alps aligned with predictions of island biogeography. We focused on the lower canopy of pine (evergreen) and larch (deciduous) trees, collecting data in Spring, Summer and Autumn. We mapped forest patches and single trees and calculated insularity metrics (e.g. distance to neighbouring trees). We modelled variation of spiders within each tree (α–diversity, measured with Hill numbers) and between trees (β–diversity) against insularity metrics (e.g. tree height, distance to neighbouring trees, distance to the ecotone), tree phenology, sampling season and elevation. We explored diversity patterns for the entire spider assemblage and for two distinct functional groups: web–weavers and non–web–weavers. Insularity metrics contributed to explaining species α–diversity patterns of the canopy–dwelling spider assemblages. However, web–weavers were not affected by any of these variables. Seasonal fluctuations were associated with host tree needle phenology: evergreen trees consistently hosted higher spider diversity than deciduous ones, especially in Spring before leaf–out. At the onset of the season, dissimilarity in spider assemblages across trees (β–diversity) was mostly explained by geographic distance and insularity predictors, with the effect of geographic distance becoming negligible towards Autumn. These findings suggest that trees in alpine treeline ecotones can constitute habitat islands for specialised canopy–dwelling spiders and these effects are mediated by both sharp seasonal and phenological phases as well as by specific hunting strategies of spiders. Trees can be perceived as habitat islands and alpine treeline ecotones as habitat archipelagos, by spiders; The response to insularity differs between spider functional groups as defined by hunting strategies; Host tree leaf phenology (deciduous larch vs. evergreen pine) and seasonality also affect spider diversity; Our findings stress the usefulness of functional island biogeographic studies examining plant–animal interactions.