Is metapopulation size important for the conservation of understory plants and epiphytic lichens?

For a given species, reducing the extent, connectivity and health status of source populations will diminish the probability of colonizing suitable habitats. In this study, we evaluated if metapopulation size helped predict species presence in suitable habitats, by using boreal understory plants and epiphytic lichens as case studies. We sampled a network of 792 vegetation plots located across a 240,000-km2 section of pristine boreal forests. Landscape-level species abundance (LLSA), a proxy for metapopulation size, was quantified in a 50 km radius surrounding each plot. The effects of local environmental factors and LLSA on species presence were modelled with general linear models (GLMs). Forty-eight vascular plant species and 8 pendulous epiphytic lichens were abundant enough to be considered in the statistical analyses. Habitat variables explained the presence of 30 vascular plants and 4 lichen species to a relatively strong degree (Tjur's R2 > 0.15). Among these species, 16 vascular plants and 4 lichens were significantly influenced by LLSA, independently of local habitat characteristics. Vascular plant species whose presence was significantly affected by LLSA did not exhibit particular morphological traits or habitat preferences compared with those that were not affected. By contrast, the 4 epiphytic lichen species that were affected by LLSA were all associated with older forests. No species exhibited obvious metapopulation size thresholds below which the probability of species presence in suitable habitats dropped drastically. In terms of conservation, these results suggest that coarse-filter approaches are probably more robust and easy to implement than species-specific threshold approaches in this kind of ecosystem.