Soft range limits shape sensitivity to forest cover more strongly than hard range limits

Land-use change is a major threat to biodiversity, yet there remains
considerable unexplained variation in how it affects different populations
of the same species. Here, we examine how sensitivity to forest cover
changes depending on proximity to different limits of a species’ range. By
comparing responses as species approach their coastal (‘hard’) and inland
(‘soft’) range limits, we aim to provide insight into the relative
influence of mass effects, as compared to abiotic and biotic environmental
suitability in shaping population sensitivity. We combine data from
several large databases to obtain a dataset of 2,543 bird species surveyed
across 116 studies, spanning six continents. Using expert-verified range
maps, we calculated the position of populations relative to their species’
nearest inland (‘soft’) and coastal (‘hard’) range limits and categorized
the inland limits as equatorward- or poleward-facing. We investigated how
distance to range limits and forest cover, derived from a 30m-resolution
global dataset, affect the probability of species’ incidence. We found
that bird populations are more sensitive to forest cover when located
closer to their species’ inland (‘soft’) range limits, whereas this was
not the case at coastal (‘hard’) range limits. The heightened sensitivity
to forest cover at soft range limits was similar regardless of whether the
range limit faced equatorward or poleward. These results highlight how
populations close to the soft limits of their species’ ranges are at
higher risk of extirpation resulting from loss of forest cover. This
suggests that environmental conditions (e.g., climate), which become more
challenging away from the core of the species’ range, drive variability in
sensitivity to forest cover.