The effect of habitat loss and fragmentation on isolation-by-distance and time

Throughout Earths natural history, habitats have undergone drastic changes in quality and extent, influencing the distribution of species and their diversity. In the last few hundred years, human activities have destroyed natural habitats at an unprecedent rate, converting continuous habitat into fragmented and isolated patches. Recent global metanalyses suggest that habitat loss and fragmentation (HL&F) has negatively impacted the genetic diversity of many taxa across the world. These conclusions have been drawn by comparing present-day genetic patterns from populations occurring in continuous and fragmented landscapes. In this work, we attempted to go beyond pattern and investigate through simulations some of the processes that influence genetic variation in the context of HL&F. Since most species have a geographically restricted dispersal (known as "isolation-by-distance", IBD), we studied the impact of HL&F on isolation-by-distance. We characterised the behaviour of IBD in the case of i) instantaneous HL&F, ii) gradual (two-steps) HL&F, and iii) instantaneous HL&F following range expansion. In addition, we propose a spatially-explicit theoretical framework by modifying the original theoretical results on isolation-by-distance (Slatkin, 1991; Slatkin, 1993) and apply them to a toroidal stepping-stone model in the context of HL&F. Our results suggest that isolation-by-distance can be maintained for relatively long time after HL&F, thus pointing to the long-term importance of spatial genetic structure in species genetic diversity. In addition, our results may explain why present-day fragmented population still show significant IBD pattern although being disconnected.