Time-lag in the interacting effects of habitat amount and fragmentation on genetic diversity in red squirrels (Sciurus vulgaris)

Genetic diversity is important for adaptation and persistence of populations. It is influenced by many scale-dependent factors including demographic history and interactions with surrounding landscape, including the availability of habitat. We aim to determine the independent and interactive effects of habitat amount and habitat fragmentation on red squirrel (Sciurus vulgaris) genetic diversity. We also assess the spatial and temporal scales at which habitat amount and fragmentation have the strongest impacts on genetic diversity. We deployed over 1100 hair tubes across five 16 × 16 km study areas in the south of France to gather red squirrel hair samples, and genotyped 443 individuals at 16 microsatellite loci. Using moving windows, we calculated the proportion and number of patches of forest habitat around sample locations across time periods from 1900 to 2019 and within distances between 4 and 18 km. We fit mixed-effects models correlating genetic diversity to these metrics. We found a significant positive interaction between the amount of forest habitat and the number of patches when modeling genetic diversity. We present evidence that genetic diversity is most closely related to landscape conditions roughly 40 generations (40 years) prior to sampling, and at broad spatial scales (12 km radius or larger). Our research provides additional insight into the effects of habitat fragmentation per se (i.e. independently of habitat amount) on biodiversity, in addition to the scales of effect. Understanding the scales at which habitat loss and fragmentation impact genetic diversity is important for predicting and mitigating future biodiversity impacts.