Relative Effects of Habitat Amount and Fragmentation Per Se on Genetic Diversity of the Glanville Fritillary Butterfly

Habitat loss and fragmentation are considered key drivers of biodiversity loss. Understanding their relative roles is difficult as habitat loss and fragmentation tend to co-occur. It has been proposed that the total habitat amount available in the local landscape mainly drives species richness, while fragmentation per se—the breaking apart of habitat independent of habitat amount—has a negligible or even a positive effect on biodiversity. Several studies support this at the species richness level. Yet, the potential effects of fragmentation per se on genetic diversity at the landscape scale are understudied. Using the Glanville fritillary butterfly metapopulation in the Åland islands, we tested the effects of fragmentation on genetic diversity using a landscape-based approach and 2610 individuals genotyped at 40 neutral SNP markers. We assessed the independent effect of habitat amount and fragmentation (i.e., number of patches, habitat aggregation) within the local landscape on focal patch genetic diversity. The amount of habitat in the local landscape had a positive effect on genetic diversity. Fragmentation measured as the number of patches within the landscape had a negligible impact on genetic diversity, whereas habitat aggregation had a negative effect on genetic diversity when the available amount of habitat in the landscape was low. Focal patch size increased genetic diversity, whereas focal patch structural connectivity had no impact. Our results highlight that all fragments are important to contribute to the total amount of habitat available, and that the impact of habitat fragmentation matters more when the total amount of habitat available is low.