Landscape structure is an important determinant of the sizes and connectivity of spatially structured populations. Many demographic and evolutionary processes are influenced by spatial structure of populations. Inbreeding may increase in frequency and cause loss of genetic diversity in fragmented landscapes. Previous studies have shown that inbreeding can reduce fitness of individuals (inbreeding depression) and increase the extinction risk of local populations. However, it has not been studied whether individual- and population-level effects of inbreeding depression is consequential at the metapopulation level, because substantial inbreeding is expected only in relatively small subpopulations. The objective of this study is to scale up the effects of inbreeding depression to the metapopulation level and to examine the effects of landscape structure on the frequency of inbreeding, genetic diversity, and metapopulation persistence. Because this question is difficult to investigate empirically, we used a modeling approach. We developed a genetically explicit individual-based model of a metapopulation parameterized with extensive data and knowledge available for the Glanville fritillary butterfly in the Åland Islands in SW Finland. We applied this model to 18 semi-independent networks of habitat patches in a NE region of the mainland Åland.
Results/Conclusions
The results show that substantial inbreeding occurs in the metapopulations, even in the ones occupying large habitat networks. The extinction risks of metapopulations living in small but not the smallest networks increase due to inbreeding depression, compared with those in smallest and sufficiently larger networks. In smallest networks, stochasticity dominates the dynamics such that inbreeding depression had a negligible effect. On the other hand, large networks contained large populations that are not extinction prone and also acted as a reservoir of genetic diversity. In the intermediate sized networks, inbreeding depression caused small local populations to go extinct more often, and the metapopulations lose critical connectivity for their persistence. These outcomes are largely attributable to the interactions between the mating and oviposition behavior of the butterfly and a large number of small patches composing the habitat networks. We conclude that landscape fragmentation may reduce persistence of metapopulations not only by the loss of habitat amount and connectivity but also by eroding genetic diversity through increased inbreeding in this species.