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, including extinction, colonization, gene flow, and genetic drift. Inbreeding may increase in frequency in landscapes characterized especially by small, less connected populations. 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 the extinction risk of metapopulations living in small, more fragmented networks increased disproportionately due to inbreeding depression, compared with those in sufficiently larger networks. This indicates that landscape structure and inbreeding depression interact to further disconnect subpopulations in metapopulations in this study system. Inbreeding depression caused small populations to go extinct more often, and metapopulations living in small networks were more negatively affected by the extinction of small subpopulations providing critical connectivity for their persistence. The frequency of inbreeding was about four times higher in smaller networks, but substantial inbreeding occurred also in large networks. We conclude that landscape fragmentation may reduce persistence of metapopulations not only by the loss of habitat and connectivity but also by increasing the degree of inbreeding and inbreeding depression.