The effects of fragmentation per se on patch occupancy are stronger and more positive in a landscape with a higher quality and more homogeneous matrix

Habitat fragmentation per se - independent of habitat amount - often increases patch occupancy, possibly because patches are closer together in landscapes with higher fragmentation per se, which should increase dispersal success. Here, we ask whether this effect is influenced by the quality and/or heterogeneity of the landscape matrix, i.e. the non-habitat portion of the landscape. Specifically, we expect the positive effect of fragmentation per seshould be accentuated when matrix quality is high, reducing dispersal mortality. In contrast, when matrix quality is low, high dispersal mortality should lead to fewer colonisations, and accumulation of extinctions across the smaller patches in a more-fragmented landscape could lead to negative effects of fragmentation per se. Additionally, matrix heterogeneity could obscure fragmentation effects, as the link between habitat spatial distribution and between-patch dispersal becomes less predictable. We test these ideas using Glanville fritillary butterfly Melitaea cinxia occupancy data for 4291 habitat patches in the Åland Islands, Finland. Habitat patches for the study species are discrete and well-defined areas where at least one of the two host species occurs. Adult individuals disperse from habitat patches, spending time in the landscape matrix while searching for new habitat patches. Our predictions were mostly supported. Fragmentation effects were more strongly positive when matrix quality was high; however, we did not see the predicted negative effect of fragmentation per se in landscapes with low matrix quality. As predicted, fragmentation effects on patch occupancy were weaker in landscapes with a more heterogeneous matrix. Our findings may explain why fragmentation effects are often weak. They also suggest that the moderating effects of matrix quality and heterogeneity should be explicitly considered when interpreting effects of habitat fragmentation per se on species distributions.