Intraspecific variations in dispersal ability of saproxylic beetles in fragmented forest patches

The extrapolation of metapopulation concepts to saproxylic insects suggests that the occupancy of forest patches and the colonization of ephemeral deadwood substrates are driven by micro-evolutionary processes that are related to adaptive plasticity and intraspecific sex-dependent polymorphism of dispersal traits. We hypothesized that forest fragmentation could favor more mobile individuals within populations, but little empirical data have been published on the potentially sex-biased response of insect populations to habitat availability. We selected 88 fragmented woodlots in two European agricultural landscapes to cover different degrees of spatio-temporal fragmentation, from small, isolated and recently established woodlots to large, inter-connected ancient woodlots. In line with our hypothesis, the average wing loading (WL), used as a proxy for dispersal ability, for each of nine flight-dispersing saproxylic beetle species should be lower in recent, small, isolated woodlots than in ancient, large, inter-connected woodlots, respectively (i.e. ancient vs. recent, small vs. large, isolated vs. connected). Forest patch size did not significantly influence the average dispersal ability of beetle colonizers. However, WL of one-third of the tested species did significantly respond to forest ancientness or connectivity. Significant patterns were sex-biased, probably due to the contrasting role of males and females in species colonization dynamics. WL was lower in recent than in ancient forest plots for Melandrya barbata males, and it was lower in isolated than in connected woodlots for Tetratoma ancora and Phymatodes testaceus males. Contrary to expectations, we did not observe any decrease in polymorphism of dispersal abilities with decreasing woodlot size or increasing isolation. Our findings give support to the usefulness of gender consideration in insect conservation ecology studies.