Habitat fragmentation impacts mobility in a common and widespread woodland butterfly: do sexes respond differently?

Abstract

Background: Theory predicts a nonlinear response of dispersal evolution to habitat fragmentation. First, dispersal will be favoured in line with both decreasing area of habitat patches and increasing inter-patch distances. Next, once these inter-patch distances exceed a critical threshold, dispersal will be counter-selected, unless essential resources no longer co-occur in compact patches but are differently scattered; colonization of empty habitat patches or rescue of declining populations are then increasingly overruled by dispersal costs like mortality risks and loss of time and energy. However, to date, most empirical studies mainly document an increase of dispersal associated with habitat fragmentation. We analyzed dispersal kernels for males and females of the common, widespread woodland butterfly Pararge aegeria in highly fragmented landscape, and for males in landscapes that differed in their degree of habitat fragmentation.

Results: The male and female probabilities of moving were considerably lower in the highly fragmented landscapes compared to the male probability of moving in fragmented agricultural and deciduous oak woodland landscapes. We also investigated whether, and to what extent, daily dispersal distance in the highly fragmented landscape was influenced by a set of landscape variables for both males and females, including distance to the nearest woodland, area of the nearest woodland, patch area and abundance of individuals in the patch. We found that daily movement distance decreased with increasing distance to the nearest woodland in both males and females. Daily distances flown by males were related to the area of the woodland capture site, whereas no such effect was observed for females.

Conclusion: Overall, mobility was strongly reduced in the highly fragmented landscape, and varied considerably among landscapes with different spatial resource distributions. We interpret the results relative to different cost-benefit ratios of movements in fragmented landscapes.

Figure 1

Butterfly detection probabilities. Relationship between detection probability rates of males (bold line) and females (dotted line) and the number of recapture surveys.

Figure 2

Negative exponential dispersal kernels. Sex-specific cumulative probabilities to move a given distance (km): P(D) (black bold line) ± SD (dotted lines) in the different landscape types.

Figure 3

Butterfly recapture distances. Mean habitat availability proportion ± SD (black bold line) and butterfly recapture proportion (dotted black line) in relation to distance (meters) from individual capture locations.

Figure 4

Study sites. A. Map of the Île-de-France region (Paris, France), and B. locations of the eight woodland patches sampled in the northern part of the Parc du Sausset.

Figure 5

Available woodland habitat calculation. Proportion of available woodland for a butterfly at distance d of the capture point (d varied from 0 to 50 meters).