We showed that P. aegeria individuals (both males and females) had a more sedentary daily movement pattern in a highly fragmented urban landscape compared to males in fragmented agricultural and more continuous woodland landscapes (data for females were not available in these two landscape types). We showed that movements of P. aegeria in fragmented urban areas corresponded to a very large α value, whereas we expected the opposite due to the high fragmentation level of the landscape, and hence, the scattered and heterogeneous distribution of habitat resources. More generally, we observed that daily distances were inversely related to the distance to the nearest woodland patch and they differed between sexes. Daily distances flown by males were related to the area of the woodland capture site, whereas no such effect was observed for females. Finally, we showed that habitat availability and sampling design had not biased results, on the one hand by constraining daily dispersal movements and on the other hand by leading to under-sampling one of the sexes.
Distances of daily movements of P. aegeria decreased with increasing distance to the nearest woodland habitat patch. Thus, inter-patch distances play a role for these movements and nearby landscape cues may facilitate moving across patches. Indeed, it has been shown that individuals of fragmented agricultural landscape populations are able to orient toward forested habitat, but only so from a distance of 100 m on average.
In P. aegeria, daily distance flown differs between males and females[40, 43] in highly fragmented landscapes. Here, males, but not females, had higher probabilities to stay in large compared to small woodland patches. Indeed, male individuals are known to adopt territorial behaviour in sunspots on the forest floor[44, 45]. But males may adopt one of two different mate-locating tactics: males actively search for females (i.e. patrolling) or wait for them in aggressively defended territories (i.e. territorial perching)[37, 46, 47]. These two strategies can generate high local abundances in woodland patches which may lead to density-dependent dispersal, as demonstrated for other butterfly species[48, 49]. However, in our study, there was no evidence for a relationship between local patch abundance and movements. Our results only suggest that in large woodland areas, males show shorter daily movements. Two reasons may explain such a pattern: (1) it might be a consequence of decreasing patch boundaries with increasing patch area, or (2) males adopt perching behaviour more successfully in large woodland patches. We cannot neglect that these results may also be linked to the carrying capacity of woodland patches and be influenced by higher woodland patch quality. Indeed, landscape composition (i.e. quantity and quality of habitat resources present, e.g.) and landscape configuration (i.e. spatial arrangement and connectivity of habitat resources) are key factors that influence dispersal processes and have a strong impact on local populations.
Compared to males, females had a higher probability of moving longer distances in our highly fragmented landscape, which may be due to the advantage of distributing eggs over a large area. Moreover, single individuals of P. aegeria crossing open fields in Britain were all females, which also suggests that when females cross boundaries between woodland patches, they are more likely to continue their flight across the landscape than males. Based on these results and by considering that males and females differ in the degree of long-distance dispersal, we expect the same differences in dispersal patterns between males and females to apply in fragmented agricultural and more continuous woodland landscapes. The low recapture percentage for females could be explained by two reasons. Firstly, females show more cryptic behaviour than males and secondly, females are thought to be much more significant for long-distance dispersal in P. aegeria than males. Males have a larger propensity to return into a habitat patch in the butterfly Speyeria idalia. In our study, there was no relationship between female movements and the variables related to the departure site or to landscape features. Hence, this suggests that males and females interact at different spatial scales with their environment. Due to their mating behaviour, males are strongly influenced by their immediate environment; more precisely, their ability to detect and pursue a female depends on the acuity of their eyes, the motion of the object, the background and the ambient level of illumination in the butterfly Asterocampa leilia. Field studies on male behaviour in P. aegeria have frequently observed fast types of flight, with high levels of acceleration from a resting posture to passing objects as a typical component of the behavioural repertoire of territorial males[44, 55]. Females will show this type of powerful, explosive flight much more rarely than do males (perhaps only to escape from predator attacks) and they have very different flight patterns altogether. They alternate between fluttering inspection flights above potential host grasses and dispersal flights that are regularly interrupted by basking stops[51, 56]. Thus, at the landscape scale, females have a higher probability to move further compared to males. Female movements are mainly driven by mating and mediated by costs of the searching males (e.g. energy expenditure, time lost and enhanced predation risk, and spreading their offspring).
However, even if females were better able to cover wider distances than males in the highly fragmented landscape, the high α values showed that average distances in the highly fragmented urbanised landscape were small for both males and females compared to α values for males in fragmented agricultural landscapes (Boshoek/Rillaar) and the landscape dominated by deciduous oak woodland (Meerdaalwoud). Indeed, compared to values of 31 other species reviewed in and compared to values of male P.aegeria in fragmented agricultural landscapes (Rillaar and Boshoek) or in a woodland landscape (Meerdaalwoud), the parameter scaling the exponential negative distribution of dispersal distances showed high values. Compared to the value of 24.3 in the localized skipper butterfly Hesperia comma,, our study on P. aegeria females showed a similar α value (i.e. 26.3), whereas males had higher α values (i.e. 46.2). Such differences between sexes in dispersal kernels are rarely tested. Our results indicated that dispersal kernels should not be considered as species-specific, but rather as the results of the context- and condition-dependent dispersal process (e.g.). Only British Plebejus argus showed a higher α value (126.6), but this is due to specificities concerning populations extremely isolated in habitat islands within the British landscapes. Many factors may partly affect α values. The main bias occurred for males. Due to their behaviours (they can either defend a territory and adopt a waiting strategy, intercepting females passing through their territory, or instead may actively search for mates), they have a very high probability of capture, and hence are usually over-represented in MRR datasets. This bias was not controlled for and may explain the large value of α measured, notably compared to the females.
In the context of dispersal modelling, recent studies have aimed to analyse dispersal kernels in various landscapes (e.g. for seeds[61, 62]). Such models are currently used to study highly complex dispersal patterns. The evolution of dispersal kernels, which are themselves shaped by the environment, provides a valuable indication of selection acting upon species traits. In this context, our study fits recent modelling studies where dispersal kernels emerged from movement rules[62, 63]. More precisely, our study provides a novel extension to these recent modelling developments because our results show that individuals do not use a single species-specific fixed movement rule but rather that sex-specific rules may apply too. These results are in agreement with other modelling studies[62, 64, 65].
Finally, the aim of this study was to assess dispersal evolution in relation to habitat fragmentation. Even if we showed here that individuals move within the landscape, and that males in fragmented habitats move less, we still have no idea whether they are ‘able’ to move more or whether the movement differences are due to some other factors than fragmentation (e.g. dispersal evolution).