Factors affecting dispersal in a butterfly - A comparison between core and edge populations

In the current era of anthropogenic climate change is the long-term survival of all organisms dependent on their ability to respond to changing environmental conditions either by (1) phenotypic plasticity, which allows species to tolerate novel conditions, (2) genetic adaptation, or (3) dispersal to more suitable habitats. The third option, dispersal, allows individuals to escape unfavorable conditions, the colonization of new areas (resulting in range shifts), and affects patterns of local adaptation. It is a complex process serving different functions and involving a variety of underlying mechanisms, but its multi-causality though has been fully appreciated in recent years only. Thus, the aim of this doctoral thesis was to disentangle the relative importance of the multiple factors relevant to dispersal in the copper butterfly Lycaena tityrus, including the individual condition (e.g. morphology, physiology, behavior) and the environmental context (e.g. habitat quality, weather). L. tityrus is a currently northward expanding species, which makes it particularly interesting to investigate traits underlying dispersal. In the first experiment, the influence of weather and sex on movement patterns under natural conditions was investigated. Using the Metatron, a unique experimental platform consisting of interconnected habitat patches, the second experiment aimed to examine the influence of environmental factors (resources, sun) on emigration propensity in experimental metapopulations. Human-induced global change (e.g. climate change, agricultural intensification) poses a substantial challenge to many herbivores due to a reduced availability or quality of feeding resources. Therefore, in the third experiment, the impact of larval and adult food stress on traits related to dispersal ability was investigated. Additionally, the effect of different ambient temperatures was tested. In the fourth experiment, core (Germany) and recently established edge (Estonia) populations were compared in order to explore variation in dispersal ability and life history traits indicative of local adaptation. Dispersal is often related to flight performance, and morphological and physiological traits, which was investigated in experiments 2-4. Butterflies were additionally subjected to behavioral experiments testing for the individual’s exploratory behavior (experiments 3 and 4). Males and females differed substantially in morphology, with males showing traits typically associated with a better flight performance, which most likely result from selection on males for an increased flight ability to succeed in aerial combats with rivalling males and competition for females. This pattern could be verified by mobility measures under natural conditions and flight performance tests. Interestingly, although females showed traits associated with diminished flight performance, they had a higher emigration propensity than males (though in a context dependent manner). Reasons might be the capability of single mated females to found new populations, to spread their eggs over a wide range or to escape male harassment. Conditions indicative of poor habitat quality such as shade and a lack of resources promoted emigration propensity. The environmental context also affected condition and flight performance. The presence of resources increased the butterflies’ condition and flight performance. Larval and adult food stress in turn diminished flight performance, despite some reallocation of somatic resources in favor of dispersal-related traits. These detrimental effects seem to be mainly caused by reductions in body mass and storage reserves. A similar pattern was found for exploratory behavior. Furthermore, higher temperatures increased flight performance and mobility in the field, demonstrating the strong dependence of flight, and thus likely dispersal, on environmental conditions. Flight performance and exploratory behavior were positively correlated, probably indicating the existence of a dispersal syndrome. The population comparison revealed several differences between edge and core populations indicative of local adaptation and an enhanced dispersal ability in edge populations. For instance, edge populations were characterized by shorter development times, smaller size, and a higher sensitivity to high temperatures, which seem to reflect adaptations to the cooler Estonian climate and a shorter vegetation period. Moreover, Estonian individuals had an enhanced exploratory behavior, which can be advantageous in all steps of the dispersal process and may have facilitated the current range expansion. In summary, these findings may have important implications for dispersal in natural environments, which should be considered when trying to forecast future species distributions. First, dispersal in this butterfly seems to be a highly plastic, context-dependent trait triggered largely by habitat quality rather than by individual condition. This suggests that dispersal in L. tityrus is not random, but an active process. Second, fast development and an enhanced exploratory behavior seem to facilitate the current range expansion. But third, while deteriorating habitat conditions are expected to promote dispersal, they may at the same time impair flight ability (as well as exploratory behavior) and thereby likely dispersal rates. For a complete understanding of a complex process such as dispersal, further research is required.