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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.
Recent climate change and its consequences for living organisms constitute one of the greatest problems of our century. Global warming entails an increase in mean temperature and the frequencies of extreme weather events. Those changes in environmental conditions affect both plants and animals. Because of their inability to escape from unsuitable environments, plants have evolved a wide spectrum of molecular programs to protect themselves against changing conditions. Responding on altered environmental conditions will change plants chemical composition and therefore also affect plants interaction with other species (e.g., predator-prey or symbiotic relationships). For instance, changes in the chemical composition of plants may influence the survival of associated herbivores. In other words, these herbivores will be affected indirectly by climate change due to changes in the suitability / quality of their food. The aim of this doctoral thesis was to discover the effects of climate change within the relationship of the butterfly Pieris napi and its host plant (Sinapis alba used here as host plant), including individual conditions (e.g. chemical compositions of plants; morphology, physiology of the butterfly) and behavior of female butterflies and larvae. In the first experiment, the influence of simulated climate change on the chemical composition of the plant Sinapis alba was investigated. The second experiment aimed to examine the influence of changes in plant composition on the butterfly P. napi. Glucosinolates (secondary compound of plants) are known to have an important effect on the preference and performance of herbivores. Therefore, in the third experiment, the impact of glucosinolates on the preference and performance of P. napi was investigated in order to see if these plant compounds had the most important influence on this butterfly. Furthermore, in the fourth experiment, it was explored whether there is a latitudinal gradient within the species´ responses to changes in its host plant. The fifth and last experiment aimed to examine, if there are general principles across species regarding indirect effects of climate change.
Climate change, simulated by different combinations of temperature and water regimes, had an effect on the plant chemistry. The combination of temperature and water availability changed plant composition substantially. Especially the amount of carbon and glucosinolates (here above all sinalbin) in S. alba plants varies between the different treatments and therefore between the different combinations of temperature and water regimes. Regarding glucosinolates, elevated temperatures increased their concentration in leaves, whereas water deficit in combination with higher temperature reversed this pattern. For carbon content, all plants, except those of the control group, showed a decreased amount of total carbon. However, simulated heat waves had no effect on plants, leading to the assumption that the plants were able to recover from heat stress sufficiently during the control phases. Changes in plant composition affected both larvae and females of the butterfly P. napi. Therefore, changed host-plant chemistry alters the plant quality for this herbivore, meaning that plants of different treatments represent different plant qualities defined by their composition. Females of P. napi may be able to differentiate between plant qualities and even show a direct preference. Therefore, glucosinolates seem to act as oviposition stimulants. However, preferring another plant quality with lower amount of glucosinolates suggest that females of this butterfly species were attracted by more than high levels of glucosinolates alone. Larvae fed with different plant qualities performed differently, indicated by smaller wings (lighter bodies) and prolonged development when fed with plants contained higher amount of the glucosinolate sinalbin. It can be assumed that a higher amount of sinalbin decreases the quality of the host plant and therefore lead to these responses. Probably larvae need to shift their resources from growth to detoxification and therewith survival. Furthermore, drought conditions during plant growth seem to reduce the overall negative effects of higher temperatures, lead to an increase of host plant quality. Larvae seem to benefit from feeding on these “double-stressed” plants. Comparison between the results of the preference and performance tests suggests that there might be a mismatch between female preference and larval performance. It seems that the stimulating effect of high concentration of glucosinolates, in this case sinalbin, misdirects females´ decision to less suitable host plants, meaning that the advantage of less competition for larvae come at costs through detoxification. Using Brassica napus plants with genetically fixed glucosinolate levels, it could be demonstrate that there must be other plant components influencing females´ oviposition behavior been seen in the choice experiment with S. alba. The comparison of German and Italian populations to changes in host-plant quality showed fewer differences between countries as expected. However, German and Italian individuals differed in their reaction to altered plant quality, at least in developmental time and larval growth rate. It seems that Italian larvae benefitted from plants grown under higher temperatures, whereas drought-stressed plants affected them negatively. German individuals in contrast seem to benefit only from water stress during plant growth. With regard to the sexes of P. napi, it seems that females respond differently than males to changes in plant quality. Furthermore, the results of the performance test on Bicyclus anynana showed that there might be some general principles for the respond of butterflies to changes of its host plant. B. anynana responded in a similar way to different host plant qualities as P. napi did, meaning that plants grown under higher temperatures and drought conditions seem to be beneficial for the larval performance.
In summary, these findings may have important implications for the indirect effects of climate change on this butterfly in natural environments. First, climate change seems to have an impact on the chemical composition of plants. Second, changes in plants caused by increasing temperature and droughts seem to influence the preference and performance of this butterfly. However, there are differences between populations, which seem to be induced by former adaptation. And third, there might be some general principles for the respond of butterflies to changes in their host plants. This thesis focuses only on possible indirect effects of climate change. However, there are direct effects, which may alter the responses of herbivores to changes in their host plant as well. Therefore, further investigations in this linkage and in other plant-herbivore relationships will be necessary to explore how climate change may alter the relationship between herbivores and their hosts.