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Mutualisms are ubiquitous in nature and shape whole ecosystems. Although species benefit by interacting with each other, they permanently act selfishly. As a consequence, the involved partners must balance gaining the maximal benefit while accepting a certain amount of costs. Changes in the environment, however, may alter selection pressures and lead to a shift in the relative costs and benefits for both involved species. Due to this complexity, many mutualisms and their underlying processes, such as the dependence of the involved species on each other, are only poorly understood. Moreover, in several so-called mutualistic interactions it is unclear if they are in fact beneficial for all partners because detailed cost-benefit analyses are missing. The aim of my thesis was to contribute to a better understanding of the basic principles of mammal-plant mutualisms with special emphasis on the interdependence of the involved species. Using the interaction between an insectivorous bat species (Kerivoula hardwickii) and carnivorous pitcher plants (genus Nepenthes) as a model system, I conducted a detailed cost-benefit analysis to test if the partners interact mutualistically and are strongly dependent on one another. I hypothesised that pitchers of these plants serve as high quality roosts for the bats while the bats in turn fertilise the plants via their nutritious faeces. For the involved species the costs of the interaction should be lower than the gained benefits, but general costs should increase in the absence of the partner. Over the course of my field research, I found the bats roosting in three Nepenthes species, but the bats occupied intact pitchers of only one species, Nepenthes hemsleyana. In Nepenthes bicalcarata and Nepenthes ampullaria, the bats used senescing or damaged pitchers whose high amount of digestive fluid had drained off. Thus, only N. hemsleyana was potentially able to digest bat faecal matter, and thereby benefit from the bats. My cost-benefit analysis showed that N. hemsleyana plants strongly benefited from their bat interaction partner: In feeding experiments the plants gained between 34% and 95% of their nitrogen from bat faeces, which significantly improved their growth, photosynthesis and survival. In contrast, plants without access to faeces could not fully compensate the induced lack of nutrients by using arthropod prey. Field observations revealed no obvious costs for the pitcher plant. N. hemsleyana pitchers occupied by bats did not differ in their lifespan from unoccupied ones as bats did not injure the plants’ tissue. The interaction was also advantageous for K. hardwickii because N. hemsleyana offered high quality roosts with a favourable microclimate and low parasite infestation risk. Consequently, bats roosting in N. hemsleyana pitchers were in better condition than those roosting in dead N. bicalcarata pitchers. Although N. hemsleyana pitchers are rare in the natural habitat, bats could easily find and identify them due to an echo reflector, which reduces time and energy costs for roost detection. Most N. hemsleyana plants continuously provided at least one intact pitcher meaning bats could return to the same plants over a period of several months or even years. The interaction between K. hardwickii and N. hemsleyana can be classified as an asymmetric facultative mutualism with stronger dependence of the plant partner. N. hemsleyana has outsourced arthropod capture and digestion to its mutualistic bat partner while arthropod attraction is strongly reduced. Contrastingly, several populations of K. hardwickii frequently use alternative roosts. Strong selective pressure on the plants could be the consequence to attract bats with a potential stabilising effect on the interaction: N. hemsleyana has to outcompete the involuntarily offered roosts of the other Nepenthes species in terms of quality and accessibility. My thesis revealed complex interdependencies in an animal-plant mutualism. This study exemplifies that rigorous cost-benefit analyses are crucial for the classification of interspecific interactions and the characterisation of how the involved species affect and depend on each other.
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.