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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.
Carbon dioxide (CO2) is one of the most important factors of the Earth’s carbon cycle. Peatlands are well-known to be a long term sink for atmospheric carbon dioxide. Under changing environmental conditions, the carbon balance and hence the CO2 fluxes can be significantly changed, and peatlands may even become a significant atmospheric carbon source. To be able to predict the changes in climatic conditions and their effects on ecosystems, it is important to understand the contemporary CO2 exchange of the ecosystems. Many studies on peatland CO2 fluxes have been conducted in the boreal zone of North America and Scandinavia. Still little scientific evidence is available from peatland ecosystems of boreal Russia. This dissertation presents the detailed investigation of CO2 dynamics and the relevant processes and environmental factors from the boreal peatland site Ust-Pojeg (61°56'N, 50°13'E) in Komi Republic, northwest Russia. On the small spatial scale (microform), the investigated peatland was characterised by high variability in vegetation composition and coverage as well as in water table level which resulted in large variability in CO2 fluxes not only between the microform types but also within one microform type. The cumulative flux over the investigation period for the different microforms ranged from strong CO2 sources to CO2 sinks. An area-weighted estimate for the entire peatland showed that it was a CO2 source for the investigation period, which was characterised by average conditions in terms of precipitation and temperature. The CO2 fluxes were measured at different scales: by the closed chamber method at the microform scale and by the eddy covariance technique at the ecosystem scale. Three different upscaling methods were used to compare the fluxes. Irrespective of the upscaling methods, the discrepancies between the estimates based on the upscaled chamber measurements and estimates based on measurements by the eddy covariance technique were high. The high spatial heterogeneity of the vegetation and the water table level and thus of the CO2 fluxes were recognised as reasons for high potential errors when upscaling CO2 fluxes from the microform to the ecosystem level. Large discrepancies were also observed in comparison between measured CO2 fluxes and CO2 estimates based on the mechanistic ecosystem model LPJ-GUESS. Insufficient model forcing may have led to errors in the timing of the onset and the end of the growing season, and the modelled vegetation did not always reproduce the observed vegetation. These two factors may have led to the discrepancies in the model-measurement comparison. Although the closed chamber technique is widely used for measurements of CO2 fluxes between ecosystems and the atmosphere, the errors which might occur during the measurement itself or which are associated with the used measurement devices as well as the flux calculation from chamber-based CO2 concentration data are still under discussion. The study showed that the CO2 fluxes measured by the closed chamber method can be overestimated during low-turbulence nighttime conditions and can be seriously biased by inappropriate application of linear regression for the flux calculation. The methodological studies were conducted at the boreal peatland Salmisuo in eastern Finland (62°46'N, 30°58'E). The methods developed in this dissertation could contribute significantly to improved CO2 flux estimates. VI