Dokument

Summary:
Natural habitat loss and exotic plant invasions are two major drivers of global change in subtropical and tropical ecosystems. These two drivers lead to a loss of biodiversity and alter species interactions, which may imperil vital ecosystem functions and services such as pollination and seed dispersal by animals. Yet, additive and potential interactive effects of natural habitat loss and plant invasion on mutualistic interactions are poorly understood. I therefore studied the combined effects of these two drivers on plant–pollinator and plant–frugivore interactions in a heterogeneous forest landscape in South Africa. The studies presented in this thesis demonstrate that natural habitat loss and exotic plant invasions profoundly alter and partly disrupt plant–animal mutualistic interactions. Specialized species and interactions are especially vulnerable, which may not only affect their own survival but also that of their mutualistic partners. Here, it should be noted that my findings indicate that both studied mutualisms, pollination and seed dispersal, are equally likely to be significantly altered by global change, despite marked differences between them such as the specialization between mutualistic partners. Furthermore, the community approach applied in all of my studies showed that the overall effects of the two global change drivers on the studied mutualism are generally negative, but also allowed me to separate the responses of different species to the two drivers. Here, I showed that these responses were in many cases directly related to the functional traits of species. In contrast, species richness was a poor representative of changes in the functional composition of species communities. Consequently, understanding and predicting ecological processes under global change considerably benefits from functional trait-based approaches. Furthermore, I found that different drivers of global change affect communities via different response traits. Results obtained from one functional trait therefore may not be representative for those from another trait. Similarly, in all of my studies I found marked effects of the two investigated global change drivers on mutualistic interactions and the composition of species communities. However, effects were often specific to one driver, and many results would have been overlooked in single-driver studies. Consequently, my results highlight the need to include multiple global change drivers when studying ecosystem processes on a community scale. Here, I found that the two drivers can be complementary in their negative effects on specialized species and interactions. While these additive effects are already worrying, I also found evidence for negative synergistic effects between the two drivers on specialized species. Such synergisms considerably increase the risk of species’ extinctions and thus of a permanent disruption of essential ecosystem processes.

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