Natural and human-induced dynamics in plant–animal mutualistic networks
Species interactions are an integral part of ecological communities. Collectively, these interactions form complex and highly dynamic networks. The structure of these networks varies due to geographic and temporal variation in the abundance and co-occurrence of interacting species and due to species...
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|Zusammenfassung:||Species interactions are an integral part of ecological communities. Collectively, these interactions form complex and highly dynamic networks. The structure of these networks varies due to geographic and temporal variation in the abundance and co-occurrence of interacting species and due to species gains and losses after anthropogenic perturbation. In Europe’s last relict of old-growth lowland forest (Białowieża, Poland), I studied the outcomes of these natural and human-induced dynamics in highly diversified mutualistic networks of plants, pollinators and seed dispersers. These mutualistic interactions between plants and free-living animals are of great importance, as the flowers and fruits of many plant species are critical resources for a variety of animal species, which in turn contribute significantly to the regeneration of plant communities. As part of my studies, I was able to show that plant–animal mutualistic networks are highly dynamic systems that respond collectively to changing biotic context and human-induced perturbation. Observed shifts in facilitative and competitive interactions among plants sharing mutualistic partners show that biotic context is a strong determinant of the outcome of interspecific interactions. The use of network analyses, thereby, allowed me to identify some of the mechanisms that shape species interactions and their outcomes. For example, my studies show that a change in the population density of one species suffices to trigger cascading effects on the interactions and populations of other species. This finding highlights that species interactions may have a pervasive effect on the assembly and disassembly of ecological communities. Even more importantly, I could show that these community-wide dynamics were in all cases linked to consumer-resource relationships, which are key determinants of plant–animal mutualisms. Thus, changes in the foraging behaviour of animals in response to variation in the density of plant resources and competitors affected the structure of mutualistic communities. This underscores that despite the evolutionary conservatism in ecological interactions, biotic context determines to which extent these coevolved interactions are realized. The fact that the sharing of mutualistic partners among plant species was reflected in their co-occurrence demonstrates that the above-mentioned dynamics in ecological networks may also determine community assembly processes and species co-existence. Importantly, the comparison of several types of species interactions revealed how biotic context in its various forms can shape land-use effects on species interactions. I found that the mutualism between plants and seed dispersers was more susceptible to habitat degradation than the mutualism between plants and their pollinators. This finding highlights that a high degree of generalization, such as in the seed dispersal mutualism, does not necessarily buffer ecological communities against the loss of species. This becomes even more important if a few species have a disproportionate effect on a given target function and if species are particularly vulnerable to ecological perturbation, such as habitat specialists or large-bodied frugivores. Furthermore, I observed that shifts in the abundance of plant resources in degraded habitats can amplify land-use effects on plant–animal mutualistic interactions. Importantly, changes in the density of plant resources explained about 40 to 70 percent of the variation in land-use effects on interactions between plants and their pollinators and seed dispersers. This demonstrates that a consideration of biotic context (e.g. in the form of resources) may considerably improve predictions of the magnitude of land-use effects on species interactions. Thereby, the correlated responses of pollinators and seed dispersers to the shifts in plant population densities in degraded habitats highlight that these dynamics are not restricted to single types of interaction, but potentially operate at the level of ecosystems. Studies that only focus on subsets of species or interaction types may be unable to identify the consequences of human land-use that have been shown here. In principle, the results of the presented studies may also be valid for other types of mutualistic and antagonistic interactions that are based on consumer-resource relationships. Altogether, the results of my thesis suggest that natural and human-induced dynamics in plant–animal mutualistic networks follow similar principles. In the worst case these dynamics might have cascading effects on the functioning and integrity of ecosystems through a parallel loss of multiple animal-mediated ecosystem services after habitat degradation.|