| dc.description.abstracteng | Biodiversity in terrestrial ecosystems is declining due to increasing anthropogenic pressure. Urban and agricultural expansion lead to shrinking natural or seminatural habitats. In addition, management intensification of existing agricultural land further reduces the quality of agroecosystems as habitat for many species. Both, local reduction of plant diversity and the regional degradation of seminatural vegetation have consequences for consumer species. The consequences of plant species loss for the diversity of consumer species and ecosystem functioning have been addressed over the past decades in numerous studies, ranging from controlled experiments with manipulated plant communities to systems with natural occurring gradients in plant diversity on a local and landscape scale. These studies have found that plant and habitat diversity can have cascading effects on the faunal community, shift its trophic structure and influence species-mediated ecosystem-processes, such as predation. Although it is clear that interspecific interactions are the drivers of such community shifts and ecosystem processes, our knowledge on how changing diversity of basal resources impacts species interactions is currently still limited.
The major aim of my thesis was to contribute to a deeper understanding of the impact of plant diversity on the multitude of interactions that species in complex communities are involved in. This thesis focuses on interactions of mobile generalist omnivores and predators that can be found across the whole gradient from natural to strongly modified habitats. Although this group of invertebrates may not be particularly threatened by the loss of plant diversity, they are of great interest as model organisms, because their dietary plasticity allows them to interact with different sets of co-occurring species in their local environment. A further aim of my thesis was to expand the focus from trophic to non-trophic interactions by including microorganisms in my studies that are, despite their tremendous diversity and importance for many ecosystem functions, rarely considered in the analysis of aboveground interactions.
In the first research chapter (chapter 2), I studied the effects of landscape context on the gut bacterial community and body condition of predatory insects (lady beetles) in a mensurative experiment in the Midwest of the USA. Insects were sampled across a landscape complexity gradient (increasing amount of cropland), and across two field types with a pronounced difference in plant diversity (soybean monocultures vs. restored prairies). Unexpectedly, predators collected in soybean fields had a more diverse set of gut microbes than predators from prairies. However, predators from soybean had lower fat content than predators collected in prairies, suggesting greater resource availability in prairies. Whether the lady beetle species were native or exotic to the USA influenced the effect of landscape context, suggesting differences in foraging preferences between these groups at a landscape scale. Another key finding was that lady beetle species have distinct microbial communities. Overall, my study highlights complex interactions among gut microbiota, predator identity, and landscape context.
The studies in chapter 3 and 4 were conducted within the framework of a biodiversity experiment (The Jena Experiment) with manipulated taxonomic and functional plant diversity to study if plant diversity per se has an impact on microbial and trophic interactions involving invertebrates. In a pilot study (chapter3), I evaluated metabarcoding of gut contents of invertebrates as a novel approach to analyze biotic interactions in species-rich communities. In an extended study spanning the full experimental design (chapter 4), I subsequently used this approach to analyze DNA of plants, animals, fungi, and bacteria in gut contents of three invertebrate species that vary in their degree of omnivory. The results the richness and composition of detected taxa is only little affected by plant diversity directly and mainly driven by indirect effects of plant diversity via the performance of the plant or animal community. A key finding was that vegetation cover shifts the trophic position of omnivores but the direction of the effects depended on the species identity of the omnivore. Further, the consumers were associated with different sets of animal and microbial taxa, reflecting their different food preferences.
The final research chapter (chapter 5) assessed the efficiency of pitfall trapping, which is one of the most frequently used approaches to assess aboveground invertebrate diversity, under different scenarios by employing an ecological simulation approach. An individual-based model for simulating the movement and pitfall trap sampling of arthropods was developed and factors that are assumed to affect the trapping efficiency were systematically assessed at the species and community level. Body mass, temperature, and pitfall trap number strongly increased the sampling efficiency. This has implications on the study of communities, as the strong impact of body mass could result in an overestimation of large-sized species in the arthropod community and imply wrong conclusions about its trophic structure. It is therefore proposed to conduct a bias correction and a correction factor that requires only information on species body mass is provided to derive reliable abundance estimates from pitfall trap sampling.
This thesis revealed that species interactions are driven by a multitude of direct and indirect effects of plant diversity on a local and landscape scale. This is further complicated by the contrasting responses of consumer species that are often treated as one functional group and highlights the need to further investigate the response of individual key species instead of focusing solely on whole communities. Overall, my thesis is a first step to integrate novel approaches that allow the empirical assessment of multi-level species interactions into biodiversity research. | de |