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For centuries, humans extensively used and profoundly altered ecosystems at a global scale, which is assumed to have serious implications for ecosystem functioning and human-well being. Amongst others, it has been suggested that deforestation and the associated process of forest fragmentation have severe and multi- faceted consequences entailing an overall loss in biodiversity, the disruption of trophic interactions and impaired functioning of forest ecosystems. Eventually, consequences of forest fragmentation may threaten ecosystem stability and ecosystem services of forests. Insect herbivores are known to play a key role in all plant-based ecosystems, i.e. they affect growth, fitness and reproduction of plant individu als and thus, have been suggested to influence plant species persistence as well as the structure and composition of plant communities. Hence, changes in insect herbivore communities due to forest fragmentation, particularly increased insect herbiv ore abundances, may cause an overall increase in the susceptibility of plants to insect herbivory with severe consequences for forest ecosystems. So far, there is no consensus regarding the implications of forest fragmentation for plant-herbivore interactions. Findings of previous studies indicate inconsistent responses of insect herbivores to forest fragmenta ion and the ultimate degree of insect herbivory in fragmented forest landscapes varies correspondingly. With this thesis, I aimed to unravel the discrepancy in the above findings by addressing the context-dependency and the complex nature of antagonistic plant-herbivore interactions, which may both cause spatial variability. To address the context-dependency of plant-herbivore interactions, I conducted two field studies in a subtropical forest landscape in southern KwaZulu-Natal (South Africa). The first field study aimed at disentangling potential interactive effects of forest fragmenta tion on the landscape scale and local tree diversity on plant-herbivore interactions and the associated process of insect herbivory. With the second field study, I examined the trophic control of herbivorous insects through insectivorous birds along a gradient of increasing forest fragmentation including ultimate consequences for the degree of insect herbivory. Finally, to address the complexity of plant-herbivore interactions, I performed a comprehensive meta-analysis on plant responses to insect herbivory and thus, feedback effects on insect herbivores as well as the potential of plants to mediate the outcome of plant-herbivore interactions. Overall, findings obtained in the three studies support the assumption that both context-dependency and the complexity of plant-herbivore interactions may contribute to the discrepancy in findings of empirical studies on plant-herbivore interactions in fragmented forests. In more detail, underlying mechanisms of the effect of forest fragmentation include complex interactive effects of co-occurring environmen tal drivers as well as multitrophic cascades which mediate the properties of plant-herbivore interactions in fragmented forests. Hence, without considering the environmental context of plant-animal interac tions, attempts to unravel the impact of human-driven landscape modifications such as forest fragmentation are prone to lead to biased conclusions. Similarly, plant responses to herbivory have the potential to mediate the outcome plant-herbivore interactions through compensatory growth and induced defence. More specifically, full compensatory growth may blur differences in the feeding pressure of in sect herbivores on plants and thus, studies on plant- herbivore interactions that solely monitor the de gree of herbivory may easily overlook differences in insect herbivore abundances. Moreover, herbivo ry-induced production of defence compounds may create feedback effects and thus, alter the compo-sition of insect herbivore communities with potential consequences for the degree of insect herbivo ry. Hence, plants have to be considered as active counterparts of insect herbivores and thus, have to be incorporated in considerations on effects of human-driven landscape modifications on plant-herbivore interactions. Findings of the two field studies further show that forest fragmentation has a major impact on forest ecosystems and that the consequences are multi- faceted. In addition to shifts in the community composition and species loss, my results demonstrate that forest fragmentation further interferes with trophic interactions involving multiple trophic levels. In more detail, increasing forest fragmentation altered the community composition of insect herbivores and thereby, diminished the significance of patterns in local tree diversity for insect herbivores. Further, increasing forest fragmentation triggered a trophic cascade beginning with the loss of insectivorous birds, disrupting the trophic control of insect herbivores and ultimately, resulting in increased levels of insect herbivory, which may have serious implications for plant communities. The latter finding additionally reveals that species with similar ecological functions are not necessarily redundant. In contrast, I argue that it is highly likely that species loss is tightly linked to a loss in the ecological function of species. Moreover, I conclude that we have to consider that disturbance-resistant species may not necessarily compensate for the loss of species and maintain the ecological function. Altogether, I could show that forest fragmentation poses a serious threat to forest communities and trophic interactions and thereby, puts ecosystem functioning and services of forests at high risk. In terms of conservation management, I argue that it is essential to reduce forest fragmentation to a minimum and maintain a network of continuous forests that are well-connected with smaller forest remnants at the landscape scale. This in turn, will benefit species persistence, species migration and recolonization as well as trophic interactions and thereby, ensure species and ecosystem functioning. Likewise, considering the patterns in the findings derived from the meta-analysis may offer man agement implications, e.g. for grassland and forest ecosystems. For instance, alleviated herbivory allows plants to fully recover from herbivory (or artificial defoliation), but may simultaneously maintain plant diversity of grasslands. Additionally, despite short-term benefits for plant growth, high nutrient availability and thus, fertilization or increased nitrogen deposition may not necessarily mitigate effects of herbivory. To conclude, holistic research approaches that view species and their trophic interactions from different angles as well as consistent advances in ecological research tools (e.g. interactive effects, communitiy-level and landscape scale approaches, multitrophic network approaches, meta-analyses in ecology) may contribute to a more comprehensive understanding of the dynamics that structure communities and trophic networks. Both a more holistic view as well as methodological progress in turn, will help to develop effective management implications in order to sustainably maintain functioning and stability of forest ecosystems as well as the services they provide in a human-modified world. |
