Plant-Insect-Pathogen Interactions on Local and Regional Scales

Abstract

In many plant communities, weeds are important drivers of ecosystem processes, but natural enemies may control their growth and population dynamics. These enemies may directly reduce plant growth by grazing or infection, or indirectly enhance plant susceptibility to other enemy species. The role of insects as vectors of plant pathogens has thereby often been underestimated but appears to be widespread and important. Crucial for the understanding of such plant–enemy interactions is the consideration of processes on local and regional scales. We analysed the effects of herbivore and pathogen attack and their interactions using the weed creeping thistle, Cirsium arvense, as a model system. Local thistle patches were limited by the thistles’ low competitive ability in secondary succession, by pathogens and insects and by the disturbance or management of habitats. The regional dynamics of C. arvense were mainly driven by human activities and showed a mosaic of increasing and decreasing local patches with low persistence. High patch turnover rates and subsequent small average patch sizes limited the success of herbivore populations. Furthermore, we analysed thistle–herbivore–pathogen–parasitoid interactions on different spatial scales in a landscape context. Interactions between C. arvense and either insect herbivores or rust pathogens were related to landscape context at large spatial scales (3,000 m radius of landscape sector), whereas herbivore–parasitoid interactions were influenced at smaller scales (750 m). Hence, species at higher trophic levels appeared to have a smaller range of dispersal than those at lower levels. These marked spatial differences emphasize the need to consider both local and landscape management in biological control. In general, such an approach may help to explain the dynamics of plant populations in dependence on possible control by insects and pathogens which can affect the plants’ influence on ecosystem processes. However, the potential of facilitation among the plants’ antagonists via plant-mediated indirect interactions or with insects as vectors of pathogen spores and their dependence on species-specific spatial scales need much more experimental evidence.