Measuring natural pest suppression at different spatial scales affects the importance of local variables

The role biodiversity plays in the provision of ecosystem services is widely recognized, yet few ecological studies have identified characteristics of natural systems that support and maintain ecosystem services. The purpose of this study was to identify landscape variables correlated with natural pest suppression carried out by arthropod natural enemies, predators and parasitoids. We conducted two field experiments, one observational and one experimental, where landscape variables at broad and local scales were measured and related to natural pest suppression. The first experiment measured natural pest suppression at 16 sites across an urban to rural landscape gradient in south central Wisconsin. We found natural enemy diversity positively affected natural pest suppression, whereas flower diversity negatively affected pest suppression. No relationship was found between natural pest suppression and broad scale variables, which measured the percentage of different land cover classes in the surrounding landscape. In the second experiment, we established small (2- by 3-m) replicated plots that experimentally varied flower diversity (0, 1, or 7 species) within a plot. We found no significant relationship between natural pest suppression and the different levels of flower diversity. The fact that we only found differences in natural pest suppression in our first experiment, which measured natural pest suppression at sites separated by larger distances than our second experiment, suggests the more appropriate scale for measuring ecosystem services performed by mobile organisms like insects, is across broad spatial scales where variation in natural enemies communities and the factors that affect them become more apparent.