Interacting effects of management and environmental variability at multiple scales on invasive species distributions.
The distribution and abundance of invasive species can be driven by both environmental variables and land management decisions. However, understanding these relationships can be complicated by interactions between management actions and environmental variability, and differences in scale among these variables. The resulting 'context-dependence' of management actions may be well-appreciated by ecologists and land managers, but can frustrate attempts to apply general management principles. In this study, we quantify the effects of land management and environmental variability at different scales on the occurrence and abundance of Hieracium pilosella, a major agricultural weed in New Zealand. We used a hierarchical study design and analysis to capture relevant scales of variation in management actions and environmental heterogeneity, and test hypotheses about how these factors interact. We show that fertilizing and grazing interact with environmental gradients at the scale of management application (farm paddocks) to influence the establishment and local abundance of H. pilosella. We further show that H. pilosella's relationships with fine-scale abiotic and biotic factors are consistent with expected mechanisms driven by larger-scale management actions. Using data on occurrence and local abundance, we tease apart which factors are important to establishment and subsequent local spread. Synthesis and applications. A major challenge for environmental scientists is to predict how invasive species may respond to ongoing landscape modifications and environmental change. This effort will require approaches to study design and analysis that can accommodate complexities such as interacting management and environmental variables at different scales. Management actions will be more likely to succeed when they explicitly account for variation in environmental context.