Consumers and nutrients alter colonization of an old field community independently and by distinct mechanisms

Abstract

Abstract As communities assemble, species establishment can be driven by multiple factors. Two such factors, nutrients and consumers, are predicted to interact, but few studies have explored whether this interaction results from synergistically altering the same biotic and abiotic mediators or complementarily altering different mediators. To test this, I added seeds of multiple native species to old field communities in which I experimentally manipulated soil nutrients, leaf litter and consumers (insect herbivores and fungal pathogens). Using Bayesian hierarchical modelling, I evaluated the degree to which changes in light and water availability, fungal disease and insect herbivory, and community functional composition influenced seedling establishment and the richness of colonizing species and were driven by experimental treatments. I then examined whether nutrients, litter and consumers interactively determined the success of colonizing species with different resource allocation strategies. Contrary to expectation, nutrient addition and consumer exclusion reduced, and litter removal increased, seedling establishment and colonizer richness independently and via different pathways. Also contrary to expectation, seedling resource allocation strategy did not interact with nutrient addition and consumer exclusion to alter establishment success. Rather, as community damage (foliar fungal disease + insect herbivory) increased, seedlings with high % leaf nitrogen (a proxy for an acquisitive resource allocation strategy) become increasingly disadvantaged relative to seedlings with low % leaf nitrogen (a proxy for a conservative resource allocation strategy). Synthesis: These results indicate that seedling establishment is unlikely to be interactively impacted by nutrients and consumers (fungal pathogens + insect herbivores) because these two drivers act on different environmental pathways to influence community assembly.