Empirical Implications of Food Web Constraints for Metacommunity Assembly

Empirical metacommunity investigations remain scarce and biased toward simplified laboratory experiments or observation studies at single trophic levels, hindering generalizations of mechanism in natural systems. There is a need to expand metacommunity research into more realistic scenarios by integrating different type of biotic interactions at a range of spatial resolutions and under a range of environmental conditions. Especially, food web interactions have well-documented impacts on local and regional diversity patterns that have been untested in metacommunity research. In this thesis, I investigate empirical implications of food web constraints for insect community assembly in space. The Trophic Island Biogeography theory is a promising and parsimonious theoretical approach to integrate elements from food web theory into a metacommunity perspective. In the first chapter of this thesis I explore the implications of this theory for the establishment of arthropod feeding guilds in experimental grassland islands. I demonstrate that neither diet-based constraints nor landscape characteristics can predict the early structure of locally assembling food webs, with their establishment deriving instead from interactions between both processes as a function of diet specialization. Past work has made the connection between habitat fragmentation and the alteration of trophic control. However, mechanistic linkages with multiple stressors are needed to accurately describe potential effects of global change on consumer diversity regulation. The second chapter of this thesis tests how multiple stressors (habitat loss, fertilization, and stand perturbation) potentially spread through the food chain to affect insect herbivore diversity and spatial turnover. It shows that despite the potential for interactive effects of multiple stressors on each trophic level, plant-mediated processes mostly determined the decline in insect herbivore abundance, richness and spatial turnover, while predator-mediated effects were negligible. In complementarity with these results, the third chapter of this thesis illustrates how bottom-up trophic dependencies and dispersal-related drivers of consumer assembly can loop back to trigger higher top-down damage from consumers on plants, here an endangered native tree species of Ontario suffering from recruitment collapse. Taken together this thesis illustrates the importance of trophic dependencies for local community establishment, with important implications for biodiversity in altered landscapes.

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