The effect of landscape structure on insect herbivory and biodiversity: Implications for forest ecosystem services in the Monteregie, Québec

Anthropogenic disturbances are fragmenting forested areas around the world, fundamentally changing the structure of landscapes in a manner that has poorly understood consequences for biodiversity, ecological processes, and ecosystem services. This lack of understanding makes it difficult for land managers to make optimal decisions that balance environmental benefits and costs of land use change. In this thesis, I address this gap in our understanding by using a combination of theoretical and empirical approaches to quantify how and when fragmentation affects ecosystem processes that have distinct impacts on forest ecosystem services. Specifically, I test the role that insect herbivory plays in mediating the effects of landscape structure on ecosystem services in forest ecosystems in the Montégerie in southwest Quebec. Insect herbivory is a good model system to test these effects because it is sensitive to landscape structure, can have strong effects on ecosystem services, and can be either beneficial or harmful depending on the herbivore species and the ecosystem context. In this thesis, I quantify the effects of landscape structure on insect herbivory and biodiversity, and assesses the implications of these effects for forest ecosystem services in four steps: 1) In the second chapter examine the known relationships between landscape structure, herbivory and ecosystem services through a semi-quantitative literature review and show that herbivory plays an important role mediating effects of connectivity on ecosystem services. The results of this review show assessing the mechanisms that regulate the response of insect herbivores to landscape structure would allow for better predictions regarding this process. 2) In the third chapter, I quantify the relationship between landscape structure and herbivory, and test potential mechanisms (i.e. predation pressure) driving the response, using a manipulative field experiment in the Montérégie. I find that landscape structure alone does not explain levels of herbivory, even though there was strong vertebrate predation pressure on herbivores that varied with landscape structure. Because these results show structure at the landscape scale does not explain patterns of herbivory, my next step was to determine whether insect herbivory is spatially heterogeneous at a finer scale. 3) In the fourth chapter, I measure the spatial heterogeneity of insect herbivory in remnant forest patches at both the landscape and patch level, quantifying herbivore damage on sugar maple trees at the edge, interior, and canopy of remnant forest patches that differ in size and connectivity. I find that patterns of herbivory are affected by the interaction of landscape and patch level sources of spatial heterogeneity. 4) Finally, to better understand the mechanism by which these finer scale patterns occur, I measure the effects of landscape structure and location within a forest patch (i.e., edge, interior, canopy) on arthropod functional and taxonomic biodiversity. I find that canopies of fragmented forest patches are important reservoirs of arthropod biodiversity, even in fragmented forest ecosystems. Human activities, such as fragmentation or restoration, continue to affect the structure of forest ecosystems, with unknown consequences for the ecosystem services provided by forests. My thesis advances our understanding of the effects of landscape structure on ecosystem services by using herbivory as a model system to provide a conceptual framework with which to balance the social and environmental costs and benefits associated with altering landscape structure.

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