Human-driven land-use change is rapidly transforming ecosystems worldwide, with profound consequences for biodiversity and the ecological interactions that underpin agricultural production. In agroecosystems, shifts from complex, heterogeneous landscapes to simplified, intensively managed systems alter not only species composition but also the traits of organisms that persist within them. Among these traits, body size plays a central role in shaping ecological function, influencing processes such as pollination, herbivory, dispersal, and reproductive capacity. However, while community-level responses to land-use change are well documented, much less is known about how intraspecific trait variation may mediate ecological outcomes. This dissertation examines how landscape simplification influences intraspecific body size variation in arthropods, and explores the consequences of these changes for ecosystem function. My first chapter investigates how landscape simplification in the Finger Lakes region of New York affects body size in both a key pollinator and herbivore pest of strawberry, and how this variation impacts fruit quality and potential yield. I demonstrate that larger pests occur in simpler landscapes, which in turn reduces fruit size, with potential to substantially decrease crop production. This chapter highlights that even small differences in body size, impacted by landscape complexity, can have significant downstream impacts on crop production. Chapter two scales up on this work, looking at a generalist herbivore across New York State. I examine how landscape simplification shapes not only body size, but also phenotypic plasticity of traits across different populations. I show that individuals from highly simplified agricultural landscapes exhibit reduced size and reproductive output, but increased plasticity in response to resource quality. I also show that survival did not change between populations, indicating that species can utilize different trait strategies under contrasting environmental conditions to persist in human-modified landscapes. My final chapter takes a global approach to questions of intraspecific trait variation, synthesizing data across 173 taxa. I determined whether landscape simplification has a filtering effect on intraspecific body size, then further examined if the relationship between landscape complexity and intraspecific body size could be explained by latitude, functional group, or diet breadth. My analyses revealed no consistent effects on intraspecific body size from either landscapes or latitudes, for any grouping of taxa. This work illustrates the need to integrate large-scale gradients with fine-scale ecological context in order to accurately anticipate if and how species will respond to landscape simplification, fragmentation, and shifts in climate. Together, this dissertation demonstrates that locally focused, trait-mediated approaches remain essential for understanding ecological responses to land-use change. This work furthers our understanding of the drivers behind changes in arthropod communities and the ecosystem services they provide, from the local level all the way to the global scale.