Pollination in a changing world: function and resilience

Abstract

Human activities have far-reaching impacts on ecosystems worldwide. Agricultural expansion and intensification is a primary driver of terrestrial biodiversity loss, disrupting important ecosystem functions. Paradoxically, agriculture relies strongly on these same ecosystem functions. Animal-mediated pollination is critical for the functioning of natural and production systems, but is strongly affected by agriculture, largely due to impacts on wild pollinators that require natural habitat. However, although more biodiverse communities often provide greater functional performance and stability, specific effects of agriculture on pollination is poorly understood. In this thesis, I used landscape and small-scale experimental studies to investigate the impacts of agricultural intensification on insect pollinator communities and pollination function. I investigated (1) whether pollinators show response diversity (differential changes in abundance) to agricultural intensification by collecting pollinator species from flowering fields along an agricultural gradient. I also quantified functional redundancy (species with similar functional roles) by measuring pollinator functional traits. I measured (2) changes to pollination services with agricultural intensification by quantifying pollen deposition rates. I then investigated (3) how plant and pollinator numerical evenness affects pollination function, using a mesocosm experiment. I developed (4) a method for measuring pollinator hairiness and investigated its predictive power for pollinator effectiveness. In my landscape study, I found that (1) although there was strong community-wide response diversity to agricultural intensification, functionally redundant pollinator species mostly lacked response diversity. Exotic species enhanced response diversity but did not functionally replace natives. Furthermore, (2) pollination services from native species declined with agricultural intensification, whereas pollination from exotic species increased, which increased overall pollination services. However, agriculture reduced evenness of pollination service delivery from different species. In mesocosms (3) seed production was highest where a few plant and pollinator species dominated interactions, but where interaction frequency was relatively equal. Hairiness (4) strongly predicted pollinator effectiveness, with hairier species depositing more pollen. Thus, agricultural intensification can drastically alter pollinator communities, and their robustness to future anthropogenic disturbances. In high intensity landscapes, exotic species provide compensatory pollination services, but native species declines likely increase vulnerability to future disturbances. As anthropogenic modification of the biosphere continues, progressing our understanding of how these changes impact ecosystems is imperative.