Effects of population size on plant reproduction and pollinator abundance in a specialized pollination system

1. Many plant species are currently experiencing negative consequences of habitat fragmentation as a result of reductions in population size and disruptions in pollination services. Plants in specialized pollination systems might be especially vulnerable to changes in plant population size and density resulting from land-use changes. 2. Representing such a system, we chose the globeflower Trollius europaeus L. with its pollinating fly Chiastocheta, which also acts as a seed predator at its larval stage, to investigate the effects of small plant population size on reproductive success and pollinator abundance. 3. Reproductive output of T. europaeus declined with increasing plant population size, while Chiastocheta abundance within T. europaeus flowers was independent of plant population size. However, at the local level, Chiastocheta numbers within flowers were inversely correlated to local T. europaeus flower density. We further found that increasing floral densities increased plant reproductive success at the population level. 4. Chiastocheta abundance was the main driver of reproductive output of T. europaeus through its dual role as an obligate pollinator and seed predator: at least some Chiastocheta flies were needed to secure pollination, but a continued increase in Chiastocheta flies within a flower incurred seed predation costs that greatly reduced reproductive success. Thus, high local flower density contributed positively to per capita reproductive output by diluting Chiastocheta abundance within flowers independently of overall population size. 5. Synthesis. Our findings highlight that plant population size is not always the main determinant of reproductive success for populations, but that other factors such as plant density and the specific ecology of a pollinator and its interplay with other population parameters can be more important in determining the fate of a population. Furthermore, the effects of plant population size and floral density on pollinator visitation in T. europaeus vary across scales, with implications for plant fitness. It is therefore important not to focus solely on pure plant population size in determining population viability. Thus, from a conservation perspective, even small and isolated T. europaeus populations may be viable and resistant to pollination-associated vulnerabilities depending on plant density at local (subpopulation) scales.