Consequences of plant population size and density for plant-pollinator interactions and plant performance

1 Habitat fragmentation and the resulting decline in the local abundance of plant species can affect biological interactions. We examined the effects of abundance on plant-pollinator interactions by observing the pollinator service and subsequent reproductive output of a mostly outbreeding, but self-compatible, plant, Lychnis viscaria, in experimental populations of different sizes (number of individuals) and densities (distance between individuals). 2 Bumblebees, the main pollinators of L. viscaria, preferred larger populations, but visitation rates were higher in sparser populations. Pollinators were attracted to the larger inflorescences in sparse populations, which were also more visible due to their larger area for a given size. 3 Bumblebees probed more flowers within plants in sparse populations, probably due to the larger inflorescences and longer flight distances between individuals. 4 Subsequent reproductive success (capsule production) was higher in sparse populations, due to differences in pollination success and resource competition, and their interaction. In self-compatible species, such as L. viscaria, reproductive success may be determined more by resource availability, whereas self-incompatible plants may be more sensitive to changes in pollinator abundance. 5 We conclude that plant-pollinator interactions are sensitive to changes in both the size and spatial arrangement of plant populations, which can affect their demography and genetics. In this study, species density had a greater effect than size and the unexpectedly beneficial effects of low density may be due to greater resource availability.