Edge effects and mating patterns in a bumblebee-pollinated plant

Researchers have long assumed that plant spatial location influences plant reproductive success and pollinator foraging behaviour. For example, many flowering plant populations have small, linear or irregular shapes that increase the proportion of plants on the edge, which may reduce mating opportunities through both male and female function. Additionally, plants that rely on pollinators may be particularly vulnerable to edge effects if those pollinators exhibit restricted foraging and pollen carryover is limited. To explore the effects of spatial location (edge vs. interior) on siring success, seed production, pollinator foraging patterns and pollen-mediated gene dispersal, we established a square experimental array of 49 Mimulus ringens (monkeyflower) plants. We observed foraging patterns of pollinating bumblebees and used paternity analysis to quantify male and female reproductive success and mate diversity for plants on the edge versus interior. We found no significant differences between edge and interior plants in the number of seeds sired, mothered or the number of sires per fruit. However, we found strong differences in pollinator behaviour based on plant location, including 15 % lower per flower visitation rates and substantially longer interplant moves for edge plants. This translated into 40 % greater pollen-mediated gene dispersal for edge than for interior plants. Overall, our results suggest that edge effects are not as strong as is commonly assumed, and that different plant reproduction parameters respond to spatial location independently.
Many flowering plant populations have irregular shapes that increase the proportion of plants on the edge. This reduces the number of neighbours a plant has, which may reduce mating opportunities and lead to decreased fitness. Plants that rely on pollinators may be particularly vulnerable to edge effects. Christopher et al. examined whether fitness via male and female function as well as pollinator foraging patterns differed between plants on the edge or the interior of an experimental population. They found no differences between fitness components. However, pollinator visitation rates were significantly lower for edge plants. This suggests that plant reproduction parameters respond independently to spatial location.