Edge effects and mating patterns in a bumblebee-pollinated plant

Abstract

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.

Keywords: Edge effects; Mimulus; gene dispersal; mate diversity; monkeyflower; paternity; pollination; seed set; siring success; spatial location.

Figure 1.

Schematic of the experimental array, a 7 x 7 square grid of plants spaced 0.8 m apart. Edge plants are shown in orange; interior plants in blue. A focal plant on an edge of the population (shown in grey) has five adjacent neighbours. A focal plant in the interior of the population (shown in pink) is surrounded by eight near neighbours.

Figure 2.

Mean number of seeds (±1 SE) mothered on, or sired by, edge and interior plants. N per bar = 96 for edge and 100 for interior plants. The number of seeds sired is slightly lower than the number of seeds mothered due to the estimation error of paternity shares.

Figure 3.

Mate number per fruit for edge versus interior plants. Mate number determined by paternity exclusion for 5 seeds per fruit. N = 92 plants for edge and 95 for interior.

Figure 4.

Pollinator movements and pollen-mediated gene dispersal for edge and interior plants. X-axis categories are abbreviated; true ranges are 0–0.999, 1–1.999, etc. N for pollinator moves = 73 for edge plants and 136 for interior. N for gene dispersal = 385 for the edge and 457 for the interior.