Optimal foraging theory has revealed a conflict in the plant-pollinator mutualism, especially for self-incompatible plants. Apocynum cannabinum (Apocynaceae) is self-incompatible and clonal and therefore requires extrinsic pollen to effect fruit production. Its insect pollinators are unlikely to make such patch-to-patch movements frequently due to prolific flower production and the energetic cost of flight. To investigate relationships between plant population density, spatial configuration of patches, and pollinator behavior, we studied visitation patterns and resulting fruit production in a natural setting.
The study was conducted at Bath Nature Preserve in Bath Township, Ohio, USA. The preserve is a mosaic of habitat types in a post-agricultural area. A. cannabinum is restricted to old-fields and mowed areas within the 400-acre preserve. All patches of A. cannabinum were mapped, and three populations at each of three relative densities were selected for continued study. The populations were located in three distinct areas: Round Top (RT), North Fork (NF), and Hillside (HS). Density bins were: low (≤ 1.0 plants/m2), medium (1.1-2.0 plants/m2), and high (≥ 2.1 plants/m2). Floral characteristics were measured, and the abundance and diversity of insect pollinators was observed. Fruit production per patch served as a measure of interpatch pollinator movement.
Results/Conclusions
Total fruit set per plant (n=30) was highest in medium density patches (10.8 ± 17.5 fruits), followed by high density (5.0 ± 9.3 fruits) and low density (2.0 ± 3.9 fruits) patches. A two-way ANOVA resulted in a statistically significant difference between density groups (F(2, 289)=10.21, p<0.0001) and an interaction between density and location (F(4, 289)=6.08, p=0.0001). Mean nearest neighbor distance (Euclidean, range 1.0 – 57.1 m) did not have a significant effect on fruit set.
The most frequent visitors to A. cannabinum were insects in the order Hymenoptera (74.5%), followed by Lepidoptera (17.2%). Pollinator species composition varied by location, corresponding to differences in fruit production. A greater proportion of visits to RT and HS patches were from solitary bees – 69% and 68% of Hymenoptera, respectively, compared to 43% at NF. NF patches received a greater proportion of visits from honeybees – 29%, compared to 7% (RT) and 14% (HS). RT and HS patches experienced greater Lepidoptera visitation than did NF.
Results suggest there is a more complex mechanism contributing to fruit set than increased visitation due to forage availability. In contrast to the literature, one or more species of solitary bee seems to be an efficient pollinator of A. cannabinum.