Frequency dependence of pollination - from individual behaviour to community dynamics
Ecology and Biodiversity of Plants and Ecosystems
Final Report Abstract
A central question in ecology is how species which compete for the same resources can coexist. This question is especially relevant for plant species, since all plants depend on water, light and a small number of nutrients. Previous theoretical work showed that stable species coexistence is possible when rare species have greater fitness (survival and reproductive success) than common species, i.e. when fitness declines with increasing frequency (negative frequency dependence). Conversely, when fitness increases with frequency (positive frequency dependence), rare species can quickly become extinct. Beside their resource requirements for survival and growth, many plant species depend on animals as pollinators of their flowers in order to reproduce. Previously published models, experiments and field data suggested that the number of flower visits and amount of pollen that flowers receive generally increase with plant species’ frequency. However, before this project it was unclear under which conditions such positive frequency dependence occurs and how factors such as pollinator foraging behaviour and traits of plants and pollinators influence the effect of plant species frequency on flower visitation and pollination success. The objective of this project was to to fill this research gap through a combination of methods: computer simulations of foraging pollinators, a laboratory experiment with bumblebees foraging on artificial flowers and a field study in a species–rich plant community. In contrast to previous studies, my collaborators and I found that under certain conditions flower visitation rates and pollen receipt of plant species can be negatively frequency–dependent. In our computer simulations, the rarer of two plant species was favoured when the overall density of flowers was high and pollinators had limited information about the distribution of floral rewards such as nectar. While in the laboratory experiment bumblebees showed on average a slight preference for the more abundant artificial flower type, both experimental results and computer simulations suggested that a rare plant species can at least reduce or even overcome this disadvantage by increasing its floral reward (nectar sugar concentration), even if the increase is temporary and followed by a period with low reward. In addition to the highly detailed simulations of foraging pollinators, we developed a statistical model to predict networks of interactions such as those between multiple plant and pollinator species based on their traits, abundances and shared evolutionary history. While this model is less mechanistic compared to simulations of individual pollinators, it can be applied to many existing data sets. Thus, the model enables us to study the relative importance of plant and pollinator traits and abundances for the distribution of flower visits under a wide range of conditions. In a field study in the Cape Floristic Region of South Africa, we collected data on plant–pollinator interactions as well as flower frequencies at two spatial scales, flower visitation rates and seed production. As we had hypothesized based on theoretical considerations, we found a hump–shaped relationship between flower frequency and number of flower visits, with a peak at intermediate frequency. For plant species with a larger number of visitor species, this peak was higher and occurred at higher flower frequency, indicating that such generalized species are less limited by pollinator availabiliy. Overall, the results of this project show that the effect of plant species frequency on flower visitation and pollen receipt is more complex than previously thought, and that both positive and negative relationships are possible, depending on plant and pollinator traits and environmental conditions. These findings suggest that plant–pollinator interactions can contribute to plant species coexistence under some conditions, but may impede the persistence of rare plant species in other cases. Further research on the link between pollination and plant population dynamics is needed to fully understand the implications of our findings for the maintenance and preservation of species-rich plant communities.
Publications
- 2018. Adaptive foraging of pollinators can promote pollination of a rare plant species. – American Naturalist 192(2): E81–E92
Benadi, G., and Gegear, R. J.
(See online at https://doi.org/10.1086/697582) - 2018. Frequency dependence of pollinator visitation rates suggests that pollination niches can allow plant species coexistence. – Journal of Ecology 106(5): 1892–1901
Benadi, G., and Pauw, A.
(See online at https://doi.org/10.1111/1365-2745.13025) - 2022. Quantitative prediction of interactions in bipartite networks based on traits, abundances, and phylogeny. – American Naturalist 199(6): 841–854
Benadi, G., Dormann, C., Fründ, J., Stephan, R., and Vázquez, D. P.
(See online at https://doi.org/10.1086/714420)
