Abstract: Widespread declines in pollinator populations have prompted concern that many flowering plant species will decline in tandem. However, demographic life-history theory shows that the population growth rate of iteroparous perennial plants is largely insensitive to perturbations in fecundity, suggesting that changes in pollination may not have extensive population-level consequences in these species, particularly if resources can be reallocated to other life history functions that more strongly affect population growth. To link changes in pollination to population growth rates, the entire plant life cycle must be considered. Therefore, we conducted an experimental demography project to examine how three pollination treatments affect population growth rates of iteroparous perennial plants: reduced pollination, increased pollination, and an unmanipulated control. We bagged 50% of flowers to reduce pollination and added supplemental outcross pollen to stigmas to increase pollination for three study species—Delphinium nuttallianum, Hydrophyllum fendleri, and Potentilla pulcherrima—at the Rocky Mountain Biological Laboratory in Colorado, USA from 2018–2022. We used field censuses to parameterize integral projection models (IPMs) with demographic field data to understand how the pollination treatments affect finite rate of increase (λ). We then used Life Table Response Experiments to attribute the treatment differences in λ to changes induced in the underlying vital rates.
We find a range of outcomes across our three study species. For D. nuttallianum, experimentally-imposed changes in pollination caused population growth rate changes via seed production. Plants experiencing reduced pollination reallocated resources to increased vegetative growth, but this was insufficient to offset population growth losses attributed to reduced seed production. For P. pulcherrima, the pollination treatments did not affect population growth rates, likley because this species is capable of autogamous self pollination, which might buffer population growth from pollinator declines. For H. fendleri, increased pollination caused higher seed production, but at the cost of decreased vegetative growth and survival. The net result was a decline in population growth. Taken together, our results demonstrate that altered pollination can affect population growth via vital rates other than reproduction, which to our knowledge has not been shown before. Our results also show that species vary in their capacity to buffer against fitness losses when pollination declines. We argue that viewing the impacts of pollination on plants in the context of the whole life cycle offers a powerful lens for predicting and managing plant populations in a changing world.
