The effects of pollination on plant demographic vital rates other than reproduction are largely unknown, which greatly limits our ability to predict how changes in pollination may affect plant population dynamics. This knowledge gap is especially relevant in light of declining pollinator populations and a growing need to understand their ecological consequences. Pollinator-mediated changes in seed production are unlikely to have straightforward effects on plant population growth rates, especially in iteroparous perennials where costs of reproduction and resource reallocation are common. Indeed, demographic life history theory indirectly suggests that altered pollination may have negligible effects on population growth in perennial species. The first step toward understanding the consequences of altered pollination on plant population growth is to assess its effects on vital rates in addition to reproduction. Therefore, we examined how multiple demographic vital rates responded to three treatments: reduced pollination, increased pollination, and an unmanipulated control in a long-lived perennial plant species, Hydrophyllum fendleri, at the Rocky Mountain Biological Laboratory in Colorado, USA. We bagged 50% of flowers to reduce pollination and added supplemental outcross pollen to stigmas to increase pollination. We measured survival, vegetative growth, flowering, and seed production on tagged individuals across the three treatments.
Results/Conclusions
As expected, pollination treatments affected seed production (total # seeds per plant; X2 = 24.2, p < 0.0001). Plants produced fewer seeds in the reduced pollination treatment compared to the control and increased pollination treatments. There was no difference in seed production between hand pollinated and control plants, showing that seed production was not pollen limited. Pollination treatment also affected plant growth from year t to t + 1 (X2 = 8.0, p = 0.045). Growth was higher in the decreased pollination treatment compared to the increased treatment, consistent with costs of reproduction. Pollination treatment also affected the probability that a plant will flower in the following year (X2 = 119.8, p < 0.0001), with a reduced probability of flowering in the controls relative to the other two treatments. Pollination treatment did not affect the probability of survival. Here we find that altered pollination levels do indeed affect vital rates other than reproductive output in the present year. These results suggest that changes in pollination levels may affect plant population growth rates via vital rates other than reproduction, emphasizing the importance of considering the entire plant life cycle.