Effects of pollinators, seed predators, and vertebrate herbivores on female and hermaphrodite sex morphs and demography of the gynodioecious plant, Polemonium foliosissimum

Background/Question/Methods

In gynodioecious plant species, in which there are female and hermaphrodite sex morphs, females require a fitness advantage over hermaphrodites to be maintained since they do not sire seeds. In the past decade, the importance of ecological context, and species interactions, especially of mutualism and antagonism, in providing the requisite advantage to maintain females has come into the limelight. Despite this, few studies have assessed the lifetime fitness consequences of interactions with mutualists and antagonists using demographic modeling. I am conducting experimental field manipulations to assess the effects of bee pollinators, fly seed predators, and deer herbivores on the long-lived Rocky Mountain plant, Polemonium foliosissimum, and using demographic modeling to project the long-term consequences for population growth and sex morph fitness.

Results/Conclusions

In Polemonium, females set significantly more seed than hermaphrodites (mean for females = 851, hermaphrodites = 672; t =2.399, df = 385, p= 0.017) and experience lower fruit predation rates (t = -4.096, df = 368, p < 0.001). Females may also be more tolerant of seed predation than hermaphrodites, having an ability to re-allocate resources from damaged to undamaged fruits that hermaphrodites lack (as indicated by significant correlations between seeds per fruit and predation rates for females but not hermaphrodites). To test the possibility of greater tolerance to predation by females, and to assess the extent to which reallocation is enhanced by supplemental pollen, in 2011, I conducted an experiment in which I applied two levels of hand pollination (open pollination vs. pollen addition) and seed predator egg removal (ambient numbers of eggs vs. eggs removed) in a fully factorial design.

Preliminary results based on comparisons of vital rates of plants with >50% versus <50% of stems browsed suggest that herbivory increases the rate at which medium reproductive plants regress to the small reproductive class (32% vs. 36%), decrease the rate at which they grew to large reproductive class (15 vs. 10%) and increased their rate of mortality (5 vs. 7%; α² = 1.984, df = 1, p = 0.113). Modeling these preliminary results, herbivory appears to reduce the relative lifetime fitness of females by shifting plants out of the largest size classes where the female fertility advantage is greatest. I will use demographic modeling to compare the outcome of the effects of herbivores (via alterations to growth/survivorship vital rates) and seed predators/pollinators (via alterations to fertility) to population growth and relative sex morph fitness.