The effect of herbivory and pollen limitation on a declining population of Trillium grandiflorum

In long-lived organisms, it is often difficult to determine which environmental factors will have the largest effects on population dynamics. In this study, I incorporated the results of short-term experiments and observations, into a demographic matrix model to determine the effects of both herbivory and pollen limitation on the dynamics of a declining population of the perennial herb Trillium grandiflorum. While pollen supplementation experiments in both 1999 and 2000 revealed that plants produce fewer seeds as a result of pollen limitation, this pollen limitation had almost no effect on the growth rate of the population. White-tailed deer (Odocoileus virginianus) consumed nearly half of the reproductive plants in this population in 2000 and 2001. Herbivory causes reproductive and large nonreproductive plants to regress in stage and have lower fecundity. In the absence of herbivory, demographic projections suggest that the population would shift from declining to growing, the proportional representation of new recruits would rise, and plants in larger stage classes would have higher reproductive values. Such shifts in the reproductive values and stable stage distribution result in shifts in elasticities. A life table response experiment demonstrated that, while herbivory affected six vital rates, two of these contributed disproportionately to the change in the population growth rate: reproductive plants remaining reproductive and large nonreproductive plants remaining nonreproductive. In this population, herbivores contribute more to its decline than pollinators. Thus, active long-term management of deer populations appears necessary for the conservation of understory herbs such as Trillium grandiflorum.