The effect of vegetative herbivory on the demographic importance of selfing and outcrossing in Impatiens capensis.
Steets, Janette *,1, Knight, Tiffany2, Ashman, Tia-Lynn1, 1 University of Pittsburgh, Pittsburgh, PA, USA2 Washington University in St. Louis, St. Louis, MO, USA
ABSTRACT- Vegetative herbivory is known to affect plant-mating system, which in turn may reduce population growth rate and alter population genetic structure. In two natural Impatiens capensis populations, we manipulated insect herbivory to investigate the effects of leaf damage on population growth and the demographic importance of selfed relative to outcrossed seeds. Specifically, I. capensis produces both both cleistogamous (obligately selfing) and chasmogamous (facultatively outcrossing) flowers. We created a mating system-explicit periodic matrix model for I. capensis in which the demographic rates of cleistogamous and chasmogamous individuals were considered separately. We found that the population growth rate was most sensitive to changes in vital rates of the cleistogamous individuals regardless of herbivory environment, indicating the demographic importance of selfing for this species. Analysis of the vital rates revealed that cleistogamous and chasmogamous individuals differed in important ways. Cleistogamous individuals had higher germination and survival probabilities and lower fecundities than chasmogamous individuals. We found that a 35 - 50% reduction in herbivory caused the population growth rates to increase by approximately 100 - 130%. The reduction in population growth rate with increased herbivory was primarily due to effects on the survival of cleistogamous individuals and the cleistogamous fecundity of cleistogamous and chasmogamous adults. Our study is among the first to consider the demographic consequences of mixed mating and to address the combined effects of herbivory and mating system on population dynamics.
Key words: population growth rate, mixed-mating system, demographic matrix modeling, herbivory
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