CONDITION-DEPENDENT LIFE HISTORY STRATEGIES

Many organisms evolve condition-dependent life history strategies to maximize their lifetime fitness in response to intrinsic and extrinsic processes. I investigated a sequentially flowering plant’s strategy to allocate resources to retain flowers versus grow existing basal fruits to a larger size, using the plant Yucca glauca. The sink strength hypothesis suggests basal fruits are nutrient sinks depriving distal flowers of resources and reducing their probability of retention. A low probability of retention of distal flowers can also be explained by the architectural effects hypothesis. This hypothesis posits inherent positional differences in structures along an inflorescence such as flower size and amount of vascular tissues decrease flower retention with increasing flower position, independent of the number of basal fruits. I experimentally showed that the presence of basal fruits decreased the probability of retention of distal flowers, which supports the sink strength hypothesis. Further, in the absence of fruits, plants retained distal flowers at a probability similar to that of basal flowers, which is inconsistent with the architectural effects hypothesis. Next, I developed a stochastic dynamic programming model to examine the conditions under which decreasing flower retention in response to existing basal fruits is optimal for sequentially flowering plants. The model predicts that plants should decrease flower retention with increasing number of basal fruits when large fruits produce more viable seeds than small fruits (fruit size-dependent viability benefit). Finally, I tested if a higher probability of flower abortion in the presence of basal fruits affects the life history strategy of insects that lay eggs in flowers. Yucca glauca flowers are egg-laying sites for seed-eating insect Tegeticula yuccasella. Flowers that have a high probability of being aborted are low quality egg-laying sites for T. yuccasella because all eggs in aborted flowers die. I experimentally showed that when basal fruits were present, T. yuccasella were less likely to lay eggs in flowers. These investigations help identify mechanisms underlying condition-dependent plant and animal life history strategies that contribute to intra-population variation in life history strategies. Adviser: Brigitte Tenhumber