Herbivory affects patterns of plant reproductive effort and seed production

Insect herbivory can have a major influence on plant reproduction, and potentially drive selection for strategies that reduce or resist herbivore effects. I used a combination of field experiments and ecological modeling to examine how modifications in the patterns and timing of reproductive investment might ameliorate the consequences of herbivore damage for plant reproduction. I performed experiments to examine how changes in reproductive effort after apical damage and reduction of insect herbivory affected seed production in two thistles native to Nebraska. I then used field data to parameterize a life history model predicting the resource allocation among buds and size and timing of flowering that would optimize fitness under a continual risk of herbivory. In monocarpic Cirsium canescens, insect herbivores had a severe impact on plant seed production. Plants did modify reproductive effort in response to apical damage. High seed production from a large apical head, as well as increased flowering and seed production with apical damage from later flower heads, played a role in improving seed production. However, changes in flowering and investment patterns were insufficient to compensate for high insect damage; plants had lower seed set under ambient herbivory. We found similar effects in the iterocarpic Cirsium undulatum, although plant responses were not consistent between years. The combination of these two experiments allowed us to quantify the influence of plant reproductive response on the consequences of insect damage, and how it varies between plants with different life history strategies. To better understand how the risk of insect herbivory might shape optimal plant allocation patterns, I constructed a stochastic dynamic programming model (SDP) to examine the optimal allocation between flower heads through time, and the size and time at which buds should flower to maximize fitness. The model predicts optimal allocation patterns should vary with survival risk, and plants should favor strategies that reduce the duration of risk. Both the model and experiments demonstrate the pressure insect herbivores can exert over plant reproductive strategies, and broaden our understanding of how ecological interactions can affect influence basic life history decisions. Advisors: Svata Louda & Brigitte Tenhumber