Plant Responses to Joint Effects of Herbivores and Pollinators

Plants are fed upon by a range of insect foragers, including herbivores and pollinators. Because herbivores damage plant parts and pollinators transfer pollen among plants, plants generally benefit by avoiding herbivores and attracting pollinators. Through interactions with their host plants, herbivores and pollinators can influence the expression and evolution of plant traits. While there is a substantial body of research on plant-herbivore and plant-pollinator interactions, and increasing appreciation for the joint effects of herbivores and pollinators on plants, there are still aspects of plant-pollinator-herbivore interactions that warrant attention, including the persistence of effects beyond the year in which the interaction occurred, the effects of variability in herbivory and pollination on the evolution of plant traits, and the effects of herbivores and pollinators on asexual reproduction in plants. Ecological interactions between foragers (pollinators and herbivores) and host plants make possible a network of feedbacks in which foragers both influence and respond to plant traits. These feedbacks can link the plant traits to which foragers respond, and thus may help explain observed variation in plant traits. Interactions among plants, herbivores, and pollinators have been well documented, but addressing interactions among all three across years in a single system is rare. Across year effects are particularly important because single-year studies might misinterpret plant responses to their environment. In chapter two I describe two experiments using the perennial plant Chamerion angustifolium that address plant-forager interactions. In one I manipulated herbivory and pollen receipt to quantify forager effects on plant traits and in another I manipulated plant size and flowering phenotype to quantify forager response. I found pathways of interaction between plants and insects both within and across years, suggesting the potential for feedback between foragers and plant traits. Results suggest that while pollinators prefer plants with more flowers, and pollen receipt results in smaller plants, herbivores cause size overcompensation and flower reduction. Together these effects of both herbivores and pollinators may help maintain intermediate values of size and flowering traits. Environmental conditions can have a profound influence on plant fitness, and can vary substantially in time. When environmental variability is unpredictable, that is, when plants have no cues as to upcoming environmental conditions, they should evolve a bet-hedging strategy to deal with environmental variability. In chapter three, I constructed a simulation model to address the evolution of the timing and pattern of resource allocation (allocation schedules) in annual and perennial plants under stochastic variability in herbivory and pollination. Both herbivory and pollination can be highly variable in space and time, but we don't fully understand how this variability influences the evolution of plant traits. I found that annual plants flower later in the growing season and perennial plants have early and prolonged flowering under high environmental variability. I found that across-year variability selected for late flowering in annuals and for early and prolonged flowering in perennials, suggesting that populations of annuals and perennials should evolve different types of allocation schedules under variable herbivory and pollination. The biotic environment plays an important role in whether a species will be able to invade a new habitat. Herbivores and pollinators may influence the spread of plant species by influencing allocation to reproduction. Many plants reproduce both sexually and asexually, and the mode of reproduction a plant expresses can influence colonization of and establishment in new habitats. In chapter four, , I describe a series of studies using Eichhornia crassipes (water hyacinth) to examine the effects of pollination and simulated (manual) larval and adult herbivore damage on plant growth, sexual and asexual reproduction, and herbivore resistance. Herbivores and pollinators are known to influence a wide variety of plant traits, but despite the importance of mode of reproduction for plant population dynamics, we know little about how clonal plants respond reproductively to herbivory or pollination. I conducted surveys of natural populations to assess differences in allocation pattern, herbivore damage, and herbivore resistance, and used a common garden experiment to determine if differences in allocation pattern or resistance are due to environment or among-population genotypic differences. I found that the damage mimicking larval feeding generally shifted plant responses toward asexual reproduction, while adult-type damage and pollination had no effect, demonstrating that plant foragers and forager identity can have important, but as yet incompletely understood, effects on asexual plant reproduction. Although there is a large body of research on plant-herbivore and plant-pollinator interactions, there remain complexities in these interactions that warrant further research. Results from my dissertation demonstrate that (1) multi-year studies on perennial plants are necessary to understand the effects of foragers on perennial plants, (2) forager variability might have important effects on the evolution of plant traits, and (3) foragers can have important effects on plant asexual reproduction.