Phenology and plant defenses: optimizing complex strategies in a dynamic world
Edwards, Collin B
Tic-Tac-Toe, played by placing pieces on a 3x3 grid, is a solved game. The game Go, played by placing pieces on a 19x19 grid, is not. In a simple world with simple rules, organisms might succeed by following simple strategies. If plants must always fend off exactly one type of herbivore, and exactly one defense trait is effective against that herbivore, the optimal defense strategy will be to use that trait. If an insect’s fitness is based solely on emerging from hibernation at the right time of year, and the weather patterns are identical every year, there will be a best to day to emerge, and the optimal strategy is to emerge on that day. But much as Go is more complex and high-dimensional than Tic-Tac-Toe, the real world is far more complicated and dynamic than these simple examples, and organismal strategies often involve balancing expression of multiple traits in complex ways. In studying ecological strategies, ecologists and evolutionary biologists often work backwards, observing complex patterns of behavior or trait expression, and seeking to understand what factors could make these observed patterns beneficial. For my dissertation I have taken two such observed patterns – plant defense syndromes, and divergent phenological responses to climate change – and taken empirical, mathematical, and computational approaches to uncovering the mechanisms underlying them. Most plants employ multiple defenses against herbivores simultaneously. For example, plants in the milkweed genus often either have high levels of toxins, or high levels of defensive hairs and sticky latex, or especially tough and highcarbon leaves. A number of hypotheses have been put forth to explain why plants should use multiple defenses, and why some combinations of defenses are more common than others, but these hypotheses have been difficult to test. Chapter 1 is an empirical study of herbivore performance on common milkweed (Asclepias syriaca), which found support for three hypotheses that could explain defense syndromes in the Asclepias genus: (a) multiple herbivores, (b) bet hedging, and (c) trait synergies. Chapter 2 is a modeling study, in which I used simple models of plant and herbivore dynamics, parameterized from the empirical literature, to explore why multi-trait defense strategies are so ubiquitous. This study found that while diminishing marginal returns, bet hedging, and trait synergies can each alone only occasionally favor multi-trait defense strategies, when combined these mechanisms overwhelming favor multi-trait defense strategies. While it is an ecological disaster that will likely shape the challenges faced by the next several generations of humans, global climate change provides a unique wide-scale observational study of organismal phenology. Many plants and animals have shifted the timing of their life history events when exposed to novel climate conditions; surprisingly, these responses are often very inconsistent even among nearby or closely related organisms. I developed a simulation model of the evolution of phenological cueing strategies, incorporating real weather data from across North America and Hawaii, to explain the observed variation. This study revealed that for many climate regimes, different strategies for responding to environmental cues could lead to very similar behavior and thus very similar fitness outcomes. As a consequence, simulated populations experiencing the same climate evolved dramatically different strategies for responding to the climate – and while these populations behaved similarly when exposed to “familiar” climate, their phenology diverged when they were exposed to simulate climate change. Taken together, these studies demonstrate the role that temporal variability and interplay between related traits can take in driving the evolution of organismal strategy. By combining empirical and theoretical tools and a bit of luck – we can uncover underlying ecological processes from observed patterns, as was done here to explain plant defense syndromes and shifting phenologies.
phenology; plant defenses; plant-insect interactions; theoretical ecology
Ellner, Stephen Paul
Agrawal, Anurag; Geber, Monica Ann; Hochachka, Wesley M.
Ecology and Evolutionary Biology
Ph. D., Ecology and Evolutionary Biology
Doctor of Philosophy
dissertation or thesis