Plants constantly adopt strategies to optimize their chances of survival. Those strategies can span the lifetime of one plant or eons of evolution for a family of plants. In the end, whole ecosystems can be impacted by something as seemingly small as a bush flowering a few weeks earlier in the spring.
“The species within ecosystems are so tightly connected that changing one little aspect of one species can reverberate throughout the system,” says Tyler C. Coverdale, Cornell Presidential Postdoctoral Fellow. “For instance, it’s easy to point to the rapid evolution or rapid adaptation of a plant species and say, ‘this is why this species will persist even when the climate changes.’ But if a plant gets cues that spring has arrived and it starts flowering but its pollinators aren’t responding to the same cues, then there’s nothing to pollinate the flowers and the system breaks down.”
Plant Defense Systems and Their Ecosystems
Coverdale came to Cornell in the Summer of 2018 as one of the inaugural group of Cornell Presidential Postdoctoral Fellows to work with Anurag A. Agrawal, Ecology and Evolutionary Biology/Entomology. Over three years, Coverdale will carry out research projects building on his dissertation work looking at the defensive strategies of plants and the tradeoffs they must make when they choose to defend themselves. His overall goal is to gain insight into how particular ecosystems work and to try to answer broader questions in ecology and evolution.
“The species within ecosystems are so tightly connected that changing one little aspect of one species can reverberate throughout the system.”
“There’s a long history in the field of ecology, and especially in plant defenses, of trying to understand the evolutionary pressures faced by plants—how they respond to different herbivore regimes and climatic conditions and so on,” Coverdale says. “We can gain those insights by looking at patterns of plant defense within species and across species.”
Thorns, Spines, Prickles
In one project, Coverdale is taking a deep evolutionary look at the physical defenses—spines, thorns, or prickles—of the Solanaceae family, which includes potatoes, tomatoes, and tobacco. “Closely related species of plants can produce different types of physical defenses,” Coverdale says. “One will produce a spine, and one a prickle, for instance. That’s similar to the difference between horns and antlers—two very different physiological structures that serve the same purpose.”
Coverdale will use a suite of statistical methods to compare across the family of 3,000 species to pinpoint when and where they produce the different types of defenses. “I will be asking why these differences exist,” Coverdale says. “Are there certain environmental conditions that favor one defense over another? Why do some plants have no defenses whereas others living in the same conditions are heavily defended?”
Big Leaves versus Small Leaves, Shade versus Sun
In a second project, slated to begin in the 2019 growing season, Coverdale will subject several closely related species of milkweed to controlled experiments in greenhouses and in the field, exposing the plants to varying amounts of light. His goal is to learn how plants adapt to individual stressors, in this case shade, over different time spans and to understand the constraints that control their responses over time. “When you put a sun-loving plant in the shade, one common response is to produce bigger leaves,” he explains. “They make themselves as big as possible to capture as much light as possible. Plants that are actually adapted to the shade produce smaller leaves than closely related plants that live in the sun. So these two different strategies have evolved: one over long evolutionary periods and one over the individual lifespan of a plant.”
Coverdale works with Agrawal, who is an expert on the milkweed family and the milkweed–monarch butterfly ecosystem. They are comparing the responses of two pairs of milkweed species—two that grow in full sun and two that grow in more shaded areas. The researchers will put the plants in their opposite habitats to see how they handle the shift in light.
“An individual plant in the course of its life can change a lot of things: the size and shape of its leaves, how much it invests in defense, how much it grows vertically versus horizontally, how much it grows above ground versus below ground,” Coverdale explains. “But there’s a tradeoff, too, because the plant can’t do everything all the time. If it’s making bigger leaves, then it can’t invest as much in its own defense, for instance. Plants that move from sun to shade tend to become more vulnerable to herbivores.”
Plants’ Toxic Chemicals
To test vulnerability, Coverdale and Agrawal look at chemical changes in the milkweed, which produces toxic substances in its sap as a defense strategy against herbivores. In a co-evolutionary adaptation, the caterpillars of the monarch butterfly, which eat milkweed exclusively, are able to take up the toxic chemicals and sequester them, using the plant’s defense against herbivores as their own defense against predators.
“We’ll measure the chemistry of the sap in the plants directly, and we’ll let caterpillars feed on the leaves to see how they do over time,” Coverdale says. “The caterpillars are a great bioassay for how well-defended the plants are.”
Interwoven: Evolutionary and Natural History, Chemical Ecology, Experimental Research
Coverdale first met Agrawal about four years ago when Agrawal gave a lecture at Princeton University where Coverdale was a PhD student. “I was blown away by his talk because there are only a few people in my field who understand their system from every perspective like he does,” Coverdale says. “To see someone bring in deep evolutionary history, chemical ecology, natural history, comparative approaches, and great experimental work—that is very rare. One of my big goals in coming to Cornell is to learn from Anurag what it takes to really know your system and to be able to approach it from every perspective. That’s a way to increase the impact of my work and my ability to gain insight into how ecosystems work.”
Cornell also offers a wide range of perspectives and expertise that Coverdale values. “The Agrawal group is unique in my field in that Anurag is co-appointed in ecology and evolutionary biology, and entomology,” he says. “Cornell also has maybe a half dozen departments looking at these same plant-herbivore interactions that I’m interested in from other perspectives, such as the School of Integrative Plant Science and Molecular Biology and Genetics. Getting this fellowship, which comes with the flexibility to work with people across departments, means the projects I’m working on can expand in any number of directions. The expertise is here, the facilities are here, and the willingness to collaborate is top notch.”