Central to ecology and evolution is an understanding of the myriad ways in which organisms influence each other through their interactions. A hallmark of plant-herbivore interactions, however, is that they are spatially variable, with outcomes of interactions often depending on the environmental conditions where the interactions play out. This variability, or context-dependence, hinders the development of general predictive models of plant-herbivore interactions and therefore represents an important area for investigation. The goal of the plant-herbivore interaction lab is to develop a framework for predicting how environmental factors influence the magnitude of impact that plant-herbivore interactions have on the evolution of traits, population dynamics, and community composition for both plants and insects. We work in natural and managed ecosystems at scales from local neighborhoods to populations and communities distributed across regional environmental gradients. Below are the major themes of my research program.

Evolutionary ecology of plant defense against herbivory

Plants have evolved many defensive traits, such as chemical toxins, thorns, or sticky latex, to protect themselves from the damaging effects of herbivory. Producing defenses has been shown to come at a cost of other functions, such as growth and reproduction, across species but not within species (Hahn & Maron 2016). Understanding of what drives the substantial population-level variation in plant defense remains poor, but is critical for developing a synthetic framework for understanding the ecological and evolutionary consequences of plant defense. We are currently addressing the question of how the abiotic environment mediates plant growth, defense, and insect herbivore pressure among populations of co-occurring grassland plant species. To date, our data support a trend emerging in the literature that under certain environmental conditions, growth and defense correlations can often be positive (Hahn et al. 2019, Hahn et al. 2021) rather than negative (i.e., tradeoffs). Additionally, our work is also showing that the impact of herbivores on plant fitness, a notoriously variable process, varies predictably across environmental gradients and with species traits (Hahn et al. 2021, Palmer et al. 2022, Calixto et al. 2023). Thus, our results challenge the idea of ubiquitous tradeoffs between growth and defense and demonstrate the importance of environmental variation for shaping plant-herbivore interactions.

Funding: NSF DEB-1901552

Linking functional traits across trophic levels

A classic question in ecology concerns how species in diverse communities partition resources to coexist. Trait matching, which involves identifying traits that mediate linkages between trophic levels, is a potentially useful method for revealing the mechanisms by which consumer species partition resources. Herbivore species may require a specific assortment of functional traits to consume certain plant species based on their functional traits (i.e., trait matching). The main objective of this project is to identify matching traits between plant and insect herbivore communities (i.e., grasshoppers) that explain the assembly of their communities. Our central hypothesis is that variation in plant community traits selects for herbivores with traits that enable them to capitalize on those plant resources. We are using experimental mesocosms and field surveys, and metabarcoding of grasshopper gut contents to explain how trait matching between plants and grasshoppers structures communities.

Funding: UF Research – DeLuca Jumpstart Award

Interactions between biocontrol herbivores

Invasive plants cause substantial ecological damage  and costs billions of dollars in economic losses and control per year in the United States. Classical biological control is the practice of introducing natural enemies from the native range with in the intention that they will reduce the population size of the invasive species.  However, the effectiveness of biocontrol agents is highly variable, with some agents essentially eradicating the invader, while others have little to no impact . Thus, management of invasive plants typically takes an integrated approach, where mechanical, chemical, and biological control, including the release of multiple biocontrol agents, are often combined to increase the success of control efforts. We are investigating whether interactions between the two beetles released to control the invasive plant air potato (Dioscorea bulbifera) could be synergistic, where the feeding by one beetle species may stimulate more vigorous feeding by the second beetle. Alternatively, interactions could be antagonistic, where feeding by one beetle actually deters feeding by the second. We have previously shown that feeding by the initial biocontrol agent, Lilioceris cheni, induces the production of toxic chemicals (saponins) in the plant. Because L. cheni is highly specialized on air potato, the chemicals act as a feeding stimulant and the beetles feed more heavily on previously damaged plants. The second biocontrol beetle is also highly specialized on air potato, and thus may also respond to previously damaged plants by feeding more voraciously. Thus, this scenario would result in a synergism between the two biocontrol agents where each performs better in the presence of the other. Alternatively, the newly approved biocontrol beetle may be more susceptible to the chemical toxins induced by the first beetle, which would result in antagonistic interactions. We are investigating these potential synergistic and antagonistic interaction using a combination of lab and field experiments. 

Funding: USDA