My work seeks to understand interactions among plants and antagonists as an avenue for identifying mechanisms responsible for evolutionary radiations, community assembly, and the maintenance of diversity in changing landscapes. I combine molecular and ecological assessments to examine population dynamics of plant-insect interactions across landscapes that change over space and time. My goal is to inform our theoretical understanding of evolutionary processes shaping trophic relationships and provide valuable information for the applied conservation of habitats threatened by anthropogenic forces.
Background. The study of interactions between plants and animals is a major pursuit in ecology and evolutionary biology, as it contributes to the emergence of biological diversity. For example, plant-animal interactions can lead to specialization and subsequent diversification in herbivores, demonstrated by the exceedingly speciose orders of herbivorous insects. As plants have evolved to utilize new niches, phytophagous insects are thought to have subsequently diversified by exploiting newly available resources from plant hosts. Both pair-wise and diffuse interactions between plants and their animal associates can give rise to evolutionary responses, including reciprocal co-evolutionary dynamics. For example, plant-antagonist interactions can give rise to “evolutionary arms races,” where reciprocal genetic changes occur as plants evolve defenses such as secondary metabolites and herbivores evolve mechanisms to overcome these defenses.
Incorporating students and outreach into research. I instill my students with the exciting awareness that, although anthropogenic activity has tamed much of the charismatic megafauna, the insect world is still just as wild as it has ever been. I have had great success in engaging student researchers and citizen scientists of diverse backgrounds and ages in all aspects of the research described below, including field collections and observations, ecological and molecular laboratory assessments, data analysis and co-authorship of grants. Plant-insect interactions provide a highly accessible and teachable model of ecology and evolution that has been a powerful tool for both my scientific and pedagogical pursuits.
Factors affecting the emergence of novel interactions. Biological invasions are a major threat to biodiversity, and studies of natural enemies in these systems can function as large-scale laboratories for understanding the ecology and evolution of emergent species interactions, informing conservation policies. Colonization of novel environments can lead to intraspecific evolutionary changes that result in geographic patterns of phenotypic variation. This variation often translates to novel interactions with other organisms, including the emergence of interactions through reciprocal evolutionary changes among species occupying distinct trophic levels. The Chinese tallow tree (Triadica sebifera) and a newly discovered species of lepidopteran herbivore of unknown origin in the family Gracillariidae (Caloptilia triadicae) comprise a system that affords an exceptional opportunity to investigate the emergence and evolution of species interactions. Much is known about evolved variation in invasive Triadica populations, but little is known about how these post-invasion phenotypic changes affect associations with and evolutionary processes in Caloptilia, the only known specialist herbivore in the North American range. In particular, the extent to which stochastic and deterministic factors shape post-invasion species interactions remains unclear in this and many other systems involving invasive species. To characterize how Triadica-Caloptila associations vary according to geography and host genetics, I conducted an observational field study, incorporating a range of genetically variable Triadica populations (intrinsic deterministic factors), climatic variability (extrinsic deterministic factors), and isolation by distance (stochastic factors). I found that stochastic isolation by distance and genotypic variability in the tree were the most important drivers of leaf toughness and caterpillar abundance (figure 1). To confirm the role of host genetics on this interaction, I also conducted a common garden experiment to assess the role of geographically structured genetic variation of Triadica in shaping interactions with Caloptilia, while controlling for confounding climatic gradients and long-distance stochastic processes. The common garden study reinforced the conclusion that intrinsic deterministic factors are important drivers shaping this interaction, but also revealed that seasonal variation mediates the relative impact of host genetics and microhabitat variability. Importantly, genotypic variation was also a significant predictor of traits associated with competitive ability in Triadica. Overall, this work has exciting implications for the role of host genetic variation in shaping mosaics of species interactions and informs our understanding of how this variation influences the potential biocontrol efficacy of this newly discovered herbivore.
Phylogeographic mosaicism. Responses of plants to variation in environmental conditions, even at fine spatial scales, can drive adaptive genetic structure in herbivores. To understand how these factors are affecting the emergence and evolution of this novel species interaction, I am currently examining the population genetic structure of Caloptilia and Triadica across North America. Using multiple measures of genetic variation, including mitochondrial and chloroplast sequence data, and nuclear microsatellites, I am conducting congruence analyses to determine if the ecological patterns I observed in my field and common garden studies emerge as patterns of phylogeographic congruence between host and herbivore. Results suggest that, stochastic geographic dispersal is the most important driver the emergent patterns of herbivore genetics. However, North American Triadica exhibit genetic structure between early and late introductions, as well as within late introductions, and this variation significantly affects genetic variation in Caloptilia. Conclusions from the ecological work indicate a primary role for intrinsic deterministic factors in shaping the ecology of this novel interaction, such that some Triadica populations appear to be resistant to Caloptilia. Therefore, I predict that future Caloptilia populations will continue to diverge, reflecting patterns of Triadica genetic structure, warranting continued tracking of this new model system of emergent evolution.
Background. The study of interactions between plants and animals is a major pursuit in ecology and evolutionary biology, as it contributes to the emergence of biological diversity. For example, plant-animal interactions can lead to specialization and subsequent diversification in herbivores, demonstrated by the exceedingly speciose orders of herbivorous insects. As plants have evolved to utilize new niches, phytophagous insects are thought to have subsequently diversified by exploiting newly available resources from plant hosts. Both pair-wise and diffuse interactions between plants and their animal associates can give rise to evolutionary responses, including reciprocal co-evolutionary dynamics. For example, plant-antagonist interactions can give rise to “evolutionary arms races,” where reciprocal genetic changes occur as plants evolve defenses such as secondary metabolites and herbivores evolve mechanisms to overcome these defenses.
Incorporating students and outreach into research. I instill my students with the exciting awareness that, although anthropogenic activity has tamed much of the charismatic megafauna, the insect world is still just as wild as it has ever been. I have had great success in engaging student researchers and citizen scientists of diverse backgrounds and ages in all aspects of the research described below, including field collections and observations, ecological and molecular laboratory assessments, data analysis and co-authorship of grants. Plant-insect interactions provide a highly accessible and teachable model of ecology and evolution that has been a powerful tool for both my scientific and pedagogical pursuits.
Factors affecting the emergence of novel interactions. Biological invasions are a major threat to biodiversity, and studies of natural enemies in these systems can function as large-scale laboratories for understanding the ecology and evolution of emergent species interactions, informing conservation policies. Colonization of novel environments can lead to intraspecific evolutionary changes that result in geographic patterns of phenotypic variation. This variation often translates to novel interactions with other organisms, including the emergence of interactions through reciprocal evolutionary changes among species occupying distinct trophic levels. The Chinese tallow tree (Triadica sebifera) and a newly discovered species of lepidopteran herbivore of unknown origin in the family Gracillariidae (Caloptilia triadicae) comprise a system that affords an exceptional opportunity to investigate the emergence and evolution of species interactions. Much is known about evolved variation in invasive Triadica populations, but little is known about how these post-invasion phenotypic changes affect associations with and evolutionary processes in Caloptilia, the only known specialist herbivore in the North American range. In particular, the extent to which stochastic and deterministic factors shape post-invasion species interactions remains unclear in this and many other systems involving invasive species. To characterize how Triadica-Caloptila associations vary according to geography and host genetics, I conducted an observational field study, incorporating a range of genetically variable Triadica populations (intrinsic deterministic factors), climatic variability (extrinsic deterministic factors), and isolation by distance (stochastic factors). I found that stochastic isolation by distance and genotypic variability in the tree were the most important drivers of leaf toughness and caterpillar abundance (figure 1). To confirm the role of host genetics on this interaction, I also conducted a common garden experiment to assess the role of geographically structured genetic variation of Triadica in shaping interactions with Caloptilia, while controlling for confounding climatic gradients and long-distance stochastic processes. The common garden study reinforced the conclusion that intrinsic deterministic factors are important drivers shaping this interaction, but also revealed that seasonal variation mediates the relative impact of host genetics and microhabitat variability. Importantly, genotypic variation was also a significant predictor of traits associated with competitive ability in Triadica. Overall, this work has exciting implications for the role of host genetic variation in shaping mosaics of species interactions and informs our understanding of how this variation influences the potential biocontrol efficacy of this newly discovered herbivore.
Phylogeographic mosaicism. Responses of plants to variation in environmental conditions, even at fine spatial scales, can drive adaptive genetic structure in herbivores. To understand how these factors are affecting the emergence and evolution of this novel species interaction, I am currently examining the population genetic structure of Caloptilia and Triadica across North America. Using multiple measures of genetic variation, including mitochondrial and chloroplast sequence data, and nuclear microsatellites, I am conducting congruence analyses to determine if the ecological patterns I observed in my field and common garden studies emerge as patterns of phylogeographic congruence between host and herbivore. Results suggest that, stochastic geographic dispersal is the most important driver the emergent patterns of herbivore genetics. However, North American Triadica exhibit genetic structure between early and late introductions, as well as within late introductions, and this variation significantly affects genetic variation in Caloptilia. Conclusions from the ecological work indicate a primary role for intrinsic deterministic factors in shaping the ecology of this novel interaction, such that some Triadica populations appear to be resistant to Caloptilia. Therefore, I predict that future Caloptilia populations will continue to diverge, reflecting patterns of Triadica genetic structure, warranting continued tracking of this new model system of emergent evolution.
Anthropogenic Drivers of Urban Biodiversity
As natural environments are rapidly converted to urban landscapes, study of the interface between ecology and humans is increasingly vital to understanding the mechanisms that will influence future conditions for our species and the biodiversity of the planet. Since 2011, I have worked on a collaborative project through the Center for Biological Research at Tulane and Xavier Universities, the Stockholm Resilience Center and the US Forest Service to assess vegetation structure and functional traits relative to historical geography, socioeconomic factors, politics, and infrastructure in post-Katrina New Orleans. Follow the links below to read our recent publication and to learn more about our startling findings- that the post-Katrina urban ecology of New Orleans is shaped more by socioeconomic disparities than actual storm damage.
As natural environments are rapidly converted to urban landscapes, study of the interface between ecology and humans is increasingly vital to understanding the mechanisms that will influence future conditions for our species and the biodiversity of the planet. Since 2011, I have worked on a collaborative project through the Center for Biological Research at Tulane and Xavier Universities, the Stockholm Resilience Center and the US Forest Service to assess vegetation structure and functional traits relative to historical geography, socioeconomic factors, politics, and infrastructure in post-Katrina New Orleans. Follow the links below to read our recent publication and to learn more about our startling findings- that the post-Katrina urban ecology of New Orleans is shaped more by socioeconomic disparities than actual storm damage.
Diversity of Interactions and Climate Change
To understand complex ecosystem dynamics and how they are responding to global change, it is imperative to understand, not only the biodiversity in a system, but, more importantly, the diversity of the interactions in a community. Since 2007, I have worked extensively to examine trophic diversity in research sites ranging from the swamps of Louisiana, to the mountain passes of Arizona and Northern California, to the deserts of Nevada. Using plants, caterpillars and parasitoid enemies as a model system, my collaborators and I use large, long-term rearing databases to determine the effects of environmental gradients on the diversity of interactions and to make predictive models for these systems. Please follow the links below to read some of our published findings and to learn more about the "Caterpillars and Climate Change" project.
To understand complex ecosystem dynamics and how they are responding to global change, it is imperative to understand, not only the biodiversity in a system, but, more importantly, the diversity of the interactions in a community. Since 2007, I have worked extensively to examine trophic diversity in research sites ranging from the swamps of Louisiana, to the mountain passes of Arizona and Northern California, to the deserts of Nevada. Using plants, caterpillars and parasitoid enemies as a model system, my collaborators and I use large, long-term rearing databases to determine the effects of environmental gradients on the diversity of interactions and to make predictive models for these systems. Please follow the links below to read some of our published findings and to learn more about the "Caterpillars and Climate Change" project.