My research currently centers around three main projects. My current post-doctoral research is focused on sub-individual plant trait variability, shifts in this ‘trait’ with domestication selection, and consequences to herbivores (1). I am also wrapping up publications from my dissertation, which explored effects of natural variation in soil type on diverse multi-trophic communities (2). I am also working on a collaborative project investigating ecological correlates of color and pattern evolution in larval Lepidoptera (3).
(1) How do variable plant-scapes influence insect herbivores?
For an herbivorous insect, moving within a plant is like walking (or hopping!) across a landscape made of food; however, not all parts of the terrain underfoot are equally tasty. For example, individual leaves within a single plant can vary greatly in their nitrogen and water levels, as well as in the amount of defense (such as toxic chemicals in the leaf, or the density of tiny leaf hairs). These small differences among leaves mean that herbivores may have to adjust their physiology or behavior as they munch from leaf to leaf. Such adjustments can scale up to increase (or decrease) the overall growth rate of an individual herbivore, or the population growth rate of an herbivore population.
For my post-doctoral research, I am studying how such sub-individual variability might influence herbivore performance on plants. Crop domestication provides a particularly interesting scenario in which to address these questions, for at least two reasons: 1) humans exert direct selection on homogeneity of plant parts (e.g. uniform fruit size); and 2) domestication increases or decreases trait means (e.g. greater protein content; lower defensive chemistry), and means are often positively correlated with the amount of variability (see below). Because many herbivores complete their development within a single plant, it is this scale of variability that may matter most within the lives of many insect pests. To address these questions, I am using a common garden of wild and domestic alfalfa plants at Kellogg Biological Station (KBS). By quantifying physical, nutritive, and chemical traits of individual leaves within plants, I hope to better understand how domestication has shifted levels of sub-individual variability, and link such shifts to the amount of herbivory experienced by plants.
(2) How does abiotic context shape multi-trophic communities?
Insect herbivore communities are shaped by two non-exclusive mechanisms: from the 'bottom-up', host plant traits can act directly on herbivores by changing preference of ovipositing females and/or larval mortality; simultaneously, natural enemies exert predation pressure from the 'top-down'. In addition to their independent effects, these two forces can work together to alter insect assemblages. When host plants are of low quality, herbivores may develop more slowly and be more available to their enemies in time. Alternatively, low quality plants often harbor lower abundances of herbivores, and may be passed over by predators and parasitoids in favor of higher quality plants with more prey.
In collaboration with my PhD advisor, Dr. Sharon Strauss (UC Davis), I used a natural mosaic of two soil types — serpentine (low resource) & non-serpentine (higher resource) to address these questions. I quantified traits and resistance of host plants in each soil type, and collected and reared over 3,000 lepidopteran caterpillars (ca. 80 species) to understand whether soil effects could cascade to higher trophic levels. I found that host plants were more resistant to herbivores in serpentine soils, and were associated with smaller and less diverse herbivore assemblages than higher resource non-serpentine soil. Structure of the plant-herbivore food web also differed with soil type: generalist herbivore species fed on fewer plants in serpentine soils, creating a more modular ecological network than in neighboring non-serpentine soils. Soil type also influenced interactions between caterpillars and (some) natural enemies: parasitism — but not bird predation — was higher in serpentine than non-serpentine soil. Together, these results show cascading effects of soil type on community structure and species interactions, across multiple trophic levels.
(3) Do host plants drive macroevolution of herbivore coloration?
Warning coloration (aposematism) and camouflage (crypsis) are two strategies employed by animals to avoid predation. These divergent visual tactics are often related to species’ palatability: distasteful or noxious organisms use bright colors and disruptive patterns as a visual warning to would-be predators, while edible species conceal themselves by blending in with their background. For plant-feeding insects, host plant characteristics may critically influence palatability, as herbivores incorporate plant toxins into their own bodies. In addition, traits of the herbivores themselves – such as their degree of dietary specialization – may shape the efficacy with which they co-opt plant chemistry for their own defense.
In collaboration with Dr. Sharon Strauss (UC Davis) and Dr. Marjorie Weber (Michigan State U.), we are exploring how herbivore diet breadth and host plant traits shape the evolution of coloration, using the remarkably rich and morphologically diverse larval Lepidoptera. To do this, we are using a large dataset of over 1800 North American moth and butterfly species for which larval photographs are available. By using phylogenetically-informed methods, we can ask whether the macroevolution of color and pattern is correlated with the growth form of caterpillar host plants, as well as dietary specialization of caterpillar species.