Honors Theses

Date of Award

5-2026

Document Type

Undergraduate Thesis

Degree Name

BS

Department

Biology

Faculty Mentor

Jeremiah Henning, Ph.D.

Advisor(s)

Jonathan H. Perez, Ph.D. and Laura Frost, Ph.D.

Abstract

Plant-soil-microbe interactions are integral components of ecological networks and drive ecosystem function from the bottom-up. Whereas herbivores regulate ecosystem function from the top-down, having opposing and potentially complementary effects on ecosystem function. My thesis sought to understand how below-ground microbial communities contribute to herbivore preference and plant biomass across varied soil substrates. My thesis asked: how do substrate and soil microbial communities influence herbivore preference and plant biomass accumulation on swamp milkweed (Asclepias incarnata)? To address this question, seedlings of A. incarnata were sown in 2.5 cm wide × 10 cm deep pots filled with either natural beach sand or with sand produced from crushed glass bottles (cullet), which is a man-made substrate being utilized in small-scale coastal restoration projects. Additionally, we crossed our soil substrate with a native microbial community treatment for a full factorial 2×2 (substrate × microbial addition) experiment that was conducted in a controlled greenhouse environment. Each combination of four treatments was spatially blocked and contained within an insect rearing cage, which was replicated nine times. We measured bi-weekly plant height and leaf count and assessed which plants larval butterflies were feeding on to evaluate the effects of the treatment factors. At the end of our experiment, we also measured the final leaf, stem, and root biomass of each plant. I hypothesized that microbial addition would enhance plant vigor by increasing nutrient absorption, hence enhancing herbivore preference and growth. Conversely, without microbial addition, glass sand was expected to inhibit plant growth and reduce herbivore preference through compromised nutrient availability. To analyze our data, we conducted generalized linear mixed-effects models to accommodate repeated measurements throughout our experiment. We found that microbial inoculation significantly increased belowground biomass, while substrate type had no significant effect on final plant growth metrics. However, intense herbivory eliminated aboveground differences among treatments. This research elucidated (1) a complex interrelationship between bottom-up abiotic soil characteristics and microbial communities and top-down aboveground herbivore–plant interactions, and (2) how changes in restoration substrate may affect plant–herbivore interactions. These results helped guide the development of future methods to restore ecological functioning across trophic levels in nutrient-limited, coastal ecosystems.

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