Document Type

Thesis

Degree

Master of Science

Major

Biology

Date of Defense

11-7-2011

Graduate Advisor

Robert J. Marquis, PhD

Committee

Ricklefs, Robert

Kellogg, Elizabeth

Abstract

There have been few studies that examine the influence of soil water content on the outcome of interactions between ants, insect herbivores, and extrafloral nectary (EFN)-producing plants despite the potential of soil water to alter such interactions. Here I asked two questions: 1) Does water availability mediate the phenotypic expression of EFN, chemical and physical defenses? 2) Does water availability influence the outcome of tritrophic interactions between ants, herbivores, and EFN producing plants? Plants of Chamaecrista fasciculata (Fabaceae) growing naturally in a remnant prairie were randomly assigned to water addition or control and ant removal or control in a two factorial crossed design. Rainfall was below the 95% confidence interval for a 16-year average throughout the season resulting in low water availability for control plants. Plants subjected to ambient water conditions produced a higher concentration of hydrolyzable tannins in leaves than plants in the water addition treatment. There were no other differences in EFN, chemical, or physical defenses among treatments, either water addition or ant exclusion, or their interaction. Plants subject to ambient water had fewer herbivores, greater leaf area per plant, and produced more fruit and seeds than plants in the water addition group. Herbivores in the control for water addition ingested more leaf area per plant and per individual insect than those plants in the water addition treatment yet this did not reduce fruit or seed production. Ant presence alone increased plant survivorship early in the season but had no significant impact on leaf damage, leaf production or fruit and seed production of the survivors. Among all experimental plants with ants present, synergy between chemical defense and ant defense may have lowered leaf area lost to herbivores and increased seed production suggesting possible synergy between chemical and ant defenses. Overall these results demonstrate the potential for a direct influence of water availability on chemical defense and an indirect effect of water availability on tritrophic interactions between ants, herbivores, and plants.

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