Date of Award

2018

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biology

First Advisor

Alison K. Brody

Second Advisor

Josef Gorres

Abstract

Ants are ubiquitous in most communities and many form opportunistic mutualisms with honeydew-producing hemipterans (e.g. treehoppers). Hemipterans excrete honeydew, a carbohydrate rich substance, that ants harvest and, in return, ants protect their honeydew-producing partners from parasitoids, predators, and competitors. Given the efficacy of tending ants in removing hemipteran antagonists, and the strong roles that ants play within their communities as predators, competitors, and seed dispersers, surprisingly little is known of the effects of ant-hemipteran mutualisms (AHM) on the invertebrate communities in which they are embedded or on the plants that host AHM. Using observational and manipulative field experiments, I examined the long-term effect of AHM on their host plant’s, honey mesquite (Prosopis glandulosa), reproductive potential and quality. In addition, I measured how the presence of AHM affects the abundance, richness, diversity, and composition of the invertebrate communities living on honey mesquite.

Plants hosting AHM may indirectly benefit (through the removal of herbivore arthropods) or suffer (through the loss pollinators) due to the defensive behavior of tending ants. To determine the effects of AHM on their host plant, I established a four-year press experiment in which I removed AHM from 50 randomly trees, while leaving 50 as controls. In addition, I marked and followed 30 trees from which AHM were naturally absent. To assess if mesquite quality differed between trees hosting AHM and trees in which AHM were naturally absent, in 2012 I assayed foliar condensed tannin concentrations, a secondary defense compound, and, in 2015, I measured foliar nitrogen, phosphorous, potassium, magnesium, and iron as they are essential for growth and reproduction. I compared the reproductive potential between AHM present and removed trees by counting flowers and fruits across all 4 years of the study. Mesquite that hosted AHM contained significantly less condensed tannins and significantly higher concentrations of N%, Mg, and Fe. Furthermore, over the duration of the study mesquite hosting AHM contained significantly more flowers than those from which AHM were removed or naturally absent. My results indicate that AHM select trees of high quality and their continued presence is associated with high levels of reproductive potential.

Most studies that have evaluated community-level effects of AHM compare total abundance and species richness in communities (or host plants) with and without AHMs. However, both measures are dependent on sampling effort, complicating comparisons across different studies. To examine the effects of AMH on the arthropod community in mesquite, I first compared family richness and alpha diversity using standardized rarefaction and extrapolation curves. I then measured beta diversity and turnover in community composition from one year to the next. The removal of AHM increased invertebrate diversity and significantly altered community composition. Although treatments did not statistically differ in turnover rates, replacements occurred among treatments at the family level which may be biologically meaningful. Furthermore, herbivore and predator populations increased, and pollinator populations decreased following the removal of AHM. These results suggest that the presence of AHM can alter the composition of arthropod communities and food-web dynamics. However, these effects were significant in some years and not others, suggesting the importance of temporal variation in drivers of communities. Overall, my work demonstrates that AHM can be drivers of community composition and illustrate the importance of examining their effects across multiple seasons.

Language

en

Number of Pages

128 p.

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