Doctor of Philosophy (Ph.D.)
Degree Granting Department
Biology (Integrative Biology)
Peter Stiling, Ph.D.
Marc Lajeunesse, Ph.D.
Earl McCoy, Ph.D.
Henry Mushinsky, Ph.D.
biodiversoty, ecology, herbivory, saurochory
Herbivory produces direct and indirect effects on plants and at different spatial scales will have varied consequences. Consumption of plants by vertebrate grazers may affect the plants on an organismal level through direct mortality, on a community level by changing species composition or by altering the rate of succession, and even at a whole ecosystem level by altering nutrient cycles.
The majority of the scientific literature has focused extensively on herbivory by mammals and birds. With regard to mammals, studies have shown how folivory affects individual plants, plant populations, and communities of plants.
Mammals, as well as birds, also ingest and disperse seeds. This dispersal of seeds by animal consumption, known as endozoochory, allows the non-motile parent plant to disperse seeds to otherwise unreachable locations. Endozoochory may also have an effect on the germination of seeds.
Attempts to quantify the effects of non-avian reptiles on plants and plant communities through folivory and frugivory is lacking, despite the prevalence of some form of herbivory in many non-avian reptile groups.
I examined the effect of the gopher tortoise (Gopherus polyphemus) herbivory on plant community composition on Egmont Key, Florida by direct experimentation with folivory and frugivory (chapter 1). This species was chosen because of its relatively large size, high density on the proposed field site, and because it is easy to locate due to the presence of large conspicuous burrows in the ground. These burrows provide a central location by which the tortoise begins and ends a foraging activity. Tortoise exclosure plots were used to directly measure the effects of grazing on plant counts and biomass, species richness, biodiversity, and species dominance and evenness. At the conclusion of these exclosure experiments, cafeteria-feeding trials were conducted to relate dietary preference to the plant abundance results found in the field. Seeds from the plant species found on Egmont Key were also fed to tortoises to determine the effects that passage through the tortoise gut had on seed germinability (chapter 2). Seeds that passed through the gut were collected from scat, and planted alongside control seeds that were not consumed by tortoises. Each treatment also was planted with and without tortoise scat to determine if altered germinability post ingestion was from physiological effects from gut passage, or simply being in the presence of scat. Finally, a meta-analysis was conducted looking at the effects of gut passage on seed germinability across different animal taxa and several other moderators (chapter 3).
Tortoise exclosure plots had lowered species richness, and significantly lowered diversity and evenness, but significantly higher dominance than in controls. Heliotropium polyphyllum, the most highly preferred local species by tortoises, was the most dominant plant in exclosure and control plots and became even more dominant in exclosure plots. The abundance and biomass of the next two most common plant species, Fimbristylis cymosa and Polypremum procumbens, which are not preferred by tortoises, were reduced in the exclosures, probably due to increased competition with Heliotropium. Several rare plant species were eliminated in the exclosure plots. I conclude that tortoise herbivory may directly influence plant community assembly by reducing preferred plant species and promoting the growth of non-preferred species.
Ingested seeds from two fleshy-fruited plant species germinated in greater proportions and in less time than control seeds. The presence of scat also increased germination proportion following gut passage and decreased time to germination in both these species. Germination proportions in five dry-fruited native species were either not affected, or lowered, and time to germination was either not affected, or slower following gut passage. Similarly, the effects of scat had no effect on germination percentage in these species, or decreased it, and had no effect on time to germination, or increased it. One nonnative dry-fruited species was also tested and while germination percentage following gut passage was unaffected, time to germination decreased. Similarly, while the addition of scat increased germination percentage, there was no effect on time to germination. In summary, only seeds from fleshy-fruited plant species were consistently positively affected by passage through gopher tortoise guts and by gopher tortoise scat.
Utilizing meta-analyses, I found non-flying mammals, but no other herbivores, enhance the percentage of seeds germinating compared to uningested seeds, and non-avian reptiles were found to decrease the time to germination. Seeds from fleshy fruits, despite being evolved for vertebrate herbivore consumptions, did not germinate faster or in greater proportions than did seeds from dry-fruited plant species. Seeds dispersed by animals native to the same areas as the plants displayed enhanced germinability compared to uningested seeds, as did those dispersed by herbivores or omnivores, and seeds passed by herbivores and omnivores cause increased germination proportions in faster times than those passed by carnivores.
Scholar Commons Citation
Richardson, Jason C., "The Effects of Gopher Tortoise (Gopherus polyphemus) Herbivory on Plant Community Composition and Seed Germination, and The Effects of Gut Passage on the Germinability of Seeds: A Meta-Analysis" (2018). Graduate Theses and Dissertations.