Gopher tortoise herbivory increases plant species richness and diversity

Mammalian herbivores often alter plant species richness and diversity, but such impacts have not been much investigated in reptiles. This study examined the effects of gopher tortoise (Gopherus polyphemus) herbivory on species richness, Gini-Simpson diversity, and dominance, plant abundance, and biomass. Tortoise herbivory was eliminated in five areas through the use of exclosure plots for a period of two years and was compared to five similar areas where tortoises were allowed to feed. Cafeteria feeding trials were also used to quantify dietary preference. Tortoise exclosure plots had lowered species richness, and significantly lowered diversity, 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. We conclude that tortoise herbivory may directly influence plant community assembly by reducing preferred plant species and promoting the growth of non-preferred species.