The Effects of Livestock Grazing on the Reproductive Ecology of Sabatia grandiflora (Gentianaceae) in South-Central Florida, USA.

Abstract

Strategic timing is crucial for the successful reproduction of flowering plants. In order to maximize their production of viable seeds, plants must flower when mutualists (e.g. pollinators) are abundant, while avoiding antagonists. Herbivory has been found to reduce a plant’s reproductive success by depleting its resources for growth. This can cause a delay in the plant’s flowering, compromising its ability to flower while pollinators are abundant. It can also reduce the plant’s reproductive output, resulting in a smaller seed set.

While most studies thus far have investigated herbivory either by naturally-occurring vertebrate and invertebrate herbivores, or by experimentally simulating grazing, few studies to date have considered the disparate impact that herbivory by livestock may have on flowering phenology and plant reproduction. This study aimed to understand if grazing by cattle challenges the survival of an annual, facultative wetland plant species, Sabatia grandiflora, by having an antagonistic effect on its reproduction.

To address this question, I investigated the effect of herbivory on flowering phenology, total reproductive output and growth characteristics such as plant height and depth of standing water. As expected, grazed plants produced fewer flowers per plant and had a smaller reproductive output than ungrazed plants. However, grazed samples had greater densities of S. grandiflora plants which flowered more synchronously and, as a result, produced a greater overall floral display. These results suggest that, while grazing may detract from an individual’s capacity to produce reproductive structures, grazing by livestock ultimately makes changes to the environment which benefit the population.

Livestock grazing has been found to select against tall, palatable plant species which are often strong competitors. In this way, grazing can disrupt competitive relationships between plant species, increasing species diversity and evenness and improving the habitat for weaker competitors like S. grandiflora. Livestock grazing is often considered to be harmful to natural habitats and biodiversity. However, this study supports the idea that, at sustainable levels, grazing can be an important disturbance factor which serves to moderate the intensity of competition between plant species and facilitates the success of weaker competitors. This is an important finding for understanding the ecology of grassland-embedded wetland habitats, which are important reservoirs of plant biodiversity and refugia for rare species.