Drivers of Bird-Dispersed Exotic Plant Species in the Southeastern United States

Author

William Sipek, Southern Illinois University CarbondaleFollow

Date of Award

12-1-2021

Degree Name

Master of Science

Department

Plant Biology

First Advisor

Battaglia, Loretta

Abstract

Exotic species present a substantial threat to the systems that they occupy. They outcompete native species and can grow to dominate new ecosystems where their natural predators and competitors are absent. In highly biodiverse areas like the northern Gulf Coast, their impact can be catastrophic. Many plants and animals here exist in highly connected networks. Many bird-dispersed plants fleshy fruits to attract frugivores, which eat the fruit and then spread the seed elsewhere. If exotic plants also produce fruits for consumption, then they may begin to make up a substantial portion of frugivore diets, enabling their own spread while reducing that of native plants. The factors affecting what plants are dispersed where are numerous, including the amount of fruit available on an individual plant and the number of species of fruiting plants surrounding it. After these plants have spread and become established, many factors affect their growth. One major obstacle they may face is the presence of disturbances, such as the frequent variations in precipitation and annual tropical storm season. As well as simply impacting water availability, these disturbances can flood typically freshwater-immersed plants with saltwater during storm surges an batter them with high winds. These disturbances act to maintain high biodiversity in the Southeastern United States by creating new openings in existing habitats for various plant species to take hold. Their impact on exotic species, however, is mixed as they may kill off existing exotic plants but also open up new sites for their establishment. In order to explore the dispersal and establishment of exotic species, a two part study was conducted; one exploring contagious seed dispersal and one examining the impact of disturbances on an invasive tree. Seed traps were set up beneath native and exotic plants to determine what diaspores were being deposited beneath them. The fruiting neighborhood surrounding each focal plant was measured, as was the canopy cover above the seed trap and the fruit remaining on each focal plant during each diaspore collection. The findings from this study indicate that exotic species have more diaspores beneath their canopies but that having more neighbors surrounding a plant will decrease the number of diaspores beneath its canopy. Specifically, exotic focal plants have more exotic and native diaspores beneath their canopy but the more exotic neighbors it has, the fewer exotic diaspores are found. The number of native neighbors, however, has a positive relationship to the number of native diaspores beneath a focal plant’s canopy. These results imply that, while exotic species act as hubs for exotic diaspores, the impact is diluted by nearby conspecifics. The opposite neighborhood effect is true of native neighbors and native diaspores The second study involved taking nearly 150 cross-sections of the exotic tree Triadica sebifera from different patches extending from the shoreline inland. These samples were sanded down and treated with phloroglucinol in order to count and measure the trees’ growth rings. These measurements were then tested against years of significant tropical storm seasons and years of drought to determine the impacts that these disturbances have on the growth of this tree. It was found that disturbances have no effect on the high-growth rate of this invasive tree. It was found, though, that most trees germinated immediately after significant tropical storm seasons, indicating that these disturbances are acting to open up areas for its establishment.