Population dynamics and invasion rate of an invasive, tropical understory shrub, Ardisia elliptica

Populations of invasive, non-native plant species readily establish new populations, maintain high growth rates, occur at high densities, and exclude populations of native species. Because population explosion is the first step in biological invasions, considerable insight into the processes that contribute to the success of invasive species can be gained from population level studies. The goal of this research was to understand the factors and processes that are contributing to the demographic success of an invasive tropical shrub (Ardisia elliptica) within local patches and across the landscape in Everglades National Park. I used matrix models to evaluate which life-history stages may be contributing the most to population growth and spread in southern Florida and to evaluate how different types of dispersal agents may contribute to its spread. A. elliptica is an understory shrub that can form dense thickets and exclude native plants. Results from this study suggest that water availability places a limit on seed survival and germination strongly influencing the distribution of A. elliptica and the potential impact it will have on native communities. A habitat dominated by A. elliptica supports an impoverished bird community and favors dominance by Gray Catbirds. Consistent with the enemy release hypothesis, A. elliptica suffered significantly lower foliar herbivory rates than its native congener, A. escallonioides, however, results from a simulated herbivory experiment indicated that even moderate levels of leaf area loss over a year did not have any significant impacts on its growth or potential reproductive output. Population dynamics of A. elliptica varied spatially and temporally. For populations that had the highest growth rates, elasticity structure was evenly distributed throughout the life cycle. Despite the abundance of catbirds in habitats dominated by A. elliptica and their fondness for A. elliptica fruits, catbirds may not have contributed much to the rate of spread of A. elliptica across the study region. Instead, infrequent long-distance dispersal by raccoons may have been more important to its invasion rate. Simulations indicated that only an integrative approach to management that targets multiple life history processes will result in a substantial reduction in the population growth rate of A. elliptica.