The overwhelming majority of plant species introduced into a new range never become invasive. Consequently, identification of factors allowing the small fraction of successful invaders to naturalize, increase in abundance, and displace resident species continues to be a key area of research in invasion biology. Of the considerable number of hypotheses that have been proposed to resolve why some plant species become noxious pests, the enemy release hypothesis (ERH) is one of the most commonly cited. The ERH maintains that invasive plants succeed in a new range because they are no longer regulated by their coevolved natural enemies, and this reduction in enemy pressure imparts a competitive advantage over native species, which continue to be negatively impacted by top-down processes. Alternatively, the ability of invasive plant species to outperform their counterparts, rather than escape from enemies, may be key in conferring invasion success. The importance of preadapted traits and release from natural enemies in successful invasion remains unclear, likely owing to a lack of empirical studies comparing their effects on relative performance and population growth of closely related species that differ in origin and invasiveness. A system of co-occurring native, introduced non-invasive, and invasive Eugenia congeners exists in south Florida, providing an opportunity to address deficiencies in our understanding of plant invasions by investigating the factors leading to invasion success for Eugenia uniflora. This approach is novel because very few studies have simultaneously incorporated both native and introduced non-invasive congeners into tests of these hypotheses, and no others have done so using this system of Eugenia congeners. The first study in this dissertation tested the ERH using an insect herbivore exclusion experiment in the field to compare the effects of natural enemies on the performance and population growth of Eugenia uniflora and its native congeners. The results showed that E. uniflora sustained more herbivore damage than its native counterparts, and that the effects of herbivores were sufficient to have negative impacts on performance and population growth. In sum, these findings contradict the ERH. Surprisingly, the vast majority of damage to E. uniflora was caused by the recently introduced Sri Lankan weevil (Myllocerus undatus), with which it shares no coevolutionary history. The second study compared seedling performance among native, introduced non-invasive, and invasive Eugenia congeners to determine if the success of E. uniflora can be attributed to superior performance traits. Invasive E. uniflora was found to outperform its native and introduced non-invasive counterparts in a number of seedling traits, including emergence, growth, and survival, in spite of sustaining higher levels of herbivore damage in the field. This result was consistent across years and sites, suggesting that superior performance may be an important factor in invasion success by E. uniflora. The final experiment investigated the role of enemy release on performance of native, introduced non-invasive, and introduced invasive Eugenia seedlings using an insect herbivore exclusion experiment in the field. In this study, the invasive E. uniflora was again found to sustain more damage by foliar herbivores compared to its native and introduced non-invasive counterparts. However, in spite of higher levels of herbivore damage, E. uniflora continued to outperform its congeners in terms of stem growth, and its congeners did not outperform E. uniflora in any attribute. Insect herbivores negatively affected survival of all species, but were found to have little effect on growth. In combination, the results of these studies indicate that the ability of E. uniflora to outperform its native and introduced congeners at the seedling stage, and not release from insect herbivores, may contribute to its success as an invader. Additionally, E. uniflora exhibits relatively low resistance to herbivory in the new range, and instead may possess an ability to tolerate moderate levels of damage. The implications of this study are that enemy release may not be important in determining invasion success in some systems, and that the accumulation of new enemies may mitigate the effects of invasive plants over time. The paucity of studies investigating interactions among invasive plants and herbivores that share no coevolutionary history warrants further research. Finally, this system of Eugenia congeners provides valuable opportunities to test additional hypotheses and to further explore factors leading to invasion success.

Knowledge of the impacts of white-tailed deer (Odocoileus virginianus; hereafter deer) as dominant herbivores throughout the Southeastern United States of America is lacking. To address this, three paired experimental units of exclosures and controls were constructed in 2000 on three Wildlife Management Areas across Mississippi within the ecoregions of the Upper Coastal Plain, Lower Coastal Plain, and the Mississippi Alluvial Valley. Vegetation was sampled in the summers of 2000, 2005, and 2021 including vegetation structure, canopy coverage, basal area, and species composition. Additionally, in 2005 and 2021, biomass was sampled to determine potential impacts on nutritional carrying capacity. Among the three study sites, vegetation metrics followed successional trends and were not influenced by herbivory or lack thereof. Additionally, regional differences in nutritional carrying capacity seemed to be of greater importance than herbivory. This research illustrates the difference in effects of deer by region and forest type.

Field and laboratory studies were used to elucidate the factors affecting temporal and spatial variations of species abundance and structure of macroalgal assemblage and environmental variables between fish farming (FFA) and non-fish farming (NFFA) areas in Hsiao-Lu-Chiao island, a coral island in southwestern Taiwan. Four experiments have been approached: 1. field surveys of macroalgal assemblage structure on 5-m and 10-m depth at 3 sampling sites at FFA (FFA1, FFA2 and FFA3) and 1 sampling site at NFFA from September 2004, January 2005 and April 2005; 2.the relationship between abiotic (monthly maximum air temperature, monthly minimum air temperature, monthly mean air temperature, monthly cumulative precipitation, monthly cumulative irradiance, seawater temperature, light extinction coefficient, water motion, and nutrient (NO3-, NO2-, NH4+, SRP, DON, and DOP) and biotic (seaurchin density) factors and spatio-temporal variations in macroalgal structures analyzed by non-parametric multivariate model; 3. Factors affecting macroalgal abundance and structure: (1). Comparison of growth temperature ranges in different species to field temperature fluctuation; (2).Comparison of growth irradiance ranges in different species to field irradiance fluctuation; 4.Herbivore pressure: (1). Spatio-temporal variations of sea urchin abundance and structure of assemblage; (2). Gut contents and food preference of sea urchin experiment; (3). Herbivore exclusion experiment. Macroalgal %cover, biomass, species richness, diversity (H¡¦) and evenness (J¡¦) showed temporal and spatial variations, low values in January 2005 and also low values in the 5 m- and 10 m-depth areas of FFA1 and the 10 m-depth areas of FFA2. The data of k-dominance curve, hierarchical cluster and ANOSIM tests indicate that macroalgal assemblage is different between 4 sampling sites, between 2 depths and between 3 seasons. Ceratodictyon spongiosum is the most important species that separates September and January assemblages from April assemblage and separates the FFA1 and FFA2 assemblages from the FFA3 and NFFA assemblages. BVSTEP analysis shows that nutrients (NO3-, DON, DOP), temperature, monthly cumulative precipitation, and sea urchin density are the factors corresponding to variations of macroalgal assemblages, this correlation is more significant for 5 m-depth assemblage. Fish farming area FFA1and FFA2 assemblage are affect by sea urchin density, temperature and DON. Sea urchin influnces macroalgal abundance and assemblage structure in FFA1 and FFA2. Macroalgal %cover in 5 m-depth area shows a reversal relationship with sea urchin density; however, this relationship is not observed for 10 m-depth area. FFA1 and FFA2 are belong to high grazing pressure sites as indicated by high sea urchin density and exclusion experiment. Sea urchin gut contents and feeding preference test show that sea urchin has strong food selectivity with Hypnea charoides and Gracilaria coronopifolia as the most preferred species. Herbivore exclusion experiment shows that Hypnea charoides and Gracilaria coronopifolia are the species recruited in the cages. Ceratodictyon spongiosum had high biomass in FFA1 and FFA2 in January, which was ¡¥low DOP/high DON¡¦. The coindicence of temporal variations in FFA3 assemblage structure with a change from ¡¦Halimeda opuntia and Boodlea compostia¡¦ ¡÷ ¡¦Amphiroa fragilissima, Corallina phhulifera and Galaxaura oblongata¡¦ ¡÷¡¦Halimeda opuntia and Boodlea compostia¡¦ with low nitrogen/ high phosphorous¡¦ ¡÷¡¦ high nitrogen/ low phosphorous¡¦ ¡÷ ¡¦low nitrogen/ high phosphorous¡¦ suggest a role of ¡¦low nitrogen (NO3-)/high phosphorous (DOP)¡¦ for FFA3 structure modification. NFFA assemblage is controlled by temperature and monthly cumulative precipitation. Monthly cumulative precipitation in September was higher than January and April, in which Boodlea compostia and Gracilaria coronopifolia were dominant algae in September. The temperature growth responses of algae using the continuous-flow outdoor laboratory tank culture system fit their seasonal growth, reflecting the temperature-dependent manner of seasonal variations in abundance. It could be concluded from the present investigation that the structure of benthic macroalgal assemblage in Hsiao-Lu-Chiao island in southwestern Taiwan is affected by predicted natural and pulse disturbances. Temperature fluctuations involve in overall temporal variations in structure. Sea urchin herbivory and nutrient as pulse nutrient modulate the structure in fish farming area while monthly cumulative precipitation is associated with algal structure in non-fish farming area.