Differential seed mortality among habitats limits the distribution of the invasive non-native shrub Ardisia elliptica

During the early phases of biological invasions, both limited dispersal and habitat preference may contribute to observed patterns of distribution for non-native species. In a disturbed area of Everglades National Park, the non-native Ardisia elliptica (Myrsinaceae) does not occur in all habitat types nor is it evenly distributed among habitats where it does occur. One goal of this study was to determine whether the patchy distribution of A. elliptica may be due to differential effects of habitat on seed fates and germination. Screen mesh bags containing seeds of A. elliptica were buried in the surface litter of five habitats and were later sampled periodically over a three month period to measure seed germination, survival and death. Seed mortality and mean germination speed was higher in open exposed habitats relative to closed canopied forests. During the study period, which coincided with the dry season, there was no germination of seeds that had been placed in pine forests or in Schinus thickets that were highly disturbed pine forests. Because hydroperiod is a critical determinant of habitat type in the Everglades ecosystem, a second goal of this study was to determine whether water availability was correlated with seed germination and seedling survival in five sites where A. elliptica has invaded. Seeds were sown under mesh boxes and monitored for germination and seedling survival for ten months. Estimates of water availability taken as predawn water potentials on A. elliptica were positively correlated with percent seed germination but not with percent seedling survival. Additionally, pre-dawn water potential of A. elliptica differed among sites. This study suggests that differential water availability among habitats may have influenced the patchy distribution of A. elliptica in Everglades National Park. Lack of sufficient moisture availability for seeds in exposed habitats such as pine forests and wetland prairies during the dry season has contributed to its distribution. Because some of the habitats where A. elliptica currently occurs were pine forests and wetland prairies prior to disturbance, this study suggests that substrate disturbance and canopy formation have altered the soil microenvironment, favoring the establishment of A. elliptica.