Barriers to invasion : experimental analysis of mechanisms that prevent plant invasions in Hawaiʻi

Abstract

Invasive species are a serious threat to biodiversity and human well-being world-wide, yet why only a small proportion of introduced species become invasive is still poorly understood. Introduced plant success could be limited by barriers to 1) seed production, 2) dispersal, 3) germination, 4) seedling and juvenile survival and 5) growth. I investigated these barriers in 29 species, classified as invasive, casual escapees, or non-invasive in Hawaiʻi, belonging to three plant families (Acanthaceae, Apocynaceae, Bignoniaceae). I conducted three sets of experiments to examine these barriers: pollen viability staining and hand-pollinating species that do not regularly produce seeds, outplanting seeds at two field sites with and without competition and monitoring survival and growth of the resulting seedlings through one year, and growing seedlings in the lab to measure species traits. I found that 1. Four species had inviable pollen, as measured by pollen viability staining, and failed to produce seeds. Calotropis gigantea (crownflower) produced seeds only when hand-pollinated, indicating that natural seed set is prevented by the lack of a pollinator. 2. Lack of dispersal mutualists limits the success of one species, and possibly two additional species. 3. Germination was not a barrier for any of the species studied. 4. Competition with the naturally-assembled plant community reduced survival in some non-invasives or casuals. Damage from herbivores in the field was correlated with lower survival of some species in the Acanthaceae and Bignoniaceae, but not the Apocynaceae. 5. Seedlings of invasive species grown in pots had significantly higher RGR and photosynthetic rates than non-invasives. In field seedlings, invasives had higher RGR, greater specific leaf area in the absence of competition, and, in the presence of competition, higher photosynthetic rates. 6. Three species in this study, though currently not invasive, had no demonstrable barriers to invasion. These studies are some of the first to explicitly document barriers to introduced plant success, by comparing phylogenetically controlled invasive and non-invasive ornamental plants in the introduced range. My work contributes to our understanding of invasions by documenting previously hypothesized, but poorly tested, barriers associated with missing mutualists, biotic resistance due to competition, and species traits.