Impact of Alien Slugs on Native Plant Seedlings in a Diverse Mesic Forest, Oʻahu, Hawaiʻi, and a Study of Slug Plant Food Preferences

Abstract

Introduced species have the potential to cause serious ecological disruption, particularly on oceanic islands. When introduced species invade natural areas, endemic species may be threatened, especially when the invasive species represent guilds or functional groups that were previously lacking. Hawai‘i has no native slugs, but over a dozen species are now established. Slugs are important seedling predators in their native habitats, and in introduced habitats they can cause major shifts in the abundance some plant species. In order to better investigate slug impacts on native plants in Hawai‘i, I carried out research which 1. identified differences in the acceptability of five native plant species to five alien slug species 2. assessed the effect of slug herbivory on the growth and survival of three native and two alien plant species, and 3. measured changes in seedling regeneration due to slug herbivory. Results from feeding assays indicated a significant difference in palatability among plant species, but no statistical difference in overall feeding preference among slug species. Urera kaalae (Urticaceae) was found to be significantly more palatable than the other four plant species and, thus, is predicted to be more vulnerable to slug herbivory in the field. I tracked the fate of planted seedlings and natural germinants from the seed bank in both slug-excluded and slug-accessible plots in diverse mesic forest in the Wai‘anae Mountains on the island of O‘ahu. Among seedlings that survived to the end of the experiment, there was no significant difference between slug herbivory treatments in growth index measurements. There was little germination from the seed bank, with no statistical difference in total number of seedlings between treatments. However, two of the three native species, Schidea obovata (Caryophyllaceae) and Cyanea superba (Campanulaceae) had significant reductions in survival of 49% and 53%, respectively, in the slug-exposed treatment. Survival of two invasive species, Clidemia hirta (Meslastomataceae) and Psidium cattleianum (Myrtaceae) was not significantly affected by slugs. This study demonstrates that slugs may pose a serious threat to native plant species by reducing their survival and thereby facilitate the success of certain invasive species.