Plant Invasion Success: Investigating the Roles of Herbivory and Plant-Soil Feedbacks

Abstract

Understanding why some non-native plants become successful invaders while most fail to invade can help in assessing risks of invasion. Plants that have evolved defensive traits to resist and/or tolerate herbivory may be more successful invaders since herbivory can potentially prevent species from establishing or regulate existing populations. Plant-soil feedbacks may also enhance dominance of invasive species through allelopathy, changes in soil chemical properties, and interactions with soil biota that promote their own growth and suppress recruitment and growth of co-occurring species. I investigated herbivory as barrier to post-establishment invasion success by conducting three sets of experiments that compared invasive and non-invasive woody species in Hawaii. I measured: seedling acceptability to two generalist herbivores in laboratory no-choice feeding trials, foliar herbivory on seedlings in a field common garden, and seedling tolerance to simulated foliar herbivory in a greenhouse. I also investigated the role plant-soil feedbacks play in the dominance of an invasive tree in Hawaii, Ardisia elliptica, by conducting greenhouse soil feedback experiments to determine if A. elliptica positively impacts its own growth and suppresses growth of two co-occurring species. I found that: 1. There was no general difference in resistance to herbivores between invasive and non-invasive species in feeding trials or the common garden experiment. 2. There was no general difference in tolerance to simulated herbivory between invasive and non-invasive species. 3. There was no evidence of positive soil feedbacks promoting A. elliptica growth. 4. Soil taken from beneath A. elliptica did not suppress growth of two co-occurring species. Thus, herbivory and plant-soil feedbacks do not appear to be playing a strong role in post-establishment invasion success of the species I investigated, suggesting that we still lack a comprehensive understanding of what factors drive invasion success or failure.