Submitted to: Journal of Ecology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/18/2010
Publication Date: 7/26/2010
Citation: Huang, W., Siemann, E., Wheeler, G.S., Zou, J., Carrillo, J., Ding, J. 2010. Resource allocation to defense and growth are driven by different responses to generalist and specialist herbivory in an invasive plant. Ecology. 98(5):1157-1167. Interpretive Summary: Understanding mechanisms that contribute to biological invasions is crucial for managing invasive species. Different ideas have been proposed to explain invasion success of exotic plants. Among these, the enemy release hypothesis proposes that exotic plants are successful because of the absence of suppressive natural enemies from their native range. Additionally, when these exotic plants are released from natural enemies they evolve to become more competitive; they grow and reproduce more with reduced insect defenses. We examined two populations of the invasive weed, Chinese tallow and we compared the ability of plants from the native range (China) and the invasive range (North America) to resist and tolerate insect damage. Two insect species were used, one was a generalist that fed on many plant species; the other was a specialist and only fed on a few. The specialist grew more rapidly and ate more of the plants from the invasive population. This suggests that these invasive plants have reduced defenses compared with plants from the native range of Chinese tallow. Further these defenses are most effective against specialist insects. Also, these invasive plants had greater tolerance to insect damage as they grew more rapidly following damage. These results suggest that in the more than 200 years that Chinese tallow has grown in North America with little specialist insect damage, this invasive plant population has developed reduced resistance to specialist insects. Additionally, these invasive plants have developed greater tolerance to insects, especially toward the generalist species. These invasive plants with reduced resistance will be highly nutritious to specialist biological control agents however the plants’ tolerance to damage may reduce their control.
Technical Abstract: Invasive plants often have novel biotic interactions in their introduced ranges. These interactions, including less frequent herbivore attacks, may convey a competitive advantage over native plants. However, herbivores vary in their responses to different plant defenses and plants vary their defense strategies over ecological and evolutionary time periods. In order to tease apart the relationships among these factors, we examined resistance and tolerance of Chinese tallow tree (Triadica sebifera) populations from the introduced and native ranges to generalist (Cnidocampa flavescens) and specialist herbivores (Gadirtha inexacta) in the native range. In a lab bioassay of resistance, caterpillars of each species were raised on excised leaves and we collected data on leaf mass consumed and larval development. Overall, the specialist developed more rapidly, and produced more biomass with higher consumption compared to the generalist. For the generalist, we found that the amount of leaf mass consumed, development time, and caterpillar mass were comparable for caterpillars fed leaves from invasive versus native populations. However, the specialist consumed more mass of invasive populations, had higher growth rate and grew larger on leaves from invasive populations. This suggests that plants from invasive populations have altered chemistry that has a larger impact on specialist resistance than on generalist resistance. In a field test of herbivore tolerance, we subjected plants to a defoliating herbivory event then allowed them to regrow for 100 days. We found that plants from invasive populations grew more rapidly in the absence of herbivory and that they had greater herbivore tolerance than native populations, especially for tolerance to generalists. Together the results of these experiments indicate that differences in selective pressures between ranges have caused dramatic reductions in resistance to specialist herbivores; results from herbivore feeding trials with excised leaves suggests that changes in plant secondary chemistry underlie these differences. Moreover, we found an increase in tolerance to herbivory that appears to at least partly reflect an increase in growth rate in the introduced range but the greater tolerance to generalist herbivores suggests the intriguing possibility of selection for traits that allow plants to tolerate generalist herbivores more than specialist herbivores.