REDUCED HERBIVORE RESISTANCE IN INTRODUCED SMOOTH CORDGRASS (SPARTINA-ALTERNIFLORA) AFTER A CENTURY OF HERBIVORE-FREE GROWTH

We compared resistance to insect herbivory in two introduced populatio ns of smooth cordgrass (Spartina alterniflora) differing in their hist ory of herbivory. One population in Willapa Bay, Washington, has sprea d in the absence of herbivory for more than a century, while another p opulation in San Francisco, California, was introduced 20 years ago an d is fed upon by the Spartina-specialist planthopper, Prokelisia margi nata. The planthopper is a sap-feeder common on the Atlantic and Gulf coasts of North America, where smooth cordgrass is native. Smooth cord grass plants from Willapa Bay (WB), San Francisco Bay (SFB), and Maryl and (the source of the SFB introduction) were exposed to P. marginata herbivory over two consecutive summers in a common greenhouse environm ent, and their growth was compared with that of control plants that we re grown herbivore-free. The planthoppers had relatively little effect on the growth of SFB plants, with plants exposed to herbivores averag ing 77% and 83% of the aboveground biomass of herbivore-free controls after the first and second season of herbivory, respectively. The grow th of plants from Maryland was similarly little-affected by the planth oppers, with the plants exposed to herbivores averaging near 100% of t he biomass of herbivore-free controls after two seasons. In contrast, the growth of the WB plants was greatly reduced by the planthopper, wi th the plants exposed to planthopper herbivory averaging only 30% and 12% of the aboveground biomass of herbivore-free controls after the fi rst and second seasons of herbivory, respectively. By the end of the s econd season of herbivory, 37% of the WB plants exposed to herbivory h ad died, while none of the SFB plants exposed to herbivores had died. Among WB clones, there was variation in resistance; one WB clone suffe red 0% mortality while another suffered 100% mortality when exposed to herbivores. Short-term herbivory experiments with the putative founde r clone for the WB population suggested that the WE founder was simila r to the more resistant WB clones in its susceptibility to planthopper herbivory. Nitrogen analyses of green leaf tissue indicated that WB p lants, including the WB founder clone, averaged 70% more total leaf ni trogen than SFB and Maryland plants. In a planthopper choice experimen t, more planthoppers were observed on WB plants than SFB plants after 95 days of exposure to herbivory. Planthopper preference for WB plants may have contributed to the lower resistance of WB plants to herbivor y; however, even before planthoppers had become more abundant on the W B plants, the proportion of leaves with 50% or more dead tissue averag ed significantly greater on the WB plants, suggesting a difference bet ween populations in tolerance to herbivory as well. Multiple factors, including a founder effect, further loss of herbivore tolerance, and h erbivore preference for WB plants, appear to account for the reduced p lanthopper resistance in the WB population.