Spartina alterniflora was introduced to Pacific U.S. estuaries from the Atlantic coast along a biogeographic difference in the range of one of its specialist herbivores, Prokelisia marginata. This gradient of herbivore history allows testing of two prominent invasion hypotheses: enemy release and evolved increased competitive ability. A factorial greenhouse experiment was conducted in 2006 with source population, P. marginata presence/absence, and nitrogen fertilization (high/low) as fixed factors. Five S. alterniflora populations were included, with at least one representative from each possible herbivore history (native, invasive but never separated from P. marginata, invasive and separated). Changes in nitrogen supply rates (obtained with Plant Root Simulator probes) and plant performance traits were measured.  Comparisons among S. alterniflora populations suggest that plants from invasive populations tend to be larger and more asexually fecund, but are equally tolerant and resistant to herbivory. Flowering rate was equally low among all populations and was not significantly affected by nitrogen levels or herbivory. Total nitrogen and nitrate supply rates were similar for native and invasive populations, suggesting comparable nitrogen uptake rates, but populations varied in their responses to the combined effects of fertilization and herbivory. These results imply that rapid evolution for larger, more fecund plants with slightly increased nitrogen utilization may have occurred in invasive populations, but they do not suggest that herbivory was the primary driver. A second experiment, comprised of competitive plantings between populations with and without herbivores, will provide a link between predicted and actual competitive ability.