Do Populations of an Invasive Weed Differ Greatly in Their Per-Gram Competitive Effects?

. Quantifying an invasive species' negative impacts across its introduced range will be quite challenging if the impacts vary unpredictably from site to site or from population to population. Little emphasis, however, has been placed on quantifying such interpopulation variation in the impacts of individual invasive species. We studied the response of a native grass (Festuca rubra) to competition from 4 geographically dispersed invasive plant (Melilotus albus) populations in order to determine if some populations of this invader have greater competitive impacts than others. Despite the relatively large number of experimental units in our greenhouse study, we did not obtain evidence that competitive effects per gram of biomass varied by invader population. Therefore, in some cases it should be possible to estimate the effects of invasive weeds with simple competition models that ignore some forms of phenotypic variation, as long as the models control for invader biomass per unit area (i.e., invader yield).