Species level traits affect environmentally mediated, interspecific interactions.  To generate predictions of how variation in the environment can lead to altered species interactions, the response of each of the species of interest to a set of environmental changes must be measured under controlled conditions.  A controlled greenhouse experiment was conducted examining the responses of three grass species which co-exist in a semi-arid grassland in southern Arizona, U.S.A.  Two of the species are exotic, and one, a native (Digitaria californica).  One of the exotics has grown to abundance on the landscape (Eragrostis lehmanniana), and the other has maintained populations but remains uncommon (Eragrostis culvula).  The physiological responses of all three species were measured to changes in availability to soil moisture to assess the performance of each of the species under these variable conditions.  The experiment consisted of both a dry down and pulse addition experiment.  All three species had their best performance under conditions of greatest water availability, but had varying responses to both drying and rewetting conditions.  The most abundant species was neither the species that had the greatest potential performance nor the species that was most resistant to low resource availability.  Examination of the variation in responses of similar co-occurring species allows for competition and coexistence to be addressed in both a mechanistic and predictive manner.  By combining the understanding generated about competition and coexistence from this method within a variable environment, predicting success within real plant communities becomes an attainable goal.