A temporal dimension to the stress gradient hypothesis for intraspecific interactions

Space and time are the two fundamental drivers of ecological dynamics. Studies exploring the Stress gradient hypothesis (SGH) - which predicts that the patterns of interspecific interactions shift from negative to positive with increasing environmental severity - conceptualize environmental severity predominantly from a spatial perspective. Here, from a temporal perspective and for intraspecific interactions, we asked: do the predictions of the SGH at the intraspecific level apply to seasonal change in environmental severity? We conducted a field experiment, which was complemented by a two-year field survey of natural populations of the non-native biennial forb Alliaria petiolata at the Koffler Scientific Reserve, Ontario, Canada. In both experiment and field survey studies we found statistically significant negative density-dependent survival in the productive summer period and positive density-dependent survival over the severe winter period. Effects were stronger in the field experiment than in the survey of natural populations. We suggest that the SGH at the intraspecific level may be applicable to seasonal variation in environmental severity, though our ability to detect its effect in natural communities may depend on other factors such as species dominance and environmental heterogeneity.