Space and time are the two fundamental drivers of ecological dynamics. Yet, the Stress Gradient Hypothesis (SGH)—which predicts that interactions shift from competition to facilitation along a gradient from permissive to stressful environments—theorizes stresses from merely spatial perspective. We asked: do the predictions of the SGH at the intraspecific level hold if environmental stress relates to time, such as seasonal variation, and do the intraspecific interactions in natural communities vary with varying species dominance? We conducted a density removal experiment, which was complemented by a two-year field survey, involving Alliaria petiolata at the Koffler Scientific Reserve, Ontario, Canada.
Results/Conclusions
We found statistically significant evidence for competition (negative density effects on survival) in summer and facilitation (positive density effects on survival) over winter. Overall patterns were stronger in the experiment than in field populations, where A petiolata occurred in varying dominance with other species. We suggest that increasing density may minimize per-capita metabolic cost in winter, and thus higher density increases winter survival. Our results suggest that the SGH is applicable to seasonal variation in environmental stress, and that species dominance strongly moderates species interactions.