Intensity and temporal variability as components of stress gradients: implications for the balance between competition and facilitation

The mechanisms regulating switches in species interactions along gradients of stress are yet to be fully elucidated. In particular, the role of temporal variability in environmental severity or consumer pressure has been not explored either empirically or theoretically. Here, through a spatially explicit model (i.e. a two-dimensional lattice), we show that variations in the temporal variance of environmental stress can be as important as those in the mean intensity in regulating the spatial distribution and coexistence range of species differing in their relative competitive ability and tolerance to stress, as well as the direction and magnitude of their interactions. In addition, our simulations suggest that enhanced temporal fluctuations in environmental stressors can enhance absolute levels of stress perceived by interacting species. This study shows that tests including both the mean and temporal variance of environmental stress will be key to forecast changes in species interactions under different scenarios of climate change.