Biotic forcing: the push–pull of plant ranges
Scientists now recognize the importance of species interactions for range shifts, but lack general predictions about when and how species interactions influence shifts. The ‘biotic envelopes’ of plant species are defined by inter-specific interactions that influence their range limits. Two prominent hypotheses describe the biotic envelopes of plants by predicting that the outcome of inter-specific interactions is determined by climate, especially temperature and aridity. The first hypothesis posits that species distributions are structured by a trade-off between competitive ability and cold tolerance, so plant species exposed to warming climates will have trailing range edges that are limited by competitive interactions. The second hypothesis proposes that the effects of competition and facilitation from neighbouring plants change within a species range, such that facilitative interactions dominate in more environmentally stressful conditions; these facilitative interactions define leading range edges in a warming climate. We incorporate these hypotheses into a common framework that allows us to identify when mismatches in dispersal rates will lead to range expansion or contraction for a focal species. We provide general predictions about the biotic envelopes of plants, and how climate change will alter these envelopes, while highlighting uncertainties in applying these predictions beyond range edges.