Understanding the factors that determine the sign and strength of interspecific interactions may shed light on community composition. Competition is thought to lose importance with stress even giving way to facilitation, as stated in the stress gradient hypothesis (SGH). On the other hand, closely related species are thought to compete strongly (competitive-relatedness hypothesis), while facilitation is expected between distant relatives. To test these ideas, we measured pairwise species interactions between 36 herb species along a stress gradient in a semiarid grassland. There, soil depth is heterogeneous and related to water availability such that thin soils are dry and stressful but deep soils are less so. Interactions were estimated using a population growth model that included competition and facilitation, allowing interactions to change over soil depth. The model was parameterized using changes in species abundance over 14 years. We then compared the interactions between close and distant relatives over the soil-depth gradient.
Results/Conclusions
We estimated interactions for 246 species pairs, out of which 67 changed with soil depth. In the latter, we found strong competition at both extremes of the gradient. Strong competition in deep soil, as expected in SGH, was observed more frequently between close relatives. This could be due to a high niche similarity between closely related species, which thus compete strongly even when resources are abundant. Competition in shallow soils may be expected if stress is caused by resource scarcity (water in our system), and was observed mostly between distant relatives. Facilitation occurred both between close and distant relatives. For some species pairs, competition shifted into facilitation, but, contrary to the SGH, facilitation occurred in deep soil. This was more common between distant relatives, probably because their niches differ more. Intraspecific competition was usually stronger than interspecific interactions. This may have important consequences for species coexistence because it allows for niche differentiation as species limit themselves more strongly than they do others. Our results suggest that the classic versions of the stress-gradient and competitive-relatedness hypotheses do not fully explain interactions between species when stress is caused by lack of resources.