Strong facilitation in mild environments: the stress gradient hypothesis revisited

1. The idea that the role of facilitative interactions increases as environmental conditions become more stressful has become a ruling paradigm in ecology. Here, we review three reasons why positive interactions may actually be more prominent than generally thought under moderately stressful rather than under extreme conditions. 2. First, there is evidence that in some communities the net effect of amelioration of shortage of a limiting resource, such as water under the canopy of nurse plants, may be beneficial under moderate conditions whereas it can be overruled by increased competition for the same resource in very harsh environments. 3. Secondly, we show that even in situations where the relative role of facilitation increases monotonically with stress, the absolute effect should as a rule be largest at intermediately stressful conditions. This is because under the harshest conditions, facilitative amelioration of conditions is insufficient to allow growth altogether. Therefore, while facilitation will expand the range of conditions where an organism may occur, the largest absolute effects on biomass will always occur under less stressful conditions. 4. A third reason why facilitation may be more important under moderate conditions than previously thought is that in any ecosystem, the suite of organisms is adapted to local conditions. This implies that even under conditions that appear benign, facilitation may play an unexpectedly large role as organisms are simply more sensitive than those found under harsher overall conditions. 5. Synthesis. We argue that while facilitation will extend the range of conditions where an organism can occur, it should also boost performance of the species well into the more moderate range of conditions. Broadening our search image for facilitative effects towards milder environments will reveal wider than expected prevalence of positive interactions and their effects on stability and diversity in nature.