1. The stress-gradient hypothesis (SGH) predicts that the frequency of facilitative and competitive interactions will vary inversely across abiotic stress gradients, with facilitation being more common when abiotic stress is high. The effect of competition intensity on species richness is generally thought to be negative, but tests along true stress gradients are lacking. This body of research has primarily focused on vascular plants and could be well informed by broadening to other communities.
2. We report the first regional-scale test of the SGH using biological soil crusts dominated by mosses and lichens, a key multi-functional community of arid and semi-arid ecosystems worldwide. We examined the intensity of facilitative or competitive interactions at the level of the community and among species pairs along an aridity gradient. Along this gradient we also examined the relationship between competition intensity and species richness.
3. All evidence strongly suggested that negative species interactions are prevalent in this study system and much more common than expected by chance. At the community scale, we found that abiotic stress associated with aridity was positively related to and explained 6–56% of the variance in indicators of facilitation or competition, dependent on the index and algorithm used. Despite this intriguing contradiction of the SGH at the whole community scale, we found scant dependency of species-level interactions upon abiotic stress. However, the sign and intensity of these interactions proved to be species-specific. We also found that the relationship between competition and species richness, usually negatively affected by competition, was positively related to richness at low abiotic stress, and negatively related to richness at high abiotic stress.
4. Synthesis. We propose that the response of species interactions to stress gradients may partially depend upon the particular modes of facilitation and competition, in addition to type of stressor and life-history strategies of species involved. We also hypothesize that because stress can act as a filter, a greater number of species interactions are possible under low stress conditions. We believe that this may render intransitivity dominant over niche segregation and expansion, potentially resulting in positive effects of competition on species richness in low stress situations.