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 this tests along a true stress-gradient are lacking. This body of research has primarily been conducted using vascular plants, and may be well-informed if research were broadened to other communities.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.  

Results/Conclusions

All evidence strongly suggested that negative species interactions are prevalent in this study system and more common than expected by chance. At the community scale, we found that abiotic stress associated with aridity was positively related to indicators of competition, explaining 6 to 56% of the variance, dependent on the index and algorithm used. Despite this intriguing result 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. 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 acts as a filter, a greater number of species interactions are possible under low stress. We believe that this may render intransitivity dominant over niche segregation, potentially resulting in positive effects of competition on species richness in low stress situations.