Co-existence in a species-rich grassland: competition, facilitation and niche structure over a soil depth gradient

QuestionNiche differentiation may promote species co-existence because it precludes competitive exclusion. However, in stressful environments, facilitation may predominate, and large niche overlaps may result from niche widening and positive spatial associations. We analysed the niche structure of a species-rich system over a soil depth gradient (a niche axis positively correlated with water availability) and tested for various patterns expected from competition or facilitation. LocationSemi-arid grassland, Oaxaca, Mexico. MethodsWe recorded species abundance and soil depth in 1200 0.1x0.1m quadrats from four sites. Community structure was described through correspondence analysis and correlated with soil depth. We measured niche breadth (Levine's index), position (mean depth) and overlap (Hurlbert's index). We used a null model to estimate the expected overlap if interactions play no role in determining community structure. We tested whether overlap changed along the depth gradient. We also analysed if niches become narrow with higher competition. ResultsData supported three predictions derived from competition: most species' niches were relatively narrow and differed in their position along the gradient, a pattern reflected in a changing community structure; realized niches became narrower where competition was stronger; and niche overlap was smaller than expected by chance. Facilitation had no support: large niche overlaps expected from stress amelioration were as frequent as predicted by chance. Overlap increased in the most stressful (shallow) soils, as expected if facilitation predominates there, but also in the deepest ones where evidence for competition was strong, suggesting an artifact at the extremes of the gradient. ConclusionsIn this grassland, competition seems to restrict species distribution, abating richness in the deepest soils and relegating poor competitors (succulent and poikilohydric species) to the shallow soils. There, drought tolerance rather than facilitation determined community composition and richness. Our results support the idea that hydrological niches play an important role in maintaining plant diversity because (1) differentiation in hydrological requirements (small niche overlaps over the water-use axis) reduces competition, and (2) spatial heterogeneity and stress create competition-free patches. Our results also support the hypothesis that facilitation is rare when stress is caused by resource (in this case, water) scarcity.