Diversity maintenance in communities is determined by species’ ability to coexist with each other and to overcome environmental stress that may act as an environmental filter. Coexistence through stabilizing mechanisms arises from stronger intra- than interspecific competition, making species limit their own population more than others limit them, as happens when there is niche differentiation. Additionally, stress may affect stabilizing mechanisms because it determines the strength of species interactions. We analyzed how stabilizing mechanisms change along a hydric stress gradient by comparing intra- and interspecific interactions between six annual plant species that we sowed in a field experiment. We also determined if hydric stress was an environmental filter preventing species establishment. We used multivariate linear mixed models to assess species’ response to the soil depth gradient and the number of intra- and interspecific competitors. These response curves were then used to calculate interpretable measures of environmental constraints, competition and stabilizing niche differences along the soil depth gradient.
Results/Conclusions
Stronger intra- than interspecific competition was more common where stress was more intense. We also found facilitation in two species and it stabilized coexistence. Some species were negatively affected by stress, while others where favored by it perhaps because it eliminates soil pathogens. All species could coexist stably with the others at least at some portion of the gradient except for one. The single species that lacked stabilizing mechanisms seems to be a transient member of the community, maintained by source populations found in the surrounding areas. At a larger scale, niche differentiation also promoted coexistence by allowing species to persist at different soil depths. Our results also suggest that niche overlap was reduced with stress. This is consistent with published evidence for reduced niche breadth in organisms that inhabit extreme conditions. Stabilizing mechanisms were slightly more common with stress. However, because our species sample was small and there were exceptions to the general pattern, further research is necessary to establish if this is a common occurrence in nature.