There is an increasing recognition that facilitation is as important species interactions as competition in ecological communities, and especially so in stressful environments. Compared to rich theoretical literature on competitive communities, there is much less theoretical research on ecological communities with facilitative interactions. Here we develop a simple two-species mathematical model in which competitor and stress-tolerant species interact through competition and facilitation. The competitor species is competitively superior to the stress-tolerant species, and can competitively exclude the stress-tolerant species in benign environments. The stress-tolerant species has facilitation effects through habitat amelioration on itself and the competitor species. The model is analyzed by a combination of analytical and numerical techniques.
Results/Conclusions
The model displays rich behavior along the stress gradient. At low stress levels, the competitor species excludes the stress-tolerant species. At moderate stress levels, they coexist at a locally stable equilibrium or on limit cycles. The limit cycle mimics the trajectory of plant succession, in which the initial increase of the stress-tolerant species facilitates the increase of the late-successional competitor species and the increase of the competitor in turn causes the decline of the stress-tolerant species. At higher stress levels, coexistence breaks down, either through the extinction of the competitor species or through increasing amplitudes of limit cycles causing the collapse of both competitor and stress-tolerant species. These results suggest that ecological communities of competition-facilitation interactions can be inherently unstable and destined to collapse in increasingly stressful environments.