Facilitation- vs. competition-driven succession: the key role of resource-ratio.

Symbiotic nitrogen (N)-fixing plants are abundant during primary succession, as typical bedrocks lack available N. In turn, fixed N accumulates in soils through biomass turnover and recycling, favouring more nitrophilous organisms. Yet, it is unclear how this facilitation mechanism interacts with competition for other limiting nutrients such as phosphorus (P) and how this affects succession. Here, we introduce a resource-explicit, community assembly model of N-fixing species and analyze successional trajectories along resource availability gradients using contemporary niche theory. We show that facilitation-driven succession occurs under low N and high enough P availabilities, and is characterised by autogenic ecosystem development and relatively ordered trajectories. We show that late facilitation-driven succession is sensitive to catastrophic shifts, highlighting the need to invoke other mechanisms to explain ecosystem stability near the climax. Put together with competition-driven succession, these results lead to an enriched version of Tilman's resource-ratio theory of succession.