Symbiotic nitrogen (N)-fixing organisms such as actinorhizal plants and legumes tend to thrive during primary succession, as typical bedrocks lack available N. Through biomass turnover and recycling, fixed N accumulates in soils, benefiting the whole community. This facilitation mechanism by N-fixers is thought to be a main driver of primary succession in some environments. However, plants also compete for other essential nutrients, like phosphorus (P). How do competition and facilitation interact, and under what condition is succession mostly driven by facilitation? What are the conditions under which succession is reasonably ordered and predictable, and how does ecosystem stability change during succession? We addressed these questions theoretically using a resource-explicit, community assembly model of N-fixing species competing for N and P. These species differed in their relative abilities to acquire P and fix N. The model was analyzed using a recently developed extension of graphical resource competition theory to community assembly. We studied and characterized succession trajectories along gradients of nutrient availability in the abiotic environment.
Facilitative succession only occurs under very low N availability. It relies on the initial invasion of the bare substrate by the most efficient N-fixing strategies, sequentially displaced by colonizers that are more competitive but whose establishment relies on soil N accumulated by previous strategies. This leads to autogenic ecosystem development, as vegetation biomass and soil nitrogen increase during succession. Facilitative succession comes with two characteristic signatures. First, the very strict order in which species can replace each other filters out the inherently random colonization process, leading to relatively ordered succession trajectories. Second, the late successional ecosystem presents alternate stable states, making late succession inherently prone to catastrophic shifts. Conversely, high environmental N availabilities inhibit N fixation, leading to competition-dominated succession, with trajectories that are very sensitive to the colonization order. Put together, these results contribute to an enriched version of Tilman’s resource-ratio theory of succession.