Facilitation is ubiquitous, but its consequences on biological communities are still unclear. It has been suggested that facilitation and diversity stabilize communities, but there is also support for the opposite. The issue is further obscured by unclear definitions of stability, and because facilitation often occurs among species consuming the same resources. The positive effects of such intraguild facilitation would be overwhelmed by negative ones at high interactor densities due to resource depletion. We analyzed in silico the effects of intraguild facilitation on diversity, and the effects of both on different stability properties: elasticity (rate of return to stable conditions), resilience (attractor basin’s width, a measure of how much disturbance the system withstands before losing its capacity to return to its former stable state), constancy (a property of point attractors as opposed to quasiperiodic, resonant or chaotic ones where communities change constantly), number of alternative stable states and resistance to invasion. To do so, we modified a competition model to include facilitation. Sets of ten species (hereafter biotas) where generated in three scenarios: no facilitation, facultative and obligatory facilitation. We then searched for alternative stable states (hereafter stable communities) and measured their richness and stability properties.
Results/Conclusions
Facilitation had complex effects on species richness: Community diversity (α diversity) in constant communities and the number of alternative stable states (which promote β diversity) were smaller when facilitation occurred. However, facilitation slightly reduced the frequency of constant communities in a biota, favoring diversity because richness was far greater in quasiperiodic than in point attractors. In line with previous findings, species richness had negative effects on most stability properties. After controlling for this effect, facilitation increased elasticity but reduced resistance to invasion and had complex effects on resilience. The facilitative and competitive components of intraguild facilitation had different effects on stability: elasticity depended more strongly on the strength and type (intra vs interspecific) of competition, whereas resilience was determined in a greater extent by facilitation. Moreover, these differences seem to change in a strongly non-linear way with interaction intensity. This suggests that stability depends strongly on the underlying mechanisms that drive the interspecific interaction. Thus, there is no easy answer to whether facilitation stabilizes communities or not: it all depends on the stability property analyzed and on the kind of interaction.