Many mutualisms are context-dependent, typically benefiting both partners only when conditions are favorable. This context-dependence could buffer complex communities from rapid reductions in the availability of resources, or could instead amplify sensitivity by creating positive feedbacks leading to accelerating degradation. Mathematical models have quantified how community structure and complexity can lead to resilience, the ability to tolerate and bounce back from disturbances ranging from loss of biomass, changes in resource availability, or invasions or outbreaks of consumers or pathogens. We derive a new mathematical model of an interacting plant, a resource-collection mutualist based on mycorrhizae, and a pathogen to study how the system responds to disturbances. This model naturally extends to many interacting species, where measures of resilience include the restoration of biomass, the maintenance of species, and ability to buffer against long-term variation in environmental forcing. We seek to understand how resilience depends on community structure, and the resilience of that community structure itself.
Results/Conclusions
We find that mutualists allow plants to persist when resource supply rates fall below the limiting value for plants in isolation, harm plants at higher resource supply rates, and can be lost from the system at very high rates. The effects of pathogens on plant populations can be reduced by the presence of mutualists. With multiple species, the effects of pathogens on the degree of coexistence depends on the detailed structure of the plant-mutualist and the plant-pathogen networks, with higher degrees of specialization promoting more coexistence. Mutualists favor plant species with lower intrinsic resource collection abilities at low resource supply rates, and determine a series of plant species replacements as resource supply rates or pathogen pressure increases. In this way, mutualists buffer the system against resource or pathogen perturbations by promoting a diversity of responses that can maintain the diversity of the plant population. The degree of this buffering depends sensitively on the structure of the network. By considering more mechanistic interactions among species, these models illuminate how a full suite of measures of resilience respond to the perturbations characteristic of global change, including increased rates or variability of resource supply, and invasions of pathogens or plants.