A mutualism–parasitism continuum model and its application to plant–mycorrhizae interactions

We consider interactions between a symbiont and its host in the framework of the familiar Lotka–Volterra predator–prey model, modified to allow the symbiont to benefit the host. The model includes both benefits and costs to the interaction and spans the mutualism–parasitism continuum. We use this model to explore the shift from mutualism to parasitism in plant–mycorrhizae interactions across gradients of soil fertility. We demonstrate two mechanisms by which increased soil fertility may cause interactions to change from mutualistic to parasitic: as relative benefits to the plant decrease with increasing soil fertility, the interaction between the fungus and its host turns parasitic; with two fungal species—one a mutualist and the other a cheater—increasing soil fertility can favor the cheater if mutualists have faster growth rates than cheaters.