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.