Interacting coexistence mechanisms in annual plant communities: Frequency-dependent predation and the storage effect.

We study frequency-dependent seed predation (FDP) in a model of competing annual plant species in a variable environment. The combination of a variable environment and competition leads to the storage-effect coexistence mechanism (SE), which is a leading hypothesis for coexistence of desert annual plants. However, seed predation in such systems demands attention to coexistence mechanisms associated with predation. FDP is one such mechanism, which promotes coexistence by shifting predation to more abundant plant species, facilitating the recovery of species perturbed to low density. When present together, FDP and SE interact, undermining each other's effects. Predation weakens competition, and therefore weakens mechanisms associated with competition: here SE. However, the direct effect of FDP in promoting coexistence can compensate or more than compensate for this weakening of SE. On the other hand, the environmental variation necessary for SE weakens FDP. With high survival of dormant seeds, SE can be strong enough to compensate, or overcompensate, for the decline in FDP, provided predation is not too strong. Although FDP and SE may simultaneously contribute to coexistence, their combined effect is less than the sum of their separate effects, and is often less than the effect of the stronger mechanism when present alone.