Background/Question/Methods
The keystone predation concept has been highly influential in the development of community theory. Field studies suggest that a keystone predator can promote the coexistence of many prey species by predation on competitive dominants. In models, this idea is represented as a competition-predation tradeoff, but in the absence of other coexistence mechanisms only permits coexistence of two prey species. Here we show how a keystone predator interacts with a well-known coexistence mechanism for desert annual plants, the storage effect (SE). The SE arises when annual plants partition resources temporally in a variable environment, and is capable of strongly stabilizing species coexistence. However, if there are strong fitness inequalities between species, few species may coexist despite the presence of the storage effect. So we ask whether the introduction of a predator with a strong preference for the dominant species can equalize fitnesses and allow many species to coexist.

Results/Conclusions
We confirm that without the storage effect, a keystone predator allows coexistence of at most two species through predation on the competitive dominant. Moreover, we find that a keystone predator reduces the stabilizing tendency of the SE. However, despite this reduction in magnitude, the SE can allow multispecies coexistence in the presence of a keystone predator provided the keystone predator reduces the fitness differences sufficiently. When fitness differences remain substantially unequal, dominance by a single species is still predicted, but due to interactions between nonlinear biology and environmental fluctuations, the usual R* and P* rules developed for equilibrium do not apply. Instead, predictions from scale transition theory are used.