Interacting coexistence mechanisms: frequency-dependent predation in a variable environment

We study the interaction between environmental fluctuations and frequency-dependent predation in a model of competing annual plant species subject to seed predation. Three coexistence mechanisms arise in these circumstances, which combine to promote species coexistence. In the absence of frequency-dependent predation, the storage effect arises from the interaction between competition and environmental fluctuations, but the in the presence of frequency-dependent predation, this mechanism splits into two mechanisms: the storage effect due to competition (SEc) and the storage effect due to predation (SEp). With FDP in a fluctuating environment, another mechanism, nonlinear competitive variance (NCV) arises, but does not promote coexistence. Instead, it reduces the effectiveness of FDP. SE depends on the presence of environmental fluctuations and their interactions with competition (SEc) or predation (SEp). Predation weakens SEc, but coexistence does not suffer if predation is sufficiently strongly frequency dependent. On the other hand, environmental fluctuations leading to SEc weaken FDP and strengthen NCV. However, with high survival of dormant seed, SEc can be strong enough to compensate, or overcompensate, for the decline in FDP and the increase in NCV, so that coexistence is strengthened when both FDP and SEc are both present, even though each is harmed by the other. With more than two plant species, the new mechanism, SEp, arises and further strengthens coexistence.