Models of seed predation and coexistence of desert annual plant species.

摘要

This work provides a few steps in uncovering the various effects of predation in species coexistence.; Chapter one investigates the effect of seed predation on the coexistence of competing annual plants in a constant environment, examining both fluctuation-dependent and fluctuation-independent coexistence. Predation in the model is density-dependent through the numerical response of the predator to prey density. Predation can be selective but not frequency dependent. Fluctuations occur in the model through instabilities in population dynamics. The coexistence mechanisms in a constant environment are based on life-history tradeoffs in which strong equalizing components are present, i.e. components that reduce average fitness differences between species.; Chapter two investigates the effect of seed predation on the coexistence of competing annual plants in a variable environment. In a variable environment, coexistence arises from a mechanism called the storage effect. In the presence of predation, the distinction between high and low density species is reduced. In particular, the presence of predation reduces covariance between environment and competition because it reduces prey density and thus resource competition. As a consequence, a species at low density has less advantage over higher density species and coexistence due to the storage effect is weakened.; Chapter three compares two multispecies coexistence mechanisms, frequency-dependent predation (FDP) and the storage effect (SE). They both promote coexistence by providing benefits to species that fall to low density. FDP works by focusing predation on more abundance species, while neglecting species with low densities. This chapter studies the interaction between FDP and SE. Generally predation appears to weaken SE. However, coexistence will not suffer if the frequency-dependent component of predation is strong enough. Similarly, environmental fluctuations can reduce the contribution of FDP to coexistence because they reduce predator densities. However, with high survival of dormant seed, SE can be strong enough to compensate, or more than compensate, for the decline in FDP, so that the coexistence region remains the same or is enlarged.