Plant-animal interactions in random environments: habitat-stage elasticity, seed predators, and hurricanes

When environments change stochastically, the question arises how to evaluate the effects of a plant-animal interaction on the fitness of the plant, where plant fitness is measured by the stochastic growth rate. We develop the concept of habitat-stage elasticity, Eβ S, which gives the proportional sensitivity of the stochastic growth rate to perturbations of stage transition rates in each state (β) of the habitat. We employ it to understand why a specialist gall-making seed predator has relatively low impact on the fitness of a subtropical shrub. The plant lives in a forest characterized by patchy, recurrent disturbances caused by hurricanes. Both predation rate and plant demography vary with canopy openness. In the most closed-canopy state, the seed predator destroys 90% of the fruits, and demographic quality, the dominant eigenvalue of that state's matrix, is low, while in the most open-canopy state, predation is negligible and demographic quality high. The seed predator is locally extirpated by strong hurricanes, recolonization taking several years. The effect of the predator on the stochastic growth rate is negligible at both low and high hurricane frequency. Its effect peaks (6%) at an intermediate hurricane frequency. The stochastic growth rate varies in its sensitivity to the predator in different states of the habitat due to a product of two factors: the frequency of the state in the environment and the contribution of fecundity to its elasticity. The latter factor encapsulates the expected sequence of future states of the habitat. In our system, the contribution of fecundity to elasticity of the darkest state increases with hurricane frequency, even though the probability of encountering that state decreases, because today's dark habitats are more likely to become lighter ones. The contribution of fecundity to the two lightest states does not vary with hurricane frequency. In contrast, its contribution in intermediate states at intermediate hurricane frequencies is most dynamic, since uncolonized states may become colonized states and insensitive states may become sensitive states. The effects of an animal on plant fitness is determined by the disturbance regime in addition to its impact on vital rates in each environmental state.