Predicting extinction: Progress with an individual-based model of protozoan predators and prey

Despite the importance of understanding persistence, there are few direct t ests of the ability of models to predict predator and prey population persi stence. We tested whether an individual-based model could forecast the dyna mics and time to extinction in aquatic microcosms of a protist predator and prey: predatory Didinium nasutum and bacterivorous Colpidium striatum. By addressing both persistence and dynamics, the model increases the testabili ty of mechanisms of extinction. Population-level equations modeled the func tional response and prey growth. For individual predators, we simulated tim e since dividing and feeding, and number of prey consumed; these influenced the timing of division and death. We tested the model by comparing simulat ed dynamics to data from three experiments.: (1) an experiment initiated wi th low predator-prey ratios in 30-mL bottles; (2) an experiment similar to Experiment 1, but in which immigrant predators, prey, or both were added du ring the first density cycle; (3) an experiment in 30-mL bottles, initiated with various predator-prey ratios. Using only nine parameters measured in independent experiments, simulations gave satisfactory predictions of the period and amplitude of cycles of pre dator and prey densities, and predator and prey densities through time for Experiment 1. Adding stochasticity to the model also allowed it to reproduc e observed prey and predator persistence and the proportion of replicates w ith prey extinctions. We used the improved model to forecast the results of Experiments 2 and 3. In Experiment 2, persistence changed with immigration . The model qualitatively reproduced these changes but underestimated their magnitude. Increasing the initial predator-prey ratio reduced persistence in Experiment 3. Simulations failed to qualitatively reproduce these result s for 30-mL microcosms, unless we raised initial prey density. This study demonstrates the use of an individual-based model to help identi fy and test mechanisms of extinction in predator-prey interactions. The com bination of individual-based and population-level formalisms can maintain b oth model tractability and a close working relationship between models and accessible data.