Agent-based modeling of the dynamics of mammal-eating killer whales and their prey

The role of mammal-eating, or transient, killer whales Orcinus orca in the decline of various marine mammal populations in Alaska is controversial and potentially important in their recovery. Classical predator-prey models are insufficient to describe the dynamics of a single predator on the number of prey types known for these predators, and there are few population-level data that could be used to parameterize such models. As an heuristic framework for this more complicated system, we developed an agent-based model of killer whales with plausible energetics and behavior. We calibrated and validated the model using single-prey scenarios (a community of generic 'Seals') against published expectations for prey consumption rates, killer whale group dynamics, and demography. We then explored the emergent properties of single-prey models and of 3-prey models using the 'Seals' as primary prey, a generic small population of 'Sea Lions', and seasonally available large 'Whales'. The single-prey model gave results that were intuitively reasonable and responsive to underlying parameters but were also sensitive to encounter/killing rates, as expected in classic predator-prey models with similar parameters. However, the dynamics included long time lags (similar to 30 yr) with strong shifts in predator age structure and vital rates. In multi-prey scenarios in which the importance of seasonally available large whale prey was manipulated, large whale prey had the potential to augment killer whale numbers somewhat but had a minimal effect on the overall dynamics, whereas perturbing the carrying capacity of the primary prey created strong numeric shifts in killer whale population size and consequent indirect effects on both alternate prey. No predictive utility is suggested due to the absence of such elements as spatial realism, explicit prey-switching and more realistic prey structure, but the models suggest that we consider more complicated numerical dynamics of killer whales in discussions of their impact on prey.