Understanding the complex trophic interactions among species in an ecosystem involves solving the puzzle describing the ecosystem structure and functioning. The choice of interaction formulation fundamentally influences ecosystem model conclusions and therefore the usefulness of management
advice. I reviewed existing methods for modeling species interactions in marine ecosystems, ranging from process models to statistical models that are fitted to data. An Antarctic multispecies simulation model was used to explore the responses of predators to perturbations in prey, as well
as the poorly understood phenomenon of abrupt prey switching. Relatively simple tractable models that had been ground-truthed with data and quantified the slope of the interaction relationship showed considerable potential for elucidating the outcomes of species interactions that are difficult
to measure directly in the field. Models must integrate changes in vital rates through to population and community-level changes (the scale needed for management). Abrupt prey switching by a predator to prey species A when prey species B decreased to a threshold level could mediate a dramatic
decline in prey species B, as well as greatly facilitated the subsequent recovery of prey species A. Whether or not prey switching occurred, and whether it was step-wise or gradual, had a marked impact on the rate of recovery of the prey species, as well as interlinked predators. Recovery
times predicted by the multispecies model were longer than those from single-species models. Increasing understanding and quantification of species interactions is crucial to advancing an ecosystem approach to fisheries management.
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