Apex predators and trophic cascades in large marine ecosystems: Learning from serendipity

Abstract

The global loss of large predators is undeniable. However, the effects of predator depletion on the structure and functioning of ecosystems are far from resolved, especially as they apply to large pelagic marine ecosystems. Much of what we know about how marine predators function in ecosystems comes from small-scale studies on relatively small, slow-moving, seafloor-feeding predators that are easy to manipulate. Scaling up to consider pelagic (ocean) ecosystem effects from large predatory fish has been challenging for several reasons. For one thing, predators have been functionally removed from many marine ecosystems due to unsustainable fishing that occurred decades or centuries ago. Also, studies often rely on correlations showing increases in prey populations as predators decline, but these correlations can be confounded if coincident oceanographic factors such as ocean warming control prey abundances. What is needed is an ecosystem-scale, large predator addition experiment for which most biotic components of the ecosystem are monitored before and after predator addition. Such an experiment is exactly what Casini et al. (1) report in PNAS.