How humans shape marine ecosystems by altering predator populations
Humans have been harvesting predators and other creatures from the global oceans for millennia. More recently, conservation measures, such as marine reserves, have been established to restore populations depleted from this harvest. Both predator harvest and replenishment alter the top-down forces that predators exert on lower trophic levels over large spatial and temporal scales. This global-scale ‘experiment’ has led to a proliferation in recent years of research aimed at understanding how human alteration of predator populations shapes marine ecosystems. Two overarching pathways of cascading predator effects, known as trophic cascades, have been identified: lethal and risk-based. Striking examples exist of lethal trophic cascades caused by fisheries, where predator declines have caused increases in prey (e.g., herbivore) abundances and decreases in prey resources (e.g., algae). These examples have led to a general expectation that fisheries, and conversely marine reserves, may cause such changes. To test this assumption, my collaborators and I looked across hundreds of sites over many years for lethal trophic cascades resulting from both a) predator declines due to fishing and b) recovery due to marine reserves. Counter to conventional wisdom, our results provide cross-ecosystem field evidence that lethal trophic cascades in oceans that are sustainably managed are, in fact, rare. Conversely, only a few examples of risk-based trophic cascades due to fishing have been documented, yet a consensus is emerging that such effects should be common. We therefore synthesized existing data from a remote coral reef archipelago with a gradient of fishing pressure to explore whether lethal or risk-based trophic cascades are more likely to occur as a result of fishing. We found that overfishing can lead to distinct, cascading risk effects in natural ecosystems whose magnitude exceeds that of presumed lethal effects and likely accounts for previously unexplained findings. Most recently, we have explored if and how risk-based trophic cascades can fundamentally alter coral reef ecosystem structure. We found that this occurs over large spatial and temporal scales by affecting primary producer (e.g., algae and seagrass) distribution and, consequently, carbon storage. Our results collectively suggest that risk effects may provide a powerful lens through which to understand and predict human impacts in oceans – from fisheries to climate change.