Loss of predation risk from apex predators can exacerbate marine tropicalization caused by extreme climatic events

Extreme climatic events (ECEs) and predator removal represent some of the most widespread stressors to ecosystems. Though species interactions can alter ecological effects of climate change (and vice versa), it is less understood whether, when and how predator removal can interact with ECEs to exacerbate their effects. Understanding the circumstances under which such interactions might occur is critical because predator loss is widespread and ECEs can generate rapid phase shifts in ecosystems which can ultimately lead to tropicalization. Our goal was to determine whether loss of predation risk may be an important mechanism governing ecosystem responses to extreme events, and whether the effects of such events, such as tropicalization, can occur even when species range shifts do not. Specifically, our goal was to experimentally simulate the loss of an apex predator, the tiger shark Galeocerdo cuvier effects on a recently damaged seagrass ecosystem of Shark Bay, Australia by applying documented changes to risk-sensitive grazing of dugong Dugong dugon herbivores. Using a 16-month-field experiment established in recently disturbed seagrass meadows, we used previous estimates of risk-sensitive dugong foraging behaviour to simulate altered risk-sensitive foraging densities and strategies of dugongs consistent with apex predator loss, and tracked seagrass responses to the simulated grazing. Grazing treatments targeted and removed tropical seagrasses, which declined. However, like in other mixed-bed habitats where dugongs forage, treatments also incidentally accelerated temperate seagrass losses, revealing that herbivore behavioural changes in response to predator loss can exacerbate ECE and promote tropicalization, even without range expansions or introductions of novel species. Our results suggest that changes to herbivore behaviours triggered by loss of predation risk can undermine ecological resilience to ECEs, particularly where long-lived herbivores are abundant. By implication, ongoing losses of apex predators may combine with increasingly frequent ECEs to amplify climate change impacts across diverse ecosystems and large spatial scales.