There is growing recognition that the effects of climate change disturbances on ecosystems will be mediated in part by biotic interactions. Apex predators are likely candidates to be “biotic multipliers of climate change” partially because of their ability to widely generate top down control through both consumptive and non-consumptive pathways. While the effects of predator losses on top-down control receive much attention, other factors such as prey body condition also mediate the strength of predator effects. As a result, abiotic stressors that reduce prey body condition directly or indirectly may play a critical role in mediating top-down control in ecosystems by increasing risk-taking of energetically-stressed individuals. This has implications for the strength and nature of top-down control in ecosystems where resource loss is great.
Here, we use a climactic extreme event (marine heat wave), associated catastrophic seagrass loss, and a 13 year pre-loss dataset as a natural experiment to investigate the impact of widespread resource loss on the relative abundance of six groups of air-breathing marine megafauna in Shark Bay, Western Australia. Furthermore, we investigated whether such widespread seagrass loss weakened previously demonstrated risk-sensitive habitat use patterns of megafauna (driven by locally abundant tiger sharks) in this system.
Results/Conclusions
Tiger shark catch rates were not significantly different after the seagrass decline, suggesting that mesoconsumers did not experience lower predation risk after the decline. In contrast, most mesoconsumer populations (dugongs, sea snakes, cormorants, and some turtles) declined significantly as food and refuge resources were reduced. For those individuals that remained, three species of mesoconusmers (dolphins, cormorants, and sea turtles) significantly increased use of dangerous but profitable shallow seagrass beds following the die-off. Because the distribution of their food resources was similar pre- and post-seagrass decline this pattern is consistent with increased risk-taking and a relaxation of the strength of predator risk effects generated by tiger sharks.
These findings at population and ecosystem level scales are consistent with earlier theoretical and small-scale experimental work (i.e. the "asset protection principle"). Furthermore, this work suggests that resource reductions can alter anti-predator behavior and reduce the strength of non-consumptive predator effects at large scales even when apex predator communities remain stable. This weakening of the landscape of fear may become increasingly common under situations where resource species continue to be disturbed by climactic extremes globally; a major challenge going forward is determining the contexts under which such weakening is expected to occur.