![]() First, particularly hungry herbivores may be more likely to engage in risky foraging behaviour, even when the predator is nearby, which weakens the cascading effect. Importantly, the strength of a trophic cascade may weaken when the prey are hungry. The details of this relationship are also governed by the hunger level of the predator and its herbivorous prey and the productivity at the base of the food chain. The strength of a predator can be owing to direct consumption of its prey (a consumptive effect), or through behavioural changes in the prey owing to the ‘landscape of fear’ that the predator imparts on the system. ![]() ![]() Strong predators of herbivores can trigger a trophic cascade, providing an indirect benefit to primary producers by reducing grazing rates of herbivorous consumers. When predators have a significant influence on the populations or behaviour of herbivores, they can exert top-down control into food webs. The recovery of this important predator to densities commonly found prior to SSWD, whether through natural means or human-assisted reintroductions, may therefore be a key step in kelp forest restoration at ecologically significant scales. These results highlight the importance of Pycnopodia in regulating purple sea urchin populations and maintaining healthy kelp forests through top-down control. Pycnopodia seem unable to chemically distinguish starved from fed urchins and indeed have higher predation rates on starved urchins owing to shorter handling times. purpuratus d −1, and our model and sensitivity analysis shows that the magnitude of recent Pycnopodia declines is consistent with urchin proliferation after modest sea urchin recruitment, and even small Pycnopodia recoveries could generally lead to lower densities of sea urchins that are consistent with kelp-urchin coexistence. We used experiments and a model to test whether restored Pycnopodia populations may help recover kelp forests through their consumption of nutritionally poor purple sea urchins ( Strongylocentrotus purpuratus) typical of barrens. The recent collapse of predatory sunflower sea stars ( Pycnopodia helianthoides) owing to sea star wasting disease (SSWD) is hypothesized to have contributed to proliferation of sea urchin barrens and losses of kelp forests on the North American west coast. ![]()
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