Sunflower sea star predation on urchins can facilitate kelp forest recovery

<p>The recent collapse of predatory sunflower sea stars (<em>Pycnopodia helianthoides</em>) due 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. We used experiments and a model to test whether restored <em>Pycnopodia</em> populations may help recover kelp forests through their consumption of nutritionally poor purple sea urchins (<em>Strongylocentrotus purpuratus</em>) typical of barrens. <em>Pycnopodia</em> consumed 0.68 <em>S. purpuratus</em> day⁻¹, and our model and sensitivity analysis shows that the magnitude of recent <em>Pycnopodia</em> declines is consistent with urchin proliferation after modest sea urchin recruitment, and even small <em>Pycnopodia</em> recoveries could generally lead to lower densities of sea urchins that are consistent with kelp-urchin coexistence. <em>Pycnopodia</em> seem unable to chemically distinguish starved from fed urchins and indeed have higher predation rates on starved urchins due to shorter handling times. These results highlight the importance of <em>Pycnopodia</em> in regulating purple sea urchin populations and maintaining healthy kelp forests through top-down control. 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.</p>