Sunflower sea star predation on urchins can facilitate kelp forest recovery

The recent collapse of predatory sunflower sea stars (Pycnopodia helianthoides) 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 Pycnopodia populations may help recover kelp forests through their consumption of nutritionally poor purple sea urchins (Strongylocentrotus purpuratus) typical of barrens. Pycnopodia consumed 0.68 S. purpuratus day−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. Pycnopodia seem unable to chemically distinguish starved from fed urchins and indeed have higher predation rates on starved urchins due to shorter handling ti..., 1) Sea urchin conditioningData: gonads.xlsxCode: Gonad Index Analysis.RMethod: Our interest was in whether Pycnopodia will prey on  S. purpuratus that exist in sea urchin barrens, which have minimal gonad content and low nutritional value to predators. However, S. purpuratus are rare in the San Juan Islands and do not currently form urchin barrens. We therefore used a conditioning protocol to create urchin-barren analog urchins by starving and spawning out urchins and kelp forest analog urchins by feeding the urchins with kelp ad libitum prior to the predation trials, summarized in Supplementary Table 1. We conducted 3 collection dives over July 18-20 at known populations of purple urchin sites near Cattle Pass, Lopez, and San Juan Islands. We collected 300 S. purpuratus from rocky subtidal habitat with healthy bull kelp forests and high flow, in roughly the interquartile range of the sizes we observed in the field; we avoided the largest and smallest of the urchins. The sizes of the ur..., R and R studio