New Delhi, June 6 -- The ochre sea stars are back. At eight sites along the Oregon coast monitored continuously for 23 years, population numbers have climbed to or above pre-epidemic levels, the animals have grown large enough to eat California mussels at rates not seen since before 2013, and something that ecologists rarely get to say out loud is now defensible: a keystone predator has pulled back from the brink. The question scientists cannot answer is why.

the animals driving that recovery are conspicuously small, populations are less stable year to year than before the epidemic, and nobody has a satisfying explanation for the baby boom that made it possible.

Meanwhile, the sunflower sea star, the ochre's larger, more ecologically cr...