Sea otters returned to California estuary; eelgrass later expanded 600%

When sea otters returned to California’s Elkhorn Slough in 1984, the estuary was already facing severe environmental pressure. The coastal wetland had high nutrient levels linked to its agriculture-dominated watershed, while eelgrass had fallen to an all-time low of just 2 hectares.Decades later, researchers found that the return of the predators had effects far beyond their own population. Since sea otters recolonised the estuary, the extent of eelgrass beds increased by 600%. A later study also found that erosion of creek banks and marsh edges in areas with large otter populations slowed by an average of 69%.The two studies show how the recovery of a top predator can affect different parts of an ecosystem. The first study, published in PNAS in 2013, examined the relationship between sea otters, crabs, smaller grazers and eelgrass.