Crab Feast
Elkhorn Slough marine reserve (credit: NOAA)
Most everyone loves a crab feast and not just humans. Sea otters have become effective ecosystem engineers by consuming an invasive crab species that is disrupting the coastal marine ecology along the Pacific coast. They are restoring the ecology of a marine estuary in central California by setting off an ecological trophic cascade of positive feedback loops as they devour the invaders.
According to a report published by Biological Invasions, staff at the Elkhorn Nature Reserve near Monterrey and researchers with the University of California Santa Cruz and the National Oceanic and Atmospheric Administration (NOAA), otters in this estuary are keeping populations of the invasive green crab at bay.
Green crabs (Carcinus meanas) first appeared in the slough in 1994. The crustation, a native of Europe, was unintentionally introduced to the San Francisco Bay area hitching a ride on a merchant ship in 1989. Since then, it has been an aggressive invader in bays and estuaries from Elkhorn Slough to Alaska. It damages seagrass beds by borrowing under them and consuming small prey important to migratory shorebirds. Once established, their populations persist with very high numbers. In Oregon, no limits are set on how many green crabs can to taken by crab fishing pots.
California otter eating green crab (credit: Elkhorn Slough marine reserve)
However, in Elkhorn Slough, the crab population have drastically declined from its previous highs. At the same time, the sea otter population has increased significantly in the slough. Otter numbers are at their highest while the green crab population is at its lowest. Using US Geological Service data on otter foraging the investigators calculates that up to 120,000 green crabs/year are being consumed by the otters.
Sea otters were once hunted to near extinction for the fur hat trade in the early 20th Century. This rebound along the California coast is good news. The new ecological study also provides further evidence that recovery of a top predator, such as this keystone species, has multiple knock-on benefits by not only controlling an invasive species but also restoring an ecosystem in the process. The otters are having a tasty crab feast as well. WHB