If you take a class in ecology, sea otters inevitably come up. They’re a classic example of a keystone species — that is, an animal or plant or other organism that has a critical role in the ecosystem such that, if you take it out of the system, everything collapses. Sea otters are a keystone species in the kelp forests off California. If the sea otters aren’t around to munch on sea urchins and keep the urchin population in check, urchins proliferate and graze the kelp to death. Without the kelp, other animals that depend on the vegetation — including crabs and geese — suffer. All that happens because there are no sea otters.
Now a group of scientists from California have found that sea otters play a similar keystone role in another coastal California ecosystem — the seagrass beds of Elkhorn Slough. The study was published this week in PNAS.
Elkhorn Slough is an estuary near rich fields of strawberries and artichokes. Those farm fields are a problem because when water drains off them into the estuary, it carries nutrients (from fertilizer) and pesticides. Nutrient loading is a particular problem in marine ecosystems as it changes the abundance of certain organisms and sets everything out of whack in the food web. Some critters thrive, but many suffer. And in Elkhorn Slough, seagrass beds have been on the decline.
Scientists suspected, however, that sea otters may play an important role in this area. When the researchers looked at a 50-year record of nutrient levels, seagrass and sea otters, they noticed a pattern. There were no sea otters in the area between 1965 and 1984. During that time, when nutrient levels started to increase, the seagrass beds declined. But when the sea otters were reintroduced to the area, the seagrass beds again thrived, even though the nutrient levels continued to increase.
The researchers suspected that the link between the sea otters and seagrass was not sea urchins, as in the kelp forests, but crabs: The sea otters eat the crabs. Without so many crabs around, grazing herbivore species thrive. Those grazers eat algae that lives on and harms the seagrass, and as a result, the seagrass thrives.
But when there are no otters around, this all falls apart. The crabs outcompete the algae grazers, letting the algae proliferate. And with the nutrients boosting the algae population, the seagrass beds suffer.
“This estuary is part of one of the most polluted systems in the entire world, but you can still get this healthy thriving habitat, and it’s all because of the sea otters,” study author Brent Hughes of the University of California, Santa Cruz, told BBC News. “So it’s almost like these sea otters are fighting the effects of poor water quality.”
This new pattern is a bit more complex than the classic sea otter–kelp story, but it’s yet another example of how important top predators are and the disaster that can follow their removal.
Image courtesy of flickr user Blake Matheson