Otters can help rejuvenate sea grasses, a vulnerable natural resource that protects the coastline and provides habitat for fish, according to research published on Monday.

Scientists examined how the sea grasses in one area of California rebounded when otters returned to the area, and found that the otters helped by eating crab populations.

By keeping the crab numbers low and the sizes of the remaining crabs smaller, the otters removed a key threat to the sea slugs which feed on algae and keep sea grass leaves clean and healthy.

This meant that the presence of a top predator helped save the smallest players in the ecosystem and rendered it healthier.

"Our findings depart from a view of nature built largely around bottom-up control, which has been the dominant predictor in explaining sea grass loss for more than three decades," said the study in the Proceedings of the National Academy of Sciences.

The research spanned several decades of records on sea grass and otter populations in the Elkhorn Slough, one of California's largest estuaries along the central coast.

Key periods were compared, including from 1971 to 1976 when there were no otters, to 2005-2009, or about two decades after otters were reintroduced to the area.

They found that eelgrass rebounded in the presence of otters.

These water weeds are where juvenile fish like Pacific herring, halibut and salmon make their homes. They also protect coastlines from storms and heavy waves, and soak up harmful carbon dioxide from the atmosphere.

When water runoff carries harmful nutrients from agricultural operations and city living, algae grows quickly and the sea grass leaves falter because they don't get adequate sunlight.

"These are important coastal ecosystems that we're losing, and mostly that's been associated with bottom-up effects like nutrient loading," said Brent Hughes, a PhD candidate in ecology and evolutionary biology at the University of California, Santa Cruz.

"This study shows that these ecosystems are also being hit by top-down forces due to the loss of top predators.