The return of sea otters has caused a remarkable recovery of endangered sea grasses which grow in estuaries, bays and inlets and provide food and shelter for a wide variety of marine wildlife, a study has found.

Sea otters are considered to be top predators and it has surprised scientists to find that they have caused the growth of vegetation at the very bottom of the food chain. But the discovery is one of number of recent observations pointing to the importance of animals at the top of the ecological hierarchy to restore the overall health of a threatened habitat.

Sea grasses, also known as eel grass because of their long, thin leaves, are one of the few flowering plants that live in the sea and provide a rich habitat for a range of animals, such as fish, wildfowl and even large mammals such as manatees, or sea cows.

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Sea-grass beds can be found in sheltered bays in south-west England, Wales, Scotland and western Ireland and in similar locations on European coasts. Vast underwater meadows of sea-grass skirt the coasts of southern Europe, serving as nurseries for pipefish and seahorses.

In Britain and elsewhere in the world sea-grass beds are threatened mainly by nutrient pollutants such as nitrates and phosphates which run off from the land into the sea causing an explosion in algae, which grow on the leaves of sea-grass, blocking out sunlight.

A study by researchers monitoring the sea-grass beds of Elkhorn Slough, a tidal salt marsh in Monterey Bay, California, found that a decline in sea-grass could be reversed when sea otters returned to the area even when nutrient pollution remained high.

Further research found that the sea otters caused a marked decline in the number and size of crabs, their main source of food, which in turn led to an increase in slugs and sea-snails which fed on the algae growing on the surface of the sea-grass.

Without sea otters in the bay, crabs became so large and numerous that they had a direct impact on their main source of food – snails – which in turn led to an explosive growth of algae on the surface of sea-grasses, the scientists found.

Brent Hughes of the University of California, Santa Cruz, said that the connection was not obvious at first but it is now clear that sea otters can have a direct effect on the health of the vital sea-grass beds on which a whole ecosystem depends for its survival.

“When we see sea-grass beds recovering, especially in a degraded environment like Elkhorn Slough, people want to know why. The sea-grass is really green and thriving where there are lots of sea otters, even compared to sea-grass in more pristine environments without excess nutrients,” Mr Hughes said.

Previous attempts to restore sea-grass beds have focussed on trying to curb nutrient pollution but the latest findings, published in the journal Proceedings of the National Academy of Sciences, suggests that restoring the other end of the food chain can be just as important.

“This provides us with another example of how the strong interactions exerted by sea otters on their invertebrate prey can have cascading effects, leading to unexpected but profound changes at the base of the food web,” said Tim Tinker, a member of the research team at Santa Cruz.

“It’s also a great reminder that the apex predators that have largely disappeared from so many ecosystems may play vitally important functions,” Dr Tinker said.

Other studies, for instance, have shown that the re-introduction of wolves into Yellowstone Park in Wyoming has led to the re-growth of young saplings resulting from decreased browsing by elk, the main prey of the wolf.