LONDON (Reuters) - Seagrass, a vital but largely overlooked component of the world's oceans, is being destroyed by ignorance and inaction, threatening millions of people and many species of marine animals, according to a new report published on Wednesday.

The vast sub-aquatic meadows that grow on shallow shelves around the continents are in their own way as important to coastal waters as trees are to the above ground environment, says the report from the United Nations Environment Program World Conservation Monitoring Center.

They purify the water, protect the soil and provide breeding and feeding grounds for many species, it added in a World Atlas of Sea grasses.

"The grasses are the basis for many marine food chains, are good refuges for young fish, stabilize the sandy soils in which they grow and filter the water," Ed Green, co-editor of the atlas, the first comprehensive study of the world's sea grasses, told Reuters.

Sea Turtles and Dugongs depend on the sub-sea meadows and many other species live and breed there, while thousands of subsistence fishermen and their families in Africa and Asia pluck their living from among the green fronds.

Furthermore, the livelihoods of millions more people who make their living from tourism are being put at risk from the destruction by leisure craft of the sea grass meadows whose dense rooting systems halt coastal erosion.

But sea grasses, a group of 60 flowering plants that live exclusively under water, are the poor cousins of marine conservation, coming a distant third behind coral reefs and kelp beds in the planning and execution of protected zones. "They are very fragile indeed. They are sensitive to eutrophication -- nutrient pollution from the land from farming, industry and housing -- which causes algae to bloom and effectively suffocates the sea grass beds," Green said. "They are also destroyed by land reclamation projects such as coastal housing developments and new airports."

The atlas estimates that the global area covered by sea grasses is just 177,000 sq km -- two-thirds the area of the United Kingdom -- and that it has shrunk 15 percent in the past 10 years.

"But that average hides huge variations. In some areas the loss is a lot less, but in others it is huge," Green said.

"In some of these places it is already too late. In others like Canada, Australia and the United Arab Emirates the situation is not too bad if action is taken now," he added.

The seas around Denmark have been virtually denuded of sea grass meadows over the past 40 years through pollution and land reclamation, the Mediterranean off the southern coast of France has likewise suffered badly as has the eastern seaboard of the United States. "Politicians have tended to react to public pressure -- and coral and kelp have held their attention to the detriment of sea grass," Green said. "We are not asking for them to be forgotten and attention to be focused only on sea grass. What we want is a holistic approach, seeing them all as linked."