Coral reefs and mangroves are worth protecting for economic reasons, contributing as much as $1m per sq km to tropical economies.
That is the conclusion of a new United Nations report which calculates the value of reefs and mangroves to fishing, tourism and coast protection.
Intact mangroves protected areas of coastline during the 2004 Asian tsunami, reducing the death toll.
Reefs are under threat from pollution, overfishing, and ocean acidification.
"Day in and day out, and across the oceans and seas of the world, nature is working to generate incomes and livelihoods for millions if not billions of people," said Klaus Toepfer, executive director of the United Nations Environment Programme (Unep).
"I hope the financial facts contained in this study will radically change the attitude and behaviour of governments, industry, local authorities and individuals so that they better prize and conserve these natural assets."
Pollution, climate change and insensitive development are not only damaging ecosystems but undermining the economic basis for coastal communities worldwide, Unep says.
The report concludes that the value of reefs and mangroves varies between different regions but is generally high.
Overall, it finds, reefs are worth between $100,000 and $600,000 per sq km per year, rising to more than $1m in parts of southeast Asia where reef-based fisheries alone generate incomes of £2.5bn annually.
Coral prevents pounding surf from eroding valuable coastline
Mangroves can be even more valuable, it says, with their worth to Thailand estimated at $3.5m per sq km.
Coral and mangrove ecosystems act as fish nurseries and prevent waves from washing sand off beaches.
In some tropical countries including Indonesia reefs are quarried for building material, resulting in the need for ugly concrete breakwaters to protect the remaining beach.
Unep cites studies from Sri Lanka concluding that one sq km of reef can prevent 2,000 cubic metres of coastal material from being washed away each year.