By Alex Kirby BBC News Online environment correspondent

Natural disasters often wreak unnatural levels of havoc, a US-based conservation group says.

We have altered so many natural systems so dramatically that their ability to protect us from disturbances is greatly diminished

Janet Abramovitz, Worldwatch

This is because human activities frequently intensify the damage caused by nature, the group claims.

It says the huge growth in human numbers and in buildings, roads and other development also mean many more people are in danger. Yet preparing for disaster is far cheaper than the cost of recovery afterwards.

The group, the Worldwatch Institute, based in Washington DC, makes the claims in its report Unnatural Disasters.

Worse than war

In the 1990s, it says, natural events like hurricanes, floods and earthquakes affected more than two billion people and caused more than $608bn (�425bn) in economic losses.

Hurricanes cause immense damage

More people are displaced by natural disasters than by war, the report says.

But more and more of the resulting devastation is the consequence of ecologically destructive practices and an increasing number of people living in harm's way.

The report's author, Janet Abramovitz of Worldwatch, said: "By degrading forests, engineering rivers, filling in wetlands and destabilising the climate, we are unravelling the strands of a complex ecological safety net.

"We have altered so many natural systems so dramatically that their ability to protect us from disturbances is greatly diminished. Dunes, barrier islands, mangrove forests and coastal wetlands are natural 'shock absorbers' that protect against coastal storms.

"Forests, floodplains and wetlands are 'sponges' that absorb floodwaters."

Disproportionate impact

The report says "the enormous expansion of the human population and the built environment" is also worsening the problem.

Praying for cyclones to stop

One person in three now lives within 100 kilometres (62 miles) of a coastline, and 13 of the world's 19 mega-cities (those with more than 10 million people) are in coastal zones.

The report says the entire Mediterranean coast is vulnerable to rising sea levels, as are the Atlantic and Gulf coasts of the US.

Worldwatch says the impact of "unnatural disasters" falls disproportionately on poor people, who have fewer resources for coping and who are likelier to be living in vulnerable areas.

In 1998, Hurricane Mitch caused damage to central America totalling $8.5bn (�590m) - more than the combined gross domestic product of the two countries worst affected, Honduras and Nicaragua.

Erosion toll

The report says that preparing for disasters, or acting to reduce their impact, is usually far cheaper than clearing up after them: "On average, $1 invested in mitigation can save $7 in disaster recovery costs."

It gives examples of countries that are acting to lessen the impact of disasters:

• China, which recognises that forests are worth 10 times more for controlling floods and supplying water than as timber, and has halted logging in the Yangtze river watershed;
• Vietnam, which has restored 2,000 hectares of mangroves to provide a buffer against coastal storms;
• Bangladesh, whose emergency warning scheme is believed to have saved 30,000 lives in the 1991 cyclone.