NEW YORK — Rising world temperatures are disrupting ecosystems around the world, threatening fisheries and water supplies on which millions of people depend, says a report released Wednesday.

Global temperature increases may also be undoing years of conservation work, as shifts in climate make nature reserves unsuitable for preservation of the species they were designed to protect, said the report, prepared by the World Wildlife Fund.

Coral reefs, mangroves — which serve as spawning grounds for fish — and mountain forests, or cloud forests — which serve as watersheds in many parts of the world — are among the ecosystems most threatened by global warming, the fund said.

In some cases, it is already too late to prevent the loss of species and the disruption of important ecosystems, said Adam Markham of World Wildlife Fund-International in Switzerland.

“There’s no doubt that we will see species loss, probably in the coastal zone and in mountain regions,” Markham said.

The report was released to coincide with the opening of a United Nations negotiating session aimed at drafting an agreement to limit the emission of so-called greenhouse gases. Computer projections suggest that these gases, which trap the sun’s heat in the atmosphere, could lead to a global average temperature rise of 3 degrees by the end of the next century.

Negotiators are hoping to have an agreement ready for signing in June at the United Nations Conference on Environment and Development in Brazil.

The U.S. delegation has refused to agree to binding limitations on the release of greenhouse gases.

The most important of the gases is carbon dioxide, released by the burning of fossil fuels and the destruction of forests. Others include methane, nitrous oxide and chlorofluorocarbons, or CFCs, which also damage the Earth’s protective ozone layer.

The world’s temperature has risen about one-half degree Celsius (about one degree Fahrenheit) during the past century, but scientists aren’t sure whether that is because of the release of greenhouse gases or natural climate variations.

If temperatures rise three degrees during the next century, 55 percent to 80 percent of the world’s most important nature reserves would suddenly be shifted into different climate zones, threatening or destroying the species they were intended to protect, the World Wildlife Fund report said.

Robert Peters, director of the climate change program at Conservation International in Washington, D.C., applauded the report for drawing attention to the effect of climate change on natural areas. Many previous reports have focused on the consequences for agriculture or for coastal cities, for example, which would be threatened by rising sea levels, he said.

“It is high time that conservationists in general and the wider public — including policy makers — put this issue of how global climate change will affect natural ecosystems right at the top of the list,” Peters said by telephone. “I think it means profound disruption for human society.”