LONDON (Reuters) - Checking humanity's unflagging expansion is the only way to save the world's ``biodiversity hot spots,'' a U.S. researcher said Wednesday.

Looking at the connection between population density and the environment in 25 key regions and three tropical wildernesses, Robert Engelman of Population Action International said a new approach was needed.

``If you go right for the worst-case scenario, it's very hard to imagine long-term conservation of biodiversity in any of these areas if human demands don't eventually reach a plateau,'' he told Reuters in an interview.

``It clearly signals to conservationists that they should be paying attention to population trends and they might even consider taking actions that might help slow the growth of the human population.''

Nearly a fifth of the world's 6 billion people live within the most species-rich and environmentally threatened areas, Dr. Richard Cinotta, Engelman and colleagues at the Washington-based research and advocacy group wrote in the latest issue of Nature.

Beyond being just a proliferation of animals and plants, biodiversity is vital for everything from sources of medicine and climate regulation to pollination and pest control.

Areas of rich biodiversity facing the greatest risk include the Western Ghats/Sri Lanka, the Philippines, the Caribbean, the tropical Andes and Madagascar. The majority of the 25 regions and tropical hot spots straddle the poorer parts of the world.

Education The Key

Engelman, Population Action International's vice president of research, said there were indications that population growth was slowing.

Equal rights to education was a key, he added.

``One of the side benefits of educating girls is they tend not to get pregnant so early in life and they tend not to have such large families,'' Engelman said.

``We're making the further connection there that this will rebound to the benefit of tigers and panda bears and plants and insects all over the world over the long term.''

The impetus, Engelman said, must come from cooperation among governments, nongovernmental organizations and international bodies such as the World Bank -- much of it through better coordination of existing programs.

``In addition to doing the work to conserve these species, we should all be thinking about the importance of policies that end up slowing population growth,'' he said. ``That's where the hope for the permanent sustainability of these ecosystems lies.''