Most experts say the goal is out of reach.

WHAT ARE THE PROBLEMS?

* Rising human populations, pollution and climate change are threatening to cause the worst spate of extinctions since the dinosaurs were wiped out 65 million years ago, according to UN studies. About three species an hour may be going extinct.

* No one knows how many species there are. Between 1.7 and 2.0 million species have been identified, ranging from bacteria to blue whales, and the final total could be somewhere between 5 million and 30 million, according to the 2005 Millennium Ecosystem Assessment by 1,300 scientists.

WHY CARE ABOUT BIODIVERSITY?

* The diversity of agricultural crops is vital for food supplies. People rely on free services provided by nature such as insect pollinators or water purification by forests or wetlands.

* Many rare plants or creatures have unexpected value. The southern gastric brooding frog in Australia, found in the 1980s but now extinct, used to incubate its young in its stomach in a technique that could have held clues to preventing human ulcers. Hoodia, a plant in southern Africa, can act as an appetite suppressant that could help curb obesity.

* Many governments see an ethical responsibility to safeguard life on earth. Humans have done more to disrupt life than any species in history.

WHAT CAN THE UN TALKS DO?

* More protected areas: 12 percent of the world's land area is set aside for wildlife but only about 0.5 percent of the oceans -- with almost none outside national territorial waters. Developing countries say they need cash to set up protected areas.

* Fairer sharing of benefits of biodiversity: the convention has agreed to negotiate rules by 2010 for ensuring that local people get benefits from biodiversity, for instance medicines developed from plants. In some parts of the world, pharmaceutical companies have been accused of "biopiracy".

* No biofuels subsidies: the secretariat of the convention will issue a report advising against subsidies, import tariffs or other mechanisms to promote biofuels. Such fuels can threaten biodiversity, for instance if forests are cleared or wetlands drained to grow crops for fuel.

* Stop invasive species. The conference will try to prevent arrivals of "alien species", such as zebra mussels from Europe that have damaged the North American Great Lakes. Measures can include better controls of air cargo or of ships' ballast tanks.

* Credits to help protect tropical forests. Under discussion as part of an assault on global warming -- trees soak up greenhouse gases as they grow -- more forests could help safeguard biodiversity.