By Helen Briggs and Victoria Gill

Science and environment correspondents

It's being called a last chance for nature - 100 countries backing calls to protect 30% of the planet.

The aim is to reach this goal by 2030 and conserve forests and other vital ecosystems in order to restore the natural world.

The "30x30" target is the key ambition of the?UN biodiversity summit, COP 15

But as the talks in Montreal, Canada, move into their final days, there is division over this and many other targets.

Biodiversity refers to all living things, from polar bears to plankton, and the way they fit together to sustain life on Earth.

• 'Magical marine species' pushed toward extinction

• Reverse nature's decline or there is no future - UN

• COP15: Call for biodiversity 'peace pact with nature'

What's wrong and how do we fix it?

Scientists have warned that with forests and grasslands being lost at unprecedented rates and oceans under pressure from pollution and over-fishing,?humans are pushing the Earth beyond safe limits.?This includes increasing the risk of diseases, like SARs CoV-2, Ebola and HIV, spilling over from wild animals into human populations.

Under the proposed agreement, countries would sign up to targets to expand protected areas, such as nature reserves. It draws inspiration from the so-called "father of biodiversity", the biologist Edward O Wilson, who called for half of Earth to be protected.

But there is debate over how much land and sea to include, and some scientists fear the targets may be diluted.