"If we keep degrading biodiversity there will be a point where it's very difficult to support agriculture"
* Human activity cuts diversity of animals and plants
* Biodiversity falls below "safe" limits-Science study
* Grasslands, savannahs, forests, woodlands at risk
By Alister Doyle
OSLO, July 14 (Reuters) - An expansion of farmland has damaged nature beyond a "safe" limit on 58 percent of the world's land surface, threatening natural services such as crop pollination by insects, scientists said on Thursday.
Grasslands, such as in United States, Argentina, South Africa or Central Asia, are among natural systems most affected by declines in animals and plants caused by human activities, they wrote in the journal Science.
Northern pine forests and tundra are least affected, they said.
Overall, the study said the diversity of animals and plants on 58 percent of the world's land area, home to 71 percent of all people, had fallen below a safe threshold, driven mainly by an expansion of farmland as well as by roads and bigger cities.
They defined "safe" as places where the local abundance of species was at least 90 percent of levels in comparable regions untouched by human activity. They based the conclusions on 2.38 million records for 39,123 species at 18,659 sites.
The declines raise risks for natural services such as pollination of food crops by insects, production of nutrients by soils, or the ability of forests to absorb carbon dioxide as a natural brake on climate change.
"If we keep degrading biodiversity there will be a point where it's very difficult to support agriculture," lead author Tim Newbold of University College London told Reuters.
Still, the study said there was uncertainty about the 90 percent threshold for damage - some other scientists believe nature can withstand bigger declines.
"Given the stakes it's best to be precautionary," said Newbold, who previously worked at the U.N. Environment Programme. Intact natural systems are most resilient to shocks such as droughts, floods, disease or global warming.
"Decision-makers worry a lot about economic recessions, but an ecological recession could have even worse consequences," Andy Purvis, another author of the study at the Natural History Museum, London, said in a statement.
"Until and unless we can bring biodiversity back up, we're playing ecological roulette," he said.
The report complements work by a separate group of scientists seeking to outline planetary boundaries, or safe limits for human prosperity in areas such as biodiversity, climate change, ocean acidification and freshwater use.
Sarah Cornell, who coordinates research into planetary boundaries at the Stockholm Resilience Centre, welcomed Thursday's findings as a step to pin down links between biodiversity and ecosystems.
Loss of biodiversity "means a much more fragile system," she said.
(Reporting by Alister Doyle; editing by Mark Heinrich)
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