Biodiversity has dropped below the "safe limit" across 58 per cent of the Earth's surface, according to the most comprehensive analysis of global data to date.

Key points: Planet's "safe limits" exceeded if less than 90 per cent abundance of original species or 80 per cent of those species is retained

Planet's "safe limits" exceeded if less than 90 per cent abundance of original species or 80 per cent of those species is retained Analysis found average local abundance of species across the world was 84.6 per cent

Analysis found average local abundance of species across the world was 84.6 per cent Other experts say study is a warning but more work needed to clarify

The international study, published today in the journal Science, suggests that the degree of lost biodiversity across more than half the world's surface is substantial enough to question the ability of many ecosystems to support human societies.

"We've found that across most of the world, biodiversity loss is no longer within the safe limit suggested by ecologists," said lead researcher Dr Tim Newbold of University College London.

"We know biodiversity loss affects ecosystem function but how it does this is not entirely clear.

"What we do know is that in many parts of the world we are approaching a situation where human intervention might be needed to sustain ecosystem function."

Other environmental experts said the research was impressive in its scope. But they said it is too soon to say we are on the verge of an ecological disaster and more work is needed at a local level.

Previous work has proposed a threshold whereby continued loss of biodiversity may impair an ecosystem's ability to function.

The "safe limit" in this framework is deemed to be crossed once the abundance of original species in a habitat drops below 90 per cent of what it was before human land use.

"The main implication of crossing the safe limit of biodiversity intactness is that the ability of biodiversity to support important ecosystem functions — things like food production, nutrient cycling and pollination — becomes uncertain," said Dr Newbold.

To assess the extent to which these limits had been crossed, Dr Newbold and a team of international researchers analysed a database of some 2.3 million records covering more than 39,100 species living in 18,600 sites.

They then applied land use data, modelled by researchers at CSIRO, to see how biodiversity in every square kilometre of the globe has changed due to human modification of the habitat.

Total abundance of species occurring in primary vegetation (areas above safe limit in blue) ( Newbold et al/Science )

Their global map (above) reveals that the average abundance of original species in much of the world has fallen to 84.6 per cent.

The analysis suggested at least seven of the 14 terrestrial ecosystem types — or biomes — have crossed the suggested safe limit for biodiversity.

Grasslands, savannahs and shrublands were most affected by biodiversity loss.

"This is where a lot of the world's farmland is currently," explained Dr Newbold.

"But many tropical biomes have also surpassed safe limits, and there is likely to be more farming in these biomes in the future."

Tundra and boreal forests were the least affected.

According to the global map, much of Australia is below the safe limit.

"Generally the most change in biodiversity has occurred in places where there are more people, but this isn't always so," said Dr Newbold.

"In the case of Australia, the maps of the different land uses … suggest that a lot of Australia's area is used for grazing livestock.

"This probably reflects reality to some extent, but is perhaps an overestimate [as] the extent of grazing land has proven difficult to estimate by global models."

'Warning light' but more studies needed

Professor David Bowman of the University of Tasmania said the research is significant because it provided a global estimate and it harmonised with what conservation biologists were seeing in ecosystems.

The study was a very well done statistical analysis, he said, but it is indicative rather than predictive.

"The best available data in terms of mapping the biome and species abundance and richness is woefully inadequate given the magnitude of the extinction crisis," Professor Bowman said.

"[The study] is a warning light saying there's something pretty dramatic happening down there on planet Earth but more global studies, better data are needed to make sense of what's going on."

It is unclear, he added, how many species could be lost from an ecosystem before it becomes vulnerable.

"We don't know what is happening with our ecosystems concerning the novel species that humans are spreading around.

"We don't know if they'll be able to step up and do the work of the old species that are being deleted. This is a huge question with ecological and evolutionary science."

Maps need to be combined with actions

Professor Hugh Possingham, director of the ARC Centre of Excellence for Environmental Decisions at the University of Queensland, said the link between biodiversity and species abundance within ecosystems is still not well understood.

"I wouldn't like to think because we've crossed these safe operating boundaries it's a disaster" he said, adding that many ecosystems around the world had experienced significant losses of original species but had not collapsed.

Professor Possingham said it was important to quantify biodiversity loss, but the question is what to do about it.

"If you were a real pessimist you'd look at the red bits on the map, which is most of Australia, and say we're already stuffed."

"But I don't know how to use these maps unless we combine them with actions, because you can only prioritise actions, you can't prioritise places."

He said conservation decisions needed to be based on factors such as what actions could be taken to restore the abundance and richness of species; how much those actions were likely to cost; were they likely to succeed; and whether or not there is a net benefit.

"Until you knew all that you wouldn't know where these maps are telling you to act or not."