We’ve passed the threshold for biodiversity loss, even without accounting for invasive species.

Tim Newbold of University College London and his team conducted what may be one of the most comprehensive biodiversity analyses to date. They analyzed a database of more than 2.3 million records of more than 39,100 species living in 18,600 sites, representing a far more comprehensive dataset than those included in previous studies of the state of global biodiversity.

"We focused on species abundance because loss of abundance from ecological communities is likely to impair the functioning of ecosystems," explained Newbold.

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Measures of biodiversity intactness. (A) Total abundance of species occurring in primary vegetation; (B) Richness of species occurring in primary vegetation; (C) Total abundance of species not present in primary vegetation; (D) Richness of species not present in primary vegetation. [] | Newbold et al., Science (2016)

Using this dataset, they calculated the global Biodiversity Intactness Index (BII), an index used to describe changes in the number of species present, or "species abundance," in a given region. Generally, the safe limit is placed at a precautionary 10% reduction in BII, meaning that species abundance within a given habitat is 90% of its original value in the absence of human land use. (Some researchers say that reductions can safely be as much as 70%, however.)

Their BII map reveals that global biodiversity has fallen to 84.6%, which is below the threshold that the authors deem safe for maintaining healthy ecosystems. The impact of land-use pressures on biodiversity varies: Grasslands like those found near the Cape in South Africa are most affected, and tundra and boreal forests like those that cover the high northern latitudes of North America and Russia appear to be the least affected.

These results do not account for species that may have appeared in a given habitat over recent years. "Species not native to an area or not found in the natural habitat of an area will have no effect on the Biodiversity Intactness Index — they are intentionally excluded from the index," explains Newbold. "But how they might affect ecosystem functioning is uncertain. They might make some contribution, make no contribution, or they might even impair ecosystem function."