Grasslands like the South American Pampa have suffered some of the most severe species losses. | Andrés Soliño/ Flickr/ CC BY-NC-ND 2.0
The variety of plant and animal species across more than half of the Earth's surface has dropped below what some scientists consider the safe limit, a new study in the 15 July issue of Science reports.
Ecosystems around the world could suffer as more and more species disappear and biodiversity levels drop, scientists note. Healthy, functioning ecosystems provide a range of services that are critical to humans, such as crop pollination, waste decomposition, and regulation of the global carbon cycle. However, extensive land use by humans can wipe out or alter species' habitat, putting increasing numbers of plant and animal species in jeopardy.
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.
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. [View the full-size image] | 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."
The team found that some high biodiversity wilderness areas, such as Amazonia and the Congo Forests, which are assumed to be relatively intact, are approaching the proposed safe limit for biodiversity loss. "Preventing these areas from crossing the proposed boundary will require the preservation of remaining natural habitat," said Newbold.
Next, his team plans to study which types of species are lost most often following land-use change. Such insights are important, since some types of species contribute more than others to the functioning of ecosystems.