A new study argues that just 2.8% of Earth's terrestrial area is "ecologically intact," about 10 times lower than previous estimates, which scientists use to help prioritize areas for conservation.

In the study, published April 15 in Frontiers in Forests and Global Change, researchers developed a new definition of ecological intactness that includes the presence and abundance of animal species in a region, in addition to estimates of human modification in an area, the main focus of previous studies on the topic. The study also suggests that up to 20% of land could be restored to intactness by reintroducing up to five species to these areas.

Intact areas are important reservoirs of biodiversity, so identifying these places for conservation is key for avoiding species losses, according to lead author Andy Plumptre, the head of the Key Biodiversity Areas Secretariat, a U.K.-based partnership of 13 conservation organizations. 

There's no clear definition of intactness, but other studies have used remote sensing such as satellite data to suggest that 20% to 40% of Earth's land area is intact, defined by low human footprint in terms of agriculture, population, settlements, roads and light pollution. But supposedly intact areas may not have complete animal communities, Plumptre said. 

"We know that there's hunting that goes on under canopies; forest canopy can look intact, but you've actually lost a lot of the big species, which play important ecological roles in the functioning of that ecosystem," he explained.

One example is forest elephants in the Congo Basin. 

These animals "have a big impact on opening up the understory by browsing on young saplings and dispersing the seeds of many of the big fruiting trees that wouldn't be dispersed otherwise," Plumptre said. "Where elephants are lost, you see a reduction in the fruiting and reproduction of those trees. They change the actual composition of the forest."

To address this disconnect, Plumptre and his colleagues' new measure of ecological intactness includes three measures: habitat, faunal and functional intactness. 

An area was considered to have habitat intactness if it was at least 10,000 square kilometers and had a human footprint, measured on a scale of 0 to 50, of four or less. The team defined faunal intactness as having no local extinction of species since 1500 A.D., a baseline used by the International Union for Conservation of Nature's Red List of Threatened Species. To measure local extinctions, the team used data for all animal species recorded in the Red List, plus additional data on mammal species from a previous study. 

To calculate intactness on a global scale, the researchers first developed a map of habitat intactness. In agreement with other studies, they found that 28.5% of land met their criteria of low human impact. But when they overlaid a second map of faunal intactness, the amount of land area that qualified as intact dropped to just 2.9%. 

Because a species could be present in an area but at such low numbers that it cannot carry out ecological roles such as seed dispersal or predation, the researchers also estimated functional intactness in terms of animal density for 16 large mammal species that are often lost from areas due to human activities. When they added this third map to the analysis, the estimate of intact land area fell to 2.8%.

According to Plumptre, these findings are useful for scouting potential places that qualify for conservation as Key Biodiversity Areas under his program, which protects "sites of importance for the global persistence of biodiversity or globally important sites for species and ecosystems." One of the 11 criteria for selecting such sites is "ecological integrity," which stipulates that they have intact ecological communities and recommends that they be larger than 10,000 square kilometers. 

The researchers say the study is a starting point to identify potential Key Biodiversity Areas based on the ecological integrity criterion, but note that much closer analysis of these areas will be required to evaluate truly intact areas. 

"The data is based on range data, which are drawn relatively crudely. So within those ranges, there'll be places that the species occurs and the species doesn't occur," Plumptre said. "As you look at more fine-grained resolution, you'll get better measures of the impacts of people and better measures of the distribution of species. My guess is that you're actually going to get reduced estimates of ecological intactness, because you'll discover more human impacts … You'll find more roads, tracks and footpaths that aren't detected from satellites. So if anything, the true number could be even less than 2.8%."

Climate change could further alter ecosystems and mammal populations in ways not examined in the study. For example, recent research found that trees whose seeds are dispersed by forest elephants produce 80% less fruit than three decades ago, which the authors say is likely due to climate change. With elephants also declining in these regions, "it's a double whammy on their ability to reproduce," Plumptre said.

The researchers did identify one silver lining in their analysis. They suggest that places with intact habitat that have lost just a few animal species could be restored to ecological intactness.

"What we showed was that if you reintroduce up to five species, you could potentially regain faunal intactness in at least up to 20% of land around the world," he said. "That's partly because there's quite a lot of intact habitat left, so thinking about how to best manage that intact habitat should be a focus."

Although bringing back species to an area can improve the health of an ecosystem, such reintroductions, of beavers or wolves, for example, can also stir up controversy. There are also challenges with reintroducing animals to the wild if they have been reared in captivity, Plumptre said, "But certainly I think it's worth investigating."

2021 is the first year of the United Nations' "Decade on Ecosystem Restoration," an effort to encourage governments to repair habitats for the conservation of wildlife and human health. 

But according to Plumptre, "Everyone's talking about restoring degraded habitats, and not thinking about restoring ecological integrity in existing intact habitat," something that he said should also be a focus of restoration efforts. 

"I think it's important to have some things that we can see that remind us how the world used to be," Plumptre said. "One of the things that people have published about is that each generation, as we grow up, what we see, we tend to accept as natural. And so there's acceptance of that decline in nature around us because that's what we've grown up with, we don't actually realize what we've lost. What these places give us is a sense of what the world used to be like." 

The study, "Where might we find ecologically intact communities?," published April 15 in Frontiers in Forests and Global Change, was authored by Andrew J. Plumptre, KBA Secretariat and Cambridge University; Daniele Baisero, KBA Secretariat; Travis Belote, The Wilderness Society; Ella Vázquez-Domínguez and Carlos Luna-Aranguré, Universidad Nacional Autónoma de México; Soren Faurby, University of Gothenburg; Wlodzimiuerz Jedrzejewske, Instituto Venezolano de Investigaciones Científicas; Henry Kiara, International Livestock Research Institute; Hjalmar Kuehl, Max Planck Institute for Evolutionary Anthropology and German Centre for Integrative Biodiversity Research; Ana Benítez-López, Estación Biológica de Doñana; Maria Voigt, University of Kent; Serge Wich, Liverpool John Moores University and University of Amsterdam; William Wint, Oxford University; Juan Gallego-Zamorano, Radboud University Nijmegen; and Charlotte Boyd, International Union for Conservation of Nature (United States).