The authors of the research, published by the scientific publication "Frontiers", consider worrying that only 11 percent of the sites assessed in the study are in protected areas, and explain that many of the identified areas coincide with territories managed by indigenous communities, who play a key role in their maintenance.

The areas identified as functionally intact include Eastern Siberia and northern Canada for the boreal and tundra biomes, and part of the Amazon rainforests and the Congo basin and Sahara desert.

The authors of the study recall that for more than 30 years natural areas, which have not been considerably modified by man, have been identified as priorities in conservation and protection actions, something that is recognised by the United Nations Convention on Biological Diversity.

"We know that intact habitat is increasingly being lost and the importance of intact habitat has been demonstrated for both biodiversity and people," said Andrew Plumptre, from the University of Cambridge (UK), lead author of the study.

The research concluded that many of the habitats considered intact have species missing, either by human action or due to invasive species or diseases, he warned.

Although there is no common definition for habitat integrity the maps created so far estimated that between 20 and 40 percent of the Earth's surface remained free of major human disturbances (such as housing, roads or light or noise pollution).

With a different approach, assuming that an intact ecological community contains the species likely to occur in a given location, without loss, and with reference to an earlier time (year 1500), the authors also assessed the integrity of fauna and its density.

Despite the numbers, the authors say that up to 20 percent of the land surface could be restored in order to maintain the integrity of the fauna, through the reintroduction, and protection, of some species.