Even some common animals are decreasing in number

Dozens of mammals on Earth have lost a third of their geographic range since the start of the Industrial Revolution, while thousands of species have seen their populations and ranges decline as well, according to a new study.

The takeaway: Even some common species, despite relatively large numbers, are at risk as their habitats shrink. Those local declines in the population can signal eventual extinction long before an animal is considered endangered — and it may be too late.

"The Earth is not only experiencing accelerated human-driven species extinctions, but also population declines," the researchers wrote.

Why it matters: The research is the first of its kind to comprehensively look at how changes in the environment have begun to sharply limit the range of thousands of species. Past studies have focused on species extinction as a measure of human-driven changes in the environment, but those changes also have an impact on the ranges of species that has been understudied.

The big debate: whether we are beginning a sixth mass extinction on Earth. The study's researchers warn we are but others disagree.

The researchers mapped the ranges of 27,600 birds, amphibians, mammals and reptiles — half of all known vertebrate species on Earth —on a 6,000-mile scale and analyzed the geographic ranges of 177 mammals between the years 1900 and 2015.

What they found: The mammals studied have lost 30% of their range in the past century. A significant number of mammals (more than 40%) lost 80% of their range. The tropics had the greatest number of declining species.