In Disney's live-action remake of "The Jungle Book," young human Mowgli is still palling around with bears and panthers. In reality, however, the world has changed since Rudyard Kipling's tales first hit shelves more than a century ago.

Speaking figuratively, biodiversity's bag of Skittles has not only gotten smaller, it now has fewer flavors.

Just how different are things? One expert puts it this way: If Mowgli were around today, he would most likely be raised by cows, goats and chickens instead of wolves and panthers and orangutans. If he were really unfortunate, his compatriots could be even worse.

"Maybe even rats and cockroaches, if things go badly," said Charles Barber, former forest chief at the U.S. Department of State's Bureau of Oceans and International Environmental and Scientific Affairs, in an interview with CNBC.

The problem, according to some scientific experts, is that humans have changed the world so dramatically that it has also altered the diversity of life on Earth. "Most of these changes represent a loss of biodiversity," analysts wrote in the Millennium Ecosystem Assessment in 2005, a report that chronicled the effects of human activity on nature produced by the United Nations and the World Resources Institute, where Barber now works.

Among the Millennium Assessment's findings were that humans have "changed ecosystems more rapidly and extensively than in any comparable period in human history," due to food, fresh water and fuel needs. The spillover from those changes has contributed to big gains in humanity's development, but "have been achieved at growing costs in the form of the degradation of many ecosystem services," researchers wrote at the time.

This means that "plants and animals are now sharing the planet with a whole lot of people," Barber said, adding that "we're dealing with a fantastically different world."