Kavaan, a 29-year-old male elephant, is seen inside its enclosure at a zoo in Islamabad, Pakistan June 21, 2016. Reuters

The world's largest mammals could go extinct if humans don't do more to protect their ecosystems, according to an international team of conservation scientists. Animals including gorillas, rhinoceroses and bears could be wiped out if challenges such as expanding livestock and crop operations, illegal hunting, deforestation and human population growth aren't addressed, the scientists said Wednesday in the journal BioScience.

"The more I look at the trends facing the world's largest terrestrial mammals, the more concerned I am we could lose these animals just as science is discovering how important they are to ecosystems and to the services they provide to people," said William Ripple, distinguished professor of ecology in the College of Forestry at Oregon State University and lead author of the report entitled "Saving the World’s Terrestrial Megafauna." "It's time to really think about conserving them because declines in their numbers and habitats are happening quickly."

Animals are important not just because humans like to look at them or eat them. Large mammals can help prevent wildfires and spread plant seeds, among other benefits. To reach their conclusions, the 43 wildlife experts analyzed global trends facing wolves, lions, tigers, elephants, rhinos, zebras and other animals.

"Most mammalian megafauna face dramatic range contractions and population declines," the authors wrote. "In fact, 59 percent of the world's largest carnivores and 60 percent of the world's largest herbivores are classified as threatened with extinction on the International Union for the Conservation of Nature Red List. This situation is particularly dire in sub-Saharan Africa and Southeast Asia, home to the greatest diversity of extant megafauna."

The scientists called on world leaders to protect animals' habitat needs.

"We must not go quietly into this impoverished future," the authors wrote. "Rather, we believe it is our collective responsibility, as scientists who study megafauna, to act to prevent their decline. We therefore present a call to the broader international community to join together in conserving the remaining terrestrial megafauna."