The decline of the world's large herbivores, especially in Africa and parts of Asia, is raising the spectre of an "empty landscape" in some of the most diverse ecosystems on the planet, shows a new study.
Large numbers of animals such as rhinoceroses, zebras, camels, elephants and tapirs are diminishing or threatened with extinction in grasslands, savannahs, deserts and forests.
"I expected that habitat change would be the main factor causing the endangerment of large herbivores," said William Ripple, wildlife ecologist at the Oregon State University.
"But surprisingly, the results show that the two main factors in herbivore declines are hunting by humans and habitat change. They are twin threats," Ripple pointed out.
Ripple and his team conducted a comprehensive analysis of data on the world's largest herbivores (weighing more than 100 kg on average), including endangerment status, key threats and ecological consequences of population decline.
Ripple initiated the study after conducting a global analysis of large-carnivore decline, which goes hand-in-hand with the loss of their herbivore prey.
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"Our analysis shows that it goes well beyond forest landscapes to savannahs and grasslands and deserts. So we coin a new term, the empty landscape," Ripple pointed out.
As a group, terrestrial herbivores encompass about 4,000 known species and live in many types of ecosystems on every continent except Antarctica.
Competition from livestock production, which has tripled globally since 1980, has reduced herbivores' access to land, forage and water and raised disease transmission risks, they add.
"The market for medicinal uses can be very strong for some body parts, such as rhino horn. Horn sells for more by weight than gold, diamonds or cocaine," noted Ripple.
The causes of the decline of some large herbivores are difficult to remedy in a world with increasing human populations and consumption, said co-author Taal Levi, assistant professor at the Oregon State University.
"We hope this report increases appreciation for the importance of large herbivores in these ecosystems. We hope that policymakers take action to conserve these species," Ripple concluded.
The findings appeared in Science Advances.