Many populations of animals, such as rhinoceroses, zebras, camels, elephants, and tapirs, are diminishing or threatened with extinction in grasslands, savannahs, deserts and forests, scientists say.

Led by William Ripple of Oregon State University, an international team of 15 scientists including Professor David Macdonald and Dr Chris Sandom of Oxford University's Wildlife Conservation Research Unit (WildCRU), conducted a comprehensive analysis of data on the world’s largest herbivores (of adult body mass over 100 kilograms on average), including endangerment status, key threats and ecological consequences of population decline. The researchers publish their observations in Science Advances.

The authors focused on 74 large herbivore species – animals that subsist on vegetation – and conclude that 'without radical intervention, large herbivores (and many smaller ones) will continue to disappear from numerous regions with enormous ecological, social, and economic costs.' Professor Macdonald, founding Director of Oxford University's WildCRU, is a world expert on carnivores and said: 'The big carnivores, like the charismatic big cats or wolves, face horrendous problems from direct persecution, over-hunting and habitat loss, but our new study adds another nail to their coffin – the empty larder – it's no use having habitat if there’s nothing left to eat in it.'

Ripple initiated the study after conducting a global analysis of large-carnivore decline, which goes hand-in-hand, he said, with the loss of their prey: 'I expected that habitat change would be the main factor causing the endangerment of large herbivores,' he said. 'But surprisingly, the results show that the two main factors in herbivore declines are hunting by humans and habitat change. They are twin threats.'

The scientists refer to an analysis of the decline of animals in tropical forests published in the journal BioScience in 1992. The author, Kent H Redford, then a post-doctoral researcher at the University of Florida, first used the term 'empty forest'. While soaring trees and other vegetation may exist, he wrote, the loss of forest fauna posed a long-term threat to those ecosystems.

The new study goes further, showing that the problem extends beyond forest landscapes, to savannahs and grasslands and deserts. Professor Macdonald said: 'it was bad enough to talk about empty forests – now we’re facing empty landscapes.'

The highest numbers of threatened large herbivores live in developing countries, especially Southeast Asia, India and Africa, the scientists report. Only one endangered large herbivore lives in Europe (the European bison), and none are in North America. This, Professor Macdonald concludes 'is not because we in the more developed countries have anything to be proud of – rather it’s because we exterminated our large mammals in an earlier era.'

The authors note that 25 of the largest wild herbivores now occupy an average of only 19 percent of their historical ranges. Competition from livestock production, which has tripled globally since 1980, has reduced herbivore access to land, forage and water and raised disease transmission risks, they add.

Meanwhile, herbivore hunting occurs for two major purposes, the authors note: meat consumption and the global trade in animal parts. An estimated one billion humans subsist on wild meat, they write. 'The market for medicinal uses can be very strong for some body parts, such as rhino horn,’ said Ripple. 'Horn sells for more by weight than gold, diamonds or cocaine.’ WildCRU’s team also specialises in illegal wildlife trade and Professor Macdonald said: 'Respect for local customs is an important courtesy, but one that does not extend to tolerating the pointless extinction of wild species that are beyond financial value.'

'Almost everything in nature is connected' said Professor Macdonald, 'so eliminating large herbivores is to pull the rug out from beneath whole ecosystems.' Their loss is likely to reduce food for large carnivores such as lions and tigers; diminish seed dispersal for plants; lead to more frequent and intense wildfires; slower cycling of nutrients from vegetation to the soil; reductions in habitat for smaller animals including fish, birds and amphibians.

The consequences of large wild herbivore decline include:

Loss of habitat diversity: Large herbivores trample and consume vegetation and increase habitat diversity for plants and animals; for example, elephants maintain forest clearings whilst bison maintain and expand grasslands.

Loss of food for predators and scavengers: Large predators such as lions, leopards, and spotted hyena depend on large herbivores for food. Large herbivores also open up dense vegetation making it easier for predators to catch smaller herbivores. In Yellowstone National Park, for instance, scavengers such as coyotes, foxes, and eagles often rely on carcasses of large herbivores killed by gray wolves.

Impact on seed dispersal: Large herbivores are irreplaceable as seed dispersers as they are able to consume larger seeds, and a large volume of seeds, and disperse them in their dung over longer distances. For example, in the Congo forest elephants disperse around 345 large seeds per day from 96 species over 1km from the parent trees.

Impact on humans: An estimated one billion people rely on wild meat for subsistence. If current trends continue wild meat in African forests is expected to decline more than 80% in the next 50 years. The loss of flagship wild herbivores will also have a negative impact on tourism, a vital source of revenue in many poorer countries.

'We hope this report increases appreciation for the importance of large herbivores in these ecosystems,' said Ripple. 'And we hope that policymakers take action to conserve these species.'

To understand the consequences of large herbivore decline, the authors call for a coordinated research effort focusing on threatened species in developing countries. In addition, solutions to the decline of large herbivores need to involve local people. 'It is essential that local people be involved in and benefit from the management of protected areas,' they write. 'Local community participation in the management of protected areas is highly correlated with protected area policy compliance.'