Almost 80% of the Earth's surface has experienced a sharp fall in the number of large mammals as a result of human activities, a study suggests.
American bison are one of the species most affected by humans
By examining records dating back to AD1500, US researchers found that at least 35% of mammals over 20kg had seen their range cut by more than half.
They said urgent action was needed to protect the animals, which were being hunted or suffering habitat loss.
The findings have been published in the Journal of Mammalogy.
The research, carried out by a team of scientists from Princeton University and conservation group WWF-US, has been described as the first "measurement of human impacts on biodiversity based on the absence of native, large mammals".
"Perhaps the most striking result of our study is that [the] 109 places that still retain the same roster of large mammals as in AD1500 are either small, intensively managed reserved or places of extremes," revealed lead author John Morrison, WWF-US's director of conservation measures.
"Remote areas are either too hot, dry, wet, frozen [or] swampy to support intensive activities."
The researchers compared the current ranges of the world's largest 263 land mammals with their distribution 500 years ago.
The species that suffered the greatest loss were "habitat generalists", including tigers, leopards, lions, American bison, elk and wolves.
Geographically, Australasia fared best, holding on to 68% of its large mammals. At the other end of the scale, South-East Asia only had 1% of the mega fauna that roamed the region in AD1500.
In their paper, the scientists explained why large mammals were so important for maintaining the ecological equilibrium.
"Large carnivores frequently shape the number, distribution and behaviour of their prey," the researchers wrote.
"Large herbivores function as ecological engineers by changing the structure and species composition of surrounding vegetation.
"Furthermore, both sets of mammals profoundly influence the environment beyond direct species interactions, such as through [the food chain]."
WWF chief scientist Eric Dinerstein said he hoped the findings would help focus conservation efforts.
"We can now pinpoint places where large mammal assemblages still play important roles in terrestrial ecosystems," he explained.
"Through strategic re-introductions - such as returning wolves to Yellowstone - we can restore... places missing one or two species and recover the ecological fabric of these important conservation landscapes."