The loss and decline of animals is contributing to what appears to be the early days of the planet's sixth mass biological extinction event, scientists warn.
Since 1500, more than 320 terrestrial vertebrates have become extinct. Populations of the remaining species show a 25 per cent average decline in abundance. The situation is similarly dire for invertebrate animal life, researchers said.
While previous extinctions have been driven by natural planetary transformations or catastrophic asteroid strikes, the current die-off can be associated to human activity, a situation that the lead author Rodolfo Dirzo, a professor of biology at Stanford University, designates an era of "Anthropocene defaunation."
Across vertebrates, 16 to 33 per cent of all species are estimated to be globally threatened or endangered.
Large animals - described as megafauna and including elephants, rhinoceroses, polar bears and countless other species worldwide - face the highest rate of decline, a trend that matches previous extinction events.
Larger animals tend to have lower population growth rates and produce fewer offspring. They need larger habitat areas to maintain viable populations.
Although these species represent a relatively low percentage of the animals at risk, their loss would have trickle-down effects that could shake the stability of other species and, in some cases, even human health.
For instance, previous experiments conducted in Kenya have isolated patches of land from megafauna such as zebras, giraffes and elephants, and observed how an ecosystem reacts to the removal of its largest species.
Rather quickly, these areas become overwhelmed with rodents. Grass and shrubs increase and the rate of soil compaction decreases. Seeds and shelter become more easily available, and the risk of predation drops.
Consequently, the number of rodents doubles - and so does the abundance of the disease-carrying ectoparasites that they harbour.
"Where human density is high, you get high rates of defaunation, high incidence of rodents, and thus high levels of pathogens, which increases the risks of disease transmission," said Dirzo, who is also a senior fellow at the Stanford Woods Institute for the Environment.
"Who would have thought that just defaunation would have all these dramatic consequences? But it can be a vicious circle," he said.
The scientists also detailed a troubling trend in invertebrate defaunation. Human population has doubled in the past 35 years; in the same period, the number of invertebrate animals - such as beetles, butterflies, spiders and worms - has decreased by 45 per cent.
As with larger animals, the loss is driven primarily by loss of habitat and global climate disruption, and could have trickle-up effects in our everyday lives.