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Earth on the Brink of Modern-Day Extinction Event: Study

Jul 25, 2014 10:17 AM EDT

Earth appears to be in the early stages of the planet's sixth mass biological extinction event, according to a new study, with a 25 percent decline in populations of what species are still left.

According to the data described in the journal Science, this time around a giant asteroid or natural catastrophic event can't be blamed for the decline of species. This modern-day extinction is driven mostly by human actions.

There was a time, during the Pleistocene epoch some tens of thousands of years ago, when mammoths, giant tortoises, sabre-toothed cats and giant ground sloths roamed the planet. But natural disasters and asteroid strikes largely wiped them out.

Since 1500, more than 320 terrestrial vertebrates have become extinct.

Lead author Rodolfo Dirzo, a professor of biology at Stanford, designates the current situation as an era of "Anthropocene defaunation."

Across vertebrates, 16 to 33 percent of all species are estimated to be globally threatened or endangered. Large animals like elephants, rhinoceroses, polar bears and countless other species worldwide face the highest rate of decline, a trend that matches previous extinction events.

Although these species represent a relatively low percentage of the animals at risk, their loss would have a trickle-down effect that impacts other species, including humans.

For instance, previous experiments conducted in Kenya have isolated patches of land from species like zebras, giraffes and elephants to see how an ecosystem reacts to the removal of its large animals.

Without these massive foragers around, 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 disease-carrying rodents doubles - a threat to human health.

"Who would have thought that just defaunation would have all these dramatic consequences? But it can be a vicious circle," Dirzo said in a statement.

Humans, though they may be impacted by a mass extinction, are actually the main contributors to such an event. Human population has doubled in the past 35 years. In the same time period, the number of invertebrates - like beetles, butterflies, spiders and worms - has dropped 45 percent.

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.

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