Study Reveals Earth Heading Towards Sixth Mass Extinction

First Posted: Jul 25, 2014 06:16 AM EDT

The rapid decline of animals is contributing to what appears as the early days of the planet's sixth mass extinction event, a new study reveals.

The current biodiversity of the planet is the highest in the history of life. But, it may be reaching a tipping point. Over 320 terrestrial vertebrates have gone extinct since 1500. The existing species show an average decline of 25 percent.

Sadly, the cause of the current die-off is human activity and not natural planetary transformation or catastrophic asteroid strike that caused mass extinction earlier.  Nearly 16-33 percent of the vertebrate species are assumed to be globally endangered.

The highest rate of decline is experienced by megafauna including elephants, rhinoceroses, polar bears and countless other species worldwide. According to the researchers, the rate at which these species are declining matches the 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. Their size and meat mass make them easier and more attractive hunting targets for humans. 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," researchers explain.

Experiments conducted earlier focused on how the ecosystem reacts to the removal of its largest species. The experiments conducted in isolated areas of land from megafauna like zebras, giraffes and elephants revealed that the regions will be overwhelmed with rodents. As the number of rodents double, the rate of disease-carrying ectoparasites also increase.

"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."

In the past 35 years, there was a two-fold increase in human population and simultaneously there was a 45 percent drop in invertebrate animals. Among larger animals, it is loss of habitat and disruption in global climate that causes a decline.

Although the solution to this crisis is complicated, the researchers believe that reducing rates of habitat change and overexploitation may help lower the rate of decline.

"We tend to think about extinction as loss of a species from the face of Earth, and that's very important, but there's a loss of critical ecosystem functioning in which animals play a central role that we need to pay attention to as well," Dirzo said. 

The finding was documented in the journal Science.

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