Earth in midst of sixth mass extinction?

Washington: 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 1,500, 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 worldwidem, 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.