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The end of the elephant, rhino and polar bear? Experts say sixth mass extinction on Earth has ALREADY started – and humans are to blame

The Earth’s six great mass extinction event has already started, scientists have claimed.

They say human behaviour has already killed off over 320 species – and many more are set to follow.

The planet’s current biodiversity, the product of 3.5 billion years of evolutionary trial and error, is the highest in the history of life.

But researchers now say it has reached a tipping point.

Large animals ¿ described as megafauna and including elephants, rhinoceroses, polar bears and countless other species worldwide ¿ face the highest rate of decline.

Large animals ¿ described as megafauna and including elephants, rhinoceroses, polar bears and countless other species worldwide ¿ face the highest rate of decline.

 

HOW THEY DID IT

Researchers led by Stanford carried out a new review of scientific literature and analysis of data published in Science.

The international team of scientists cautions that while previous extinctions have been driven by natural planetary transformations or catastrophic asteroid strikes, the current die-off can be associated to human activity.

In a new review of scientific literature and analysis of data published in Science, an international team of scientists cautions
that while previous extinctions have been driven by natural planetary transformations or catastrophic asteroid strikes, the current die-off can be associated to human activity.

Lead author Rodolfo Dirzo, a professor of biology at Stanford, designates an era of ‘Anthropocene defaunation.’

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

Populations of the remaining species show a 25 percent average decline in abundance, and the situation is similarly dire for invertebrate animal life.

Across vertebrates, 16 to 33 percent 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.

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.

THE FIVE GREAT EXTINCTION EVENTS

Five times, a vast majority of the world’s life has been snuffed out in what have been called mass extinctions, often associated with giant meteor strikes.

End-Ordovician mass extinction
The first of the traditional big five extinction events, around 440 million years ago, was probably the second most severe. Virtually all life was in the sea at the time and around 85% of these species vanished.

Late Devonian mass extinction
About 375-359 million years ago, major environmental changes caused a drawn-out extinction event that wiped out major fish groups and stopped new coral reefs forming for 100 million years.

End-Permian mass extinction (the Great Dying)
The largest extinction event and the one that affected the Earth’s ecology most profoundly took place 252 million years ago. As much as 97% of species that leave a fossil record disappeared forever.

End-Triassic mass extinction
Dinosaurs first appeared in the Early Triassic, but large amphibians and mammal-like reptiles were the dominant land animals. The rapid mass extinction that occurred 201 million years ago changed that.

End-Cretaceous mass extinction
An asteroid slammed down on Earth 66 million years ago, and is often blamed for ending the reign of the dinosaurs.

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 harbor.

‘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.’

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 percent.

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 percent.

 

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 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.

For instance, insects pollinate roughly 75 percent of the world’s food crops, an estimated 10 percent of the economic value of the world’s food supply. Insects also play a critical role in nutrient cycling and decomposing organic materials, which helps ensure ecosystem productivity.

In the United States alone, the value of pest control by native predators is estimated at $4.5 billion annually.

Dirzo said that the solutions are complicated.

Immediately reducing rates of habitat change and overexploitation would help, but these approaches need to be tailored to individual regions and situations.

He said he hopes that raising awareness of the ongoing mass extinction – and not just of large, charismatic species – and its associated consequences will help spur change.

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