Mastadons in the Pleistocene Megafauna

Mastadons in the Pleistocene Megafauna

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End of the world predicted by scientists who warn Earth's animals are heading for mass extinction

Jul 25, 2014 18:12 By John Kelly

After 3.5 billion years of trial and error, the world as we know it has reached a tipping point

GettyWipe out: Scientists are warning that another global mass extinction may be imminent

The Earth could be heading for extinction ... and it is all man's fault.

Respected scientists from around the world are warning we may have reached a 'tipping point' after 3.5 billion years of evolutionary trial and error.

And the problem lies with the loss of major species such as elephants which has a knock-on effect on the world we live in.

In a new review of scientific literature and analysis of data published in Science, an international team of scientists warn the loss of such animals could trigger the planet's sixth mass biological extinction event.

Previous extinctions - including the complete disappearance of dinosaurs - are believed to have been caused by natural planetary transformations or catastrophic asteroid strikes.

But the current one, says lead author Rodolfo Dirzo, a professor of biology at Stanford, is associated with human activity.

The statistics are frightening.

Since 1500, more than 320 terrestrial vertebrates- animals with backbones - have become extinct.

Populations of the remaining species show a 25 percent average decline in abundance.

The situation is similarly dire for invertebrate animal life such as insects and worms.

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 and their size and meat mass make them easier and more attractive hunting targets for humans.

The scientists say we are at risk because of what they call the 'trickle-down' effect.

Experiments conducted in Kenya have isolated patches of land from megafauna such as zebras, giraffes and elephants, and observed how the ecosystem reacts to the removal of those species.

They noted the areas quickly 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 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 invertebrates.

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 added: "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.

"Ironically, we have long considered that defaunation is a cryptic phenomenon, but I think we will end up with a situation that is non-cryptic because of the increasingly obvious consequences to the planet and to human wellbeing."