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Early days of Earth’s sixth mass biological extinction event


An international team of scientists warns that loss and decline of animals is contributing to what appears to be the early days of the planet's sixth mass biological extinction event. The planet's current biodiversity, they claim, is the highest in history of life, but it may be reaching a tipping point.

Since 1500, more than 320 terrestrial vertebrates have become extinct and populations of the remaining species show a 25 percent average decline in abundance. The situation is similarly dire for invertebrate animal life.

And 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 of a new report published in Science – Rodolfo Dirzo, a professor of biology at Stanford, designates an era of "Anthropocene defaunation."

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.

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.

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.

"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. "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."

Souce: University of Stanford


"Defaunation in the Anthropocene" – Rodolfo Dirzo, Hillary S. Young, Mauro Galetti, Gerardo Ceballos, Nick J.B. Isaac, Ben Collen – Science 25 July 2014: Vol. 345 no. 6195 pp. 401-406 – DOI:10.1126/science.1251817

Featured image: Earth – original by Beth Scupham (CC – Flickr). Edit: TW

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  1. The earth has saved itself many times in the past and life has continued. Animals, including humans will come and go from the planet over the eons and new species will take their place. With the impending magnetic pole shift, the earth will be bombarded with cosmic radiation which will have an effect on every living thing on the planet. This is par for the course. There are no true links in the fossil record of any species, new variations just appear.Therefore the only obvious explanation is that after every magnetic pole shift mutations start to appear and then it becomes the survival of the fittest, the fastest or the most intelligent!

  2. Humankind has not woven the web of live. we are but one thread within it .watever we do to the web we do to ourselves. all things are bound together,all things connect. Not sure who wrote this but it makes a lot of sense. i myself have land that i have left to regrow. And i live with nature not against it.

  3. I think that biggest problem we have, is the way we think of earth, or don´t think. We have created a bubble, where we are the masters of everything around us, and the nature is there for us to use, form and give us what we need. Humans may have a way of seeing into the future, but when it´s looking ugly down the road, we are good at sticking the head into the sand. "Someone else will fix it, it´s ok with change if it doesn´t cost me money,comfort or my job", and so on.
    And we have a problem when it comes to looking at whole picture. "I´m just driving to work and back, what harm could that do?" " Our company can´t survive if we would cut down on pollution" "I want to eat cheap meat" What I like to, is probably what the rest of the world like. So how do we get there. Beyond comfort, wallets, quartal reports and selfish polticians. When we get there, then I belive the saving of planet earth will go quickly. Question is, will we get there?

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