In around 300 years time, 75% of all mammal species will have disappeared from this planet. That's the startling prediction from Anthony Barnosky, a palaeobiologist at the University of California, Berkeley – that’s if the current rates of extinction continue and the animals already threatened or endangered are wiped out this century.
Barnosky studies biodiversity changes and extinction rates that occurred in the deep past, and compares them to trends happening now. Since life first evolved billions of years ago and flourished, diversified, and made our planet what it is, there have been five mass extinctions. Each was triggered by a cataclysmic event and resulted in at least 75% of all species going extinct. The last of these events occurred 65 million years ago, when a meteorite slammed into Earth, throwing up persistent clouds of debris that darkened the sky for years. The resulting change in climate led to a mass extinction that included the dinosaurs.
Barnosky calculates that humans are now creating a mass extinction on the same scale – the planet's sixth one – through a combination of habitat encroachment and fragmentation, hunting, climate change, pollution, and the spread of disease and introduced species. As many as 30% of all species may be lost over the next four decades, conservationists estimate.
Extinction is actually a natural and common phenomenon – of the roughly 4 billion species estimated to have evolved on Earth, some 99% are gone. In the past, the extinction rate has been balanced by the evolution of new species, but the current, human-caused extinction is happening so fast that evolution cannot keep pace. Barnosky estimates that the current rate is 1,000 times the natural rate, putting it easily on a par with the so-called “big 5” mass extinction events.
The Anthropocene, or Age of Man, will be marked by a rapid decline in biodiversity as animals and plants disappear from the planet forever. It won't just be the individual creatures that vanish, but also their descendants on the evolutionary tree – whole lines of phyla will prematurely cease.
And the Anthropocene will also be notable for its homogeneity – what Barnosky describes as the "McDonaldization of nature". Many animals and plants have evolved to occupy specific geographical niches, such as islands or mountain lakes. As a result, it’s possible to find some endemic species that exist nowhere else on Earth, such as the giant tortoises of the Galapagos, the lemurs of Madagascar or the koalas of Australia.
Occasionally, during Earth’s history, shifting tectonic plates have forced land masses together, enabling the separated biodiversity to mix for the first time. This happened when the North and South American continents collided into each other, around 3 million years ago, for example. In the invasions that followed, South America got its first large carnivore – the jaguar from North America – which proceeded to eat much of the native fauna, resulting in the loss of many species.
Humans have been orchestrating tectonic-scale species migrations of their own, either deliberately or accidentally, which has left its mark on the living planet. Some species including rats, goats, rhododendron, wheat and eucalyptus are found around the world while many others have become rare or vanished. Many introduced species are invasive – or “weeds” – which out-compete the natives for food, light and habitat, or like the jaguar simply consume them to extinction.
We've also been spreading pests and diseases from place to place, often causing local extinctions. Isolated human populations have even been wiped out in this way, when we've introduced diseases such as flu, smallpox, HIV or malaria to places where the local people haven't developed adequate immunity.
Meanwhile, we've been artificially boosting the populations of certain select species, such as cows, dogs, rice, maize and chickens – most of which have been bred to create new varieties radically different from their wild ancestors. The combined weight of humans and the animals we've domesticated now outweighs all the wild back-boned creatures on the planet’s surface by a ratio of 95 to 5, Barnosky says. Ten thousand years ago, at the beginning of the Holocene geological epoch, the weight of humans and domesticated animals was just 0.1% of the total.
Since we have become such a dominant force on our planet, we have to decide how best to manage the situation we're creating. Many are calling for a change in the way that conservation has traditionally been practiced. Instead of battling to return ecosystems to a pre-human state, as previous efforts have tried to do, some conservationists say we should be realistic and recognise that humans are an integral part of many ecosystems now. In many cases, they argue, we should accept ecosystems that incorporate non-native species, value them and try to conserve them as “novel ecosystems” that are worth protecting.
From Hawaii to the Galapagos, conservationists are beginning to accept and implement this new approach - embracing introduced species, while trying to route out the more harmful invasives that out-compete unique flora or fauna. In the Galapagos, plagues of blackberry bushes originally from the Himalayas are simply being controlled, whereas rats and goats that eat the food of rare tortoises are being eliminated.
In other places, such as the vast monocultures we create through agriculture, efforts are already being made to restore native ecosystems, or in some cases plant non-native trees and grasses, or introduce animals to restore the functions that the pre-human ecosystem once provided – such as reducing soil erosion, pollinating flowers or controlling wildfires.
There have never been so many areas of conservation – national parks and protected zones – so you could argue that humans may be acting in time to save endangered species from extinction. But many of these areas are protected in name only – the parts of the world with the greatest biodiversity are often in the poorest and most troubled regions, such as the Congo and Borneo.
It is fairly certain that the Anthropocene will be a time of much poorer biodiversity. Once-common species will be extinct, or exist only in human-made environments like zoos or private breeding colonies far from their natural habitat, such as the lemur sanctuary in the Caribbean that Virgin boss Richard Branson proposed last year.
Paradoxically, just as we approach a tipping point for extinctions, we are beginning to understand how we could bring extinct animals back from the dead. Scientists are hopeful of cloning mammoths and even restoring our own extinct cousin, the Neanderthal. Sadly, expensive techniques like this, even if successful for individual animals, could not be applied practically to restore the intricate diversity of life that existed before humans took over the planet. Instead, in our human world, we must decide what type of ecosystems we would collectively like, and set about creating and protecting them. In the Anthropocene, we are no longer just another part of the natural world, we are the planet's gardeners. And that means we must develop our nurturing skills.
You can hear my conversations with Barnosky and others, discussing our impacts on the planet's biodiversity and geology, in a four-part series called The Age We Made, broadcast weekly on BBC World Service from Mondays at 19.32 GMT.
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