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.