Plants and Animals

Radical overhaul needed to halt Earth’s sixth great extinction event

The ConversationLife has existed on Earth for roughly 3.7 billion years. During that time we know of five mass extinction events — dramatic episodes when many, if not most, life forms vanished in a geological heartbeat. The most recent of these was the global calamity that claimed the dinosaurs and myriad other species around 66 million years ago.

Growing numbers of scientists have asserted that our planet might soon see a sixth massive extinction — one driven by the escalating impacts of humanity. Others, such as the Swedish economist Bjørn Lomborg, have characterised such claims as ill-informed fearmongering.

We argue emphatically that the jury is in and the debate is over: Earth’s sixth great extinction has arrived.

Collapse of biodiversity

Mass extinctions involve a catastrophic loss of biodiversity, but what many people fail to appreciate is just what “biodiversity” means. A shorthand way of talking about biodiversity is simply to count species. For instance, if a species goes extinct without being replaced, then we are losing biodiversity.

But there’s much more to biodiversity than just species. Within each species there usually are substantial amounts of genetic, demographic, behavioural and geographic variation. Much of this variation involves adaptations to local environmental conditions, increasing the biological fitness of the individual organism and its population.

Natural variation within two species of sea snails. Upper row: Littorina sitkana. Lower row: Littorina obtusata. Copyright David Reid/Ray Society.

And there’s also an enormous amount of biodiversity that involves interactions among different species and their physical environment.

Many plants rely on animals for pollination and seed dispersal. Competing species adapt to one another, as do predators and their prey. Pathogens and their hosts also interact and evolve together, sometimes with remarkable speed, whereas our internal digestive systems host trillions of helpful, benign or malicious microbes.

Hence, ecosystems themselves are a mélange of different species that are continually competing, combating, cooperating, hiding, fooling, cheating, robbing and consuming one another in a mind-boggling variety of ways.

All of this, then, is biodiversity – from genes to ecosystems and everything in between.

The modern extinction spasm

Cumulative vertebrate species extinctions since 1500 compared to the ‘background’ rate of species losses. G. Ceballos et al. (2015) Scientific Advances.

No matter how you measure it, a mass extinction has arrived. A 2015 study that one of us (Ehrlich) coauthored used conservative assumptions to estimate the natural, or background rate of species extinctions for various groups of vertebrates. The study then compared these background rates to the pace of species losses since the beginning of the 20th century.

Even assuming conservatively high background rates, species have been disappearing far faster than before. Since 1900, reptiles are vanishing 24 times faster, birds 34 times faster, mammals and fishes about 55 times faster, and amphibians 100 times faster than they have in the past.

For all vertebrate groups together, the average rate of species loss is 53 times higher than the background rate.

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