Three lessons about extinction
Spectacular mass extinctions have happened five times over the course of history. In this excerpt from Making Eden, David Beerling examines whether we are on the brink of a sixth and offers three important cautionary lessons for our current crisis. This excerpt has been adapted from the original.
Apocalyptic forebodings of uncertain future extinctions resonate because in spite of the acknowledged uncertainty in the numbers and time lags, plant extinctions are already upon us. Although hard to pin down, rates of plant extinction during the Anthropocene are about 10- to 100-fold higher than the natural background rate. How does this measure up against past mass extinction events, and what can we learn about the prospects for life’s survival and recovery?
Palaeontologists are always keen to point out that 99% of all species that have evolved on Earth over the past 3.5 billion years have gone extinct.
Viewed over the immensity of geological time, extinction is the norm, with species having an average lifetime of 5–10 million years, depending on the group of organism; mammal species are less durable than plants, for instance. Yet sitting far above the background level of extinction are five well-documented so-called mass extinction events that occurred during the past 540 million years since complex plants and animals populated the planet. Sharp spikes in extinction rates far exceeding background levels characterize the so-called “Big Five” mass extinctions, each representing the loss of 75% or more species. These past catastrophes in the history of life on Earth offer three important cautionary lessons for our current crisis.
Lesson 1. Draw up a list of causes for each of the Big Five mass extinctions and it soon becomes apparent that a synergy of environmental factors pushed life over the edge, rather than any single event. The end-Permian mass extinction, 250 million years ago, wiped out nearly all life on Earth and coincided with vast episodes of volcanic activity in Siberia that injected huge amounts of carbon dioxide into the atmosphere, triggering global warming, and noxious ozone-destroying sulphurous gases. The end-Triassic mass extinction, 201 million years ago, coincided with the birth of the Atlantic Ocean, as North America and Africa slowly drifted apart. The tectonic activity resulted in volcanism belching out huge amounts of carbon dioxide into the atmosphere. As the atmospheric carbon dioxideconcentration soared, the hot, parched planet that followed wiped out
Biodiversity on land and acidified the oceans, to extinguish life in the seas. Today, current rates of increase in the atmospheric carbon dioxide concentration, climate change, and ocean acidification far outstrip those experienced during any past extinction event. Humans have added to the mix with deforestation, habitat loss, and fragmentation, expansion of agriculture, over-fishing, over-hunting, and pollution.
Lesson one from Earth history is this: human activities are creating the perfect storm for the sixth mass extinction, priming the engine of species extermination.
Lesson 2. Rigorous meaningful comparisons between the magnitude of extinction events from hundreds of millions of years ago and the present are difficult. The fossil record is patchy, with much of the evidence for depletions in biodiversity obliterated, making rates of extinction hard to accurately pin down.
Nevertheless, careful estimates suggest that in terms of magnitude, the Big Five trump current losses, with each losing between 75% and 90% of species. In the 1990s, the World Conservation Monitoring Centre listed 592 plant species as having gone extinct since 1600. In 2016, The International Union for the Conservation of Nature Red List of Threatened Species listed fewer than 150 extinct species of plants. Granted the strong suspicion of undocumented extinctions, these ballpark estimates of magnitude for plants do not yet qualify as mass extinctions in the palaeontological sense. It is on the question of rates that things become concerning, with the best estimates of plant species extinction for the Anthropocene far exceeding those calculated for the Big Five, if we scale past mass extinction rates to the same window of time.
Lesson two from Earth history is that unless we relieve the pressures that are pushing today’s species to extinction, we will propel the world towards the sixth mass extinction within a few generations.
For vertebrates — mammals, birds, fish, amphibians, and others — it may already be underway, with recent declines in their diversity unprecedented in the past 65 million years. Plants have so far fared better, but may be sitting in the extinction waiting room for longer than animals, for reasons we have already discussed.
Lesson 3. Extinctions from the past provide a rough guide to the recovery time of the biosphere, following catastrophic losses. On average, it takes anything from hundreds of thousands of years to tens of millions of years for ecosystems to recover, reorganize, and bounce back to pre-extinction levels of biodiversity. After the end-Permian mass extinction, tropical forest and coral reef diversity remained depleted for ten million years or so, and global biodiversity took a hundred million years to recover. After the end-Cretaceous mass extinction event, 65million years ago, that famously saw off the dinosaurs, the diversity of life in the seas took millions of years to rebound.53
Lesson three from Earth history is that if our actions drive life towards a sixth mass extinction, we are likely sealing the fate of biological diversity for millions of years.
David Beerling is Sorby Professor of Natural Sciences and Director of the Leverhulme Centre for Climate Change Mitigation at the University of Sheffield, England. His work on the evolution of life and the physical environment was recognised by the award of the prestigious Philip Leverhulme Prize in earth sciences in 2001. His book The Emerald Planet formed the basis of a major three-part BBC television series, How to Grow a Planet. He was elected to the Fellowship of the Royal Society, London, in 2014.
