When you hear the term “sixth mass extinction,” what images come to mind? Elephants and lions being hunted down for the traditional medicine market? Sea turtle eggs being poached from the beaches of Costa Rica? Overfishing and reef destruction? The rainforest? With all of the recent and developing news concerning loss of ecosystems and species, and a widely-reported new study in Science Advances, the sixth mass extinction is increasingly penetrating the public consciousness.
Despite appearances, the sixth mass extinction is not a recent occurrence. Many years ago—60,000 or so—humanity first ventured away from its cradle in eastern Africa. The sixth mass extinction “officially” started at about 12,000 year ago at the end of the last glaciation, but there were pulses of it much earlier than that in certain parts of the world.
Most mass extinctions previous to this one (such as that of the dinosaurs) occurred because of natural catastrophes, including climate change, geological disasters, meteor impacts, and perhaps even others we haven’t identified. The sixth mass extinction may or may not have been the product of natural forces in the past, but it is increasingly a human problem in the present. In order to avert an endlessly recurring tragedy, then, humanity must learn from this often-overlooked facet of history.Ice Age Spain was populated by lions, rhinos, and mammoths. (Painting by Mauricio Antón)
It was not long ago (at least on the Earth’s timescale) that our world was host to incredible beasts of magnificent proportions. Giant cave lions, larger than those in present-day Africa, roamed North America and Europe under the walls of glaciers, side-by-side with saber-tooth tigers. Eagles as big as motorcycles in New Zealand hunted gigantic, flightless moas, which themselves could stand over 12 feet tall. Cougar-sized cheetahs prowled the Midwest in America, and were probably just as fast as those we know today. Woolly rhinos and white hippopotamuses waded through swamps in modern-day England—in fact, their bones can be found under the pavement and buildings of London. It’s just familiar enough that the alien elements make it hard to picture.
What provoked the sad and untimely demise of these incredible creatures? It deserves to be repeated that we don’t always know what caused recent extinction and ecosystem collapse in the fossil record. All we know is that extinction has been consistent since humans began to spread. Extinctions can be complex affairs, and it can be difficult to obtain adequate proof as to their causes—humans, climate change, disease, invasive species and every combination between them have been asserted as theories. The extinctions of megafauna (giant animals) have been some of the most hotly debated because of the implication of our own species as a suspect.
For instance, an entire layer of soil in many parts of Australia is charred and blackened, with chemical analysis pointing to having been burned by fire on a large scale. There is a suspicion that ancient Aborigines caused die-offs of animals by setting fires that they used to drive the animals into traps, in addition to overhunting. The species affected included the three-ton wombat, diprotodon, the marsupial lion and a crocodile-sized goanna. The theory goes that, in their attempts to take advantage of Australia’s megafauna, the newly arrived Aborigines ended up setting flame to the entire continent and devastating its plant and animal life, changing it forever.
Then again, Australia’s fiery fate could just as easily be blamed on a shifting climate that created dry tinder and lightning strikes. It could also have been some “complex combination” of both climate and humans: Did fires that humans set change the climate, creating a runaway effect, or did changing climate leave a void that humans rushed to fill, with most megafauna species in Australia already gone by the time humans arrived?
Recently, paleontological evidence has appeared to support the idea that many megafauna species were already in decline before humans arrived in Australia, and that the extinctions took much longer than initially thought. Yet, tantalizingly, these same megafauna species also went totally extinct shortly after the documented arrival of humans. We still don’t have absolute proof either way, because any dig sites with evidence show nothing conclusive or are too jumbled and confused to provide a clear picture.
The early stretch of the sixth mass extinction in other parts of the world is similarly debated. Did Native Americans kill off mega bison and dire wolves in California, or did rising temperatures do them in? Was the disappearance of lions, hyenas, mammoths and countless others from Europe due to overhunting or permanent retreat of the glaciers, or even some other mysterious factor?
It would not appear at first glance that mere temperature changes could cause such extensive species loss. These animals had endured tens and even hundreds of millions of years of climate ups and downs without vanishing, so what made the climate change during their extinction special enough to eradicate them? The only unique factor in this extinction event was the migration of humans to the six habitable continents. A recent study from Australia, for instance, provides physiological evidence of its megafauna being well-adapted to the dry conditions that might have arisen (given that they had seen dry periods before), so the theory of climate change as a cause for their collective extinction has been significantly weakened. Even if they had been in decline when humans arrived, they may have had many periods of population decline in the past, only to then have a resurgence each time.
Most would think of Africa as “The Last Bastion of Big,” but it, too, has seen many species and environments vanish. The renosterveld of South Africa, an exceptionally unique shrubbery biome, betrays a mysterious clue in its name, which is Afrikaans for “rhino field.” There are no rhinos in South Africa—at least, not anymore. At the time of European colonization, however, species were already quickly disappearing from the area, such as the bluebuck and zebra-like quagga, and poor records were kept on natural history (to say the least). Was this wilderness ecosystem named for rhinos that once lived there? If it was, no one knows what they looked like or if they were a completely unique rhino species, and with no fossil evidence forthcoming, their existence and genetics are a complete mystery.
Amid all the unknowns, there is one thing we certainly do know: The loss of “keystone” species has radically altered environments.
Mammoths, for example, maintained an entire ecosystem that could only survive as long as they did—the mammoth steppe. The mammoth steppe was a collection of grasses, shrubs and small animals that make present-day northern plains look like another planet by comparison. Once the mammoths were gone, so too was most of the mammoth steppe, and formerly livable, vibrant areas in the northern hemisphere became cold deserts. Incredibly, new evidence has been found that mammoths in some parts of the world survived as late as 3,700 B.C.—that is to say, they were alive during the time of the very early Egyptians and Mesopotamians. No one from those places would have seen them, though, as they only existed on island refuges in the frozen north.
The planet has changed drastically since our emergence; the sixth mass extinction is not a new phenomenon. There has been a complete reordering of many of the habitable biomes on Earth, and the shake-up is becoming alarmingly fast and unpredictable. However, where once there was a rich concentration of biodiversity to lean on (and perhaps abuse), there is now a wound scalped down to the bone, and the battle to hold on to what’s left is constant. Regardless of the causes of earlier extinctions, we are poised to artificially repeat such disasters many times over.
Why would modern (if not ancient) humans be so quick to destroy their own environment, especially when its importance is self-evident? Why push the envelope when we know how much has already been lost? Join us for the next part in this series as we look at the basic causes of modern mass extinction.