The Sixth Largest Mass Extinction, EverBy Branden Holmes (Surroundx) on February 25, 2012
|In the 3.8 billion years that life has existed on Earth, it has only suffered five greater mass extinctions than the one which is currently under way. In the last 100,000 years more than 2000 species have become extinct; and they are just the species which were scientifically described before they went extinct. Many, many more go extinct either before they are properly described, or even before they are known. The real extinction rate has been calculated by E.O. Wilson (The Diversity of Life, 1992) as up to 30,000 species per year, or, 3.4 extinctions every hour (on average).|
The Background Extinction Rate
The background extinction rate, or the rate at which species are expected to go extinct for purely natural reasons, is approximately one species every 5 years. Extinction is an entirely natural thing; species don't last forever. However, if E.O. Wilson is correct, the actual extinction rate is currently 150,000 times greater than this. Although most biologists aren't as pessimistic as Wilson in their figures, any way you look at the current situation, extinction is happening at a rate far above what we should expect naturally. It is for this reason that the current extinction has been labelled as the "Sixth Extinction", even though it doesn't actually yet meet the criteria for being called a "mass extinction": 75% of all genera going extinct within a 1 million year period (Richard Dawkins, The Ancestor's Tale).
Europe and Asia Spared?
If we look at the statistics, one pattern which stands out is the fact that the European and Asian mainlands have suffered relatively few extinctions, while Australasia (especially the Pacific islands), the many islands off the African coast, and the Americas (including the Caribbean) have all been devastated. Most of their large mammal and many of their bird species have become extinct, with many reptile and amphibian species also driven to extinction. The reason for this is that even though Homo sapiens sapiens was still restricted to Africa until the last 100,000 years, at least four other members of the genus Homo (H. neanderthalensis, H. floriensis, the Denisovans and an as yet unidentified fourth species) all lived in Europe or Asia until 70-13,000 yBP. Therefore the plants and animals of these regions were already adapted to the habits of man, and were better able to survive. Whereas the animals of Australia, the Americas and the pacific and African islands had never encountered man before.
Two Waves of Extinction
In many areas around the world extinction has proceeded in two waves: firstly, when the "native" people of those places (Hawaiian Islanders, Maoris, Australian Aboriginals, American Indians etc.) first settled there. And secondly, when European man subsequently colonized the natives' land (making even many of them extinct as well, such as the extraordinarily sad tale of the Tasmanian aboriginals). The natives hunted most of the so-called "mega fauna" (animals heavier than 45kg, or 100lbs) to extinction, possibly because of their hunter-gatherer lifestyles, which required them to constantly hunt. Where they lived on tropical islands, they decimated bird species, with some 2000 species of birds likely having become extinct in the recent past. When European man subsequently settled the same areas, they brought rats with them which exterminated virtually all flightless birds and edible terrestrial mammals on islands. They also had a similar, though lesser, impact on species diversity on the mainland of many countries.
The Cichlids of Lake Victoria
Lake Victoria is a huge lake in Eastern Africa, which, along with Lake Malawi and Lake Tanganyika, are collectivey known as the Great Lakes. Out of the three lakes, Victoria is the most remarkable in several different ways, and not all of them good either. Lake Victoria is the second largest freshwater lake in the world, covering some 68,600-68,900 km2. It is also the source of the Nile river, and remarkably young, having only filled up again after being completely dry, 12,400 years ago. But what Lake Victoria is most known for, at least to evolutionary biologists, is a group of remarkable fish known as cichlids (pronounced sick-lids). Virtually all of these cichlids, some 350-450 species, have evolved within a little over 12,000 years, making them of immense interest to evolutionary biologists.
But sadly, many of these species have become extinct even quicker than they evolved, as the Nile perch (Lates niloticus) has invaded their pristine habitat. One of several comepting hypotheses regarding their origin in Lake Victoria has it that many fingerlings of the Nile perch were deliberately released into Lake Victoria for commerical purposes, ironically to ease pressure on the the native fish stocks, by enticing local fisherman to target the much larger, and thereby far more economically viable perch. And indeed, their impact upon the lakes endemic species was virtually non-existence for almost 20 years. But beginning in the 1970's Nile perch numbers exploded, and have been rising ever since. Several specimens caught in recent years have weighed almost 200kg, and measured well over 2m long. They are cichlid-eating machines, and have forced a significant number of species to become extinct. It is extremely difficult to put a precise number on it, but probably around 100 different species have fallen prey to the wrath of the Nile perch, and more extinctions are likely to follow.
Birds of the Pacific Islands
The Pacific islands are some of the most beautiful places on Earth, with countless archipelagos and pristine beaches. They are evolutionary experiments, filled with odd and unusual creatures. Many islands have still never been reached by humans, and hopefully it will stay that way. From archeological digs on many of the larger islands, the pre-human faunas of these islands can be compared with the present day taxa which inhabit the region. And it is clear from this comparsion that the islands of the Pacific are now ornithologically barren. It has been estimated that some 800-2,000 species of birds have been driven to extinction by Polynesian seafarers over the past few thousand years.
Oceanic islands are by definition extremely far from the enarest significant land mass. And as a result the number of species able to colonize such islands are very few. The main types of species are insects and bats, seabirds and reptiles; mammals (excluding bats) are almost unheard of on oceanic islands. And given that most of the invasive carnivores around the world are non-chiropteran mammals, many species have evolved flightlessness, as they no longer have a need for their wings, which got them to the islands in the first place. This makes them especially vulnerable, along with their unacquiantedness with predators, to extinction.
The Five Greatest Mass Extinctions
Regardless of semantics, there have only been five greater mass extinctions during the whole of life's existence on Earth. But sadly, because the current mass extinction is still underway and with no visible end in sight, it may become one of the top five if drastic conservation measures are not taken immediately:
Many species have become extinct in the last few thousand years, as we have just seen. But these aren't all just insects or plants. Many large mammals, birds, fish and reptiles have also gone extinct. As speciesists we fell less guilt over the extinction of an insect, as opposed to, say, a bird. However, there is no justification for this discrimination, as every species has just as much right to live as any other, including ourselves.
Barlow, George W. (2000). The cichlid fishes: Nature's grand experiment in evolution. Massachusetts: Perseus Publishing.
Dawkins, Richard. (2004/2005). The Ancestor's Tale: A Pilgrimage to the Dawn of Life(paperback edition). London: Phoenix.
Eldredge, Niles. (June, 2001). The Sixth Extinction [accessed 4-2-2012]
Wilson, E.O. (1992). The Diversity of Life. Cambridge, Massachusetts: Harvard University Press.
NB: This is a greatly expanded version of an article which I first published on www.suite101.com, December 3, 2010. That article was recently deleted in a massive purge of old articles, but I have appealed to have it reinstated. One of the conditions attached to publishing the original article was that I could not publish it elsewhere for a perion of one year. After one year had elapsed I could re-publish the article elsewhere, so long as I acknowledged that the article had been published on suite101.com and provided a link to the original article. As the link to the article is now dead (unless the article is reinstated), I hope that this notice will suffice, and it must, since there is nothing more I can do to make the reader aware that this article is not wholly original.
|I am a freelance writer with a special interest in conservation biology.|
|« More articles|