Species Extinction

 

Cities span the ever-dwindling corporeal plain;

     a tumbleweed is applauded

for it's lifelike movement

in the quickening race to survive.

 

Mass extinction of biological species has occurred several times in the history of our planet. How significant these extinctions have been is illustrated by the fact that well over 99.9% of the living species to have ever inhabited the Earth are now extinct. The most notable extinctions of multicellular organisms occurred roughly at 100 million year intervals. The first took place about 550 million years ago (mya), and was actually a succession of so-called Cambrian extinctions. The second, the so-called Ordovician extinction, took place about 450 mya. The third, the Dovonian extinction, occurred roughly 350 mya. The fourth, the Permian extinction, dates back 250 mya, and the fifth, the End-Cretaceous or KT extinction, occurred 65 mya.

The causes of all but the last of these mass extinctions are still speculative. Possible causes include glaciations, volcanic eruptions and meteoric impacts. The fifth and most recent extinction (KT) is believed to have been due largely or entirely to collision with a large meteorite, about 6 miles in diameter, traveling at about 60,000 miles per hour. It left a huge crater in the Yucatan Peninsula of Mexico. The impact caused tremendous amounts of molten earth, smoke, and heat to enter the atmosphere, rendering the entire planet unsuitable for many forms of life.

Layers of sediment, containing high concentrations of Iridium, found only in the Earth's mantle and in extraterrestrial meteors and comets, have been found in numerous marine and terrestrial sites around the world. They apparently resulted from a single event that occurred at about the same time as the K-T extinction. The eventual settling of the vast quantities of materials that were released into the atmosphere following the impact of the meteor must have produced these layers. They provide a permanent record of the calamitous collision.

The K-T extinction was the second largest mass extinction event in the geological history of planet Earth with about 85% of the species perishing. It brought an end to the Mesozoic era, extinguished the dinosaurs, and ushering in the present Cenozoic era. Remarkably, a few mammals, birds, reptiles, and amphibians survived. It was from them that most of the higher animal species found on Earth today evolved. Putting our own species into temporal perspective, humans, as we know them today, evolved just one or two mya. We separated from our closest relative, the chimpanzee, just seven mya. By comparison with some ancient species of turtle that date back over 200 million years, humans and chimpanzees are most recent evolutionary inventions.

Several renowned scientists have noted the striking similarities between man and the chimp, particularly at the genetic level. Humans are 99.4% identical to the chimp in our coding DNA. In fact, scientists have concluded that these two species really belong to a single genus. Of course, many egocentric humans loathe accepting such a close relationship between themselves and another primate. For those of us who don't find this relationship upsetting, it is shocking to think that our nearest relative is a legal source of human food in many parts of the world, and that we are quickly driving this critically endangered species to extinction.

The Permian extinction, occurring about 250 million years ago, was probably the worst of all mass extinctions. It wiped out over 95% of the existing families of animals and plants. This event ushered in the Triassic period during which dinosaurs, amphibians (including frogs and turtles), flying reptiles and early mammals appeared, roughly 220 mya. These evolving animals lived through the Jurassic and Cretaceous periods of the Mesozoic era, surviving the fifth mass extinction which was somewhat less severe.

We are now entering a new period of mass extinction, the so-called Holocene epoch. In contrast to all previous extinctions, the Holocene extinction is due to the activities of a single biological species, Homo sapiens, the current agent of mass destruction. Species extermination has become important only recently, just over the past 2,000 years, with tremendous acceleration over the past 200 years. It is true, but hard to accept, that one species, our own, has already been able to alter the environment sufficiently so that hundreds of thousands of other species have been eliminated, while many more are doomed to the same fate within the next few years. The specific causes of the present mass extinction fall into three major categories: (1) habitat destruction, resulting, for example, from agriculture practices and oceanic trawling, (2) over-harvesting in both terrestrial and marine environments, thereby causing direct loss of life as well as disruption of the interdependent food chains, and (3) pollution of environmental niches (air, soil, oceans, lakes, etc.). All of these practices have dramatic effects on Earth's delicate ecosystems. For example, based on the current rate of loss that stands at about 2.5% per year, it is expected that destruction of our tropical rainforests will be completed within less than 40 years. Since one-half of all the animal species in the world live in these forests, and one-quarter live only there, when the rainforests are gone, at least one-quarter of all animal species on Earth will disappear.

Extinction is an essentially irreversible process; a species lost can never be reintroduced unless every coding segment of its DNA is duplicated thousands of times with perfect fidelity during development of the organism. Estimates of the current rates of species extinction due to the activities of the human population vary and depend on the criteria used. For known species, the estimated rate is about 8,000 per year. The background extinction rate in the absence of human activities is estimated to be less than one species per year. Extrapolating these values to all species on Earth, both known and unknown, we can assume that tens of thousands of species are going extinct every year due exclusively to the presence of Homo sapiens. One species is rendering the planet unsuitable for the lives of hundreds of thousands of species, very possibly including itself.

While we have already dramatically reduced the diversity of life on our planet, it is an interesting question to ask what fraction of the remaining species on Earth are currently seriously in danger of extinction. Current estimates of endangered species are as follows: 11% of all bird species, 20% of all reptiles, 25% of both amphibians and mammals, and 33% of all fish species. Moreover, a recent study suggests that about 40% of all plant species are endangered. Clearly the human population is having a dramatic effect on the biosphere.

Two well-known endangered species are our closest relatives, the chimpanzee and the gorilla. The human population has decimated primate habitats throughout Africa with the sole exception of western equatorial Africa, the last stronghold of the African apes. The Congo and Gabon are thought to hold about 80% of the world's gorillas and most of the chimps. Shockingly, recent surveys have shown that these populations have declined to less than half of their 1980 levels just in the last 20 years. A primary cause of primate loss of life is commercial hunting. Ape flesh, commonly called "Bush meat", costs only one-third as much as chicken, beef or fish and therefore is eaten by a significant segment of the population. Habitat destruction, due largely to mechanized logging, is also responsible for the death of large numbers of primates. The third major cause of death has recently been shown to be infection with Ebola virus, apparently reducing the ape populations to a degree comparable to hunting and habitat loss combined. Initially we might suppose that human activities were not responsible. However, the primary source of the virus seems to be infected humans. The few remaining pockets of healthy apes are in remote areas, distant from documented human Ebola outbreak sites. The current rates of loss to the gorilla and chimp populations are in excess of 5% per year. If this rate continues, by 2033, after less than two generations of these primates, the ape populations will decline by more than 80%.

We don't have to go to Africa to observe species loss. As reported in the L.A. Times (April 21, 2003), the leatherback turtle, which has outlasted the dinosaurs by 65 million years, is now losing the battle with the poachers and commercial fishermen. The total number of these ancient creatures in the entire Pacific Ocean is estimated to be fewer than 5,000, a decrease of over 95% during the past 20 years. A reduction in their numbers resulted in part from destruction of turtle nesting grounds but can also be attributed to commercial fishing. Many turtles meet death in fishing nets and on strings of baited hooks, unfurled for as much as 50 miles off the sterns of commercial long-line vessels. With their numbers drastically reduced, single animals often can't find a mate, leading experts to believe that extinction throughout the world's oceans will occur within a mere 10 to 20 years unless drastic protective and revitalization measures are taken.

A third example of an endangered species, recently rescued from the brink of extinction, is the California condor. Hundreds of thousands of these remarkable "Thunderbirds" once flew over coastal areas all the way from British Columbia in Canada to Baja California in Mexico. In 1980 less than 200 remained in the wild. These few remaining condors were captured, and a controversial joint U.S.-Mexico captive breeding program was initiated. Now, 23 years later, birds raised in captivity are being returned to their natural environment both in the Big Sur area near Monterey, California, and in the Sierra San Pedro Mortar mountain range in northern Baja where the last condors were seen in 1995. The problem is not so easily solved, however. Birds raised in captivity are less able to scavenge sparse food sources or fend for themselves against human poachers. They may also be more susceptible to disease. To overcome these and other problems, large release pens serve as "home base" so the condors can safely return to a familiar site where resting grounds and food sources are available. If disease becomes a problem, the birds can be recaptured while in the release pens and treated medically.

Only a few condors are currently being released per year, and the success of the program is by no means guaranteed. Release methods are still being tested, and learning methods, which rely on the experience of previously released birds, are not always trustworthy. It is also essential to educate the local human communities of the importance of the program so "sitting duck" condors will not be the target of people who may unwittingly find it entertaining to torment or harm the relatively tame birds. The released thunderbirds, though fierce to the eye, are nearly as vulnerable as were the gentle but now extinct dodo bird. When Portuguese sailors first landed on their isolated island of Mauritius in 1598, the funny looking birds showed no fear, having never experienced human cruelty. The sailors slaughtered them for food and sport, and a few decades later, the dodo was gone. It seems human nature includes sadistic and destructive tendencies. Thus, many potential problems remain in the international attempt to rescue the California condor from extinction.

Hundreds of endangered species are currently being protected on a small scale, but in spite of these efforts, thousands of known species go extinct every year. The well-intentioned efforts of national and international conservation groups represent only a fraction of the actions necessary to reverse the descent into mass extinction. Ultimately, we know that as the human population expands, it will continue to displace thousands of species, dramatically decreasing the biodiversity on Earth. A few hundred animal and plant species will be found only in zoos and botanical gardens, but when starvation and pestilence take their toll, these too will disappear. The thousands of extinct species will never reappear, and the evolution of new species takes millions of years. Future generations will never know the wondrous diversity of wildlife that we have come to accept as our natural heritage, just as our exposure to nature is of necessity meager compared to that of previous generations. The day may come when children will view one of the few remaining wild creatures, a cockroach or a rat, for example, and say "Look Daddy, a wild animal!"

 

 

Further Reading

Pitman, N.C.A. and Jorgensen, P.M. (2002). Estimating the size of the world's threatened flora. Science 298: 989.

Walsh, P.D., et al. (2003). Catastrophic ape decline in western equatorial Africa. Nature 422: 611-613.

Weiss, K.R. (2003). Sea turtle is losing the race. LA Times, April 21st, p. 1.

Wallace, M. (2003). California condors soar again over Baja. ZooNooz, February, 20-23.