Extinction Trends

In general, the diversity of life on Earth has been constant or increasing slightly over the past 200 million or 300 million years. The geologic record shows new species emerging and old ones dying away, resulting in broad range of forms over geologic time. On closer inspection,the geologic record also shows distinct short-term increases in the rate of extinction termed extinction events. One such event occurred 65 million years ago and marked the end of the Mesozoic Era, the age of reptiles. For years, the cause of this event puzzled scientists. However, in the980 s thin layer of unique sediment containing the element iridium was discovered in sedimentary bedrock whose age coincided with this event. Research showed that this sediment was created by an explosion on Earth’s surface, probably caused by an asteroid impact. The explosion threw a huge mass of debris into the atmosphere, which, scientists reason reduced incoming solar radiation and cooled Earth. Plants declined and the food sources for many herbivores (plant-eating animals) apparently disappeared. followed shortly thereafter by a sequence of extinctions among predators.

Many other extinction events are also identifiable in the geologic record. Some occur at 26-million-year intervals, and scientists speculate that they may be associated with extraterrestrial impacts similar to the one at the end of the Mesozoic Era. Between these events, many smaller episodes of extinction are also apparent. The influence of humans on extinction patterns. however. does not appear until about 10.000 to 15.000 years ago.

The last glaciation on Earth reached its maximum 15,000 to 20.000 years ago. At this time the northern half of North America was covered with great sheets of glacial ice. A similarly large mass of ice covered large part of northern Eurasia. Humans lived in the zone south of the ice border, where they survived by hunting and gathering. Among the animals hunted were large mammals such as mammoths and mastodons. which are now extinct. The extinction process appears to have taken place over several thousand years in the presence of humans as the ice sheets were melting and the bioclimatic environment was undergoing change. By 8,000 to 10,000 years ago, dozens of mammal species had disappeared. Scientists debate whether the extinctions were driven mainly by changing climate or mainly by human exploitation. No doubt these animals were under stress from a changing environment. but the evidence for population reduction by early hunters is also compelling. On balance, it appears certain that humans had a hand in the process. At the very least, hunting hastened the extinction process.

The next wave of extinction began about 10.000 years ago with the spread of agriculture and the growth of human populations. Before A.D. 1500 or so, much of the area taken up by sedentary agriculture in Europe and Asia was in subtropical and midlatitude environments. Many species were undoubtedly eliminated in these environments, but for most there is little or no record of their existence, especially for plants, insects, and smaller organisms. In the Mediterranean basin, on the other, where the early historical record is most complete there is sound evidence of widespread mammal extinction associated with the spread of early agriculture. Though less widely cultivated than the subtropics the wet tropics were not exempt from early agriculture. Large areas of monsoon forest and rain forest were destroyed in Asia and Africa by both crop farmers and herders. In the Americas, tropical rain forest and wetlands were also lost to agriculture. Recent evidence indicates that considerable areas of rain forest were cleared by the Aztec and Mayan civilizations in Central America. All told, the first several thousand years of agriculture undoubtedly forced the extinction of thousands of organisms, most of which we can never know about because they left no measurable record of their existence.

After A.D. 1500 and European colonization of the Americas and Africa, the rate of global extinction increased again. In North America, most of the midlatitude forests were eradicated. In the tropics, plantation agriculture, mining, and expanding subsistence farming(farming provides for the basic needs of the farmer) further reduced the tropical rain forest. Before the twentieth century, however, forest eradication was relatively minor by today’s standards: beginning around 1950, rain forest destruction became a crisis that by 1975 had reached epidemic proportions.?

1.In general, the diversity of life on Earth has been constant or increasing slightly over the past 200 million or 300 million years. The geologic record shows new species emerging and old ones dying away, resulting in broad range of forms over geologic time. On closer inspection,the geologic record also shows distinct short-term increases in the rate of extinction termed extinction events. One such event occurred 65 million years ago and marked the end of the Mesozoic Era, the age of reptiles. For years, the cause of this event puzzled scientists. However, in the980 s thin layer of unique sediment containing the element iridium was discovered in sedimentary bedrock whose age coincided with this event. Research showed that this sediment was created by an explosion on Earth’s surface, probably caused by an asteroid impact. The explosion threw a huge mass of debris into the atmosphere, which, scientists reason reduced incoming solar radiation and cooled Earth. Plants declined and the food sources for many herbivores (plant-eating animals) apparently disappeared. followed shortly thereafter by a sequence of extinctions among predators.?