Answer to Critical Thinking Activity - Chapter 27

1. The fact is, it is impossible to estimate the rate of extinction for several reasons. The paper you read says the number of species in existence is not known. How true that is. Somewhere around a million species of organisms have been classified, some say more, some say less, no one seems to really know. Estimates on the actual number of species presently on Earth run as high as 50 million, which is likely, to as much as 100 million, which is possible. We do not know how many species have become extinct in the past because by no means have all of them left a fossil record. It is probable, in fact, that the majority did not. Therefore, any estimates of extinctions per year downs through the ages, such as those given in the first paper are merely wild guesses. We do not know how many species are presently becoming extinct each year because we certainly are not aware of all of them. However, of those of which we are aware, most seem to disappear as the result of human activities. That may be simply because we are aware of them. There may be many others that become extinct for reasons having nothing to do with humanity. Nevertheless, the extinction of an organism because of human enterprise should be controllable and avoided.

2. Most recently, hypotheses centered on the new nonlinear science of complexity have emerged. Under these hypotheses species-species interactions lead to occasional instability resulting in cascades which may ripple through entire ecosystems, with potentially devastating results. {From the paper, Patterns of Extinction}

3. Glaciation, atmospheric oxygen depletion, glaciation and the resultant lowering of sea-level, meteorite or asteroid impact, the formation of Pangea, rapid warming after glaciation, volcanic eruptions, human activities. The most frequent causative agent seems to be glaciation.

4. A "minor" extinction in the Triassic allowed the dinosaurs to radiate into terrestrial niches that were previously unavailable. {From Minor Extinctions of Earth History}