Almost the entire biosphere exists in a narrow zone extending from the depth to which sunlight penetrates the oceans (about 200 m) to the snow line in the tropical and subtropical mountain ranges (about 6000 m above sea level).At the scale of the photograph in Figure 1.3, the biosphere—all of the known life in the solar system— would be in a thin layer no thicker than the paper on which the image is printed. Certainly one of the most interesting questions about the biosphere concerns the number and variety of organisms that compose it. Surprisingly, the truth is that no one knows the answer. Despite more than 250 years of systematic research, estimates of the total number of plant and animal species vary from 3 million to more than 30 million. Of this number, only 1.6 million species have been recorded. The diversity is stranger than you may think. Insects account for more than one-half of all known species, whereas there are only 4000 species of mammals, or about 0.025% of all species. Observation shows that there are more species of small animals than of large ones.The smallest living creatures—those invisible to the unaided eye, such as protozoa, bacteria, and viruses—contribute greatly to the variety of species.The biosphere is a truly remarkable part of Earth’s systems.
Posted on: Tue, 03 Sep 2013 00:01:03 +0000
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