Preserving biological richness Human beings are only one of millions of organisms sufficiently different from one another to be recognized as species. Until the 1980's, scientists estimated that there were between 3 million and 10 million species on Earth. Then, scientists began to examine populations of insects living in the high foliage of trees in tropical forest, and the experience caused them to increase their estimate to 30 million species. After scientists began to consider the populations of microorganisms in the tropics, the upper limit of their estimates rose to 100 million species. We shall probably never have an accurate count of the different kinds of organisms that share our planet.
Scientists call the variety of life forms Earth's biological diversity or biodiversity.
We also know that human activities are reducing both the numbers of species on Earth and the potential of land and water for supporting them. Biotic impoverishment is the result of chronic disturbance of the surface of Earth on land or sea. The result, no matter what its cause, is a reduction in the complexity of the form and structure of nature. Ecologists have found that large-bodied organisms that reproduce slowly are lost most readily. Small-bodied organisms with short reproductive times often survive.