A species is described as extinct when no living members remain. Scientists know from their study of fossils that dinosaurs, mammoths, saber-toothed cats, and countless other animal and plant species that once lived on Earth no longer exist. These species have "died out," or gone extinct. Once a species is extinct, there is no way to bring it back. The U.S. Fish and Wildlife Service,…
In the billions of years since life began on Earth, species have formed, existed, and then become extinct. Scientists call the natural extinction of a few species per million years a background, or normal, rate. When the extinction rate doubles for many different groups of plants and animals at the same time, this is described as a mass extinction. Mass extinctions have occurred infrequently in Ea…
Determining how many species of plants and animals are threatened or endangered is difficult. In fact, only a small fraction of the species in existence have even been identified and named, let alone studied in detail. Estimates of the total number of species on Earth range from 5 million to 100 million, with most estimates figuring around 10 million species worldwide. Of these, only about 1.75 mi…
Experts believe that the increasing loss and decline of species cannot be attributed to natural processes, but results FIGURE 1.4 Contributing factors in species endangerment instead from the destructive effect of human activities. People hunt and collect wildlife. They destroy natural habitats by clearing trees and filling swamps for development. Aquatic habitats are altered or destroyed by…
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