468 Natural Resources Systems Copyright Goodheart-Willcox Co., Inc. Why Do Species Become Extinct? Some species become extinct because of natural background extinctions. However, the vast majority of extinctions or endangerment since the evolution of human beings (Homo sapiens) are caused by humans, primarily by the following: • Habitat loss, degradation, and fragmentation • Introduction of invasive species • Overharvesting (hunting, fi shing, harvesting), Figure 19-4A • Loss of genetic variation, Figure 19-4B • Unsustainable human population growth • Climate change Habitat Loss, Degradation, and Fragmentation Habitat loss or degradation is the primary cause of species endangerment or loss. Habitat loss results when a habitat is removed completely, such as fi ll- ing in a wetland to construct a subdivision, or changed in a way that it cannot support its inhabitants. The wetland’s inhabitants must seek a new habitat or face extirpation. Extirpation occurs when a species is locally extinct and no longer found in an area it used to inhabit but is still found elsewhere. Habitat Degradation Habitat degradation changes the area in such a way that it can no longer meet the needs of its inhabitants. Degradation may be caused by factors such as pollution in the soil, water, or air, a change in water fl ow, or the killing or removal of a key species. Light pollution and noise pollution have also been shown as detrimental to habitat health. It is estimated that more than 90% of all living creatures have gone extinct over the course of history. Did You Know? tsuneomp/Shutterstock.com Figure 19-4. A—The last dodo bird was sighted in the late 1690s on its native island, Mauritius. Although the bird was hunted by sailors, it was more likely the pigs, rats, and dogs they introduced to the island that ate the dodo eggs and killed the birds. B—The last mainland (Siberia) wooly mammoths died due to climate change and human hunters. Experts say that wooly mammoths remaining on two small islands died due to genetic decline. As the genetic variation dwindled, undesirable mutations were no longer purged from the gene pool. What implications might this theory have on dwindling populations of endangered species? AuntSpray/Shutterstock.com A B