Chapter 19 Endangered Species 479 Copyright Goodheart-Willcox Co., Inc. to features that subdivide the overall area. If an area of prairie includes four dif- ferent soil types, for example, samples are taken from each of the four soil types in proportion to the amount of area covered by each soil type. Bioindicators Bioindicators are species of microorganisms, insects, fungi, and plants whose presence or absence in an environment are used to assess the quality of an environment and how it changes. Bioindicators are very sensitive to the smallest amount of pollution in their environment. If the bioindicators in an area decline, show signs of disease, leave an area, or die in large numbers, there is an issue in the ecosystem that must be addressed. These issues can be related to anthro- pogenic stressors, such as chemical contamina- tion, or natural stressors, such as drought or extreme weather changes. Many species can be used as bioindicators, including frogs, toads, salmon, prairie dogs, grizzly bear, mosses, lichens, and water lil- ies. Amphibians, such as the salamander (Uro- dela), are common bioindicators because many species breathe through their lungs and skin, Figure 19-17. Salamanders are extremely vul- nerable to environmental changes, including extreme temperature changes and poor water quality. Watching and counting populations of salamanders in a given area can be used to deter- mine if there are environmental changes that may lead to negative impacts on other species. There are more than 500 species of salamanders. Many of these species can be found in the Southeastern United States. Did You Know? Coniferous forest Deciduous forest Prairie Coniferous forest Deciduous forest Prairie Random Sampling Systematic Sampling Stratified Sampling Deciduous forest is 30% of the sample area Prairie is 50% of the sample area Coniferous forest is 20% of sample area Goodheart-Willcox Publisher Figure 19-16. Random sampling, systematic sampling, and stratified sampling are three methods used by scientists to determine the population of a species in an area. In the stratified sampling diagram, all areas are sampled based on the total percentage of land that they cover in the sample area. One sample is taken per 10% of the total area. Five samples are taken from the prairie, for example, because it is 50% of the sample area. Africa Studio/Shutterstock.com Figure 19-17. The salamander has survived for more than 160 million years. Salamanders are common bioindicators species because they cannot tolerate poor water quality or extreme temperature changes.