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A population is a group of organisms of the same species living in the same place at the same time. For example, all the rattlesnakes in a desert are a population. All the crayfish in a stream are a population. All the people in the United States are a population. The number of organisms in a population can change over time. The introduction of a pollutant into an ecosystem can cause the size of a population to decline, or get smaller. A pollutant is a substance that is harmful to the environment. It can make an area unsuitable for living things. Humans are responsible for releasing pollutants into ecosystems. Humans use fuels such as coal, oil, and gasoline for energy. However, burning these fuels releases pollutants into the air. For example, car exhaust from burning gasoline is a pollutant. Pollutants can combine with water in the air to form acid rain. Acid rain has many negative effects. Acid rain changes the chemistry of soil. This causes trees and other plants to die. Acid rain also damages the leaves of trees. Without leaves, trees cannot make the food they need to live. In some parts of the world, acid rain has killed entire populations of trees in forests. When acid rain falls into lakes, it changes the chemistry of the water. Many of the populations living in the lake die. Fish and small water organisms cannot live in water that is acidic, so populations of these organisms decline. Acid rain also causes mercury in a lake to dissolve in the water. Organisms then take mercury into their bodies through their gills. Populations of clams and snails decline because of mercury poisoning. Then, the populations of organisms that eat clams and snails do not have food to eat, and they decline, too. Another type of pollutant is oil. Humans transport oil to different places so they can use it for energy. However, oil sometimes leaks or is spilled during this transport. Spilled oil is a dangerous pollutant in ecosystems. When an oil tanker spills oil in the ocean, many populations are affected. Oil in the water can affect populations of sea birds. The oil gets on the birds' feathers. They cannot fly as well, so they are more likely to be caught by predators. Birds try to clean the oil from their feathers. In the process, they eat the oil, which can poison and kill them. Discovery Education Science © 2007 Discovery Communications, LLC Page 1 of 2 Populations and Pollution Oil spills can also affect the population of killer whales. The whales may eat fish that are covered in oil. The oil can kill the whales. Oil can also block the whales' blowholes. When this occurs, the whales cannot breathe and they die. Oil spills can also affect the population of otters. The oil gets into the otters' fur and affects the fur's ability to keep the otters warm. As a result, the otters' body temperature drops and they can die of hypothermia. Discovery Education Science © 2007 Discovery Communications, LLC Page 2 of 2