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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