Invasive species are species that are introduced into an environment but are not naturally found in that environment. One example of an invasive species is the American gray squirrel, introduced into Britain at the end of the 18th century. Until 1876 the only native squirrel in Britain was the European red squirrel, which was found in deciduous and coniferous forests. By 1940 the gray squirrel had displaced the red squirrel across most of the British Isles, and by 1984 the red squirrel was only found in isolated coniferous woodland areas. After its initial introduction, the gray squirrel population increased rapidly; however, in recent years population sizes within specific environments have become stable.

Invasive species are species that are introduced into an environment but are not naturally found in that environment. One example of an invasive species is the American gray squirrel, introduced into Britain at the end of the 18th century. Until 1876 the only native squirrel in Britain was the European red squirrel, which was found in deciduous and coniferous forests. By 1940 the gray squirrel had displaced the red squirrel across most of the British Isles, and by 1984 the red squirrel was only found in isolated coniferous woodland areas. After its initial introduction, the gray squirrel population increased rapidly; however, in recent years population sizes within specific environments have become stable.

The Great Lakes

The ecological balance of the Great Lakes has been seriously altered by human civilization, which has disrupted species in food chains from the producers (microscopic plankton) to the predator fish. This disruption has caused a reduction in the natural game fish population of the lakes.

For example, the Atlantic salmon was formerly found in Lake Ontario. The salmon's passage to Lake Erie was blocked by Niagara Falls. This fish was a prize catch for fishermen, but no Atlantic salmon have been caught in Lake Ontario since 1890. Soil that eroded from farmland covered the salmon's gravel spawning grounds with silt, trees that shaded the streams where the young salmon lived were torn down as forests were cleared, and dams built for sawmills prevented the salmon from traveling upstream to spawn.

As the population of Atlantic salmon and other deepwater fish declined, two alien marine species—the alewife and the sea lamprey—appeared in Lake Ontario. These intruders most likely entered the lakes by way of the Hudson River, the Erie Canal, and the Oswego River. The lack of predators and the abundant food supply led to a rapid increase in the populations of these species until they were the dominant species in the lake. Furthermore, the completion of the Welland Canal linking Lake Erie and Lake Ontario in 1932 gave these species access to the remainder of the Great Lakes. By the early 1970's, about half of the fish caught throughout the Great Lakes were alewives. The sea lamprey's increasing population devastated the desirable fish in the lakes, harming the fish industry.

Government agencies such as the United States Fish and Wildlife Service took action at this point to restore the game fish to the lakes and return sport fishing to this region. They developed a lampricide that proved fatal to the young sea lampreys, but had no harmful effects on other species. They also started stocking the lakes with predator game fish such as coho salmon, chinook salmon, and steelhead trout. As a result of these actions, the populations of the alewife and the sea lamprey have both decreased, and sport fishing has returned to Lake Ontario.

The Great Lakes

The ecological balance of the Great Lakes has been seriously altered by human civilization, which has disrupted species in food chains from the producers (microscopic plankton) to the predator fish. This disruption has caused a reduction in the natural game fish population of the lakes.

For example, the Atlantic salmon was formerly found in Lake Ontario. The salmon's passage to Lake Erie was blocked by Niagara Falls. This fish was a prize catch for fishermen, but no Atlantic salmon have been caught in Lake Ontario since 1890. Soil that eroded from farmland covered the salmon's gravel spawning grounds with silt, trees that shaded the streams where the young salmon lived were torn down as forests were cleared, and dams built for sawmills prevented the salmon from traveling upstream to spawn.

As the population of Atlantic salmon and other deepwater fish declined, two alien marine species—the alewife and the sea lamprey—appeared in Lake Ontario. These intruders most likely entered the lakes by way of the Hudson River, the Erie Canal, and the Oswego River. The lack of predators and the abundant food supply led to a rapid increase in the populations of these species until they were the dominant species in the lake. Furthermore, the completion of the Welland Canal linking Lake Erie and Lake Ontario in 1932 gave these species access to the remainder of the Great Lakes. By the early 1970's, about half of the fish caught throughout the Great Lakes were alewives. The sea lamprey's increasing population devastated the desirable fish in the lakes, harming the fish industry.

Government agencies such as the United States Fish and Wildlife Service took action at this point to restore the game fish to the lakes and return sport fishing to this region. They developed a lampricide that proved fatal to the young sea lampreys, but had no harmful effects on other species. They also started stocking the lakes with predator game fish such as coho salmon, chinook salmon, and steelhead trout. As a result of these actions, the populations of the alewife and the sea lamprey have both decreased, and sport fishing has returned to Lake Ontario.

The Great Lakes

The ecological balance of the Great Lakes has been seriously altered by human civilization, which has disrupted species in food chains from the producers (microscopic plankton) to the predator fish. This disruption has caused a reduction in the natural game fish population of the lakes.

For example, the Atlantic salmon was formerly found in Lake Ontario. The salmon's passage to Lake Erie was blocked by Niagara Falls. This fish was a prize catch for fishermen, but no Atlantic salmon have been caught in Lake Ontario since 1890. Soil that eroded from farmland covered the salmon's gravel spawning grounds with silt, trees that shaded the streams where the young salmon lived were torn down as forests were cleared, and dams built for sawmills prevented the salmon from traveling upstream to spawn.

As the population of Atlantic salmon and other deepwater fish declined, two alien marine species—the alewife and the sea lamprey—appeared in Lake Ontario. These intruders most likely entered the lakes by way of the Hudson River, the Erie Canal, and the Oswego River. The lack of predators and the abundant food supply led to a rapid increase in the populations of these species until they were the dominant species in the lake. Furthermore, the completion of the Welland Canal linking Lake Erie and Lake Ontario in 1932 gave these species access to the remainder of the Great Lakes. By the early 1970's, about half of the fish caught throughout the Great Lakes were alewives. The sea lamprey's increasing population devastated the desirable fish in the lakes, harming the fish industry.

Government agencies such as the United States Fish and Wildlife Service took action at this point to restore the game fish to the lakes and return sport fishing to this region. They developed a lampricide that proved fatal to the young sea lampreys, but had no harmful effects on other species. They also started stocking the lakes with predator game fish such as coho salmon, chinook salmon, and steelhead trout. As a result of these actions, the populations of the alewife and the sea lamprey have both decreased, and sport fishing has returned to Lake Ontario.