Other Causes of Extinction
One of the primary causes of extinction (already mentioned briefly) is introduction of exotic
species (alien orinvasive species). Both intentionally and inadvertently, humans have
introduced various species into habitats, which already have their own native species. As a
result, these invasive species have often had very harmful effects on the native species.
As long ago as 3500 BC, ships from Polynesian times brought crop species and domesticated
animals as well as stowaway rats and snakes. Recently cargo ships have transported zebra
mussels, spiny waterfleas, and ruffe into the Great Lakes via ballast water ((Figure25.31).
Europeans brought purple loosestrife and European buckthorn to North America to beautify
their gardens.
Figure 25.31: These zebra mussels, an introduced species, colonize most man-made and
natural surfaces, including native mussels. Here they have infested the walls of the Arthur
V. Ormond Lock, on the Arkansas River. They have caused significant damage to American
waterways, locks, and power plants. ( 19 )
Other invasive species have included the European starling, introduced by Shakespeare en-
thusiast Eugene Schieffelin to Central Park in the 1890s, because he thought Americans
should experience every bird mentioned in the works of Shakespeare. This species is a hole-
nesting species and has affected native species where it has been introduced (i.e. Australia,
North America) because of competition for nest sites. Other examples of invasive species
include the introduction of the cane toad, introduced to control the cane beetle, and the
brown tree snake (Figure25.26).
Manyoftheseexoticspecies, awayfromthepredationorcompetitionoftheirnativehabitats,
have unexpected and negative effects in the new ecosystems. Introduced species can disrupt