Encyclopedia of Evolution.pdf - Online Reading Center

Van Till, Howard J. “E. coli at the No Free Lunchroom: Bacterial flagella
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invasive species Invasive organisms enter a habitat in
which they did not evolve and experience a population explosion
(see population). While it is a natural process, species
invasion has been accelerated by human activity. The invasive
species often has a severe negative impact upon the community
of species that it enters. Species invasion has affected
the evolution of life on Earth. One well-known example is
the invasion of placental mammals from North into South
America following the formation of the Panama land bridge
(see mammals, evolution of).
When individuals of a species enter into a community
from outside, their most likely fate is death. The climatic and
microclimatic conditions in the new habitat will probably be
unsuitable for the introduced species. Even if the new species
survives, its populations may grow slowly, and it may
be eliminated by competition with the better-adapted native
species.
In some cases, however, an invasive species (also called
alien, or introduced, or exotic species) may find the new habitat
to be ideal, not only for survival but as an area in which
to proliferate wildly. The native range of the introduced species
may have been very small, while its new range is very
extensive. The Monterey pine (Pinus radiata) was native to
three small forest patches in California, but upon its introduction
to New Zealand it spread over large areas.
Human activity has had many significant worldwide
impacts, including destruction of habitats, soil erosion, pollution,
and depletion of resources. Many of these impacts are
reversible: forests grow back, soil builds back up, and pollution
degrades, after the departure of humans. However, once
a new species has been introduced, and if it becomes a problematic
invasive species, its presence is permanent and may
resist even the most focused eradication efforts. Human activities
that cause the rapid spread of introduced species include
the following:
• Most of the major American agricultural and urban weeds,
including the ubiquitous dandelion Taraxacum oficinale,
came from Europe, probably because their seeds mixed
with those of crop and garden species. Weeds thrive in
disturbed areas, and humans have not only created many
thousands of square kilometers of disturbed areas (such as
farms and cities) but carry out a regular traffic among these
disturbed areas. Many more weed species have made the
journey from Eurasia to America than vice versa because
invasive species
agriculture has existed longer and been more extensive
in Eurasia. Weeds evolved in areas of human disturbance
from ancestors that specialized in natural disturbances (see
agriculture, origin of).
• Plants and animals may be brought to a new location as
livestock, pets, or ornamentals. Ring-necked pheasants
(Phasianus colchicus) were brought from China as game
birds. Purple loosestrife (Lythrum salicaria) was an ornamental
plant, brought to America from Europe. Some of
these species have peculiarly interesting stories. Starlings
(Sturnus vulgaris) were introduced by a man who wanted
America to have all of the bird types mentioned in Shakespeare.
One of the most persistent of American weeds,
velvetleaf (Abutilon theophrasti), was brought to America
because its beautiful immature fruits could be used to
make decorations on slabs of butter, hence its other common
names “stampweed” and “butterprint.” In each case,
organisms escaped and experienced rapid population
growth. Many feral livestock animals now roam free, such
as goats and pigs in Hawaii, and donkeys in Death Valley.
• Aquatic organisms live in bilgewater in ships that cross the
ocean and escape when bilgewater is dumped. Eurasian
watermilfoil (Myriophyllum spicatum) and the zebra mussel
(Dreissena polymorpha), both of which are proliferating
in many American waterways, may have been introduced
in this way. The Asian tiger mosquito (Aedes albopictus),
a vector of viral diseases, came to North America in pools
of water that had accumulated in tires being shipped from
Asia to the United States.
• New species are sometimes deliberately released to alter the
natural environment. Australian melaleuca trees (Melaleuca
quinquinervia) were released into the Everglades in order
to dry them up for business and residential development.
In nearly all cases, the new species was introduced, while
the predators and parasites that would normally have held its
populations in check were not. Native species of the community
may accidentally help the invader to spread; for example,
muskrats help spread the underground stems of loosestrife.
The introduced species have many profound influences
on the community they invade. They may displace native species
or even drive them to extinction:
• Fire ants (Solenopsis invicta) are native to South America,
but were introduced into the United States about 1930.
They have spread throughout the southern states and
have recently entered California as well. Now called Red
Imported Fire Ants, they cause painful stings, kill some
ground-dwelling wild animals, and damage some equipment
and some agricultural crops.
• Ferocious African honeybees have displaced European honeybees
(both varieties of Apis mellifera) from the tropical
zones of the Americas, reaching as far north as southern
Texas. African “killer bees” have had a longer history of
exploitation by humans and other predators, and their evolutionary
response has been a tendency to attack potentially
dangerous animals. African bee queens were allowed to
escape from a breeding experiment in Brazil in the 1960s.