Encyclopedia of Evolution.pdf - Online Reading Center

invertebrates, evolution of
Unable to tolerate cold winters, the African bees will probably
not extend beyond the southernmost tier of states.
• The Nile perch (Lates niloticus) was introduced into Lake
Victoria in Africa, where it has driven many of the native
species of fish nearly to extinction.
• Mainland birds and other introduced animals have driven
the native birds of Hawaii away from the coastlands, and
many into extinction. The few remaining native Hawaiian
birds are largely restricted to the highlands.
• European grasses, originally spread in the droppings of
horses that escaped from the Spaniards, have displaced the
native California grasses.
• Populations of melaleuca and loosestrife (see above) outgrow
those of native plants because they produce millions of seeds.
• Introduced African grasses inhibit the germination of native
Hawaiian tree seedlings.
Invading species may create many disturbances. Feral
pigs in Hawaii create disturbances in the rain forest, in
which the native plant species cannot grow well. Plant species
on islands have few if any defenses against large browsing
or grazing animals, as large animals seldom disperse to
islands (see biogeography). However, introduced plants,
such as the strawberry guava, thrive in the disturbances created
by the feral pigs. When grasses are first introduced into
a community that previously had none, they can create a fire
hazard. Fires deplete the populations of species that are not
fire-resistant and further encourage the spread of the grasses,
which can grow back after the fires.
Invading plant species can inhibit the growth of native
species. The leaves of the wax myrtle (Myrica cerifera), which
has invaded Florida wetlands, do not decompose well and
have literally clogged up the aquatic food chain. The deep
litter of casuarina trees (Casuarina equisetifolia) introduced
into Hawaii from southeast Asia inhibits the growth of practically
everything under their canopy. Kudzu vine (Puereria
montana), introduced from Asia, drapes over and kills many
trees in the southeastern United States.
Exotic predators have disrupted whole food chains. For
instance, the brown snake (Boiga irregularis) has eaten many
of the birds on the island of Guam.
Invading species can have severe economic impacts on
agriculture and even on public health, as well as on nature
preserves. Much money and effort are spent in attempts
to control their spread, often by means that are as dangerous
as they are ineffective, such as pesticides or mass hunting
campaigns. International cargo and luggage are carefully
inspected to assure that there are no exotic plant or animal
stowaways.
Scientists have investigated biological controls as a
method of bringing escaped exotic species under control.
Loosestrife is not a pest in Europe, where its populations are
controlled by beetles. Scientists are now attempting to introduce
those beetles to North America. Other scientists are trying
to introduce parasitic flies, whose larvae develop inside
the heads of Red Imported Fire Ants and cause the ants’
heads to fall off, as a biological control agent. Biological controls,
unlike pesticides, can be permanent.
Eventually, other species (either native or themselves
introduced) will evolve ways of eating the invaders, bringing
their populations into check (see coevolution). The time
required for this process, however, is likely to make it meaningless
to human concerns.
Further Reading
Agricultural Research Service, United States Department of Agriculture.
“Biological control sends scurrying scourge scrambling.”
Available online. URL: http://www.ars.usda.gov/is/AR/archive/
may03/biol0503.htm. Accessed September 22, 2005.
Burdick, Alan. “The truth about invasive species.” Discover, May
2005, 34–41.
Cox, William. Alien Species and Evolution: The Evolutionary Ecology
of Exotic Plants, Animals, Microbes, and Interacting Native
Species. Washington, D.C.: Island Press, 2004.
National Agricultural Library. “Invasivespecies.gov: A gateway to
federal and state invasive species activities and programs.” Available
online. URL: http://www.invasivespecies.gov. Accessed April
18, 2005.
Sax, Dov F., John J. Stachowicz, and Steven D. Gaines, eds. Species
Invasions: Insights into Ecology, Evolution, and Biogeography.
Sunderland, Mass.: Sinauer Associates, 2005.
Todd, Kim. Tinkering with Eden: A Natural History of Exotic Species
in America. New York: Norton, 2002.
United States Department of Agriculture. “Animal and Plant Health
Inspection Service: Safeguarding American agriculture.” Available
online. URL: http://www.aphis.usda.gov. Accessed April 18,
2005.
Vogt, J. T., Stanley A. Rice, and Steven A. Armstrong. “Seed preferences
of the Red Imported Fire Ant (Hymenoptera: Formicidae)
in Oklahoma.” Journal of Entomological Science 38 (2003):
696–698.
invertebrates, evolution of Invertebrates are animals
without internal skeletons and backbones. Invertebrates do
not constitute a formal taxonomic group (see cladistics) but
are a category that contains all animals, except for the vertebrate
lineage within the chordate phylum (see Linnaean system).
It is a distinction that human observers, as vertebrates,
can hardly avoid making within their view of the world. As
with other groups of organisms, the evolutionary history of
invertebrates can be reconstructed by comparisons of DNA
(see DNA [evidence for evolution]), comparisons of the
structures of living forms, and from fossils.
Because of the internal skeletal system, some very large
vertebrates have been able to evolve (see dinosaurs). The
external skeleton, and the various manners by which invertebrates
breathe, have placed a size constraint on invertebrates
(see allometry). Therefore, with the exception of some
large oceanic mollusks (giant clams and giant squid), and the
Anomalocaris of the Cambrian period (see Burgess shale),
all invertebrates have been and are relatively small.
All members of the animal kingdom are believed to be
descended from an ancestral colonial protist. Because the
most primitive animals are the sponges (see below), the most
likely protistan ancestors are today represented by the choanoflagellates,
which are protists that form clusters and grab