Encyclopedia of Evolution.pdf - Online Reading Center

iogeography
their species through dispersal and subsequent evolution.
Larger islands have a greater variety of microhabitats. Moreover,
on larger islands, populations within a species are more
likely to avoid contact, resulting in greater speciation. For
both of these reasons, a greater number of species evolves on
large islands than on smaller islands. This is known as the
species-area relationship. The source of the colonist species is
the closest mainland. Islands that are closer to the mainland
receive more immigrant species, which allows the evolution
of more species native to the island. The first analysis of the
balances among immigration, evolution, and extinction on
islands, in which near versus far and large versus small islands
were contrasted, was the theory of island biogeography developed
by ecologist Robert H. MacArthur and evolutionary
biologist E. O. Wilson in 1967 (see Wilson, Edward O.).
Ice Ages
Genera and even species of plants are shared between the
arctic and alpine tundras, enough to allow these two regions
both to be called tundra. Arctic tundra is found around the
Arctic Ocean, while alpine tundra is on mountaintops. This
biogeographical pattern is explained mostly by the ice ages.
When glaciers were at their maximum extent, the arctic tundra
formed a band across what is now the northern United
States. The Rocky Mountains, Sierra Nevada, and Cascades
received tundra species from contact with this band of tundra,
and some mountains that were not in direct contact
with the tundra (e.g., the San Francisco Mountains of Arizona)
were close enough to receive tundra species by dispersal.
When the glaciers retreated northward, the tundra plants
retreated into the remaining zones of cold climate, either
northward, where they are found today in the arctic tundra,
or up the mountains, where they are today found on alpine
peaks. The alpine tundra is now stranded, sometimes in very
small patches (as in the San Francisco Mountains). This is not
the entire explanation for tundra plant species. The tundra of
the Sierra Nevada contains not only species affiliated with the
arctic, but also the evolutionary descendants of desert species
(such as the buckwheat Eriogonum). The tundra of the Sierra
Nevada not only has drier soil than other tundras but also is
close to the desert. Adaptations to cold and to drought are
often similar, and this similarity allowed some desert plants
to adapt to tundra conditions in California. This pattern is
also evident in some animals: The caribou of the cold climates
of North America is the same species as the reindeer of
the cold climates of Eurasia.
Most of the North American continent south of the tundra
was boreal forest (mainly spruce) and pine forests. Nearly
the entire area that is now the Great Plains was covered with
white spruce (Picea glauca). There was a little bit of grassland
in what is now Texas, but there was no hot desert. It
would have been an alien world to modern eyes. There were
many modern mammals, but also many mammals such as
mammoths and mastodons that have become extinct (see
Pleistocene extinction). Most of the trees and other plants
would have been familiar to modern observers, but they were
in combinations that no longer exist; for example, in some
places there were spruce trees scattered in grassland, an
arrangement uncommon today.
As the ice sheets retreated, each surviving species moved
to new locations independently, some dispersing more rapidly
than others. The glacial retreats, and the movements
of species, were not uniform; glaciers sometimes temporarily
advanced again. The forests are still moving, though too
slowly for human observers to notice without consulting
long-term records. The tallgrass prairie did not exist before
or during the ice ages. The tall, deep-rooted, warm-weather
grasses of the prairie were established during the period of
maximum warmth (the hypsithermal period, about 7,000
years ago). The prairies persisted even when conditions
became cooler and wetter (until American agriculture and
civilization destroyed most of them). The prairies persisted
because periodic fires killed any forest trees that began to
encroach. In North America, where most mountain ranges
are north-south, there were few barriers to the primarily
northward movement of plant and animal species. In Europe,
however, the Mediterranean Sea and mountain ranges such
as the Alps were barriers to northward movement. This helps
to explain why British forests have fewer species of trees
and spring wildflowers than do many American forests. The
explanation for current patterns of distribution and diversity
are rooted in the past, rather than in modern climatic conditions.
The greenhouse effect may be altering the patterns
of species movement that have been occurring for the past
few millennia.
Humans have had a tremendous effect on biogeographical
patterns. First, humans have carried species of
plants and animals from one place to another. In some
cases these plants and animals have become invasive species
that have displaced native species and greatly altered
the original environment. Humans have allowed species to
disperse, especially by ship and airplane, more effectively
than almost any species could previously have dispersed.
Second, humans have created a great deal of disturbed
habitat (such as farming, road building, and construction).
Species that specialize in disturbed areas (especially weeds)
have found a worldwide network of suitable habitats, courtesy
of humankind. Some of the most widespread species,
such as dandelions and barn swallows, live in areas altered
by human activity.
Further Reading
Bonnicksen, Thomas M. America’s Ancient Forests: From the Ice Age
to the Age of Discovery. New York: John Wiley and Sons, 2000.
Gillespie, Rosemary G. “The ecology and evolution of Hawaiian spider
communities.” American Scientist 93 (2005): 122–131.
Grant, Peter R. Evolution on Islands. Oxford: Oxford University
Press, 1998.
MacArthur, Robert H., and Edward O. Wilson. The Theory of Island
Biogeography. Princeton, N.J.: Princeton University Press, 1967.
MacDonald, Glen M. Biogeography: Space, Time, and Life. New
York: John Wiley and Sons, 2003.
Whitfield, John. “Biogeography: Is everything everywhere?” Science
310 (2005): 960–961.