Oceans
Oceans are the largest major habitat and cover 70
percent of the Earth's surface. Along marine coastal areas is a littoral
zone. Tidal action, upwelling currents, and the influx of fresh water
from rivers significantly influence the biota (flora and fauna) that can
exist. Tides circulate loose organic particles, or detritus, to detritus
feeders such as marine crabs. Detritus is consumed by oysters, zooplankters,
and barnacles. Phytoplankters thrive where upwelling currents circulate
phosphorus into the upper layers of a body of water (the euphotic zone)
in which plant growth is possible. Blue crabs have adapted to the coastal
river estuary habitat with the associated problems of regulating osmotic
pressure. Below the euphotic zone, from parts of the continental shelf
bottom down to the abyssal plain of
perhaps 5,000 m (16,400 ft), is an aphotic zone, or light-lacking zone,
that is inhabited by glass sponges, lampshells, brittle stars, and microorganisms.
Plant life, as humans know it, does not function here. The aphotic zone
nekton, or free-swimming aquatic organisms such as fishes, prawns, and
squids, have bioluminescent organs or symbiotic bacteria that supply illumination,
or both. The angler fish has huge jaws and teeth and uses a luminescent
lure that it dangles in front of its jaws. Off the southeastern coast
of North America is the Sargasso Sea. This habitat is formed by a brown
seaweed that is carried from its shoreward littoral habitat into open
water. It grows vegetatively and forms this special habitat that offers
food and shelter to many species of zooplankton. Phytoplankton productivity
is limited here, apparently because environmental minerals such as iron,
nitrogen, and phosphorous are scarce.
Freshwater Lakes
The freshwater lake, or lentic, habitat has a littoral zone that is euphotic
and hence may be heavily populated by aquatic plants. Very deep lakes
have low phytoplankton density and few rooted aquatic plants because nutrients
are scarce and the bottom is dimly lighted. The benthos (bottom) of many
lakes consists of oxygen-demanding organic detritus and is thus anaerobic.
Benthic life such as bacteria and midge larvae, however, thrive and utilize
the organics as food. With time, lakes change physically and chemically.
The classic concept of aging in lakes involves a progression of changes.
Deep, nutrient-poor oligotrophic lakes with trout, whitefish, and cisco
evolve into shallow, richly organic eutrophic lakes with blue gills, black
bass, and pike.
Stream Habitats
A current of water is the main physical feature of stream, or lotic, habitats.
As a stream flows toward sea or lake level, suspended particles and chemical
nutrients accumulate and dissolved oxygen decreases. Algae and mosses
attach to rocks and contribute some to community organic production, but
basically the lotic habitat depends on the importation of organic foods
such as tree leaves. The imported organics form the base of an important
detritus food chain populated by snails, midge larvae, and mayflies. With
a longitudinal change in physical and chemical features, the biota also
changes. Trout and various species of darters give way to bass, pike,
and catfish as the stream approaches sea or lake level.
Terrestrial Habitats
A temperature and moisture complex dictates the major terrestrial habitat
types that range from the verdant tropical rain forest to the snow-covered
polar regions. Tropical rain forests are tall and highly stratified, and
each stratum offers shelter to specialized animal life forms. Temperate
climates support deciduous forests of such trees as beech, oak, and maple.
The deciduous forest once covered eastern North America and Europe. Coniferous
forests of pines, spruces, and fir most typically occur in cool climates.
Further poleward in the Northern Hemisphere is the tundra, a marshy-plain
habitat that is frozen for much of the year. The vegetation consists of
lichens, grasses, and dwarf woody plants with species of animals such
as the lemming, snowy owl, and Arctic hare.
In North America a moisture gradient extends from the eastern deciduous woodland to the western deserts. The eastern deciduous forest type is considered mesophytic (moist) and is the home of deer, squirrels, and opossums. Ungulates such as the pronghorn and bison of North America and the zebra and antelope of Africa typify the pristine grassland habitat. Desert habitats generally receive less than 250 mm (10 in) of annual rainfall and occur in tropical and temperate regions of the world. Not all deserts are as devoid of life as the Sahara of North Africa. The desert biota has evolved strategies for utilizing scant water supplies. Some kangaroo rats of the New World deserts and jerboas of Old World deserts can actually exist without drinking water. The temperature and moisture gradients that occur latitudinally from equator to pole are somewhat duplicated in mountainous areas. As altitude changes, desert or grassland habitat can change, perhaps to tundra.
In all habitats, large or small, the biota has adapted to the physical and chemical features of the environment. Wind, moisture, salinity, concentration gradients of chemicals, and even other organisms affect the habitat and influence the success of a species trying to survive in nature.
Larry D. Caldwell
Bibliography: Brown, J. H., and Gibson, A. C., Biogeography (1994); Buckley, G. P., ed., Biological Habitat Reconstruction (1994); Cox, C. B., and Moore, P. D., Biogeography, 5th ed. (1993); Hare, Tony, Habitats (1994); Howe, H. F., and Westley, L. C., Ecological Relationships of Plants and Animals (1990); Sutherland, W. J., and Hill, D. A., eds., Managing Habitats for Conservation (1995).
