ECOLOGICAL BIOGEOGRAPHY (Who lives where and why)
The distribution of any species of organism is limited by the
distribution of suitable environments. The three main climatic controls
of the plant communities are: TEMPERATURE, SOLAR RADIATION, and
PRECIPITATION. Each of these factors affects the vegetation directly
or indirectly through its effect on the soil. Average intensity and
distribution of these factors throughout the year are both important.
In turn, the distribution of plant communities, in association with
climatic factors, determines the type of animal communities that can
live in any area.
Temperature and radiation have a familiar north-south gradient.
There is also an obvious east-west differentiation, especially through
the Temperate Zone, which is due mainly to changes in precipitation
and subsequent evaporation. There are many irregularities in this
gradient. These are all HORIZONTAL zones of climate. There are also
VERTICAL zonations, such as one finds when climbing a mountain.
As one ascends he encounters conditions similar to those at sea
level farther north. In general, we find an increase in altitude
or elevation of 1 000 feet is roughly equivalent to a loss of one
month in time of the growing season.
The physical characteristics of the environment determine the
types of organisms that can settle there initially. Winds, temperature,
amount, type, and temporal distribution of precipitation, chemical
composition of the surroundings, latitude, altitude and soil conditions
all contribute. The combination of precipitation and temperature is
fundamentally important in determining the distribution of general
vegetation, which, in turn, determines the distribution of animal life.
Temperature affects the rate of physiological processes,
reproduction, and the survival or death of plants. Soil temperature
influences the rate of absorption by the roots and the rate of root growth.
In addition, by its very presence, a particular community of organisms
gradually alter local conditions.
In the last analysis, it is the climate -- the basic action of
wind, rain, temperature -- that molds an environment. In both
hemispheres, just beyond the tropics, trade winds blow toward the
equator. They carry moisture which falls in tropical areas creating
tropical forests. But to the north and the south these same winds
leave behind patches of desert and various grassland-shrub areas,
such as savannahs. Still farther north and south, winds from polar
and subtropical zones mix, forming variable rain patterns that produce
temperate forests and grasslands.
These large terrestrial areas, dominated by particular plant
communities, are known as biomes. While biomes are repeated around the
world, each continent has its own particular version of each as well
as the animals that have adapted to it.
Terrestrial Biomes
TUNDRA
There are two types: Arctic, which is found above 60 degrees north latitude,
and Alpine which is found at elevations over 7 000 feet
The major limiting factor is water availability since water is usually
in the form of snow, ice, or permafrost.
Climate - cold, ranging from 30-700 F. on average; permafrost;
short growing season of about 60 days; frequent winds; soil often wet
with poor drainage resulting in boggy areas; rainfall averages 10-17.5 cm
(4-7 inches), usually during growing season.
Vegetation - lichens, mosses, dwarf and woody plants, e.g. dwarf willows,
heath, birches, cranberry bushes, sedges and rushes
Plant Adaptations - dwarfism; small, hairy leaves for water conservation,
ability to survive in frozen condition, vegetative propagation,
e.g. runners
Animal life - rodents, arctic hare, caribou, wolf, grizzly bear, polar bear,
snowy owl, few invertebrates - mosquitos, black flies; waterfowl may use
bog areas for breeding in summer
Animal Adaptations - white coloration, hibernation, burrowing
TAIGA (Coniferous Forest)
These areas tend to circle the northern hemisphere. They are usually found
between 50-60 degrees north latitude. The major limiting factor is light,
particularly due to dense shade created by the trees.
Climate -
cold and severe winter climate, growing season of 3-5 months;
below freezing most of the year; 60-700 F. during the summer;
rainfall of 25-75 cm (10-30 inches) per year; much of the precipitation
is snow
Vegetation - needle or scale leaved evergreen trees (spruces, firs, cedars,
pines) poor shrub and herb layer
TEMPERATE DECIDUOUS FOREST
These areas are common in the eastern United States. The major limiting
factor is light availability.
Climate - temperate, long summers, fairly constant temperatures, deep rich
soil, moderate rainfall (50-150 cm [20-60 inches]); evenly distributed
seasonally
Vegetation - deciduous trees and shrubs, large shrub and herb layer,
diverse species; cherry, oak, basswood, maple, beech, birch, hemlock
Plant Adaptations - leafless in winter, heavily foliated in summer
Animal life - all groups represented: mammals, birds, reptiles, amphibians,
many insects; white-tailed deer, wild cats, foxes, bears, arboreal birds
Animal Adaptations - sharp claws, movable scales in snakes, hibernation,
estivation, migration, various nesting habits
GRASSLANDS
Represented by prairies in North america, savannahs in Africa, and steppes
in Russia and Argentina. The major limiting factor is light availability
and seasonal distribution of precipitation.
Climate - similar to Temperate Deciduous Forest: major difference is
uneven seasonal distribution of rainfall, mostly 25-100 cm (10-40 inches)
in the spring
Vegetation -
mostly grasses, few trees, some wild flowers; grass species
dependent upon location
Plant Adaptations - deep, fibrous root system - sometimes 2 meters deep;
creates sod formation
Animal life - jack rabbit, antelope, fox, bison, coyote, wolf, cougar,
rodents, reptiles, insects
Animal Adaptations - running, leaping, burrowing
DESERTS
These areas make up about 1/7 of the total land surface. They are found
in areas of high pressure. Low altitude deserts tend to be hot, while high
altitude deserts, such as the Gobi, can be quite cold. There is a distinct
lack of organic material in the soil and high mineral salt concentrations
leading to an alkaline pH. Major limiting factor is water availability.
Climate - intense sunlight leading to hot days and cold nights;
high evaporation rate, constant winds; low annual rainfall of
2.5-10 cm (1-4 inches) occurring once or twice a year
Vegetation - xerophytic plants, annual plants, succulents, shrubs;
drought evaders with seeds that germinate quickly after a rain and have
a rapid life cycle and drought resisters with deep root systems
Plant Adaptations - hairy leaves and stems, leaflessness, water storage
in succulent stems, accelerated life cycles; phytotoxin and antibiotic
production, photosynthetic stems
Animal life - reptiles, rodents, coyote, foxes, insects, burrowing animals
Animal Adaptations - uric acid waste products allow for water retention;
nocturnal, protective coloration, estivation, hibernation
TROPICAL FORESTS
Found near equator and in lower section of temperate zones. Dependent
upon amount of rainfall, it may be a tropical rain forest or just a tropical
forest. Major limiting factor is light.
Climate - high year round temperatures, uniform length of night and day;
at least 200 cm (80 inches) of rain per year - may go as high as 1 000 cm
(400 inches)
Vegetation -
rain forests, woody vines, epiphytes, many species with few
dominants; little underbrush, poor leached soil; bamboo, orchids, mistletoe
Plant Adaptations - plants woody and grow as trees; distinct canopies
Animal life - exceedingly varied; ants, mosquitos, frogs, monkeys, birds,
few mammals
Animal Adaptations - nocturnal, fruit-feeding, hanging nest
