Biomes and the Physical Environment
•Temperature and precipitation determine ranges of species and biomes
•Atmospheric circulation patterns determine climatic zonation
•Whittaker and Walter plots summarize climatic data
•Zonation in freshwater and marine environments
Plant geographers noticed areas with similar climates had
similar looking plants, though often from unrelated families
A tree-forming cactus from
Mexico (Cactaceae)
An East African spurge
(Euphorbiaceae)
Leafless, thick, fleshy branches with spines have evolved to minimize water loss in unrelated
plants—convergence. Cactus spines are leaf petioles and euphorb spines are stem spurs.
Climate is the major determinant of plant distribution
The distribution of sugar maple—this area has the temperature
and precipitation regime that the species needs
Other maple species have different ecological tolerances and their
ranges differ somewhat
Heinrich Walter’s biome classification was based on climate zones
Whittaker plotted temperature and precipitation data for each biome
At high
temperatures,
precipitation
varies greatly
Variation in
precipitation
decreases as
temperature
decreases
Hadley cells—a circulation pattern resulting from differential heating
Warm moist air
rises in the
tropics—
constant rain
Convergence
zone—constant
downwelling of
cool dry air
Earth’s rotation deflects the surface flows in the Hadley cells because of
the Coreolis force—rotational speed of the earth is highest at the equator
20,000 km/d
35,000 km/d
Clockwise circulation
40,000 km/d
Direction of spin
Counter-clockwise
circulation
Air flowing toward the equator will be deflected west—it will be
spinning more slowly than the earth and lag behind
poleward flow will be deflected east—it will be spining faster than the earth
and surge forward
Hadley cell pattern establishes the major
deserts and rain forest zones of the world
Walter Climate diagrams
At higher temperatures plants require more water
When the blue line is
above the orange line,
precipitation is plentiful
and temperature is the
main determinant of
plant growth
Generally plants need about 20 mm of monthly ppt for every 10C
so the Precipitation scale interval is twice the temperature scale
Walter plots allow meaningful comparisons among sites
Tropical rain forest
Blue always above orange
Tropical seasonal forest
Blue below orange
during dry season
Desert
Blue always below orange
Global distribution of the world’s biomes
Climate doesn’t work nearly as well for classifying aquatic environments
Light reaches the
bottom—rooted plants
Phytoplankton are
the major primary
producers
little light—
few rooted
plants
Little wave disturbance
Sediments accumulate
The lake environment is structured around light and temperature
The dimictic mixing pattern of temperate zone lakes
In the winter the lake is
inversely stratified
The spring and fall
are mixing periods,
water column 4oC
In the summer termperate
lakes are stratified
Epilimnion—warm mixed
layer,
Hypolimnion—cold lower
layer
Thermal stratification allows both warm and cold water species to coexist
Ecological zones in the ocean
Light, the continental shelf, and tides contribute to marine zonation
The intertidal zone is exposed to air regularly when the tide recedes
Coral reef are
found in shallow
well lit seas—
highly productive
systems
The major ocean currents are driven by winds and the earth’s rotation
Upwelling zones occur mainly on the western sides of continents
Surface waters are drawn towards the equator and away from the continent by the coreolis force—
this draws deep nutrient-rich water upward
The major ocean currents “gyres” generate zonation in temperature
and productivity
A satellite image showing the gulf stream, a clockwise current in the North Atlantic