WHAT ARE THE TROPICS?
And, how do they work?
2-10-08
TROPICAL HABITATS
• Tropical lowland rainforest (=jungle, hylaea) - >100 inches of rain
per year
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Non-seasonal forest
Biodiversity is very high
Broad-leaved plants
Lianas and epiphytes abundant
Abundant and constant rainfall
Components:
• Floodplain – where rivers swell then drop (2% of the Amazon Basin)
• Varzeas – seasonally flooded with white water (2% of the Amazon Basin)
• Terre firme – high ground that does not flood (this is ca. 96% of the Amazon
Basin)
• Tropical higher-ground rainforest (Chocó of Colombia, 360 inches
of rain per year)
• Tropical moist forests
• Deciduous forests
• Dry forests
• Cloud forests – relatively high, cloud enshrouded (thus moist)
forests
• Páramos – high, cold moist plateau
As one moves north or south from the tropics, there is a
latitudinal change in biomes: tropical rainforest, temperate
forests, taiga, tundra, polar ice. As one moves up mountains, one
may see the same progression (depending on the height and
location of the mountain). Rule-of-thumb: for every 1000m one
ascends in elevation, average temperature drops 6ºC.
Another view of the same phenomenon.
Humbolt’s Law
Each 300 ft ascent up a mountain
in the tropics has the same average
temperature change as if moving
67 miles toward the pole.
TYPES OF RIVERS
White water rivers – sediment (from the
Andes) rich rivers that are turbid
(usually reddish)
 Black water rivers – drain ancient soils
with little mineral content; clear, but
usually dark due to organic materials
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– Igapó – forests that line these rivers
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Clear water rivers – low levels of
sediment and/or decomposing
vegetation
WHAT DETERMINES SEASONS
IN THE TEMPERATE ZONES?
The arrival of cold weather.
WHAT DETERMINES SEASONS
IN THE TROPICS?
Rainfall: wet season vs.
dry season.
WHAT ARE BELIZE’S RAINY SEASONS?
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Belize has two – sort of.
The real rainy season begins end of May/beginning of
June, and peaks Sept-Nov. It is characterized as short rains
that fall “in buckets.”
– It can start in Toledo in May/June (average rainfall is 190 inches)
– But in Corozal to the north it may start in July/August (average
rainfall is 30 inches)
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The other rainy season is caused by “northers” – the
remnants of U.S. winter storms that drift south.
– Usually last a couple of days as off and on long drizzly rains.
– May start in November and end in April, but the norm is January &
February and affect the whole country the same.
RAINFALL IN AMAZONIA
WHY DOES IT RAIN IN THE
TROPICS?
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Humid forests, hot weather – so, lots of
humidity in the air.
The warm, humid air rises (convection) and
forms clouds.
The higher the air goes, the cooler it gets.
Cool air can’t hold as much moisture as warm
air.
The clouds get higher, darker, then they drop
their water as rain.
The next day, the cycle repeats itself.
WHY ELSE DOES IT RAIN IN
THE TROPICS?
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The waters of the tropics are warm.
Prevailing winds are consistently from the
same direction.
They pick up moisture as they sweep across
the water.
Either the moisture laden winds get caught up
in the convection when they hit land and
ultimately drop their water, or
They hit mountains, are forced up to cooler
levels, then drop their water.
Huge amounts of rain are not
required if there is
constant/frequent cloud cover
that supplies humidity to the
plants.
Ex: cloud forests
TRANSPIRATION
Plants suck water from the soil
 When there is too much water in their
circulatory system, it evaporates via
mesophyll tissue on the underside of
the leaves
 As the water evaporates, its loss pulls
water up the plant from the soil.
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CLIMATE: 64.4ºF IS THE KEY
Tropical: January average of >64.4°F.
 Subtropical: January average fall
between 32°F and 64.4°F.
 Temperate: January average at least
sometimes falls below 32°F.
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LOCATION: The tropics are located
between the Tropics of Cancer (23.5°N)
and Capricorn (23.5°S).
BELIZE TERRESTRIAL HABITATS
GUYANA MIXED FOREST PROFILE
CONTINTENTAL DRIFT
The Permian World – 225 million years
before present (mybp). We call the single
continent Pangea. From Strahler. Physical Geography. 4 Ed.
th
The Late Triassic world – 180 mybp. Note
the northern continent – Laurasia, and the
southern continent – Gondwana (or
Gondawanaland). From Strahler. Physical Geography. 4 Ed.
th
The Late Jurassic world – 135 mybp. Gondwana has
broken up into proto-India and a single land mass that
will break into proto-Antarctica and proto-Australia.
South America and Africa are beginning to separate.
From Strahler. Physical Geography. 4th Ed.
The Late Cretaceous world – 65 mybp. Note
proto-India headed toward Asia. When it
collides, it will create the Himalayan
Mountains. From Strahler. Physical Geography. 4 Ed.
th
The world as we know it today, showing the
location of tectonic plates. From Strahler. Physical Geography. 4 Ed.
th
What happens along the edges of the tectonic
plates? Why, subduction, of course. From Strahler.
Physical Geography. 4th Ed.
The molten upper mantle oozes out
along an undersea rift and the continental
crust spreads outward.
From E.M.E. Plate Techtonics Slide Series.
1978.
New ocean floor is made as the rift
widens and the crust moves away.
From E.M.E.
Plate Techtonics Slide Series. 1978.
You can see here the various plates that
make up our world and the relative
positions of our existing continents.
The plates still move, so if you live for 70 or
so million years, you may see France
coalesced with the U.S.
This figure may be a little clearer.
From E.M.E. Plate Techtonics Slide Series. 1978.
The bottom-line is that continents
are constantly being made,
recycled via subduction, and
certainly always on the move.
Understanding these movements
helps one understand the presentday distribution of flora & fauna.
An application of our knowledge of continental drift: The
distribution of mangroves about 66 million ybp & today.
ZOOGEOGRAPHICAL REGIONS
AMAZONIA’S RAINFOREST
REFUGIA: 25,000 YBP
ECOTONE
A place where two habitats
merge (ex.: the zone along the
margins of a forest and open
field).
EDGE EFFECT
In an ecotone, there is often more species
diversity along the “edges” of the two
habitats than there is in either habitat.
This is because the edge shares aspects
of each habitat and thus offers habitat to
a greater variety of critters.
FORESTS ALONG THE EDGES
OF RIVERS
Gallery forests – forests that line
waterways
 Riparian forests – “riparian” means
along the edges of waterways, so this is
a synonym of gallery forests
 Igapó – this type of forest along black
water rivers
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MUCH MORE TO COME