Virtually all the energy in
earth's ecosystems is captured from the sun. Powered by energy
from the sun, plants convert water and carbon dioxide into sugars,
releasing atmospheric oxygen in the process. This process of photosynthesis
is usually represented as:
12H2O + 6CO2 ---> C6H12O6 + 6O2 + 6H2O
Certain purple and green bacteria conduct photosynthesis without generating oxygen. They follow two pathways, one of which generates elemental sulfur:
12H2S + 6CO2 ---> C6H12O6 + 12S + 6H2O
12H2 + 6CO2
---> C6H12O6 + 6H2O
Oxygen released into the atmosphere
during photosynthesis is derived from water; the oxygen in the
synthesized carbohydrates comes from carbon dioxide. Energy for
metabolic purposes is released when the oxygen generated can react
with the carbohydrate in the reverse process:
C6H12O6 + 6O2 ---> 6H2O + 6CO2
This is the basis for the oxygen
and carbon cycles upon which much of our biosphere operates. A
consequence of photosynthesis is a reduction in atmospheric carbon
dioxide and an increase in atmospheric oxygen. Without oxygen-evolving
photosynthesis, the biosphere (as we know it) would not exist.
There might be anaerobic life on earth, but it would be very different.
Nitrogen makes up about 78%
of earth's atmosphere, and along with oxygen, carbon, and hydrogen,
it is a vital part of organic life. Nitrogen does not combine
readily but a few organisms are able to fix nitrogen with hydrogen
to form ammonia:
N2 + 3H2
---> + 2NH3
Nitrogen-fixing microorganisms
and algae are important in the earth's nutrient cycles. One important
example is the bacterium Rhizobium, which is symbiotic with plants
of the Leguminosae. Nitrogen-fixing microorganisms play vital
roles in raising soil quality. In order to be useful to most organisms,
ammonia must be oxidized by microorganisms. When nitrates are
used by living cells, they are reduced to ammonia and then incorporated
into proteins and nucleic acids. Living animals excrete nitrogen-containing
compounds as wastes, and the tissues of dead plants and animals
contain nitrogen compounds that can be recycled.
It should be noted that a molecule
similar to nucleotides, adenosine triphosphate (ATP), is an important
source of energy in living cells. ATP is adenine bonded to a ribose
sugar which carries three phosphates. If the last two phosphates
are transferred to another molecule, the molecule yields about
8000 calories per mole, providing metabolic processes with a source
of relatively high energy.
Photosynthesizing plants and
microorganisms make the energy and many of the nutrients that
support the rest of life. Consequently they are called the "producers."
Other animals feed on them, and are then fed upon in a great nutrient
and energy chain. Plant and animal wastes and dead bodies are
recycled by decomposer organisms that convert them to nutrients
in soil or water. Thus, nutrients are recycled. Only energy is
consumed or expended.
An important interrelationship
exists between various fungi and virtually all higher plants.
The fungi invade plant root systems, forming a growth called the
mycorrhizae. The mycorrhizae mat on a plant's root systems prevents
the entry of certain pathogens, but most important, it assists
movement of nutrients from soil to the plant.
Plants and animals live in
natural communities of great complexity. The complexity is derived
by experiencing a long evolutionary history and usually by a long
history of stability that allows coevolution to occur among members.
Prey species evolve complex mechanisms to avoid over-predation
and to assure reproduction. Predators likewise adopt complex strategies.
The longer a community is stable, the more types of organisms
one will find in it, and to a great extent, the more stable the
community becomes. Some species compete with each other. Others
find ways to escape competition such as varying the use of resources
or finding a niche not exactly like the one used by the competitor.
Another type of interaction
is mutualism, a situation where different species benefit each
other. In true mutualism, both species coevolve a dependence on
each other such that neither can exist alone. Many plants have
this type of relationship with their pollinators. Another example
is the mutualism between certain bacteria that live in root nodules
of legumes, producing nitrogen-bearing substances needed by the
plant in exchange for sugar.
Photosynthesis makes earth
habitable. It converts radiant energy from the sun into biological
energy, nutrients, and organic resources (including oxygen). The
resulting biosphere stabilizes climate by releasing or removing
two important greenhouse gases, carbon dioxide and water vapor
(return to outline).
The atmosphere is traditionally
divided into strata according to variations in temperature. The
lowest level is the troposphere, where temperature drops as one
increases altitude in the air column. This layer is shallower,
7-9 km deep, at the poles than at the equator, where it rises
as much as 16 km above the earth. The troposphere contains the
familiar weather features - hurricanes, lightning, precipitation,
thunder. It also contains most of the carbon dioxide and water
vapor, the green house gases that absorb radiant heat. Temperatures
at the tropopause, the top of the troposphere reaches -55oC. The
tropopause is a region several kilometers thick where the air
temperatures remain constant. Similar boundaries separate the
major layers of the atmosphere. Above the troposphere, stratosphere
temperatures rise slowly to -46oC at a height of about 30 km,
before starting to sharply increase. The change in rate of warming
with altitude marks a division between the lower and upper stratospheres.
The boundary is also the location of the protective ozone layer.
Although the ozone layer is rarefied (about 12 ozone molecules
per million air molecules where it is most dense), it strongly
absorbs ultraviolet solar radiation wavelengths between 0.2 µm
and 0.3 µm, shielding the lower atmosphere and warming the
stratosphere. Above the ozone layer, temperatures rise sharply,
reaching about 0oC at heights of about 50 km. The layer above
50 km, the mesosphere exhibits a sharp temperature decline until
a minimum of -90oC is reached near an altitude of 80 km. This
minimum air temperature marks the mesopause, and above it the
thermosphere is marked by rising temperature. Zones of intense
ionization of oxygen and nitrogen molecules produced by solar
and cosmic radiation in the thermosphere are the basis for the
Aurora Borealis and Aurora Australis, the polar lights. These
ions also reflect and absorb radio waves used in radio communications.
Molecular oxygen absorbs ultraviolet solar radiation below 0.2
µm wavelengths, heating the thermosphere. Temperatures in
the thermosphere become very high, averaging about 700oC near
300 km. When there are periods of increased sun activity, thermosphere
temperatures can reach 1700oC (Ahrens, 1985), but the atmosphere
is too thin to conduct heat to thermometer or astronaut. It will,
however exert a drag on satellites. Hydrogen and helium in the
thermosphere escape earth gravity and diffuse into space. Water
can be separated by radiation in to oxygen which is reclaimed
by the atmospheric chemistry, and hydrogen which is lost.
The most abundant atmospheric gases in the troposphere are nitrogen, oxygen and argon in a ratio of 78:21:1. Other gases are so small in quantity that they can be considered impurities. The argon is primarily Ar40 not Ar36 noted in cosmic dust. Atmospheric Ar40 comes from the radiogenic decay of potassium-40 in earth minerals. Helium is also generated in the lithosphere by atomic decay of heavy elements but the small mass of helium and hydrogen allows them to escape the earth's gravitational field and dissipate into space from the upper atmosphere. The balance between rates of helium diffusion into space and production in the earth's mantle produces a tropospheric helium content of about 5 ppm. At higher altitudes, lighter gases become more abundant.
Abundant Gases at Different Altitudes
(From C.D. Ahrens, 1985. Meterology
Today, St. Paul:
West Publishing Company, Pp. 78)
Most
Altitude Abundant
(km) Gases
1000 He, H, O
750 He, O, H Light Gases
500 O, He, N2
300 O, N2, He
180 O, N2, O2
110 N2, O, O2 Heavy Gases
85 N2, O2, Ar
The primary constituent of
the lower atmosphere is nitrogen. About 90 percent of all nitrogen
estimated to exist in air, sea, and earth mantle, is to be found
in the air. Nitrogen, once released from minerals tends to remain
in the atmosphere. Indeed, the primary route for nitrogen to be
incorporated into geological sediment is by incorporating organic
material into sedimentary deposits. Nitrogen fixing organisms
produce nitrogen compounds that are utilized by living creatures.
At death, decaying organic material releases nitrogen, usually
in the form of ammonia, back to the atmosphere. At current planet-wide
rates of nitrogen fixation and return, a volume of nitrogen equivalent
to the total atmospheric nitrogen is cycled through living beings
and returned to air every 50 million years.
Oxygen, carbon dioxide, and
water are rapidly cycled from the atmosphere through living cells,
and life on this planet appears to act as a partial buffer to
sustain homeostasis of these molecules in the atmosphere. In the
present world, O2 is released into the atmosphere by
photosynthesis and converted into carbon dioxide by respiration
and "burning" of organic carbohydrates. Oxygen in the
atmosphere is replaced by passage through a plant cell every 2,000
years. The volume of CO2 in the atmosphere is turned
over through photosynthesis and respiration every 8 years. If
there were no reservoirs of CO2, that is, dissolved
carbonates in water, marine organic matter, land vegetation, soil
organic matter, and marine sediments, the weathering of minerals
could consume all atmospheric CO2 in only 7,000 years.
The ultimate storage of CO2 is the deposition of Calcium
carbonates at sea. Water vapor is not a major component of the
atmosphere, but it has extremely important effects on climate.
A volume of water equal to the oceans of earth, about 1.37 x 109
km3, passes through living cells every 2 million years.
Methane is generated from the
decay of organic matter , and by coal mining, or production of
natural gas. A normal cow belches several hundred liters of methane
a day. It is removed from the atmosphere by chemical reaction
with OH radicals, by diffusion into the stratosphere, or decomposition
by organisms. Atmospheric methane turnover is estimated to take
about 10 years. In the last 200 years, methane concentrations
have doubled from 0.7 ppm to 1.55 ppm. This increase is presumably
due to human actions, especially the spread of rice paddies, cattle
raising, and oil field production.
N2O and NO2
are produced by biologic action and from decomposition of methane.
NO2 is relatively unimportant in the troposphere but
it has a relatively long lifetime (150 years). It is broken down
to N2 and O2. NO which diffuses into the
stratosphere is dangerous to O3, ozone. Ozone is a
particularly important component of the stratosphere. It is a
thin layer but ozone absorbs a part of the radiation spectrum
from the sun that is dangerous to life. Furthermore it is remarkably
efficient at retaining solar energy.
The departure from vertical
(inclination) of the earth's axis produces seasonal variation
in the locations where the sun passes directly overhead. The inclined
axis and the earth's orbit combine to produce seasons, since climate
is governed primarily by the amount of sunlight reaching the earth's
surface. The northern and southern boundaries of this region are
the Tropic of Cancer and the Tropic of Capricorn, and the land
masses they delimit are called the tropics. As one approaches
the equator within this zone, there is less seasonal difference
in temperature, day and night periods are about equal, and sunlight
strikes the earth more perpendicularly, thus passing through less
atmosphere.
Warm surface temperatures cause
air to rise to the upper troposphere, and cool surface temperatures
produce sinking air masses. Consequently the atmosphere is warmed
in the tropics, where it expands and rises. Water vapor is lighter
than air, so moist air is lighter than dry air at comparable temperatures
and altitude. Tropical afternoons are characterized by afternoon
rain storms produced as the rising column of moist air cools,
allowing water to condense into clouds. Condensation adds heat
to the air, breeding thunderstorms. Thunderstorms are giant heat
pumps, pushing columns of warm air high into the atmosphere. The
column of warm air turns and moves toward the poles at the tropopause,
the break in temperature gradient that marks the boundary between
the troposphere and the stratosphere. The spin of the earth (the
Coriolis effect) deflects these columns of warm air. As the tropical
air moves toward the poles, it continues to cool and sink. Between
latitudes of 25 and 35 degrees, these sinking winds produce a
zone of high atmospheric pressure and gentle winds named the "horse
latitudes" in recognition of the many cargoes of horses who
died of thirst on ships becalmed in this zone. The great deserts
of the world generally are found in this zone, characterized by
relatively warm and dry air. Air displaced from this zone moves
toward the tropical low pressure zones, again deflected from a
straight-line motion by the Coriolis effect. These surface winds
are called trade winds, relatively steady winds from the northeast
in the Northern hemisphere and southeast south of the equator.
Thus a world wide pattern of atmospheric circulation is powered
by energy from the sun striking the tropics. Temperature gradients
between polar regions and tropics produce powerful jet-stream
winds. The greater the temperature gradient between equator and
pole, the more energy imparted to the winds. Jet streams vary
in size and position with changes in the temperature gradients,
bringing monsoons, trade winds, and the familiar seasonal changes
in weather (return
to outline).
Climate is determined primarily
by five variables:
(1) mean annual temperature;
(2) amount of temperature fluctuation;
(3) annual rainfall;
(4) timing of the rainfall; and
(5) evapotranspiration.
The Arctic tundra has a mean
annual temperature below 3oC. Cool temperate climates
fall between a mean of 6oC and 12oC. Warm
temperate climates have means above 17oC and tropical
climates have annual means above 24oC. If there is
adequate moisture, there will be plant growth, with as much as
500 g of water transpired to produce a gram of plant tissue during
growth. Given both warmth and abundant water, a rainforest will
develop if rainfall is dispersed through the year. Strongly seasonal
moisture produces a monsoon forest that experiences long periods
of little rainfall.
Characteristics of the soil, the interface between the earth and a living terrestrial ecosystem, reflect climatic history. Soil is produced by the weathering of rocks, decomposition of organic material, and organisms that serve primarily as decomposers. The uppermost soil horizon, the O horizon, contains decomposing organic litter. Below this, the A horizon or top soil, is the zone where nutrients leach from humus to the underlying soil. The B horizon, or subsoil, contains materials leached by ground water from the overlying zones. The next lower zone, the R horizon, consists of weathered and unweathered bedrock. Tundra soils are thin wet soils with high organic content overlying the permafrost. Pedalfers, the familiar soils of the temperate forests, are acid (pH 4.0-4.5) with aluminum and iron leached from the A horizon by rainfall. Latosols, the extremely leached soils found in high rainfall tropical forests generally lack an A horizon, or if present it is thin and easily eroded. Water leaching removes silica, but the B horizon is rich in oxides of iron and aluminum. Latosols are nutrient poor soils and in extreme leaching conditions they become laterites, soils that contain enough iron and aluminum that they harden to stone when dried and exposed to air. If a soil becomes waterlogged (hydromorphic), normal aeration processes are disturbed and the resulting anaerobic environment inhibits decomposition of the accumulated organic materials. Prairie and grassland soils are typically rich soils, with relatively little leaching of nutrients from the A horizon. If a soil in a semiarid area does not have proper drainage, rainwater stands and is evaporated, leaving salts leached from the soil on the surface. Desert soils are thin, and since evapotranspiration is greater than leaching effects of rainfall, there is a layer of cemented calcium carbonate on or near the surface. Organic content is nil (return to outline).
Forests are generally divided
into low altitude (below 1000 meters) and montane forests (above
1000 meters). Table 6-2 summarizes some of the differences between
these forests. As altitude increases, temperature decreases from
about 20oC at 1000 meters, to 10oC at 3000
meters, and then to about 2oC at 4000 meters. Daily
temperature fluctuations are more extreme with a rise from 1000
meters to 4000 meters. Above 1000 meters, vegetation becomes progressively
shorter and restricted in species .
Changes in the characteristics
of vegetation associated with altitudinal zonation of rainforests
on tropical mountains. Modified from Deshmukh (1986), pp. 226-227.
|
|
|
|
|
| Height of taller trees(m) |
|
|
|
| Leaf area
index* (area cm2 g-1 leaves) |
|
about 80 | about 70 |
| Net production
of wood (t ha-1a-1) |
|
about 1.4 |
|
| Buttresses on trees | Common | Rare | Absent |
| Predominant leaf size | Large | Large | Small |
| Abundance of conifers | Rare | Common | Common |
| Abundance of epiphytes | |||
| Vascular | Common | Very common | Common |
| Nonvascular | Rare | Common | Common |
| Abundance of climbing plants | Woody common |
Nonwoody common | Rare |
| *Note: Leaf area index is estimated as leaf area per unit weight. |
One's first impression of a
rain forest is that it is an endless expanse of luxuriant foliage
filled with animal life. Our subjective impression is one of great
resources, abundance, and permanence. A visitor familiar with
temperate zone agriculture might imagine the profuse vegetation
to be supported by an unusually rich and fertile subsoil. All
of these impressions are erroneous. Forest life is fragile, transient,
and continuously engaged in a life-and-death struggle. Death may
be silent and swift. The bats from a single cave complex in northeastern
Gabon consume three tons of insects a night. Death may be slow,
as a vine-like strangler fig encircles a forest tree with its
deadly covering. In the forest, individual organisms are transient,
but the community is resilient and durable. The only way to preserve
individual species in this community is to sustain the forest.
Participants in this struggle for life have developed, over millennia
of interactions, intricately complex relationships. It is the
study of these relationships that makes ecology the most fascinating
of sciences and that contributes grandeur to Darwin's view of
life.
Scientific knowledge of rain
forests is relatively recent. The first and classic synthesis
about rain forests was completed by Schimper in 1898 (for an English
edition see Schimper, 1903). The second synthesis is represented
by Richards (1952) and evolutionary thinking was introduced into
the study of rain forests by Corner (1964).
Rain forests are valuable.
They contain large stands of commercially valuable wood. They
serve important functions in retention of soil and water. Run-off
water from forested areas is clean and dependable for human use.
Large forests appear to stabilize the climate, and thus have an
impact on ecological and economic systems all over the world.
Forests are sources of numerous useful products for medicine,
contraception, spices, dyes, resins, etc. We have hardly begun
to explore the possibilities of forest products. Forest diversity
provides a pool of new useful species and a pool of genetic material
of immeasurable value. Forests are also a great source of knowledge
about biology and evolution. As communities, they are entities
of great beauty with great potential as places for education and
recreation. By discussing the ecology of rain forests, we can
illustrate some of the basics of ecology and acquire insight in
human and primate ecology.
Tropical rain forests only occur within three geographic conditions:
1. They are located between 23o30' N and 23o 30' S latitude (between the Tropics of Cancer and Capricorn). At the equator, the mean temperature at sea level is 26oC, daily fluctuation in temperature is small, about 3 to 6oC, and there are 12 hours of daylight. Near the Tropics of Cancer and Capricorn, there is greater fluctuation in temperature, about 7 to 12oC. Tropical rain forests do not occur where the mean temperature drops below 20oC in the coolest months of the year.
2. Rain forests occur below an altitude of 1000 meters. The richest diversity and the tallest trees are seen in lowland forests below 300 m. Montaine forests, with short trees and less species diversity, occur above 500 m.
3. Rain forests require at least 1800 mm of rainfall per year, and the rain must be distributed throughout the year. There can be "wet" and relatively "dry" seasons, but the dry season must still have precipitation. However, it should be noted that it is moisture in the soil and air that is important - not the actual rainfall.
|
|
|
|
|
| Lake and stream |
2 |
500 |
1.0 |
| Swamp and marsh |
2 |
2000 |
4.0 |
| Tropical forest |
20 |
2000 |
40.0 |
| Temperate forest |
18 |
1300 |
23.4 |
| Boreal forest |
12 |
800 |
9.6 |
| Woodland and shrubland |
7 |
600 |
4.2 |
| Savannah |
15 |
700 |
10.5 |
| Temperate grassland |
9 |
500 |
4.5 |
| Tundra and alpine |
8 |
140 |
1.1 |
| Desert scrub |
18 |
70 |
1.3 |
| Extreme desert, rock, and ice |
24 |
3 |
0.07 |
| Agricultural land |
14 |
650 |
9.1 |
| Total land |
149 |
730 |
109.0 |
| Open ocean |
332 |
125 |
41.5 |
| Continental shelf |
27 |
350 |
9.5 |
| Attached algae and estuaries |
2 |
2000 |
4.0 |
| Total ocean |
361 |
155 |
55.0 |
| Total for Earth |
510 |
320 |
164.0 |
* square kilometers + 0.3861
= square miles.
** grams per square meter X 0.01 = t/ha, X 0.1 = dz/ha or m centn/ha
(metric centers, 100kg, per hectare, 104 square meters),
X 10 =kg/ha, X 8.92 = lb/acre.
*** Metric tons (106g) X 1.1023 = English short tons.
Modified from Pianka (1978), p. 48.
Although they are of great
interest to science, the ecology and biology of rain forest species
are poorly known. Although rain forests have great potential for
human utilization, they are unusually fragile, and regeneration,
if possible, is too slow in terms of human economies. As a consequence,
great capital assets in timber and biological resources cannot
be harvested without serious damage. They cannot be "managed"
in the current business meaning of the term without loss of large
numbers of species, including primates. We have not developed
modern methods of exploiting tropical rain forests that retain
species diversity and until we formulate non-destructive methods,
only direct protection will retain any tropical rainforest for
the future.
How is it possible for the
world's most productive forests to occur on the world's poorest
soils? Energy that comes from the sun fuels both rain-producing
updrafts and photosynthesis. Rain forests become great nutrient
entrapment areas where nutrients are recycled within the community
of organisms. Even though the species of organisms that comprise
forests vary in different continental regions, they share certain
characteristics.
The canopy is a structure of
woody vegetation dense enough to block direct sunlight from the
ground. Here, trees and lianas are the most conspicuous vegetation.
Flowers are rare except on single isolated plants. Herbs (non-woody
plants such as orchids and ferns) occur in the upper canopy as
epiphytes, nonparasitic plants that use trees as substrata. Many
forest plants are cauliflory, that is, flowers and fruits sprout
directly from trunks or branches like the familiar cauliflower.
Trees are tall and slender, with many buttresses or prop root
bases. There are open spaces with few plants and little light
beneath the canopy.
There are three large rain
forest areas: Central and South America; Indo-Malaysia; and Africa.
Africa is the drier area, with rain forests covering only 9% of
its tropics. Furthermore, African rain forests exhibit less species
diversity than other areas. Many plant families that are pantropical
elsewhere are missing or poorly represented in African rain forests.
The relative poverty of African forests may reflect lower rainfall
and a longer history of human activities in the forest.
Consider the forest from the
perspective of a tropical tree. It must establish itself in a
suitable location, acquire nutrients and sunlight, avoid being
destroyed by predators, and, if the species is to persist, manage
to reproduce and transport its progeny to other appropriate locations.
In tropical areas of high rainfall, readily water-soluble nutrients
are leached from the soil by water action. Nutrients are available
above the ground in the form of litter. Our tree must establish
a shallow or above-ground network of roots to capture nutrients
leaching out of litter. Organisms that live in litter serve as
decomposers that speed up the nutrient cycle, adding fertility
to forest humus. Since subsurface nutrients and water are not
of primary importance to the tree, the root system is shallow,
providing only a minimal footing to support the weight and stress
of the tree. The tree must have a height, canopy spread, and architecture
of stems and leaves that capture sunlight's energy allowing photosynthesis.
To protect itself from predation, the tree poisons its more edible
components, especially the leaves. Insects are the major plant
predators in a forest, so most of the secondary compounds that
deter predation are insecticidal. Other animals who prey on plant
materials have to find ways to detoxify the leaves or cope metabolically
with the insecticides. Many additives which make foods spicy or
more palatable are secondary compounds that a plant developed
as a defense (e.g. chocolate, caffeine).
Another way a tree can reduce
predation is by withdrawing as many nutrients as possible from
its leaves, and shedding them. If this leaf fall is synchronized
among the other members of this tree species, a season occurs
when insect predators on that tree species have little to eat.
After an appropriate interval, there is a synchronous budding
of leaves on all the trees of this species, and leaves develop
faster than the growth period needed for the predator to seriously
damage the leaf crop. At an appropriate stage of development,
and before the predator population increases enough to destroy
the leaf crop, the trees pump insecticides into their established
foliage.
Some of the most interesting
coevolutionary relationships occur between trees and ants. Some
tree species provide nectaries to attract colonies of ants which
in turn, defend the tree. The ants may kill other animal predators
or prevent encroachment of competing plants by cutting away infringing
vegetation. The tree and its ant colony become a codependent community
that fights to secure its space in the canopy.
Our tropical tree must reproduce
and disperse its progeny. If it simply allows its progeny to develop
beneath its canopy, the new trees would be shadowed by the parent
and compete for nutrients. Many species have coevolved relationships
with animals to accomplish reproduction. Relatively few tropical
trees are wind-pollinated. The principal animal pollinators are
flies (Diptera), bees (Hymenoptera), moths and butterflies (Lepidoptera),
bats (Chiroptera), and birds (Aves). The tree may provide food
resources in the form of nectar or pollen to attract a pollinating
animal to its flowers, even controlling the animal's behavior
by the location and amount of nectar. The anatomy, timing, and
location of the flower are tailored to the biology of a particular
class of pollinating animals, and sometimes to a particular species.
For example, bat-pollinated trees are common in tropical America,
Asia, and Australia. Flowers of these trees are large and contain
greater volumes of dilute nectar - bats consume a lot of energy
in thermoregulation and long-distance flight. The flowers are
often dull in color, aromatic, positioned where flying bats have
access to them, and in many species, are open only at night. The
pollinating bats may also consume pollen and fruit. If the tree
is moth- or butterfly-pollinated, nectars may contain amino acids
and lipids in addition to sugar solutions since the nectar (and
in some cases pollen) may constitute the pollinator's total diet.
High rainfall areas tend to have vertebrate pollinators.
After pollination, the seed
may be incorporated in a fruit that entices another animal to
transport it away from the parent tree. Tropical wet forests have
a high proportion of tree species that produce fleshy fruits appropriate
to vertebrate seed dispersal. Some seeds are designed to appeal
to particular groups of animals who will consume them. The seeds
transit the alimentary tract and are deposited into litter with
fecal material, hopefully far from the parent tree and in an appropriate
location. The quantity of fruit and seeds produced by a tree can
be large. The portion that is not transported to a desirable location
for colonization is simply recycled as litter in the root shadow
of the parent tree. Eventually the animal carcass joins the litter
to be recycled. From the perspective of a tree, animals in a forest
are temporary stores of nitrogen and other nutrients whose behavior
is controlled and exploited by the biology of forest plants.
The top of the tree crown is
exposed to intense sunshine and weather and accumulates many micro-epiphytes,
small plants that use the tree as a substratum. Beneath this is
the protected part of the crown, the zone that receives both sunlight
and shade and which is the dominant zone of epiphytes, sometimes
large and heavy. Beneath the crown lies the driest upper part
of the trunk, encrusted with flat lichens. The lower trunk is
moist and often has a lush growth of lichens and mosses. The trunk
base, with buttresses or prop roots has many moist, shaded holes
and abundant moss growth. The roots are shallow and superficial,
providing only a minimal purchase on the substratum.
As a rainforest tree ages,
epiphytes and lianas accumulate among its overstory stems. It
grows progressively top heavy, and will eventually fall when the
shallow root bed is pulled from the ground by the stress of wind
or weight in the overstory. If the roots are well anchored in
soil, the trunk supporting the tree will eventually break. As
the tree falls, it strikes adjacent trees or is connected to them
by lianas, breaking and knocking them over like dominos, producing
a large open area in the canopy and covering the ground with tons
of litter. This scar in the canopy is called a chablis - it often
has the shape of a wine bottle with the uprooted base being the
cork in the bottle. Hurricanes, typhoons and other severe storms
provide wind energy to topple large numbers of heavily burdened
trees. As a result of frequent tree falls, tropical rain forests
have a relatively low density of mature large trees and a large
mosaic of disturbance, succession, and diversity.
A tree fall and chablis formation
begins a regeneration cycle in a forest that provides much of
the habitat complexity and species diversity. Each chablis represents
exploitable resources of space, nutrients, and sunlight. It is
first colonized by rapidly growing, short-lived, soft wood tree
species. By the time this first generation matures and falls,
slower growing, longer-lived species have established themselves
underneath. A forest tree is first a seedling in a chablis, a
sapling, a pole tree (about the same height as a human, a tree
(when the trunk is 10 cm in diameter), a coming tree (when it
reaches the canopy), a tree of the present (when it reaches into
the canopy and enlarges its crown), a tree of the past (when it
starts to die due to damage by fungi and insects), and finally
litter in a chablis.
Some tree species find alternative
ways to secure a position in the canopy. The strangler fig, for
example, begins as an epiphyte in the protected part of the crown.
The fig drops liana-like roots to the forest floor and establishes
itself as a series of vines whose roots intermingle with those
of the host tree. The fig vines enlarge, completely covering the
host trunk and the fig crown supplants the host crown. Eventually
the host dies, leaving in its place a fig tree. Decomposer organisms
break down the host trunk, producing a hollow that runs the length
of the fig trunk. Eventually the strangler fig ages and falls
to become litter in a chablis.
Rain forests also include landslides,
abandoned farms, logged regions, and other disturbed areas larger
than a chablis. If there are surrounding stands of forest to serve
as seed banks and sources of dispersing agents, the area will
be recolonized. Early pioneers will be fast growing, short-lived
"weed trees." Such forests may be impenetrable, rather
monotonous in architecture and species-poor. Later pioneer vegetation
is larger, has more species complexity, and is characterized by
longer-lived vegetation with diverse architecture. Eventually
the late pioneers are replaced by a late successional vegetation
that is diverse in architectural form and long-lived. The mature
forest is the ecological unit that has reached its maximum diversity
and number of species by containing all stages of the forest mosaic.
Chablis occur in all stages of development and make a vital contribution
to the diversity of all parts of the forest mosaic.
Part of the durability of the
rainforest community is due to the inability of any one organism
to invade and overwhelm it by overpopulation. Potential predators
that are not checked by plant defenses are controlled by other
organisms in the forest community. Parasites and disease microorganisms
protect the tree from over colonization (return to outline).
The subtropical zone, which
has an annual mean temperature between 17oC and 24oC,
usually has seasonality of rainfall. Forests in this zone tend
to be broad-leaved trees (oaks, magnolias,...) or conifers (pine,
hemlock, cedar). Usually there are few dominant tree species (often
2 or 3 and rarely over 10) and a diversified understory of shrubs
and herbs and few epiphytes and lianas. Stratification is simpler.
In the tropics, monsoon or seasonal forests, also have simpler
stratification with a deciduous canopy and an evergreen understory.
Teak is often a major tree species. Temperate rain forest ecosystems
exhibit considerably less diversity of species, but the redwood
(Sequoia) and alpine ash (Eucalyptus) at heights of more than
100 m are among the world's tallest trees. Temperate deciduous
forests have few epiphytes except for mosses, algae, and lichens.
The annual loss of leaves allows a bloom of spring flowers in
the understory prior to canopy leafing. Cold regions with high
rainfall have conifer forests with low species diversity. Humus
and A horizons in the soil are poor due to slow weathering and
decomposers, but the fauna include some species that reach large
body sizes (bear, caribou, elk, moose). The combination of cool
wet winters and hot dry summers (a Mediterranean climate) produces
forests of small evergreen trees and shrubs. Species diversity
is low and fire is a major environmental element (return to outline).
Temperate grasslands such as
the African veldt, American prairies, Eurasian steppes, and South
American pampas have reduced stratification to a single ground-level
story. The wetter grasslands have tall grass species (more than
1 m high) and a species diversity comparable to deciduous forests.
Grasslands contain some large herbivore species, but in much less
diversity than tropical savannas. If moisture is insufficient
to support grasslands the community becomes a thorn woodland where
the dominate plants and thorny trees, shrubs, and succulents (cacti).
Species diversity in both plants and animals are low.
Grasslands are called by various
names but they include a series of vegetation types characterized
by an extensive grass cover from woodland to open grassland. They
are relatively dry habitats and are maintained by seasonal dry
periods, overgrazing, and fire. A tropical savanna occurs where
the rainfall is seasonal and inadequate to support a monsoon forest.
Tree species are drought resistant and do not form a canopy. Most
of the area is covered by grasses, and grass fires are a seasonal
event. Like cold forests, tropical savannas tend to include gigantism
among their fauna (antelopes, buffalo, elephants, giraffes, lions...).
These important features of the southern continents occupy approximately
65% of Africa and 45% of South America. It is important to note
that although rain forests have existed for several hundred million
years, grasses only evolved about 50 million years ago. Savannas
are relative newcomers to our ecosystems.
Unlike most woody plants, which
grow from the shoot tip, grasses grow from below ground level,
making them less vulnerable to defoliation. Heavy defoliation
by large mammals or fire promotes grasses and suppresses woody
plants. Vegetation growth correlates with rainfall, making fluctuations
in growth more dramatic in the drier climates. In wetter savannas,
fire is an annual event that consumes as much as 50% of the annual
forage. Even though large herbivores are more obvious, termites
are the most important savanna decomposers and their biomass may
greatly exceed that of mammals.
It is interesting to note that
although Africa has primate species that use the savanna (patas,
baboons,...) and the boundaries between forest and savanna (vervet),
there are no New World Primates in this niche (return to outline).
Deserts are regions with 50%
or more of the ground without vegetation cover. They are usually
associated with a severe deficit of water (a mean annual rainfall
of less than 250 mm) which inhibits vegetation. Hot deserts occur
near 30o N and 30o S latitude as a consequence
of global air circulation. Cold deserts are found at high altitudes
above the altitudinal limit of vegetation on mountains. Biotic
deserts are areas in which human activities have destroyed plant
cover and the soil's ability to recover. Edaphic deserts are regions
where soils will not support vegetation, usually as a result of
high salt content.
Few deserts are barren. They
usually have scattered, drought-resistant vegetation and succulent
plants capable of storing water. Animals are usually nocturnal
to avoid daytime heat and have extraordinary adaptations to the
scarcity of water (return
to outline).
Table of Contents
15 Aug 2004
Department of
Department
of Anthropology, College
of Liberal Arts , UT Austin
Comments to cbramblett@mail.utexas.edu