Biogeographic processes
1. concepts
ecosystem: an ecosystem is a
self-sustaining association of living plants and animals and their nonliving
physical environment.
Ecology: the study of the
interactions between life-forms and their environment is the science of ecology
Ecosystems fall into two
major groups-aquatic and terrestrial. Aquatic ecosystems include marine
environments and the freshwater environments of the lands. We will focus on the
terrestrial ecosystems, which are dominated by land plants spread widely over
the land surface.
Two basic kinds of processes
must occur in the ecosystems: a cycling of chemical elements and a flow of
energy.
2. Energy flow
in ecosystems.
Food
chains is the linkage of who feeds on whom. More
complicated food chains are called food webs
Individuals
in a biological community can transfer energy, chemical elements, and some
compounds from creature to creature along food chains and food webs.
The
original source of energy in most ecosystems is the sun. Green plants produce
sugars through the process of photosynthesis, using only the energy of the sun
and CO2 from the air, so they are the primary producers of the food webs.
The
primary producers support the consumers-organisms that ingest other organisms as
their food source. At the lowest level
of consumers are the primary consumers (the snail, insects, and fishes). At the
next level are the secondary consumers.
Herbivores,
organisms that feed on plants, are members of the second trophic
level;
Carnivors (meat eaters) that feed directly on herbivores are in third trophic level;
Carnivores
feeding on third-level carnivores are in the fourth trophic
level; and so on.
For
examples, in a north temperate woodland food web that existed in N.America before European settlement and includes human
beings.
People
are omnivores (eaters of both plants and animals) and feed on several levels
Decomposers
also feed on detritus, or decaying organic matter,derived from all levels.
Photosynthesis: through photosynthesis co2
combined with water and light become sugar and byproduct oxygen.
Last time, we also talked about the respiration
which is essentially a reverse of the photosynthetic process:
C6H12O6 + O2 à CO2 + H2O + energy (heat).
The overall growth of a plant depends on net photosynthesis
which is the difference between photosynthetic production and respiration loss.
The amount of net photosynthesis depends on
controlling environmental factors such as light, water, temperature, soil
fertility, and the plant’s site, elevation, and competition from other plants
and animals.
Plant productivity increases as light availability
increases- up to a point. When the light level is too high, light saturation
occurs and most plants actually reduce their output in response.
Net primary productivity: the net photosynthesis for
an entire plant community us its net primary productivity. This is the amount
of stored chemical energy that the community generates for the ecosystem.
Biomass is the net dry weight of organic material.
Net primary productivity is measured as fixed carbon
per square meter per year( “fixed” means chemically
bound into plant tissues).
Net primary production tends to be highest between
20 N and 20 S at sea level and decreases toward higher latitudes and altitudes.
Precipitation also affects productivity, like
tropical rain forest area. Even though deserts receive high amounts of solar
radiation, other controlling factors are more imporatnt,
namely water availability and soil conditions.
Critical in each ecosystem is
the flow of energy and the cycling of nutrients and water in life-supporting
systems. These nonliving abiotic components set the
stage for ecosystem operations.
Light, temperature, water
and climate.
Solar energy powers ecosystems, so the pattern of
solar energy receipte is crutial.
Solar energy enters an ecosystem by way of
photosynthesis, and heat energy is dissipated from the system at many points.
Of the total energy intercepted at the Earth’s surface and available for work,
only about 1% is actually fixed by photosynthesis as chemical energy (energy
stored as carbohydrates in plants.)
The duration of Sun exposure is the photoperiod.
Along the equator, days are essentially 12 hour long year around.however,
with increasing distance from the equator, seasonal effects become pronounced.
Plants have adapted their flowering and seed germinate to seasonal changes in insolation. Some seeds germinate only when daylenght reaches a certain number of hours.
Other components are important to ecosystem
processes. Air and soil temperature determine the rates at which chemical
reactions proceed. Significant temperature factors are seasonal variation and
duration and the pattern of minimum and maximum temperatures.
a suplus
of carbonhydrates beyond what is lost through plant
respiration.
Within a community, two concepts are important:
habitat and niche.
Habitat is the type of environment in which an orgainism resides or is biologically adapted to live.
(2) Energy efficiency and transfer efficiency
Energy
efficiency is the ratio of output to input. How efficiency do living things use
energy ? the second law of
thermodynamics state that no system can be 100% efficient.
As
energy flows through a food web, it is degraded and less and less is usable.
A
common ecological measure of energy efficiency is called food chin efficiency,
or trophic level effeciency,
which is the ratio of production of one trauphic
level to the production of the next lower trophic
level.
This
effeciency is never very high. Green plants convert
only 1 to 3 % of the energy received from the sun during the year to new plant
tissue. In a natural wildness, the trophic level
efficiency of wolves is about 0.01%, because they use most of the energy they
take in from eating
for themselves, especially for moving around in the search for
prey.
In
highly managed ecosystems, such as ranches, the efficiencies may be greater.
Cattle are by far the least efficient producers, requiring around 16 lb of
vegetable matter to produce 1 lb meat. At the lower end of the scale, it
requires about 3 lb of vegetable matter to produce 1 lb of eggs or chicken
meat. On average, it takes an 7 lb of human edible
vegetable by livestock to produce 1 lb edible meat.
The
energy content of a food chain is often represented by an energy pyramid. For
the sake of simpilicity, the food chain shown here
assumes that each link in the chain has one and only one source of food.
Assume
that if a 165 lb person ate frogs, he would need ten a day, or 3000 a year
(approximately 660 lbs). If each frog ate 10 grasshoppers a day, the 3000 frogs
would require 9,000,000 grasshoppers a year to supply their energy needs, (or
about 19, 800 lbs) of grasshoppers. To sustain so many grasshoppers, it needs
about 732,600 lbs) of wheat.
If
people fed on grasshoppers rather than frogs, each person could probably get by
on 100 grasshoppers a day, the 9,000,000 grasshoppers could support 300 people
for a year, rather than only one. If, instead of grasshoppers, people ate
wheat, then 333,000 kg of wheat could support 666 people for a year.
Today,
approximately half of the cultivated land in the
This
argument is often extended to suggest that people should become vegetarians and
eat directly from the lowest level of all food chains. However, consider that
humans can eat only the parts of some plants. By eating herbivores that can eat
the parts of plants that humans cannot eat, or those plants that humans cannot
eat at all, more of the energy stored in plants becomes available for human
consumption.
The
most dramatic example of this is in aquatic food chains. Because people cannot
digest most kinds of algae, which are the base of most aquatic food chains, they
depend on eating fish that eat algae, or on those fish that eat other fish.
So,
if people were to become entirely herbivorous, they would be excluded from many
food chains.
3.biological productivity
the
total amount of organic matter on Earth or in any particular ecosystem or area
is called its biomass. Biomass is usually measured as the amount per unit
surface area (e.g.
as grams per square meter or metric tons per hectare).
The
change in biomass over a given period of time is called net production
NP
= B2 – B1
Where
B2 is the biomass at the end of the time period, B1 is the amount of biomass at
the beginning of the time period, and NP is net production.
There
are three steps in the production of biomass and its use as a source of energy.
First, an organism produces organic matter within its body; next, it uses some
of this new organic matter as a fuel in respiration; finally, some of the newly
produced organic matter is stored for future use.
The
first step, production of organic matter before any use, is called gross
production.
This
suggests another way to think about net production. Net production is what is
left from gross production after use. In these terms.
Net
production = gross production – respiration.
There
are two different kinds of biological production.
Some
organisms make their own organic matter from a source of energy and inorganic
compounds. These organisms are called autotrophs
(meaning self-nourishing). The autotrophs include
green plants, such as herbs, shrubs, and trees; algae, which are usually found in water but occasionally grow on land; and
certain kinds of bacteria that grow in water.
The
production carried out by autotrophs is called
primary production.
Other
kinds of life cannot make their own organic compounds from inorganic ones and
must feed on other living things. These are called heterotrophs.
All animals, including human beings, are heterotrophs,
as are fungi, many kinds of bacteria, and many other small forms of life.
Production
by heteotrophs is called secondary production because
it is dependent on the production of autotrophic organisms.
Once
an orgaism has obtained new organic matter, it can
use the energy in that organic matter to do things: to move, to make new kinds
of compounds, to grow, to reproduce, and so forth.
The
use of energy in organic matter in both heterotrophic and autotrophic organisms
is accompanied through respiration.
In
respiration, an organic compound is combined with oxygen to release energy and
produce CO2
4. Ecological succession
Ecological
succession occurs when older community of plants and animals (usually simpler)
are replaced by newer communities (usually more complex).
Succession
often requires an initiating disturbance. External exampls
include wind storms, severe flooding, a volcanic eruption, a devastating
wildfire, or an agricultural practice such as prolonged overgrazing.
Here
we discuss the terrestrial succession
An
area of bare rock and soil with no vestige of a former community can be a site
for primary succession.
Exampls are any new surface created by mass movement of land, areas exposed by
a retreating glacier, cooled lava flows, or lands disturbed by surface miningm clear-cut logging, land development, or volcanic
eruption.
More
common is secondary succession, which begins if the vestiges of a previously
functioning community are present. An area where the natural community has been
destroyed or disturbed, but where the underlying soil remains intact, may
experience secondary succession.
In
terrestrial ecosystems, secondary succession begins with pioneer species that form
a pioneer community.
For example, in an abondanded farm of
formerly plowed fields. Crabgrass and ragweed quickly take hold, seeded by winds and birds,
and they do well in the direct sunlight.
These
slowly are choked out by taller, sturdies grasses and
shrubs that invade and stabilize the soil, adding nutrients and organic matter.
After a quarter-century or so, pines come to dominate the land.
The
shade created by the pine forest produces conditions in which grass and shrub
seed germination necomes more difficult. But shade-tolerent, slow-growing oak and hickory hardwoods eventually
grow taller than the pine forest and shade it, so the pines slowly die back
under reduced light conditions.
A
fairly stable mature forest of oak and hickory is in place after 150 to 200
years.
However,
it is noted that a succession of communities are overlapping in time and space.