Ecosystem
A. What is an Ecosystem?
An ecosystem is a grouping of organisms that interact with each other and their
environment in such a way as to preserve/ perpetuate the grouping. Ecosystems are
therefore very complex. There may be many components (eg. the different species in a
forest). The linkages between the components may be very intricate. So we isolate aspects
of the ecosystem in order to study them, eg. the food web.
Although there is a great variety of ecosystems in existence, all of them are
characterized by general structural and functional attributes. Ecological relationships exist
between abiotic (non-living) environmental substances, eg. water, carbon dioxide CO2,
and biotic components, i.e. plants, microbes, animals.
B. The Three Major Principles of Ecosystems:
1. Nutrient cycling
It is the movement of chemical elements from the environment into living
organisms and from them back into the environment as the organisms live, grow, die
and decompose.
Organic elements include plants, animals, microbes, dead organisms, sugar,
honey, flour, wood, leather, etc.. On the other hand, inorganic elements refer to rock
minerals, metals, air, water, etc..
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2. Energy flow:
Energy is required to transform inorganic nutrients into organic tissues of an
organism.
3. Structure:
It refers to the particular pattern of inter-relationships that exists between
organisms in an ecosystem.
Energy & Mineral Movement in Ecosystems
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C. Components of an Ecosystem:
1. Abiotic components
They determine the type/ structure of ecosystem, eg. high temperature and
little rainfall result in the formation of a desert ecosystem.
The overall structure of an ecosystem may be determined by a single abiotic
factor, the limiting factor, eg. moisture or the amount of rainfall will exert its
influence on the distribution of vegetation
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2. Biotic components:
a. Producers (Autotrophs):
All green plants, ranging in size from tiny floating phytoplankton to giant
forest trees, plus a few types of bacteria. The green plants use solar energy to
photosynthesize organic compounds and living protoplasm from carbon dioxide,
mineral salts and water.
Photosynthesis
Inorganic nutrients ------------------------> Organic matter (plants)
Energy + Chlorophyll
b. Consumers:
Organisms that cannot produce their own food but must consume the
organic compounds in plant and animal tissues. Several subcategories are often
distinguished:
Herbivores (plant feeders)
- Primary Consumers
Carnivores (meat eaters)
- Secondary Consumers
Omnivores (general feeders).
c. Decomposers:
Tiny organisms such as bacteria, fungi .......etc., that break down the
complex organic compounds in dead plants and animals. They are very important
in ecosystem because they cause the continual recirculation of chemicals within
ecosystems.
Biotic Components and Food Chain
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D. Movement of Energy and Nutrients:
1. Food chain:
The particular pathway of nutrient and energy movement depends upon which
organism feeds upon another.
Autotrophs--->Herbivores--->Carnivores--->Omnivores--->Decomposers
2. Food webs:
A more complex pattern of feeding relationships among organisms. The more
species, the more complexity of the food chain, The ecosystem is more stable. The
fewer species, the simplify of the food chain, the ecosystem is unstable and fragile.
Food Web
3. Trophic levels and biomass:
The trophic structure is the organization and pattern of feeding in an
ecosystem. A trophic level means a feeding level. All producers belong to the first
trophic level; all primary consumers (herbivores) belong to the second trophic level;
all secondary consumers (carnivores) belong to the third trophic level, etc..
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Energy and Nutrients passed through the ecosystem by food chains and webs
from lower trophic level to the higher trophic level. However, only 5% to 20% energy
and nutrients are transferred into higher trophic level successfully. For this reason,
first trophic level has the largest number of organisms, and second trophic level is less
than first one; the third level is less than second level, and so on. These relationship
can be seen as a food pyramid.
Biomass means the total combined weight of any specified group of organisms.
For example, the biomass of the first trophic level is the total weight of all the
producers in a given area. Biomass decreases at higher trophic levels.
Trophic Level (Food Pyramid)
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E. Linkages and Interactions in an Ecosystem:
Plants absorb inorganic chemicals to produce nutrients. When they die, their dead
bodies are decomposed by bacteria to become inorganic matters again, and they return to
the environment. This cycling of nutrients through chemical reactions is known as
'Geo-chemical Cycles' among which the most important ones are cycles of carbon,
oxygen and nitrogen.
1. The Carbon Cycle and Oxygen Cycle:
Carbon is the basic building block of the large organic molecules necessary for
life. All life on earth is composed primarily of carbon compounds. The source of
carbon for plants is the carbon dioxide that makes up 0.03% of the atmosphere and the
much larger amount of CO2 dissolved in the ocean waters that cover 70% of the earth.
Because carbon (C) is linked with oxygen (O2) to form carbon dioxide (CO2) in the
respiration and photosynthesis processes, the carbon and oxygen cycles are
interdependent in photosynthesis. For simplicity, the photosynthesis process can be
expressed as :
Carbon Dioxide + Water + Solar Energy
---> Sugars + Oxygen + Heat
CO2
+ H2O + Photosynthesis ---> CH2O + O2 + Heat
When the plant or animal die, their dead bodies are broken down again into
carbon dioxide and water by the decomposers. This can be expressed as :
Sugars + Oxygen + Decomposition ---> Carbon Dioxide + Water + Heat
CH2O + O2
+ Action of decomposers ---> CO2
+ H2O + Heat
During these processes, energy from the sun drives the cycle and is degraded
to less useful forms (from Solar Energy to Chemical Energy to Heat) as it flows
through the ecosystem. Carbon and Oxygen are first converted from CO2 and H2O to
sugars in green plants and eventually to other organic molecules by the process of
photosynthesis.
Later the carbon -oxygen - hydrogen compounds such as sugars and other
carbon-hydrates are transferred through the food chains to herbivores and carnivores.
In each step, part of the carbon and oxygen stored in complex food or energy
molecules is broken down by the process of cellular respiration to release energy for
the organism and is cycled back to the air and water as CO2 and H2O. Finally, the
remaining carbon and oxygen that make up the bodies of plants and animals are also
returned to the air and water when their dead bodies are broken down into CO2 and
H2O by the decomposers.
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Another small fraction of the carbon from decayed plants and animals have
been incorporated by geological processes, through million of year, in the earth's crust
as 'fossil fuels' such as coal, oil and natural gas, or as 'carbonate rock formations' such
as limestone and coral reefs. These fossil fuels and rock deposits represent a
temporary storage of solar energy in concentrated chemical form.
Since the Industrial Revolution, man has been burning these fossil fuels at an
increasing rate and releasing the carbon back into the atmosphere as CO2 and H2O.
The carbon in carbonate rock formation is also eventually returned to the normal
carbon cycle as CO2 and H2O as these rocks undergo slow dissolution due to
weathering.
2. The Nitrogen Cycle:
Nitrogen is a particularly important element for human life. Many of the
body's essential functions require nitrogen-containing molecules such as proteins,
nucleic acids, vitamins, enzymes, and hormones. A lack of proteins quickly leads to a
weakened condition and poor health. For this reason world hunger is primarily protein
hunger.
Although nitrogen in gaseous form N2 makes up 79% by volume of the earth's
atmosphere, it cannot be used by most plants and animals directly. Only certain kinds
of bacteria and some blue-green algae can convert or fix nitrogen directly into useful
organic form. Animals must get nitrogen from Amino Acid (-NH groups) which are
the building blocks of proteins and other organic nitrogen molecules. Most plants
must absorb nitrogen in the form of Nitrate (NO3) and Ammonium Salts (NH4)
dissolved in soil water and taken up by the plant roots.
Most of the nitrogen in living organisms does not enter directly from the
atmosphere. Groups of nitrogen-fixing bacteria and algae in the soil, in water, and at
the roots of plants called 'Legumes' (green beans, soybeans, alfalfa and clover) can
convert gaseous nitrogen to nitrates (salt containing nitrate ions). Nitrate salts, which
are highly soluble, dissolve in soil water and are slowly taken up by the roots of plants
and converted to nucleic acids and protein. Since nitrogen is also essential for
photosynthesis, the amount of nitrate in the soil can regulate crop growth. This is the
reason why the addition of artificial nitrate or ammonia fertilizer can give greater
yields, and why countries or areas where soils are low in nitrates are likely to have
extensive malnutrition and health problems from the lack of vital protein.
When animals eat plants, some of the nitrogen is transferred to these animals.
When the plants and animals die, their nitrogen is then converted by decomposer to
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ammonia gas (NH3) and soluble ammonium salts (NH4). These are converted by other
groups of bacteria either into nitrite ions (NO2) or back to atmospheric nitrogen (N).
Some plants can absorb the ammonium salts (NH4) dissolved in soil water and
convert them to protein. Another group of bacteria can convert the nitrite ions back to
nitrate ions which can be taken up by plants to begin the cycle again.
The nitrogen cycle can be affected by man in five major ways:
1. Fertilizer production (mainly nitrates and ammonium salts) to grow more food by
increasing yields, and replenishing lost nitrogen from the soil.
2. Burning of fossil fuels in cars, power plants, and heating which puts nitrogen
dioxide (NO2)into the atmosphere.
3. Increasing animals wastes (nitrates) from more people and from livestock and
poultry grown in ranches.
4. Increased sewage flows from industry and urbanization.
5. Increased erosion of and runoff into nearby streams, lakes and rivers from
cultivation, irrigation, agricultural wastes, mining, urbanization and poor land use.
3. The Nutrient Cycle:
Chemicals needed to produce organic material are circulated around the ecosystem
and recycled continually.
In order to show the differences between ecosystems in terms of nutrients stored in
difference compartments, Gersmehl identified three storage compartments:
a.
Litter: the surface layer of vegetation which may eventually become humus.
b.
c.
Biomass: the total mass of living organisms, mainly plant tissue, per unit area
Soil: the nutrients store in soil (weathered material) and semi-weathered
material.
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A model of the Nutrient Cycle
3 Difference Nutrient Cycles
F. Environmental Limitations in Ecosystem Development:
1. Principles of limiting factors:
a. Law of the Maximum (J.V. Liebig, a German ecologist in the 19th century)
Yield of a crop could be increased only by supplying the plant more of the
nutrient present in least amount. For example, a field of wheat might have plenty of
available phosphorus for a high yield. But it might have a very poor yield because
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of insufficient nitrogen in the soil. Supplying the crop with more phosphorus would
do nothing to improve the situation. Yield could only be increased by adding
nitrogen. Nitrogen is the limiting factor in this case and the crop yield will increase
in direct proportion to the amount of nitrogen fertilizer added. Eventually, yield
would level off and there will be no increase with addition of nitrogen.
Therefore, there is an upper limit to these factors. For example, too high a
temperature is as bad in its way as too low a temperature.
b. Law of the Minimum:
Every organism requires certain amounts of several environmental factors
for optimal growth.
c. Conclusion:
If there isn't enough of a particular factor or a little too much of it, the
organism grows poorly. If the amount is too low or far too much, the organism will
fail to grow or even die.
The limiting factors are those which limit the growth, reproduction, and
therefore, the distribution of any organism by scarcity or even over-abundance of
that particular factor. (An organism-centered principle).
2. Principle of holocoenotic environment:
If one factor in an environment is changed, this change may cause shifts in
other environmental components. For example, the temperature of a greenhouse is
o
increased by 10 C. This enables the air to hold more water vapour and increase the
vapour pressure of liquid surfaces within the room. Thus, it leads to an increase in
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evaporation rates which in turn increases transpiration. Finally, it increases the
absorption of soil moisture by the plants. This reduction of free water in the soil
allows air to be drawn into the soil and increase the dryness of the soil.
In spite of the growth of an organism or community being controlled by
limiting factors, we cannot ignore the fact that the environment is really a complex
of interacting factors; if one factor is changed, almost all will change eventually.
Therefore, in 1927, a German ecologist Karl Friederich suggested that
'community-environmental relationship are holocoenotic'. This means that there are
no 'walls' or barriers between the factors of an environment and the organism or
biotic community.
As a conclusion, the ecosystem reacts as a whole. It is impossible to wall off
a single factor or organism in nature and control it at will without affecting the rest
of the ecosystem.
3. Limiting factors of an environment:
a. Light:
Light is an extremely important environmental factor because it is the vital
source of energy for ecosystems and it can also act as a control of functions such as
reproduction and migration.
i. The Quality of Light:
The amount of energy available for primary productivity will be partly
determined by the quality of light. In photosynthesis light energy is absorbed
by pigments, chlorophyll, which absorbs red and blue light but reflects green.
In terrestrial plant communities and in aquatic ecosystems tall or floating
green plants beneath will be mainly in the green wave-length band. This
means that plants living within woods or deep in water must be adapted to
surviving in conditions where there is little red or blue light.
The quality of light varies with altitude. On high mountains the invisible
ultraviolet light is intense. Ultraviolet light retards plant growth by
deactivating the hormones which cause the stems to elongate.
ii. The Intensity of Light:
This is very important as it will be a controlling factor in governing the rate f
photosynthesis. In other to grow and reproduce, plants must make more
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carbohydrates by photosynthesis than are used up in respiration. Net
productivity will be a function of these two processes. As photosynthesis only
occurs in the light, carbohydrates respired in the night-time must be replaced
before there is a net gain in organic matter each day.
The point at which photosynthesis balances the energy used up in respiration
is known as the compensation point. The following figure is shown the
tolerance between respiration and photosynthesis with varying light intensity.
The Tolerance between respiration and Photosynthesis with varying light intensity
iii.The Duration of Light:
Many aspects of behaviour in plants and animals, such as flowering, migration
and mating, are affected by day length.
Flowering in many species of plants is initiated by a certain number of hours
of darkness. Plants can be divided into three latitudinal groups on this basis:
1. long-day plants (flowering in the long days of tempera summers);
2. short-day plants (flowering in the short days of the tropics); and
3. day-neutral plants which have no definite day length requirements. This
last group is frequently found in high latitudes where there is continuous
light in the summer season.
b. Temperature:
Temperature is a universally important environmental factor both for its
directly effects on organisms and for its indirect effects in modifying other factors
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such as relative humidity and water availability. Each species has its own
minimum, maximum and optimum temperatures for life but the actual limits at
nay time will vary with such things as the age of the individual and water balances
in the body.
Generally, aquatic plants and animals have narrower tolerance ranges for
temperature than those which live on land.
Tropical plants often do not tolerate temperatures below 15oC, and most
temperate cereals are intolerant of temperatures less than -2oC, whereas evergreen
coniferous forests may withstand many degrees below freezing.
c. Water:
Water availability may often restrict ecosystem development because most
organisms need large amounts of water to survive. It not only forms a large
percentage of the tissues in plant and animal bodies but it is also essential for
photosynthesis.
Most of the water absorbed passes out of the plant in transpiration from
special pores in the leaves known as stomata. The actual rate of water loss by
transpiration will vary with relative humidity, air movement, size of the leaves and
the size of the stomata. This means that the water requirement for plants will vary
both with environmental conditions and among different species.
If there is insufficient water, plant cells lose their rigidity and the plants
may wilt. Stomata close, helping to prevent further water loss. The plant may
remain in this condition for a long time without damage, providing the
temperature is not excessively high.
Early ecologists, et. Warming in 1909, divided plants into groups based on
their water requirements and tolerance:
i. Xerophytes are plants which show morphological and physiological
features which could enable them to survive in extremely arid areas.
ii. Halophytes are plants tolerating saline conditions.
iii. Hydrophytes are flowering plants adapting to living submerged in
water.
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d. Wind:
Wind can act as an environmental factor either directly by causing
mechanical damage to plants or indirectly affecting relative humidity and
evaporation rates. High wind velocities can cause an appreciable increase in the
rate of transpiration mountain summits, coasts and open plains vegetation may be
dwarfed as a result of wind action.
e. Topography:
Topography can influence ecosystem development in three major ways.
First, by the direct effects of altitude on temperature. Temperature
decreases as altitude increases at the normal lapse rate (6.5oC/km)
Secondary, topography can act indirectly. The combination of changes in
temperature and relative humidity leads to the development of an altitudinal
zonation of ecosystems.
The third way in which topography can influence ecosystem development
is by local variation in slope orientation and angle. South-facing slopes receive
strong incident light (in the northern hemisphere) and are therefore warmer and
drier than north-facing slopes which are in the shadow for a lot of the time. Angle
of slope will be a critical factor in soil formation and drainage.
f. Soil:
The soil is a vital component of terrestrial ecosystems, particularly in
cycling nutrients without which all life would cease. Particular attributes of soils,
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such as texture, pH, soil climate and organic content operate in a closely
inter-related fashion to exert control on rates of decomposition, nutrient cycling
and plant distribution and productivity.
g. Biotic Factors:
Biotic factors are the interactions that occur between living things. Some
species are beneficial or even essential for the existence of others, whereas some
may be harmful.
One of the main forms of competition between plants is for light. The
dominant plants will be those which grow tallest and modify the light conditions
for the rest of the community. The struggle for light above ground will influence
root development and the competition for water and nutrients in the soil.
The effects of animals are primarily direct. many plants rely on animals for
pollination and seed dispersal. Conversely, many animals are directly dependent
on plants for food. Under natural conditions there will be a balance between the
biomass of autotrophs and herbivores, so that particular plant species are rarely
excluded from vegetation communities.
Man is by far the most important biotic factor. He has caused fundamental
modifications of ecosystems by fire, hunting and agriculture. More recently, since
industrialization and the intensification of agriculture, man has obliterated large
areas of natural systems and caused pollution of both terrestrial and aquatic
habitats.
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