Ecosystem - 鐘聲慈善社胡陳金枝中學 · Web viewWhen animals eat plants, some of the nitrogen is transferred to these animals. When the plants and animals die, their nitrogen

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..

Al Geography Notes (Ecosystem) Page 1

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


C. Components of an Ecosystem:

1. Abiotic componentsThey 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


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.

PhotosynthesisInorganic 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 ConsumersOmnivores (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


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..


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)


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 + HeatCO2 + 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 + HeatCH2O + 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.


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


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. Biomass: the total mass of living organisms, mainly plant tissue, per unit areac. Soil: the nutrients store in soil (weathered material) and semi-weathered

material.


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


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 increased by 10oC. 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


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


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); and3. 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


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.


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,


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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Ecosystem - 鐘聲慈善社胡陳金枝中學 · Web viewWhen animals eat plants, some of the nitrogen is transferred to these animals. When the plants and animals die, their nitrogen