- 1 Flow of Energy and Cycling of Matter in Ecosystems
- 1.1 Laws Governing Energy Transformations
- 1.2 Ten Percent Law of Energy Flow
- 1.3 Energy Flow Model
- 1.4 Stages of Energy Flow in an Ecosystem
- 1.5 Ecological Efficiency
- 1.6 Ecological Pyramid
- 1.7 Explanation of Ecological Pyramid
- 1.8 Types of Ecological Pyramid
- 1.9 Significance of Ecological Pyramid
- 1.10 Limitations of Ecological Pyramid
- 1.11 Factors Controlling Ecosystem
- 1.12 Functions of an Ecosystem
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Flow of Energy and Cycling of Matter in Ecosystems
Energy is the capacity to do work. Solar energy is transformed into chemical energy by the process of photosynthesis, and is stored in plant tissue and then transformed into mechanical and heat forms during metabolic activities. The energy, in the biological world, flows from the sun to plants and then to all heterotrophic organisms such as microorganisms, animals and man. Energy flow is the key function in an ecosystem and it is unidirectional.
‘The study of energy transfer at different trophic levels is known as ‘Bioenergetics’. Living organisms can use energy in several forms. This energy is stored in food as potential energy. The organism takes this energy by the oxidation of food matters. The bound energy in the food is transformed into solar energy. This solar energy moves to the highest consumer’s body step by step through the food chain. The flow of energy in the ecosystem is one way (unidirectional). The energy enters the ecosystem from the solar radiation. The solar energy is then converted into chemical energy by the green plants (producers). From the producer, the energy passes on and from one trophic level to the next one. The energy flow in the ecosystem is determined by two laws of thermodynamics. According to the first law, energy can neither be created nor destroyed, on the other hand, one type of energy is converted into other types.
According to the second law, every transformation of energy is associated with its loss. Hence it is presumed that a hundred percent energy transformation from one organism to another is not possible. It is always accompanied by some loss of energy in the form of heat. The energy flow of an ecosystem can pass from producer to consumers in the following manner:
- Energy fixation by the producer.
- The accumulation of energy by the producers.
- the flow of energy from producers to primary consumers.
- from primary consumers to secondary consumers.
The process in which the modified solar energy which is trapped by the producers, passes to the different trophic levels through the food chain from the producers to the consumers in an ecosystem, is known as energy flow.
Characteristics of Energy Flow
- Solar energy is the main source of energy flow in the ecosystem.
- Energy flow is unidirectional.
- The nature of energy is not rotational in the ecosystem.
- The energy gets transmitted from the producer to the trophic level step by step but once given out it cannot follow its way back.
- Energy is indestructible but it can transform itself from one to another.
Laws Governing Energy Transformations
The storage and expenditure of energy in an ecosystem is in accordance with the laws of thermodynamics (basic laws of thermodynamics). The first law of thermodynamics is the law of conservation of energy, which says that ‘energy can neither be created nor destroyed but can be transformed from one form into another’.
In biological systems, solar energy is converted into chemical energy and is stored in food materials as internal energy. If an increase or decrease occurs in the internal energy of the system, then it only indicates some work (W) is done and heat (Q) is either evolved or absorbed. It can be represented as
E (Decrease or increase in internal energy) = W (Work done by the system) ± Q (Heat given or absorbed by the system)
Hence energy is not created or destroyed in the system but is only transformed from one form to another.
The second law of thermodynamics states that processes of energy transformation will not occur spontaneously unless there is degradation of energy from a non-random to a random form. Energy transformations, which occur within the ecosystem are considered in ecological energetics. The quantity of solar energy entering the earth’s atmosphere is about 15.3 × 108 cals/m2/year (1 cal = 4.184 J). However, the average amount of solar energy (per unit area per unit time) actually available to autotrophs depends upon their geographical location. Only the Photosynthetically Active Radiation (PAR) is the energy available to autotrophs. A major portion (90-95%) of this energy is lost in the form of heat of evaporation and sensible heat.
PAR (Photosynthetically Active Radiation) is the amount of light available for photosynthesis in which the wavelength of light ranges from 400-700 nm. PAR changes seasonally and varies depending on the latitude and time of day. Around 1 to 5% is used for photosynthesis (primary production). Thus, at each transfer, heat energy (random form) is lost. Hence the energy transfer is not 100% efficient and there is degradation of energy from a non-random to a random form.
Energy conserving efficiency is 1.5% for grassland, 0.9% for savannah, 0.81% for mixed forest, 5% for modern crops, and 10-12% for sugarcane fields. Thus in an ecosystem, there is,
- Constant flow or transfer of energy from sunlight through plants (producers) to animals (consumers) in the form of food.
- A decrease in useful energy during each transformation or transfer at each successive trophic level.
- Return of entire solar energy trapped by green plants back to the environment as heat.
Ten Percent Law of Energy Flow
In the year 1942, Lindemann formulated this law. This law states about 10% of total energy is transmitted during every flow through several trophic levels. As an example in a grassland ecosystem, if a deer takes 100 kg of grass then only 10%, i.e., 10 kg is utilized for its body construction. If a deer is taken by a tiger, then only 1 kg is utilized for its body construction. Thus, a gradual domination of energy is noticed. According to the law, during the transfer of organic food from one trophic level to the next, only about ten percent of the organic matter is stored as flesh. The remaining is lost during transfer or broken down in respiration.
Plants utilize sun energy for primary production and can store only 10% of the utilized energy as net production available for the herbivores.
When plants are consumed by animals, about 10% of the energy in the food is fixed into animal flesh which is available for the next trophic level (carnivores). When a carnivore consumes that animal, only about 10% of the energy is fixed in its flesh for the higher level. So at each transfer, 80-90% of potential energy is dissipated as heat (second law of thermodynamics) where only 10-20% of energy is available to the next trophic level. The ten percent rule means when energy is passed in different trophic levels of the ecosystem, only ten percent of the energy will be passed on.
Energy Flow Model
The flow of energy through various trophic levels is given by energy flow models. Some of them are as follows:
1. Universal Energy Flow Model:
It is given by E. P. Odum. It explains the flow of energy through an ecosystem, where part of the energy is assimilated for the production of biomass, and some part gets lost due to heat or respiration.
2. Single Channel Energy Flow Model:
It shows the flow of energy in a unidirectional manner through a single channel of green producers to herbivores and carnivores. This model is one of the first published models.
3. 2-Channel Model (Y-shaped Model) of Energy Flow:
According to Odum (1956), this Y-shaped model of energy flow is known as the double channel model. Both the grazing and detritus food chains operate in the same type of ecosystem. In nature, two food chains are separated but are not so. This double-channel model shows the passage of energy through the grazing and detritus food chains. This model is more real and practical than the single-channel model. In time and space, this model separates these two types of food chains. An example is the combination of grazing and detritus food chains in nature.
Stages of Energy Flow in an Ecosystem
Energy flow in the ecosystem takes place in three stages:
1. Acquisition of Energy:
Sun is the only primary source of energy in an ecosystem. 12.3 × 1022 Kcal of solar energy reaches the earth every year. of which a major portion is reflected back and only 0.02% is absorbed by the plants. From this absorbed energy only 0.1% is utilized by green plants for photosynthesis and converts it as potential energy to be stored in the food. The total energy that is present in food is termed gross production. Plants use this food (potential energy) for their different meta¬bolic activities. The amount of energy that remains after utilization for respiration is called net production.
2. Use of Energy:
Producers play the main part in producing the energy in an ecosystem. The solar energy is trapped by the autotroph during photosynthesis. The green plants synthesize carbohydrates or other food materials within their body from CO2 and water with the help of sunlight. All animals are dependent upon plants for their food. From the producer, this energy passes from one trophic level to the next one, i.e., producers to primary consumers, primary consumers to secondary consumers, secondary consumers to tertiary consumers, and so on. Then the energy required from food is called gross energy intake or I. During energy flow, there is a degradation of energy from one trophic level to the next as in each trophic level some energy is lost as heat during metabolism. In this way, the amount of lost energy is called respiratory energy or R.
3. Transfer of Energy:
The net energy present within the producer is transferred to different levels of consumers along with the food chain. During this transference of energy in each step, 90% of total energy is lost in the form of heat and only 10% is accumulated in the body. So, from the gross energy intake, i.e., I, if we subtract respiratory energy, i.e., R, the remaining energy is what is acquired by the consumers (I-R).
Ecological efficiency describes the efficiency with which energy is transferred from one trophic level to the next. It is determined by a combination of efficiencies relating to organismic resource acquisition and assimilation in an ecosystem. Primary production occurs in autotrophic organisms of an ecosystem. Photoautotrophs such as vascular plants and algae convert energy from the sun into energy stored as carbon compounds. Photosynthesis is carried out in the chlorophyll of green plants. The energy converted through photosynthesis is carried through the trophic levels of an ecosystem as organisms consume members of lower trophic levels.
Energy transfer between trophic levels is generally inefficient, such that net production at one trophic level is generally only 10% of the net production at the preceding trophic level (the Ten Percent law, first formulated by Raymond Lindeman). Due to non-predatory death, egestion, and respiration, a significant amount of energy is lost to the environment instead of being absorbed for production by consumers. The figure approximates the fraction of energy available after each stage of energy loss in a typical ecosystem, although these fractions vary greatly from ecosystem to ecosystem and from trophic level to trophic level. The loss of energy by a factor of one-half from each of the steps of non-predatory death, defecation, and respiration is typical of many living systems. Thus, the net production at one trophic level is approximately ten percent of that of the trophic level before it.
Example: Assume 500 units of energy are produced by trophic level 1. One-half of that is lost to non-predatory death, while the other half (250 units) is ingested by trophic level 2. One-half of the amount ingested is expelled through defecation, leaving the other half (125 units) to be assimilated by the organism. Finally, half of the remaining energy is lost through respiration while the rest (63 units) is used for growth and reproduction. This energy expended for growth and reproduction constitutes to the net production of trophic level 1.
Ecological efficiency is the percentage of energy transferred from one trophic level to the next. It is the amount of resources and food converted into biomass expressed in the ratio:
% of Efficiency = (Efficiency/Total Resources) × 100
Photosynthetic efficiency = (Energy in gross primary productivity/Energy in incident solar radiations) × 100. Photosynthetic efficiency is 1 -5%.
Net Production Efficiency:
Tree species with large amounts of nonphotosynthetic biomass have lesser net production efficiency than small-sized producers. It is commonly 80%, but it may vary in small and large plants.
Net production efficiency = (Net primary productivity/Gross primary productivity) × 100.
Net production efficiency is the percentage of food energy assimilated in growth, reproduction, and storage. This ecological growth efficiency is higher in shorter lifespan organisms than longer lifespan organisms.
Net production efficiency = (Production/Assimilation) × 100
It is the percentage of total productivity available at one trophic level that is actually consumed or ingested by a trophic compartment one level up.
Assimilation Efficiency = (Food Energy Assimilated/Food Energy Ingested) × 100.
In the case of herbivores, it may be 50% but in carnivores or omnivores, it becomes 90%.
Ecological Efficiency = (Energy converted into biomass at trophic level/Energy present in biomass at lower Trophic level) × 100
It describes the efficiency with which energy is transferred from one trophic level to the next. It is the amount of food ingested, divided by the number of prey biomass.
Exploitation Efficiency = (Consumption of biomass of a trophic level/Total prey biomass) × 100
Trophic efficiency is the percentage of production transferred from one trophic level to the next. It is always less than productive efficiencies. After Lindeman’s pioneering work, it was generally assumed that trophic transfer efficiencies were around 10%.
The gradual reduction of trophic levels of the food chain of an ecosystem will produce a conical pyramid-like structure which is called an ecological pyramid. Ecological pyramids are diagrammatic representa¬tions showing the relationship between the biomass, number, and energy contents at different trophic levels where the producers make up the base and successive levels of consumers constitute the upper tiers.
Explanation of Ecological Pyramid
Trophic structure, i.e., the interaction of the food chain and the size, and metabolism relationship between the linearly arranged various biotic components of an ecosystem is characteristic of each type of ecosystem. In each ecological pyramid, the producer level constitutes the base while successive levels make up the apex. Ecological pyramids were first devised by a British ecologist Charles Elton in 1927. In the successive steps of grazing food-chain photosynthetic autotroph, herbivorous heterotroph, carnivorous heterotroph, and decay bacteria – the number and mass of the organisms in each step are limited by the amount of energy available. Since some energy is lost as heat in each transformation, the steps become progressively smaller near the top.
Types of Ecological Pyramid
There are three types of ecological pyramids
- Pyramid of number
- Pyramid of biomass
- Pyramid of energy
1. Pyramid of Numbers:
It represents the numerical relationship between different trophic levels of an ecosystem. In this type of pyramid, the more abundant species, i.e., producers (green plants) remain near the base, and less abundant species, i.e., consumers like top carnivores are near the tip. Here, the producer is ingested in large numbers by a smaller number of primary consumers (herbivores like rabbits).
Similarly, these herbivores are eaten by a lesser number of secondary consumers like snakes and lizards and these in turn are eaten by a still lesser number of tertiary consumers like predatory birds. So, the number of consumers decreases in the successive levels of pyramids from bottom to top. At the base of the pyramid, the animals are smaller in size but greater in number, and at the apex, the size is larger and the number is less. The pyramid of numbers is thus upright (Grassland and pond ecosystem). However, in a forest ecosystem, the pyramid of numbers is inverted as the producers (trees) are large-sized and less in number forming the base of the pyramid. The herbivore consumers are greater in number than the producers.
2. Pyramid of Biomass:
This pyramid indicates the total mass of each trophic level belonging to a definite food chain of any ecosystem. The main feature of this pyramid is that the biomass gradually decreases in the trophic level from the base towards the apex i.e., the pyramid is upright (forest ecosystem). The biomass of the producer is greater than that of the primary consumer; the biomass of the secondary consumer is less than that of the primary consumer and so on but in the case of the parasitic food chain, the pyramid of biomass is inverted.
3. Pyramid of Energy:
The energy flow in an ecosystem can be represented by the help of a pyramid which is called a pyramid of energy. The trophic level at the base denotes the position of the highest amount of energy whereas the amount of energy at the trophic level situated at the apex is the lowest. This is due to the reduction or loss of energy at the time of transfer of energy from one trophic level to another. For example, only 20% of the total solar energy is utilized by green plants during photosynthesis. The reduction of energy from lower to higher trophic levels is gradually increased during respiration. During the transfer of energy from one trophic level to another trophic level, only 10% is utilized, and the rest 90% of potential energy is lost as heat energy. So, a gradual decline of energy takes place from the starting point to the termination of the food chain. So this always remains upright.
Differences between Upright Pyramid and Inverted Pyramid:
|Upright Pyramid||Inverted Pyramid|
|1. The base of the upright pyramid is broad.||1. The base of the inverted pyramid is small.|
|2. The producers are the largest.||2. The producers are the smallest.|
|3. The topmost trophic level is the smallest.||3. The topmost trophic level is the largest.|
|4. It only occurs in the pyramid of energy.||4. It only occurs in the pyramid of biomass and numbers.|
Significance of Ecological Pyramid
Generally, any of the ecological pyramids can be similar in nature but it may differ in volume and periphery, because of the number of trophic levels.
Grass → Deer → Tiger
Grass → Grasshopper → Frog → Snake → Hawk
So, the pyramid of numbers, the pyramid of biomass, and the pyramid of energy will differ from one another on the basis of the nature of producers and consumers. From this thorough discussion we can conclude that the following features should remain significant in any ecological pyramid:
- Numbers of trophic level present in a food chain.
- Numbers of living organisms and their biomass at the trophic level.
- Manifestation of energy at the trophic level.
- Nature of ecosystem.
- Relation between the trophic levels.
Limitations of Ecological Pyramid
- Some species practice more than one mode of nutrition or belong to two or more trophic levels. This is particularly true in the case of consumers of higher trophic levels.
- Man in an example. He gets his food from primary producers as well as from higher trophic levels. Such organisms that feed at more than one trophic level are extremely difficult to depict in ecological pyramids.
- Saprophytes play a vital role in the ecosystem but they are not represented in ecological pyramids.
- Detritus such as litter and humus is an important source of energy and exerts considerable influence on ecosystem function, yet it is not depicted in ecological pyramids.
- Ecological pyramids do not provide any clue to seasonal and diurnal variations.
- The rate of transfer from one trophic level to another is not reflected in the ecological pyramids.
Factors Controlling Ecosystem
The ecosystem of a particular habitat is controlled by different factors. These are of two types: Physical factors and biological factors.
1. Physical Factors
- Solar Energy: Solar energy helps the producer (green plants) synthesize food substances by the photosynthetic process.
- Temperature: Temperature controls the productive activities or enzymatic activities of living organisms, which need the optimum temperature 25°C to 37°C.
- Water: The amount of water present in the earth controls indirectly the other abiotic substances of the ecosystem.
- Soil: The nature and structure of soil directly influence the ecosystem of that particular place.
2. Biological Factors
- Producer: The increasing capacity and the survival nature of the producer are directly influenced by the ecosystem.
- Consumers: Consumers are the main components of the ecosystem. So, these are directly influenced by the nature and productivity of producers.
- Decomposer: Without decomposers, the ecosystem will be incomplete. The food chain cannot be maintained in a disciplined way. So, the decomposers directly control the ecosystem.
Functions of an Ecosystem
The main functions of an ecosystem are as follows:
1. Habitat Functions:
Ecosystems provide habitat to wild plants and animals, thereby conserving biological and genetic diversity. It supports different food chains and food webs.
2. Flow of Energy:
- The radiant energy of the sun enters into the ecosystem through the photosynthetic activity of plants (producers).
- Travels via consumers and decomposers
- Ultimately disappears in space as heat energy, which never returns to the source. This phenomenon is referred to as the flow of energy which is the unidirectional flow of energy governed by different biogeochemical cycles.
3. Cyclic use of Materials:
The chemical elements composing living organisms along with nutrients are constantly recirculated within an ecosystem.
Both organisms and their environment are regulated by each other. Thus a balance is maintained in the system. The ecosystem is capable of resisting any extremes or disturbances upto a certain level. This is known as a cybernetic system. Ecosystems contribute to the maintenance of human health by providing opportunities for spiritual enrichment, cognitive development, recreation, and aesthetic experience.
5. Ecosystems not only generate fertile soil but also contribute towards the purification of air, and water and mitigation of droughts and floods.
6. Pollination of crops by various natural agents such as bees, butterflies, etc., and dispersal of seeds via birds, humans, etc.