Ecosystem — Study Notes

1. Introduction

An ecosystem is a functional unit of nature where living organisms interact with each other and with their non-living environment. The term ecosystem was coined by A.G. Tansley. An ecosystem can be as small as a pond or as large as a forest, ocean or desert. It consists of two main components: the biotic (living) component and the abiotic (non-living) component. The functioning of an ecosystem involves the flow of energy, the cycling of materials (biogeochemical cycles), and the regulation of populations.

The chapter explores the structure of ecosystems, the concept of productivity, the flow of energy through food chains and food webs, the ecological pyramids, and the biogeochemical cycles of carbon and phosphorus. It also examines the processes of ecological succession, and the services that ecosystems provide to humans. Understanding the functioning of ecosystems is essential for appreciating the importance of biodiversity and for designing conservation strategies, and it connects directly to the chapters on organisms and populations, biodiversity, and environmental issues.

2. Ecosystem Structure and Function

2.1 Components of an Ecosystem

An ecosystem has two components: - Abiotic components: The non-living factors such as temperature, water, light, soil and climate. - Biotic components: The living organisms, which can be divided into: - Producers (autotrophs): Green plants, algae and some bacteria that produce food by photosynthesis. They capture solar energy and convert it into chemical energy. - Consumers (heterotrophs): Organisms that depend on other organisms for food. They include primary consumers (herbivores), secondary consumers (carnivores that eat herbivores), tertiary consumers (carnivores that eat other carnivores) and omnivores. - Decomposers (saprotrophs): Bacteria and fungi that break down dead and decaying organic matter, releasing nutrients back into the soil. They are essential for nutrient recycling.

2.2 Ecosystem Function

The functions of an ecosystem are: - Productivity (rate of biomass production). - Decomposition (breakdown of dead organic matter). - Energy flow (through the trophic levels). - Nutrient cycling (biogeochemical cycles).

3. Productivity

Productivity is the rate of production of biomass in an ecosystem. It is expressed in terms of weight (grams per square metre per year) or energy (calories per square metre per year).

• Primary productivity: The amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis. It is the rate at which solar energy is converted into chemical energy by autotrophs.
• Gross primary productivity (GPP): The total amount of organic matter produced by photosynthesis, including the organic matter used up in respiration.
• Net primary productivity (NPP): The organic matter available to heterotrophs, given by NPP = GPP - R (where R is respiratory loss).
• Secondary productivity: The rate of formation of new organic matter by consumers.

The oceans contribute to about 50 percent of the total net primary productivity of the biosphere, because although the productivity per unit area of oceans is low, their total area is enormous.

4. Decomposition

Decomposition is the process of breakdown of dead organic material (detritus) into simpler inorganic substances. It is carried out by decomposers, mainly bacteria and fungi. The steps of decomposition are: - Fragmentation: The breaking of detritus into smaller particles by detritivores (e.g., earthworms). - Leaching: The removal of water soluble inorganic nutrients from the decomposing detritus by water. - Catabolism: The breakdown of complex organic compounds (proteins, carbohydrates, fats) into simpler inorganic substances by the enzymes of decomposers. - Humification: The accumulation of dark, amorphous, nutrient-poor organic matter called humus in the soil, which is resistant to microbial action. - Mineralisation: The release of inorganic nutrients from humus into the soil.

The rate of decomposition is controlled by the chemical composition of detritus and the climatic conditions. The detritus rich in lignin and chitin decomposes slowly, while that rich in nitrogen and water-soluble substances decomposes rapidly. Warm and moist conditions favour decomposition, while low temperature and low moisture retard it.

5. Energy Flow in Ecosystems

The energy for an ecosystem comes from the sun. Green plants capture solar energy and convert it into chemical energy through photosynthesis. This energy is then transferred through the food chain from one trophic level to the next.

5.1 Food Chains and Food Webs

• Food chain: The sequence of organisms through which energy flows, in which each organism feeds on the previous one and is eaten by the next. A grazing food chain starts with green plants (grass to deer to lion), while a detritus food chain starts with dead organic matter (dead leaves to fungi to detritivores). Detritus food chains are often longer than grazing food chains.
• Food web: A network of interconnected food chains in an ecosystem, showing the feeding relationships between organisms.

5.2 Ten Percent Law

According to Lindeman's ten percent law, only about 10 percent of the energy available at one trophic level is transferred to the next trophic level. The rest is lost as heat, in respiration and in other metabolic processes. This limits the number of trophic levels in a food chain to generally four or five.

5.3 Ecological Pyramids

The relationship between organisms at different trophic levels can be represented as ecological pyramids, proposed by Charles Elton: - Pyramid of number: Shows the number of individuals at each trophic level. It may be upright (grass-grasshopper-bird) or inverted (a single tree supporting many birds and insects). - Pyramid of biomass: Shows the biomass of organisms at each trophic level. It is usually upright, but may be inverted in aquatic ecosystems (a small standing crop of phytoplankton supports a large biomass of zooplankton and fish). - Pyramid of energy: Shows the energy content at each trophic level. It is always upright because energy always decreases at each trophic level due to the ten percent law.

6. Biogeochemical Cycles

Biogeochemical cycles are the cycling of materials (elements) between the biotic and abiotic components of the ecosystem. The two main types are the gaseous cycles (carbon and nitrogen) and the sedimentary cycles (phosphorus and sulphur).

6.1 Carbon Cycle

Carbon is present in the atmosphere as carbon dioxide. Plants fix atmospheric carbon dioxide during photosynthesis to form organic compounds. Carbon returns to the atmosphere through: - Respiration by plants, animals and microbes. - Combustion of fossil fuels. - Decomposition of dead organic matter.

The carbon in fossil fuels (coal, petroleum) is a reservoir pool, and its combustion by humans is a major cause of the rise in atmospheric carbon dioxide and global warming.

6.2 Phosphorus Cycle

Phosphorus is an essential component of nucleic acids, ATP and cell membranes (phospholipids). The phosphorus cycle is a sedimentary cycle. Phosphorus is released from rocks by weathering and erosion, enters the soil and water, and is taken up by plants. It is transferred through the food chain and returns to the soil through excretion and decomposition. Unlike carbon and nitrogen, phosphorus does not have a gaseous phase, so its cycling is slow.

7. Ecological Succession

Ecological succession is the gradual, directional and sequential change in the species composition of a community over time. It occurs until the community reaches a stable, mature community called the climax community.

• Primary succession: Begins in areas where no living organisms ever existed, such as bare rocks, newly cooled lava or newly formed ponds. It takes a long time because soil must be formed first. In primary succession on rocks, the sequence is lichens, then mosses, then grasses and herbs, then shrubs and finally trees (the climax forest). This slow process is called xerarch succession because it begins in dry conditions.
• Secondary succession: Begins in areas where the vegetation has been destroyed but the soil is present, such as abandoned farmlands, cut forests or burned forests. It is faster than primary succession because the soil and some propagules are already present. In an abandoned farmland, the sequence may be grasses, then shrubs and then trees.

The hydrarch succession (in water bodies) and xerarch succession (on rocks) both converge to a climax community that is the mesic community (moderate moisture).

8. Ecosystem Services

Ecosystem services are the benefits that humans derive from ecosystems. These include: - Provisioning services: Food, water, timber, fibre and medicines. - Regulating services: Climate regulation, water purification, flood control and pollination. - Supporting services: Nutrient cycling, soil formation and photosynthesis. - Cultural services: Recreation, aesthetic value and spiritual value.

8.1 Value of Ecosystem Services

One of the most valuable ecosystem services is pollination, which is essential for crop production. Robert Constanza and his colleagues have estimated the value of the world's ecosystem services to be between 16 and 54 trillion US dollars per year, which is more than the gross national product of the world. In India, a study showed that the natural forests of the Western Ghats provide 1.6 trillion rupees worth of ecosystem services annually. However, the aesthetic and spiritual value of ecosystems cannot be measured in money. The ecosystem services are a compelling reason for conserving natural ecosystems.

Quick Revision Tables

Table 1: Productivity Terms

Table 2: Ecological Pyramids

Table 3: Succession Types

Mind Map

Important Diagrams (SVG)

Common Mistakes

1. Students think the pyramid of energy can be inverted; the pyramid of energy is always upright because energy decreases at each trophic level.
2. The pyramid of biomass is said to always be upright; in aquatic ecosystems it can be inverted because the biomass of phytoplankton (producers) is less than that of the consumers.
3. NPP is said to equal GPP; NPP = GPP - R (respiratory loss), and it represents the organic matter available to heterotrophs.
4. Secondary succession is said to be slower than primary succession; secondary succession is faster because soil and propagules are already present.
5. Decomposition is said to be fastest in cold, dry conditions; it is fastest in warm and moist conditions, and slow where detritus is rich in lignin and chitin.
6. The carbon and phosphorus cycles are said to be of the same type; carbon is a gaseous cycle while phosphorus is a sedimentary cycle.
7. Food chains are said to be usually short because of low productivity; they are short because of the ten percent law, which limits energy transfer to about four or five levels.
8. Lichens appear in secondary succession; lichens are the pioneer species in primary succession on bare rocks.

Exam Tips

1. Learn the ecosystem components: producers, consumers, decomposers, and the functions: productivity, decomposition, energy flow, nutrient cycling.
2. Memorise GPP, NPP (NPP = GPP - R) and secondary productivity, with units of g/sq m/year.
3. The ten percent law (Lindeman) and why food chains have only 4-5 levels are guaranteed questions.
4. Learn the three ecological pyramids and remember: pyramid of energy is always upright.
5. Contrast the carbon (gaseous) and phosphorus (sedimentary) cycles, and remember that phosphorus has no atmospheric phase.
6. For succession, remember: primary (bare rock, lichens first, slow) vs secondary (soil present, faster), converging to a mesic climax.
7. Ecosystem services: pollination is most valuable; the global value estimated at 16-54 trillion dollars by Constanza; Western Ghats forests valued at 1.6 trillion rupees annually.

Conclusion

The ecosystem is the functional unit of nature, where living organisms and their environment interact through a continuous flow of energy and a cyclic transfer of materials. The chapter has described the structure of ecosystems in terms of producers, consumers and decomposers, and their functions of productivity, decomposition, energy flow and nutrient cycling. The ten percent law and the ecological pyramids reveal how energy constrains the length of food chains, while the carbon and phosphorus cycles demonstrate how materials are conserved and recycled between the living and non-living worlds. Ecological succession shows how communities develop over time toward a stable climax, and ecosystem services quantify the enormous economic value that intact ecosystems provide. This integrated understanding is essential for the study of biodiversity and conservation, where the challenge is to preserve the very systems that sustain life and human welfare.