Ecosystem

1. Chapter at a Glance

An ecosystem can be visualised as a functional unit of nature, where living organisms interact among themselves and also with the surrounding physical environment. Ecosystems vary greatly in size from a small pond to a large forest or a sea. Many ecologists regard the entire biosphere as a global ecosystem, as a composite of all local ecosystems on Earth. Since this system is too big and complex to be studied at one time, it is convenient to divide it into two basic categories, namely the terrestrial and the aquatic. Forest, grassland and desert are some examples of terrestrial ecosystems; pond, lake, wetland, river and estuary are some examples of aquatic ecosystems. Crop fields and an aquarium may also be considered as man-made ecosystems. This chapter covers ecosystem structure and function using a pond as an example, productivity, decomposition, energy flow, and ecological pyramids. NEET asks two to three questions every year, mostly on productivity values, decomposition steps, the 10 per cent law, food chains and pyramids (especially the inverted ones).

2. Ecosystem — Structure and Function

In the previous chapter you looked at the various components of the environment — abiotic and biotic. You studied how the individual biotic and abiotic factors affected each other and their surroundings. Let us look at these components in a more integrated manner and see how the flow of energy takes place within these components of the ecosystem. Interaction of biotic and abiotic components results in a physical structure that is characteristic for each type of ecosystem. Identification and enumeration of plant and animal species of an ecosystem gives its species composition. Vertical distribution of different species occupying different levels is called stratification. For example, trees occupy the top vertical strata or layer of a forest, shrubs the second, and herbs and grasses occupy the bottom layers.

The components of the ecosystem are seen to function as a unit when you consider the following aspects: (i) productivity; (ii) decomposition; (iii) energy flow; and (iv) nutrient cycling.

To understand the ethos of an aquatic ecosystem, let us take a small pond as an example. This is fairly a self-sustainable unit and rather simple example that explains even the complex interactions that exist in an aquatic ecosystem. A pond is a shallow water body in which all the above-mentioned four basic components of an ecosystem are well exhibited. The abiotic component is the water with all the dissolved inorganic and organic substances and the rich soil deposit at the bottom of the pond. The solar input, the cycle of temperature, day-length and other climatic conditions regulate the rate of function of the entire pond. The autotrophic components include the phytoplankton, some algae and the floating, submerged and marginal plants found at the edges. The consumers are represented by the zooplankton, the free swimming and bottom dwelling forms. The decomposers are the fungi, bacteria and flagellates, especially abundant in the bottom of the pond. This system performs all the functions of any ecosystem and of the biosphere as a whole, i.e. conversion of inorganic into organic material with the help of the radiant energy of sun by the autotrophs; consumption of the autotrophs by heterotrophs; decomposition and mineralisation of the dead organic matter to release them back for reuse by the autotrophs. These events are repeated over and over again. There is unidirectional movement of energy towards the higher trophic levels and its dissipation and loss as heat to the environment.

3. Productivity

A constant input of solar energy is the basic requirement for any ecosystem to function and sustain. Primary production is defined as the amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis. It is expressed in terms of weight (g m⁻²) or energy (kcal m⁻²). The rate of biomass production is called productivity. It is expressed in terms of g m⁻² yr⁻¹ or (kcal m⁻²) yr⁻¹ to compare the productivity of different ecosystems. It can be divided into gross primary productivity (GPP) and net primary productivity (NPP). Gross primary productivity of an ecosystem is the rate of production of organic matter during photosynthesis. A considerable amount of GPP is utilised by plants in respiration. Gross primary productivity minus respiration losses (R) is the net primary productivity (NPP).

GPP − R = NPP

Net primary productivity is the available biomass for the consumption to heterotrophs (herbivores and decomposers). Secondary productivity is defined as the rate of formation of new organic matter by consumers.

Primary productivity depends on the plant species inhabiting a particular area. It also depends on a variety of environmental factors, availability of nutrients and photosynthetic capacity of plants. Therefore, it varies in different types of ecosystems. The annual net primary productivity of the whole biosphere is approximately 170 billion tons (dry weight) of organic matter. Of this, despite occupying about 70 per cent of the surface, the productivity of the oceans are only 55 billion tons. Rest of course, is on land. Discuss with your teacher the main reason for the low productivity of oceans.

4. Decomposition

You may have heard of the earthworm being referred to as the farmer's "friend". This is so because they help in the breakdown of complex organic matter as well as in loosening of the soil. Similarly, decomposers break down complex organic matter into inorganic substances like carbon dioxide, water and nutrients, and the process is called decomposition. Dead plant remains such as leaves, bark, flowers and dead remains of animals, including fecal matter, constitute detritus, which is the raw material for decomposition. The important steps in the process of decomposition are:

1. Fragmentation: detritivores (e.g. earthworm) break down detritus into smaller particles.
2. Leaching: water-soluble inorganic nutrients go down into the soil horizon and get precipitated as unavailable salts.
3. Catabolism: bacterial and fungal enzymes degrade detritus into simpler inorganic substances.

It is important to note that all the above steps in decomposition operate simultaneously on the detritus.

4. Humification leads to accumulation of a dark coloured amorphous substance called humus that is highly resistant to microbial action and undergoes decomposition at an extremely slow rate. Being colloidal in nature, it serves as a reservoir of nutrients.
5. Mineralisation: the humus is further degraded by some microbes and release of inorganic nutrients occurs by the process known as mineralisation.

Decomposition is largely an oxygen-requiring process. The rate of decomposition is controlled by chemical composition of detritus and climatic factors. In a particular climatic condition, decomposition rate is slower if detritus is rich in lignin and chitin, and quicker if detritus is rich in nitrogen and water-soluble substances like sugars. Temperature and soil moisture are the most important climatic factors that regulate decomposition through their effects on the activities of soil microbes. Warm and moist environment favour decomposition, whereas low temperature and anaerobiosis inhibit decomposition, resulting in build-up of organic materials.

5. Energy Flow

Except for the deep sea hydro-thermal ecosystem, sun is the only source of energy for all ecosystems on Earth. Of the incident solar radiation, less than 50 per cent of it is photosynthetically active radiation (PAR). We know that plants and photosynthetic bacteria (autotrophs) fix sun's radiant energy to make food from simple inorganic materials. Plants capture only 2–10 per cent of the PAR, and this small amount of energy sustains the entire living world. So, it is very important to know how the solar energy captured by plants flows through different organisms of an ecosystem. All organisms are dependent for their food on producers, either directly or indirectly. So you find unidirectional flow of energy from the sun to producers and then to consumers. Is this in keeping with the first law of thermodynamics?

Further, ecosystems are not exempt from the second law of thermodynamics. They need a constant supply of energy to synthesise the molecules they require, to counteract the universal tendency toward increasing disorderliness.

The green plants in the ecosystem — terminology which makes them producers. In a terrestrial ecosystem, major producers are herbaceous and woody plants. Likewise, primary producers in an aquatic ecosystem are various species like phytoplankton, algae and higher plants. You have read about the food chains and webs that exist in nature. Starting from the plants (or producers), food chains or rather webs are formed such that an animal feeds on a plant or on another animal and in turn is food for another. The chain or web is formed because of this interdependency. No energy that is trapped by an organism remains in it for ever. The energy trapped by the producer, hence, is either passed on to a consumer or the organism dies. Death of organism is the beginning of the detritus food chain/web.

All animals depend on plants (directly or indirectly) for their food needs. They are hence called consumers and also heterotrophs. If they feed on the producers, the plants, they are called primary consumers, and if the animals eat other animals which in turn eat the plants (or their produce), they are called secondary consumers. Likewise, you could have tertiary consumers too. Obviously the primary consumers will be herbivores. Some common herbivores are insects, birds and mammals in terrestrial ecosystem, and molluscs in aquatic ecosystem. The consumers that feed on these herbivores are carnivores, or more correctly primary carnivores (though secondary consumers). Those animals that depend on the primary carnivores for food are labelled secondary carnivores. A simple grazing food chain (GFC) is depicted below:

Grass (producer) → Goat (primary consumer) → Man (secondary consumer)

The detritus food chain (DFC) begins with dead organic matter. It is made up of decomposers, which are heterotrophic organisms, mainly fungi and bacteria. They meet their energy and nutrient requirements by degrading dead organic matter or detritus. These are also known as saprotrophs (sapro: to decompose). Decomposers secrete digestive enzymes that breakdown dead and waste materials into simple, inorganic materials, which are subsequently absorbed by them.

In an aquatic ecosystem, GFC is the major conduit for energy flow. As against this, in a terrestrial ecosystem, a much larger fraction of energy flows through the detritus food chain than through the GFC. Detritus food chain may be connected with the grazing food chain at some levels: some of the organisms of DFC are prey to the GFC animals, and in a natural ecosystem, some animals like cockroaches, crows etc. are omnivores. These natural interconnections of food chains make it a food web. How would you classify human beings?

Organisms occupy a place in the natural surroundings or in a community according to their feeding relationship with other organisms. Based on the source of their nutrition or food, organisms occupy a specific place in the food chain that is known as their trophic level. Producers belong to the first trophic level, herbivores (primary consumer) to the second, and carnivores (secondary consumer) to the third.

The important point to note is that the amount of energy decreases at successive trophic levels. When any organism dies, it is converted to detritus or dead biomass that serves as an energy source for decomposers. Organisms at each trophic level depend on those at the lower trophic level for their energy demands. Each trophic level has a certain mass of living material at a particular time called the standing crop. The standing crop is measured as the mass of living organisms (biomass) or the number in a unit area. The biomass of a species is expressed in terms of fresh or dry weight. Measurement of biomass in terms of dry weight is more accurate. Why? The number of trophic levels in the grazing food chain is restricted, as the transfer of energy follows the 10 per cent law — only 10 per cent of the energy is transferred to each trophic level from the lower trophic level. In nature, it is possible to have so many levels — producer, herbivore, primary carnivore, secondary carnivore in the GFC. Do you think there is any such limitation in a detritus food chain?

6. Ecological Pyramids

You must be familiar with the shape of a pyramid. The base of a pyramid is broad and it narrows down at the apex. One gets a similar shape, whether you express the food or energy relationship between organisms at different trophic levels. Thus, relationship is expressed in terms of number, biomass or energy. The base of each pyramid represents the producers or the first trophic level, while the apex represents the tertiary or top level consumer. The three ecological pyramids that are usually studied are (a) pyramid of number; (b) pyramid of biomass; and (c) pyramid of energy.

Any calculations of energy content, biomass or numbers has to include all organisms at that trophic level. No generalisations we make will be true if we take only a few individuals at any trophic level into account. Also, a given organism may occupy more than one trophic level simultaneously. One must remember that the trophic level represents a functional level, not a species as such. A given species may occupy more than one trophic level in the same ecosystem at the same time; for example, a sparrow is a primary consumer when it eats seeds, fruits, peas, and a secondary consumer when it eats insects and worms. Can you work out how many trophic levels human beings function at in a food chain?

In most ecosystems, all the pyramids — of number, of energy and biomass — are upright, i.e. producers are more in number and biomass than the herbivores, and herbivores are more in number and biomass than the carnivores. Also, energy at a lower trophic level is always more than at a higher level.

There are exceptions to this generalisation. If you were to count the number of insects feeding on a big tree, what kind of pyramid would you get? Now add an estimate of the number of small birds depending on the insects, as also the number of larger birds eating the smaller. Draw the shape you would get. The pyramid of biomass in sea is generally inverted because the biomass of fishes far exceeds that of phytoplankton. Isn't that a paradox? How would you explain this? The pyramid of energy is always upright, can never be inverted, because when energy flows from a particular trophic level to the next trophic level, some energy is always lost as heat at each step. Each bar in the energy pyramid indicates the amount of energy present at each trophic level in a given time or annually per unit area.

However, there are certain limitations of ecological pyramids: (i) they do not take into account the same species belonging to two or more trophic levels; (ii) they assume a simple food chain, something that almost never exists in nature; they do not accommodate a food web; (iii) moreover, saprophytes are not given any place in ecological pyramids even though they play a vital role in the ecosystem.

7. Extra Reading (Not in Current NEET Syllabus)

8. Master Summary Table

9. Mind Map

10. Tips and Tricks

11. Important One-liners

• Earthworm is called the farmer's friend.
• Detritus = dead leaves, bark, flowers, animal remains and faecal matter.
• Leaching precipitates nutrients as unavailable salts.
• Humus is colloidal and acts as a reservoir of nutrients.
• Decomposition is largely an oxygen-requiring process.
• Sun is the only energy source except in deep-sea hydrothermal ecosystems.
• Decomposers are also called saprotrophs.
• Standing crop is the mass of living material at a trophic level at a given time.
• A trophic level is a functional level, not a species.
• A sparrow can be a primary or secondary consumer.
• Saprophytes are not shown in ecological pyramids.
• Crop fields and aquaria are man-made ecosystems.

12. NEET-pattern Questions with Detailed Answers

These questions are original and based on concepts that NEET UG asks again and again.

13. Quick Revision Summary