What is an ecosystem? An ecosystem may be defined as a structural and functional unit of the biosphere, comprising living organisms and their nonliving environment that interact by means of food chains and chemical cycles resulting in energy flow, biotic diversity and material cycling to form a stable, self-supporting system. The relationship between a biotic community and the nonliving environment is always a mutual one, that is, not only does the environment affect the community but the community also modifies the environment.

An ecosystem thus can be visualized as a functional unit of nature, where living organisms interact among themselves and also with the surrounding physical environment. Ecosystem varies 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 much 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.

Scope of the Study of an Ecosystem

Besides the organisms and their environment, the study of an ecosystem also includes the (i) nutritive relationships between the organisms, (ii) interactions between the organisms and the environment, (iii) cycling of matter, and (iv) flow of energy through it. The functional aspect of the ecosystem is as important as its structural aspect.

Organization (Structure) of an Ecosystem: Structural components of an ecosystem:

To sustain itself and last indefinitely, an ecosystem must have resources for supporting its resident organisms and for disposal of their wastes. Ecosystem has mechanisms for both the requirements (as shown in the figure below). The necessary components of an ecosystem are matter, (water, minerals, carbon dioxide, oxygen) and several species of organisms. An ecosystem must also receive a continuous supply of energy. The components of an ecosystem may be divided into two main types: biotic component comprising the various kinds of organisms and abiotic component consisting of environmental factors. Let’s discuss in brief about the structural components of an ecosystem herein below:  

Figure showing: Material and energy transformation is an ecosystem

Biotic Components

The living organisms present in an ecosystem form the biotic component. They are interconnected through food. Food is a substance which provides (i) materials for building, growth and repair of the body, and (ii) energy to sustain life. Thus, matter and energy are transferred in the living world as food. Regarding the mode of obtaining food, the organisms occurring in an ecosystem are classified into three main categories plants, animals and microorganisms (bacteria and fungi). These are respectively called producers, consumers and reducers or decomposers (Question asked in CSP-2013), according to their role in keeping the ecosystem operating as a stable unit.

 (i) Producers: The producers refer to the entire flora on the earth, either micro flora (photosynthetic bacteria and blue green alga; phytoplankton etc.) as well as macro flora (all higher plants) because, they produce their own food through the process of photosynthesis. Accordingly, they are of two types: photoautotrophs and chemoautotrophs.

(a) Photoautotrophs: These are the green plants, some protists, such as Euglena, and certain bacteria, such as green sulphur bacteria. They, with the help of their chlorophyll, entrap the light energy of the sun and change it into the chemical energy of a simple carbohydrate- called as glucose which is produced by them from simple inorganic compounds, namely, carbon dioxide and water. The process is called photosynthesis. From the basic organic material (glucose) so synthesized by them, the plants then form complex organic compounds such as starches, proteins and lipids. The producers utilize the self-made organic compounds for building up their bodies and for producing energy to operate the metabolic processes (respiration). As the green plants and other green organisms prepare their organic food themselves with the help of sunlight, they are known as the photoautotrophs. The producers dominate the terrestrial ecosystems, being the most abundant and massive of all groups of organisms there.

(b)       Chemoautotrophs: Some bacteria, such as colourless sulphur bacteria, iron bacteria, nitrifying bacteria, capture energy released during certain inorganic chemical reactions and prepare organic food with it. They are called chemoautotrophs, and the process is termed chemosynthesis.

      Release of Oxygen:  Producers also play a role in maintaining CO₂ - 0₂ balance in the nature. They use CO2, and release O₂ during photosynthesis.

• Noted that the producers are also called as “transducers” because, they convert the radiant energy of the sun into chemical energy of organic com-pounds synthesized by them. Conversion of energy from one form to another is termed transduction.

(ii) Consumers: These are mainly the animals or so called as fauna in the ecological terms. They are unable to synthesize their food. Therefore, they utilize materials and energy stored by the producers. They also use oxygen released by the producers during photosynthesis in their respiration. The consumers also consume other organisms or parts of other organisms which might have fed directly onto the producers and thus derive energy from the producers indirectly. They are thus also known as the heterotrophs. Animals are also called phagotrophs as they ingest food via mouth. The consumers are of 3 or 4 types in the context of ecological studies such as:

(a) Primary or First Order Consumers: These are the animals which eat plants or plant products directly. They are called herbivores. Cattle, deer, goat, rabbit, hare, rat and grasshopper are the common herbivores in terrestrial ecosystems, and snails, mosquito larvae, tadpoles and tortoises are the common herbivores in the aquatic ecosystems as they feed directly on the producers of aquatic ecosystem mainly, phytoplanktons.

      Connecting concepts: What are “Key Industry Animals”? Elton, the great Ecologist, has used the term "key industry animals" for the primary consumers because they convert the plant material into animal material. Plants that are parasites on other plants, and bacteria, protists and fungi which flourish on living plants are also primary consumers and are as such designated as key industry animals…

(b)       Secondary or Second Order Consumers: These are the animals which take herbivores as their food. They are called as carnivores in ecological parlance. Hydra, dragonfly larvae, water bugs, water beetles, frogs, small fish etc, are all secondary consumers in the aquatic ecosystems whereas, cats, dogs, and foxes etc., all constitute the secondary consumers in the terrestrial ecosystems.

(c) Tertiary or Third Order Consumers: These are larger carnivores which feed on secondary consumers. For instance, large fish and water birds in aquatic ecosystems and wolves in terrestrial ecosystems.

(d)       Quaternary or Fourth Order Consumers: These are the largest and top order carnivores which take tertiary consumers. They are not eaten by other animals. Tigers and lions are examples on land, and dolphins and whales in water. They are also called top carnivores.

(iii)       Reducers or Decomposers: These are mainly bacteria and fungi of decay. They obtain their food molecules from the organic materials of dead producers (plants) and consumers (animals) including their waste products as well. In the process of extracting nutrients and energy from these materials, they decompose the latter into (i) small organic molecules which they utilize themselves, and (ii) into inorganic compounds that are released into the environment for being reused as raw materials by the producers. The decomposers are also known as the saprotrophs. Due to their small size, the reducers are also called microconsumers.

       Noted that the balance wheel in all the components of an ecosystem is the flora and fauna that performs its individual role in different steps and makes an ecosystem to function as a well organized system of the nature.

Ecosystem – Functional perspective & role of flora and fauna in its functioning:

From functional point of view, each ecosystem has two components:

• Autotrophic component
• Heterotrophic component
• Autotrophic component: (Gr. Auto = self; trophos = feeder)
• It includes the green plants, photosynthetic bacteria and cyanobacteria which are capable of manufacturing their organic food (e.g. glucose) from simple inorganic substances (e.g. CO₂ and H₂O) by the process of photosynthesis. The energy trapped in photosynthesis is the radiant energy of sun and is changed into chemical energy. These organisms are also termed producers. These may be herbs, shrubs, large-sized trees or free-floating micro- organisms called phytoplanktons. The organic food (i.e. glucose) is used by the plants to synthesize the complex organic compounds such as starch and other carbohydrates, proteins, lipids etc.
• Heterotrophic component:(Gr. Heteros = other; trophos = feeder)
• It includes those organisms which consume either readymade organic food or decompose the complex organic substances into simple inorganic substances. These are also called consumers. Depending upon their size, heterotrophs are categorized into two categories:

(a) Macroconsumers: These include the animals which eat or ingest the producers directly or indirectly. They are further divided into four categories on the basis of nature of food:

(i)   Primary consumers or consumers of first order: This group of organisms includes the herbivores which directly eat upon the producers. Herbivores of terrestrial ecosystem include rabbit, rats, mice, deer, goat, grasshoppers, grazing cattle etc. Similarly, herbivores of aquatic ecosystem include crustaceans, molluscs etc.

(ii) Secondary consumers or consumers of second order: This group of organisms includes the primary carnivores e.g. centipedes, fishes, frogs, snakes, predatory birds, wild cats, foxes etc. which feed upon the herbivorous animals.

(iii)       Tertiary consumers or consumers of third order: This include the secondary carnivores e.g. wolves which eat upon the secondary consumers (primary carnivores). Wolves eat the foxes.

(iv)       Quaternary consumers or consumers of fourth order: These group of organisms include the tertiary carnivores e.g. lions, tigers etc. which eat upon the tertiary consumers (secondary carnivores). These are not eaten by any other animals.

(b)       Microconsumers: These are also termed as decomposers or saprotrophs (Gr. Sapros = rotten; trophos = feeder) or reducers. These break down the complex organic substances of dead plants and animals to release most of inorganic substances back into the environment for their reuse by the producers. The decomposers secrete digestive enzymes to digest the organic compounds externally. Examples of decomposers are bacteria and fungi.   (Question asked in CSP-2013).

Study of a Specific Ecosystem to appreciate the role of flora and fauna in it:

A small, freshwater pond is a classic example of an ecosystem which is compact enough to study its structure and function and hence, to witness lively the functioning of nature.

1.   Abiotic Component: Nonliving part of the pond includes : (i) water with dissolved gases and minerals; and suspended organic matter ; (ii) ''at the bottom with its minerals and organic debris; (iii) air above the surface of water containing free CO₂ and O₂, and (iv) sunlight. The abiotic environment provides raw materials and energy to the producers and habitat to all the organisms.

2.   Biotic Component: The living part of the pond comprises plants, animals and microorganisms (bacteria and fungi of decay). They play different roles in the ecosystem.

(i)   Producers: The green plants initiate the ecosystem activity. They act as the producer organisms. They do not depend on organic matter for nourishment. They utilize sun's energy directly with the help of chlorophyll and synthesize organic com-pounds from simple inorganic substances such as water, carbon dioxide and minerals. There are two types of producer organisms in a pond:

(i) large plants growing along the bank or floating in shallow water, and (ii) tiny microscopic floating plants (mostly algae) scattered throughout water as far down as light can penetrate. These minute floating plants are collectively called the phytoplankton. They are usually not noticeable unless present in great abundance, when they make the water look greenish. The phytoplanktons are more important as the food producers for the pond community than the large plants.

(ii) Consumers: The animals act as the consumer organisms by utilizing the food produced by the green plants. They include, among others, small crustaceans, worms, insect larvae, insects and fishes. There are 3 orders of consumers in a pond:

(a) Primary Consumers: The small crustaceans (water fleas, Cyclops) are herbivores. They feed on phytoplankton. They are free-floating animals and form the zooplankton. Other primary consumers are mosquito larvae, tadpoles, snails and tortoises.

(b)       Secondary Consumers: These are carnivores and feed on the primary consumers. They include Hydra, dragonfly larvae, water insects (giant waterbus, water scorpions, water skaters, water beetle) and small fishes.

(c) Tertiary Consumers: These are also carnivores. They feed on the secondary consumers. They include large fishes and water birds including water fowls).

(iii)       Decomposers: Dead producers and consumers and their wastes sink to the bottom, and form organic debris. Here, it is broken down by the bacteria and fungi of decay to simple organic and -inorganic substances. The simple organic compounds are absorbed by the decomposers themselves. The inorganic substances are released to the environment for reuse by the producers. Thus, there is a cyclic exchange of materials between the living community and the abiotic environment of an ecosystem. This maintains the ecosystem.

      From the above example therefore, we can conclude that the components (both structural & functional), of any ecosystem are seen to function as a unit when we consider the following aspects:

(i)         Productivity;

(ii)        Decomposition;

(iii)       Energy flow; and

(iv)       Nutrient cycling

Even in the pond as our simplest example of an aquatic ecosystem, all the above mentioned four basic components of an ecosystem are well exhibited. As noted above and reiterated here once again, the main abiotic component in a pond ecosystem, is the water with all the dissolved inorganic and organic substances and the rich soil deposits at its bottom. The solar input, the cycle of temperature, day-length and other climatic conditions regulate the rate of function of the entire pond. Moreover, among the biotic components, the autotrophic component (producers) is constituted by the phytoplankton, some algae and the floating, submerged and marginal plants found at the edges. Whereas, 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 the sun by the autotrophs; consumption of the autotrophs by heterotrophs; decomposition and mineralization of the dead matter to release them back for reuse by the autotrophs, these even 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.

Once we understand the real functioning as well as ethos of an ecosystem through the above example, it is now convenient to study the above aspects of an ecosystem, one by one. This will at least, corroborate the fact that how does flora and fauna hold its significance in the ecological context by first acting as producers, then consumers of the various order and finally, decomposers to keep the natural cycles working and in a continuous running condition…

What is 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^-2) or energy (kcal m^-2). The rate of biomass production is called productivity. It is expressed in terms of g^-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 utilized 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. Owing to the favourable environmental conditions, the Amazonian rain forests on land and coral reefs in aquatic ecosystems are the most productive of all ecosystems on the planet earth. This explains the reason that why do certain ecosystems are the least productive on the earth… (Question asked in the CSP-2013).

What is decomposition?

Who is not aware of the fact that why an organism like earthworm is referred to as the farmer’s ‘fiend’. 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 fragmentation, leaching, catabolism, humification and mineralization.

Detritivores (e.g., earthworm) break down detritus into smaller particles. This process is called fragmentation. By the process of leaching, water soluble inorganic nutrients go down into the soil horizon and get precipitated as unavailable salts. Bacterial and fungal enzymes degrade detritus into simpler inorganic substances. This process is called as catabolism.

It is important to note that all the above steps in decomposition operate simultaneously on the detritus (as shown in the fig. below). Humification and mineralization occur during decomposition in the soil. 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. The humus is further degraded by some microbes and release of inorganic nutrients occurs by the process known as mineralization.

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 favor decomposition whereas, low temperature and absence of oxygen inhibit decomposition resulting in build up of organic materials.