Ecological significance of flora and fauna in forming- “Ecological Pyramids”:

What are ecological pyramids?

Every one of us is familiar with the shape of a pyramid. The base of a pyramid is broad and it narrows down at the apex. We would certainly get a similar shape i.e. of a pyramid only, when we try to express the food of energy relationship between organisms at different trophic level. This, relationship between organisms may be expressed either in terms of number, biomass or energy. The base of each pyramid represents the producers or the first trophic level while the apex represents tertiary or top level consumer.

The three ecological pyramids that are usually studied in ecological studies are:

 (a) Pyramid of number;

 (b) Pyramid of biomass and

 (c) Pyramid of energy.

The logic behind the shape of each pyramid of each type may be easily understood by referring to the simple figures under a, b, c and d below:

Figure (a) below, depicts a “pyramid of numbers” which is always upright in both terrestrial and aquatic ecosystems. Because, the number of producers in both the cases is always more as compared to the consumers and thus, the producers occupy the base of the pyramid.

Figures (b and c) below, depicts a “pyramid of biomass” which is always upright in a terrestrial ecosystem because, the producers always have higher biomass (living matter) as compared to the consumers. Interestingly, it is the other way round i.e. an inverted pyramid of biomass in an aquatic ecosystem as the biomass of phytoplanktons (Producers) owning to their sheer small size as compared to the biomass of the consumers feeding upon them say, fish.

Figure (d) below, depicts “pyramid of energy” which is also always upright (10% law basis).

Any calculations of energy content, biomass, or numbers has to include all organisms at that trophic level. No generalizations we make will be true if we take only a few individuals at any trophic level into account. Also a given organisms 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. In this way, we can also easily work out how many trophic levels do human beings actually function at 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 have more biomass than the herbivores and similarly, the herbivores are more in number and in biomass than the carnivores. Also energy at a lower trophic level is always more than at a higher level.

The pyramid of biomass in sea as stated above is also generally inverted because the biomass of fishes far exceeds that of phytoplankton.

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. Hence, we can conclude that a law of thermodynamics can never be defied in nature insofar as the pyramid of energy is concerned.

However, there are certain limitations of ecological pyramids such as it does not take into account the same species belonging to two or more trophic levels. It assumes a simple food chain, something that almost never exists in nature; it does not accommodate a food web. Moreover, saprophytes 

Nobody will deny that the organisms need a constant supply of nutrients to grow, reproduce and regulate various body functions. The amount of nutrients, such as carbon, nitrogen, phosphorus, calcium, etc., present in the soil at any given point of time, is referred to as the standing state. It varies in different kinds of ecosystems and also on a seasonal basis.

What is important is to appreciate that these nutrients are never lost from the ecosystems, they in fact are recycled time and again indefinitely and hence, we call this natural process as “nutrient cycling.” This nutrient cycling takes place by way of the movement of nutrient elements through the various components of an ecosystem. Another name for this nutrient cycling is biogeochemical cycles (bio: living organisms, geo: rocks, air, water,).

 Nutrient cycles are of two types: (a) gaseous and (b) sedimentary. The reservoir for gaseous type of nutrient cycle (e.g. nitrogen, carbon cycle) exists in the atmosphere and for the sedimentary cycle (e.g., sulphur and phosphorus cycle); the reservoir is located in the Earth’s crust. Environment factors, e.g., soil, moisture, PH, temperature etc., regulate the rate of release of nutrients into the atmosphere. The function of the reservoir is to meet with the deficit which occurs due to imbalance in the rate of influx and efflux. Refer to the table below for discerning the main difference between these two types of cycles:

Differences between Gaseous and Sedimentary Cycles

Common Biogeochemical Cycles

General Features of any biogeochemical cycle: Each chemical in an ecosystem has its own characteristic biogeochemical cycle, but all cycles have steps in common. Generally, the nutrient is first taken from the environment and incorporated into the tissues of the producers (plants). If the producer is eaten by an animal, the nutrient may become a part of animal tissue. If that animal is eaten by another (carnivore), the element may be incorporated into the carnivore's body. Finally, all organisms die and their bodies are broken down by decomposers, releasing the elements into the environment for reuse by producers. Let’s now discuss some common biogeochemical cycles in nature starting with the gaseous cycles first and then we shall move on to the sedimentary cycles.

The Carbon cycle

When we study the chemical composition of any living organism, we would find that the carbon as an element constitutes as much as about 49 per cent of dry weight of organisms and is next only to water in terms of its abundance in the living bodies. If we look at the total quantity of global carbon, we find that 71 per cent carbon is found dissolved in oceans. Other than in the dissolved form, carbon dioxide is also found in the combined form in the water in the form of carbonic acid. This carbonic acid is then disassociates into hydrogen ions and bicarbonate ions. Noted that it is these bicarbonate ions dissolved in water that are used by aquatic plants for photosynthesis. Owing to this reason, the oceans are generally referred to as the “global sink” for the CO2. This oceanic reservoir regulates the amount of carbon dioxide in the atmosphere. It is then not surprising to know that our atmosphere only contain about 1 per cent of total global carbon.

Fossil fuels also represent a reservoir of carbon. Carbon cycling thus occurs through atmosphere, ocean and through living and dead organisms. According to one estimate, 7 x 10¹³ kg of carbon is fixed in the biosphere through photosynthesis annually by the producers or plants. As such, one hectare of a healthy forest absorbs about 30 tonnes of CO2 and releases some 10 tonnes of oxygen annually. A considerable amount of carbon returns to the atmosphere as CO₂ through respiratory activities of the producers and consumers. Decomposers also contribute substantially to CO­₂ pool by their processing of waste materials and dead organic matter of land or oceans. Some amount of the fixed carbon is lost to sediments and removed from circulation. Burning of wood, forest fire and combustion of organic matter, fossil fuel, volcanic activity are additional sources for releasing CO₂ in the atmosphere.    

Human activities have significantly influenced the carbon cycle. Rapid deforestation and massive burning of fossil fuel for energy and transport have significantly increased the rate of release of carbon dioxide into the atmosphere.

Nitrogen Cycle [1]

As we already know that nitrogen is a component of proteins and nucleic acids which are essential structural and functional components of living systems. Nitrogen is also a component of energy compounds, such as ATP, in all organisms.

Source of Nitrogen: The nitrate ions (NO3) of the soil and water are the principal source of nitrogen for green plants.

Processes Involved in N₂ Cycle: Six processes take part in cycling nitrogen in nature from environment to organisms and from organisms back to environment. These processes are nitrate assimilation, ammonification, nitrification & denitrification, biological nitrogen fixation and non-biological nitrogen fixation. These processes involve four types of bacteria.

Figure showing: Nitrogen Cycle in nature

(a) Nitrate Assimilation: The plants absorb the nitrate ions and form vegetable proteins from them. This process is called nitrate assimilation. The plants can also use ammonium ions (NH₄) to form proteins. The plants may be eaten. by her-bivorous animals, which convert the plant proteins into animal proteins. The herbivorous animals may be taken by carnivorous animals which synthesize their own proteins from those of the herbivorous animals.

(b)       Ammonification: Animals excrete nitrogenous waste materials, such as urea, uric acid, ammonia. Urea and uric acid are converted into ammonium compounds and carbon dioxide by the putrefying bacteria (Bacillus ramosus, B. vulgaris) and some fungi. (Please refer to the economic significance of flora and fauna for more information). The putrefying bacteria and fungi are found in the soil and in the mud at the bottom of water bodies. They also decompose the nitrogenous compounds (proteins) of the dead animals and plants into ammonium compounds, e.g., (NH₄)₃ PO₄, ammonium phosphate, carbon dioxide and water. The process is called ammonification.

(c) Nitrification: Most of the ammonium compounds formed by the putrefying bacteria and fungi are oxidized by the nitrite bacteria (Nitrosomonas and Nitrococcus) to soluble nitrites, which are further oxidized by nitrate bacteria (Nitrobacter and Nitrocystis) and fungi (Penicillium) to soluble nitrates. The process of nitrate formation is known as nitrification, and the bacteria responsible for it are referred to as the nitrifying bacteria. The nitrates so formed are added to the soil and water, thus completing the cycle.

(d)       Denitrification: Some ammonium com-pounds, nitrites and nitrates are converted by certain bacteria and fungi into molecular nitrogen (N₂) which escapes into the atmosphere or is added to water, and is thus lost from the cycle. The process is called denitrification, and the bacteria causing it are called as the denitrifying bacteria. The latter occur in the anaerobic mud of fertile ponds, lakes, bogs, estuaries, and parts of ocean floor. For example, the bacterium Pseudomonas aeruginosa reduces nitrates to molecular nitrogen. Other denitrifying bacteria are Micrococcus denitrificans and Thiobacillum denitrificans.

(e) Biological Nitrogen Fixation: It is a known fact that Nitrogen gas, N₂ as a constituent of the atmosphere, forms 78% of the air, but is not available for use as such by plants and animals. It must be 'fixed', that is, combined with some other elements, such as oxygen, for use by organisms. The atmospheric nitrogen is fixed by prokaryotes, viz., some soil bacteria (Azotobacter and Clostridium) or by certain Cyanobacteria (Nostoc, Anabaena, Aulosina, Tolypothrix), and by the symbiotic bacteria (Rhizobium legumninosarum) living in the root nodules of legumes such as peas and beans. Only combination of legume cell and bacterial cell can fix nitrogen, neither can do so alone. The bacteria take up free nitrogen from the atmosphere and convert it into soluble nitrates, such as potassium nitrate, KNO3. Some nitrate passes into the soil and is absorbed by plants which make their proteins from it. The rest is used by bacteria to synthesize their own proteins. The process is called biological nitrogen Fixation and the organisms causing it are referred to as the nitrogen-fixing organisms.

      It is no surprise that the nodule bacteria may fix 50 - 100 kg. of nitrogen per acre per year and the soil bacteria (free living nitrogen fixing bacteria)fix about 12 kg. per acre per year.

      Certain nonleguminous plants also have root nodules and some others have leaf nodules. These nodules also contain nitrogen-fixing organisms. The latter also occur in some lichens.

      When the bodies of the nitrogen-fixing organisms are decayed, their organic nitrogen compounds are decomposed into ammonium compounds, carbon dioxide and water. Am-monium compounds are disposed of as already explained.

Non-biological Nitrogen Fixation: Atmospheric nitrogen is converted into nitrous oxide (NO₂) by electrical energy of lightning and by natural ionizing radiations. These processes are respectively called as electrochemical and photochemical fixation of nitrogen. NO₂ is brought down by rain and is used by plants. The nitrogen fixed in this manner is only a fraction of what that is being fixed by nitrogen fixing organisms.

 Connecting concepts: Nonbiological nitrogen fixation is only 35 mg. per square metre per year as against biological nitrogen fixation of 140 - 700 mg. per square metre per year. The nitrogen fixing microorganisms produce about 175 million tonnes of nitrogen. This forms about 70% of our total supply of nitrogen. The remaining 30% is obtained from chemical fertilizer factories. The chemical fertilizer factories produce ammonia by combining hydrogen and atmospheric nitrogen under high pressure and temperature. This process is referred to as industrial nitrogen fixation.

Differences between Nitrogen Cycle and Carbon Cycle: There are a few significant differences between carbon and nitrogen cycles such as given below:

(a) All producers use CO₂ directly, whereas only certain prokaryotes can use N₂.

(b) Carbon dioxide is used and released by the same organisms namely, producers whereas, nitrogen is utilized and released by different organisms.

(c) Unlike the carbon cycle, the nitrogen cycle is primarily accomplished by bacteria either, eubacteria or cyanobacteria.

Oxygen Cycle

Every one of us is aware that oxygen is the sine qua non of life as it is not only required for respiration, but is also an essential component of biomolecules.  

 Sources of Oxygen:  Oxygen is available in molecular form (O₂) in the air, forming about 20.95% of it. Some oxygen is found dissolved in water as well. Oxygen also occurs as a component of water and carbon dioxide.

It is significant to note that the oxygen of the atmosphere always remains in a state of dynamic equilibrium. It is taken by animals and plants from the air or as being dissolved in water for its use in oxidative reactions (respiration). It is returned to the environment, either in combination with carbon as carbon dioxide or with hydrogen as water. The carbon dioxide and water are used by plants in photosynthesis, which liberates molecular oxygen into the environment for reuse in respiration. Thus, the cycle is completed.

Therefore, the concentrations of oxygen in the air and water are maintained by equal rates of its use in respiration and release in photosynthesis although, Oxygen is also released as a part of CO₂ by the decay of dead organic matter. In the same manner, some oxygen is added to the air as CO₂, H₂0, sulphur dioxide and nitrogen oxides during burning of fuel (wood, coal, petroleum and natural gas).

Figure showing: Oxygen cycle in nature

Effect of Human Activity on Oxygen cycle:  Oxygen was not present when the earth was formed. It was added to the atmosphere later when photosynthesis started with the evolution of photoautotrophs. Human activity has not thus affected the oxygen content of the atmosphere because, it is always replenished by photosynthesis. This signifies the urgency with which we need to maintain our forest wealth (flora) and cultivate more and more trees for this living giving oxygen.

Mineral cycles

Importance of Minerals: Certain minerals, or inorganic substances, such as calcium, potassium, magnesium, manganese, zinc, etc., are essential for both plants and animals. They mainly act as cofactors of enzymes in physiological processes. They are also components of cell organelles and bio-molecules.

Main Sources of minerals: Soil, water and rocks are the main sources of minerals for the organisms.

How does a mineral cycle operate in nature? As we already know that the starting point in any ecosystem are the producers. These producers or so called green plants absorb these minerals either from the soil or water. The herbivorous animals get minerals by eating plants, and the carnivorous animals obtain them by taking animals although, some animals do receive minerals from water also. These minerals subsequently return to soil or water by decay of dead plants and animals. Moreover, the animals do add minerals to soil and water by way of excretions (urea, ammonia or uric acid, etc.) as well through the faecal matter also. Similarly, soil and water also receive minerals from the rocks when rain water running over the rocks gradually wears away their surface and carries off minerals with it. Some of these minerals soak into the ground and some reach ponds and lakes. A large part of the minerals is, however, carried by the rivers to the sea and naturally in the sea, they become a part of the body of aquatic organisms. These minerals from the sea then reach the land surface through food chains say, when sea food is consumed by terrestrial organisms which upon their death or through excretory wastes release these minerals onto the land surface and hence, the cycle completes. In order to understand these mineral cycles in a better way, let’s undertake a study of some of the important mineral cycles in nature… 

Figure showing:  Mineral cycle in nature

Phosphorus Cycle

We know that phosphorus is a major constituent of biological membranes, nucleic acids and cellular energy transfer systems, (ATPs). Many animals also need large quantities of this element to make shells, bones and teeth. The natural reservoir of phosphorus is rock, which contains phosphorus in the form of phosphates. When rocks are weathered, minute amounts of these phosphates get dissolved in soil solution and are then absorbed by the roots of the plants as depicted in the flow chart below. Herbivores and other animals obtain this element from plants. The waste products and the dead organisms are decomposed by phosphate-solubilising bacteria releasing phosphorus. Unlike carbon cycle, there is no respiratory release of phosphorus into the atmosphere thus, on this basis; we can easily differentiate between the carbon and the phosphorus cycle in nature.

However, there are still other two major and important differences between carbon and phosphorus cycle. Firstly, atmospheric inputs of phosphorus through rainfall are much smaller than carbon inputs, and secondly, gaseous exchanges of phosphorus between organisms and environment are negligible.

Sulphur Cycle

As we know that just like phosphorous is a component of biological membranes and nucleic acids, similarly, sulphur is a component of certain proteins, some vitamins as well as enzymes.

So far as the sources of this sulphur in nature are concerned, it occurs in nature as an element and also as sulphates in the soil, water and rocks.

In nature, the sulphur cycle as usual, starts from the producers (plants) which absorb sulphates from the soil and water and incorporates the same in their proteins. The herbivores get organic sulphur from plant food and pass it on to the carnivores. Some animals get sulphur from water also. Bacteria and fungi, the main decomposers of the ecosystem, under aerobic conditions, decompose the dead plants and animals including the excretory and faecal matter of animals and thus, change the organic sulphur to sulphates which is then added to soil and water for reuse by the plants again.

 Noted that under anaerobic conditions, such as in marshes, some bacteria change this organic sulphur to sulphides. Although, this sulphide is harmful to most organisms, but the organisms like, sulphur bacteria and some fungi oxidize the same to sulphates. Some of such important sulphur bacteria include Beggiatoa and Thiobacillus thiooxidans. While, such fungi include the noted fungi like, Penicillium and Neurospora etc. Certain other bacteria like, Aerobacter and Desulphovibrio, may convert sulphates to H₂S.

Figure showing; Sulphur cycle in nature

Some of this hydrogen sulphide escapes from the marshes into the air, where it is oxidized to sulphur dioxide (SO₂). The S0₂ and also sulphur trioxide (SO₃) are released into the air by the burning of fossil fuels as well. During rainfall, SO₂ and SO₃ dissolve in water, forming sulphurous acid (H₂ S0₃) and sulphuric acid (H₂ SO₄) respectively. The H₂ SO₃ and H₂ SO₄, reaching the soil to form sulphates with metals. The sulphates are then used by plants as usual.

Furthermore, soil and water also receive sulphates from rocks. Rain water running over rocks gradually wears away their surface and carries off sulphur with it. Some of this sulphur soaks into the soil and some reaches ponds and lakes. A large part of this sulphur is however, carried by the rivers to the sea where it may get locked up in sedimentary rocks.

Just like other minerals, sulphur from the sea gets back to land as usual in 3 ways (food chains, sea sprays, and geological upheavals).

After having studied the various biogeochemical cycles in the foregoing paragraphs, the most pertinent question still remains as to the utility of studying these biogeochemical cycles after all, which may be examined in brief herein below: 

Why to study biogeochemical cycles? The study of various bio-geochemical cycles in nature holds significance from the fact that they show that intricate relationship which exist in nature between various kinds of organisms. As each type of organism in an ecosystem requires many substances, such as carbon, nitrogen, water, and for all these substances, it is always depend upon one or all of the other organisms. Plants, and thus animals, are completely dependent on the nitrogen fixing bacteria for converting atmospheric nitrogen into soluble nitrates, and these bacteria, in turn, depend on the denitrifying bacteria to return nitrogen to the environment. This type of interdependence is a part of what may be called as the “web of life”. It is nevertheless, a characteristic of all ecosystems.