Ecology and Ecosystem- energy flow, material cycle, food chain and webs

Ecology and Ecosystem


The scientific study of the processes influencing the distribution and abundance of organisms, the interactions among organisms, and the interactions between organisms and the transformation and flux of energy and matter.


Ecosystem, the complex of living organisms, their physical environment, and all their interrelationships in a particular unit of space.

An ecosystem can be categorized into its abiotic constituents, including minerals, climate, soil, water, sunlight, and all other nonliving elements, and its biotic constituents, consisting of all its living members. Linking these constituents together are two major forces: the flow of energy through the ecosystem, and the cycling of nutrients within the ecosystem.

Energy flow in ecosystem

Energy has been defined as the capacity to do work. Energy exists in two forms potential and kinetic.  Potential energy is the energy at rest {i.e., stored energy) capable of performing work. Kinetic energy is the energy of motion (free energy).  It results in work performance at the expense of potential energy. Conversion of potential energy into kinetic energy involves the imparting of motion.

Living organisms can use energy in two forms radiant and fixed energy. Radiant energy is in the form of electromagnetic waves, such as light. Fixed energy is potential chemical energy bound in various organic substances which can be broken down in order to release their energy content.

Organisms that can fix radiant energy utilizing inorganic substances to produce organic molecules are called autotrophs. Organisms that cannot obtain energy from abiotic source but depend on energy-rich organic molecules synthesized by autotrophs are called heterotrophs. Those which obtain energy from living organisms are called consumers and those which obtain energy from dead organisms are called decomposers.

When the light energy falls on the green surfaces of plants, a part of it is transformed into chemical energy which is stored in various organic products in the plants. When the herbivores consume plants as food and convert chemical energy accumulated in plant products into kinetic energy, degradation of energy will occur through its conversion into heat. When herbivores are consumed by carnivores of the first order (secondary consumers) further degradation will occur. Similarly, when primary carnivores are consumed by top carnivores, again energy will be degraded.

The producers and consumers in ecosystem can be arranged into several feeding groups, each known as trophic level (feeding level). In any ecosystem, producers represent the first trophic level, herbivores present the second trophic level, primary carnivores represent the third trophic level and top carnivores represent the last level.

Food Chain

In the ecosystem, green plants alone are able to trap in solar energy and convert it into chemical energy. The chemical energy is locked up in the various organic compounds, such as carbohydrates, fats and proteins, present in the green plants. Since virtually all other living organisms depend upon green plants for their energy, the efficiency of plants in any given area in capturing solar energy sets the upper limit to long-term energy flow and biological activity in the community.

The food manufactured by the green plants is utilized by themselves and also by herbivores. Animals feed repeatedly. Herbivores fall prey to some carnivorous animals. In this way one form of life supports the other form. Thus, food from one trophic level reaches to the other trophic level and in this way a chain is established. This is known as food chain.

A food chain may be defined as the transfer of energy and nutrients through a succession of organisms through repeated process of eating and being eaten. In food chain initial link is a green plant or producer which produces chemical energy available to consumers. For example, marsh grass is consumed by grasshopper, the grasshopper is consumed by a bird and that bird is consumed by hawk.

Thus, a food chain is formed which can be written as follows:

Marsh grass → grasshopper → bird → hawk

Food chain in any ecosystem runs directly in which green plants are eaten by herbivores, herbivores are eaten by carnivores and carnivores are eaten by top carnivores. Man forms the terrestrial links of many food chains.

Food chains are of three types:

  • Grazing food chain
  • Parasitic food chain
  • Saprophytic or detritus food chain

Grazing food chain

The grazing food chain starts from green plants and from autotrophs it goes to herbivores (primary consumers) to primary carnivores (secondary consumers) and then to secondary carnivores (tertiary consumers) and so on. The gross production of a green plant in an ecosystem may meet three fates—it may be oxidized in respiration, it may be eaten by herbivorous animals and after the death and decay of producers it may be utilized by decomposers and converters and finally released into the environment. In herbivores the assimilated food can be stored as carbohydrates, proteins and fats, and transformed into much more complex organic molecules.

The energy for these transformations is supplied through respiration. As in autotrophs, the energy in herbivores also meets three routes respiration, decay of organic matter by microbes and consumption by the carnivores. Likewise, when the secondary carnivores or tertiary consumers eat primary carnivores, the total energy assimilated by primary carnivores or gross tertiary production follows the same course and its disposition into respiration, decay and further consumption by other carnivores is entirely similar to that of herbivores.

Parasitic food chain

It goes from large organisms to smaller ones without outright killing as in the case of predator.

Detritus food chain

The dead organic remains including metabolic wastes and exudates derived from grazing food chain are generally termed detritus. The energy contained in detritus is not lost in ecosystem as a whole, rather it serves as a source of energy for a group of organisms called detritivores that are separate from the grazing food chain. The food chain so formed is called detritus food chain.

In some ecosystems more energy flows through the detritus food chain than through grazing food chain. In detritus food chain the energy flow remains as a continuous passage rather than as a stepwise flow between discrete entities. The organisms in the detritus food chain are many and include algae, fungi, bacteria, slime moulds, actinomycetes, protozoa, etc. Detritus organisms ingest pieces of partially decomposed organic matter, digest them partially and after extracting some of the chemical energy in the food to run their metabolism, excrete the remainder in the form of simpler organic molecules.

The waste from one organism can be immediately utilized by a second one which repeats the process. Gradually, the complex organic molecules present in the organic wastes or dead tissues are broken down to much simpler compounds, sometimes to carbon dioxide and water and all that are left are humus. In a normal environment the humus is quite stable and forms an essential part of the soil.

Food web

Many food chains exist in an ecosystem, but as a matter of fact these food chains are not independent. In ecosystem, one organism does not depend wholly on another. The resources are shared specially at the beginning of the chain. The marsh plants are eaten by variety of insects, birds, mammals and fishes and some of the animals are eaten by several predators.

Similarly, in the food chain grass → mouse → snakes → owls, sometimes mice are not eaten by snakes but directly by owls. This type of interrelationship interlinks the individuals of the whole community. In this way, food chains become interlinked. A complex of interrelated food chains makes up a food web. Food web maintains the stability of the ecosystem. The greater the number of alternative pathways the more stable is the community of living things.

Material cycles

The supply of nutrients other than carbon dioxide, to an eco-system comes principally from the soil, but also to a smaller extent from the air, in rain and snow, and as dust.

The supply of many nutrients is quite limited because they are in short supply in the soil and in other sources. Nutrients are cycled in such a way that they are both incorporated into plants and animals, or else are made available for plant uptake by the decomposition of dead plant and animal remains.

The pathways from sources to sinks and back to sources, are termed elemental cycles, and they differ among the various elements. We consider briefly the three most important cycles, those of carbon, nitrogen and sulphur.

Carbon Cycle

Carbon is the basis of all organic molecules. It makes up our genetic material (DNA and RNA) and proteins, which are essential for life. Carbon is so special because of its ability to bond to almost any other molecule. The major element within our bodies is carbon.

The carbon cycle is the process through which carbon is cycled through the air, ground, plants, animals, and fossil fuels. Large amounts of carbon exist in the atmosphere as carbon dioxide (CO2). Carbon dioxide is cycled by green plants during the process known as photosynthesis to make organic molecules (glucose, which is food).


This is where the nourishment of every heterotrophic organism comes from. Animals do the opposite of plants—they release carbon dioxide back into the air as a waste product from respiration. Decomposers, when they break down dead organic matter, release carbon dioxide into the air also.

Decomposers are essential because without them, all of the carbon on the planet would eventually become locked up in dead carcasses and other trash. Decay permits carbon to be released back into the food web. Carbon is also stored in fossil fuels, such as coal, petroleum, and natural gas.

When these are burned, carbon dioxide is also released back into the air. Volcanoes and fires also release large amounts of CO2 into the atmosphere. Carbon dioxide can dissolve in water, where some of it is later returned back into the atmosphere. The rest can be taken to form calcium carbonate, which builds up shells, rocks, and skeletons of protozoans and coral.

The Carbon Cycle is a complex series of processes through which all of the carbon atoms in existence rotate. The same carbon atoms in your body today have been used in countless other molecules since time began. The wood burned just a few decades ago could have produced carbon dioxide which through photosynthesis became part of a plant.

Nitrogen Cycle

Another important nutrient cycle is that of nitrogen. Nitrogen is a critically important element for all life. Proteins, which are constituents of all living cells, contain an average of 16% nitrogen by weight. Other complex nitrogenous substances important to life are nucleic acids and amino sugars. Without a continuous supply of nitrogen, life on earth would cease.

The nitrogen cycle is somewhat like the carbon cycle, but with a number of critical differences. Even though 79% of the earth atmosphere is composed of elemental nitrogen (N2), this inert gas is entirely unavailable for uptake by most plants and animals. This is in stark contrast to the small amount of Carbon dioxide (0, 03%) in the atmosphere, which is readily available for plant uptake.


Sulfur Cycle

Sulfur is an important nutrient for organisms, being a key constituent of certain amino acids, proteins, and other biochemical. Plants satisfy their nutritional needs for sulfur by assimilating simple mineral compounds from the environment.

This mostly occurs as sulfate dissolved in soil water that is taken up by roots, or as gaseous sulfur dioxide that is absorbed by foliage in environments where the atmosphere is somewhat polluted with this gas. Animals obtain the sulfur they need by eating plants or other animals, and digesting and assimilating their organic forms of sulfur, which are then used to synthesize necessary sulfur-containing biochemical.



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