Biology - 5090

Relationships of Organisms with Each Other and the Environment


Relationships of Organisms with Each Other and the Environment

Energy Flow

Plants capture light energy from the sun, which is the fundamental source of energy input in biological systems and convert it into chemical energy. This chemical energy is taken in by the animals. Eventually, it goes into the environment in the form of heat. Energy flow is a non-cyclic flow as it does not change back to light energy.

Food Chains and Food Webs

A food chain is a linear chain showing the transfer of energy from one organism/trophic level to the next, beginning with the producer. Energy is transferred between the organisms in a food chain through the process of ingestion.

Figure (i) Food Chain, Credit: K8schoollessons

A producer is an organism that produces its organic nutrients, using energy from the sun through the process of photosynthesis, for example, grass. Consumers are organisms that obtain their energy by feeding on the producers. A herbivore is an animal that obtains its energy by consuming plant species. The primary consumers are herbivores, like grasshoppers. Carnivores are animals that obtain their energy by eating other animals, like python. Decomposers are organisms that get their energy by feeding on dead, organic matter. 

The trophic level is the specific position of an organism in a food chain, food web, the pyramid of the number, or biomass. Transfer of energy from one trophic level to the other is inefficient. Producers are at the lowest trophic level and they take the energy from the sun, which they use for their own processes. Energy is lost among the food chains. When the primary consumer takes in the producer, they get less energy as producers already used up most of the energy in their life processes like respiration, metabolic reactions and in the form of heat. Some parts of the plants which used energy might be inedible so even lesser energy is transferred to primary consumers. The amount of energy passed on to the second consumer would be even lesser as the primary consumers consumed the remaining energy in many processes like excretion, egestion, sweat, etc. Thus, the tertiary consumer receives the least amount of energy.

Food chains commonly consist of less than five trophic levels. Energy loss increases as you go up the trophic level. A lot of energy is not available from the sun as it is reflected or in the wrong wavelength. Energy is lost in processes like movement, egestion, excretion and in the form of heat. The whole organism will not be digestible so even lesser energy is gained. It would not be sufficient for the organisms on the fifth level to survive easily as there is not enough energy available from the fourth level to support another level.

There is greater efficiency in supplying plants as human food, and there is relatively inefficiency in feeding crop plants to livestock that will be used as food. More energy is available on lower trophic levels. The plants gain a lot of energy from the sun, but livestock only receives some of its energy.

Pyramid of Numbers

It is the total number of organisms present at each trophic level in a designated food chain. Mostly, there are more plants than animals and more herbivores than carnivores. The size of each level represents the number of organisms at that specific step in a food chain.

As shown in the diagram on the right, lots of plants provide food for some herbivores, but the herbivores provide food for only a few primary carnivores, and so on.

Figure (ii) Pyramids, Credit:

Pyramid of Biomass

It shows the total no of mass present at each trophic level in a food chain. It gives a better idea of the actual quantity of plant or animal material present at each level. The proportions of each box represent the abundance of each kind of organism.

Pyramid of biomass gives a much better idea of the actual quantity of organisms present at each trophic level. It shows an accurate amount of energy at each level, while the pyramid of numbers does not take the size of organisms in the account.

Nutrient Cycles 

Carbon Cycle

Air contains about 0.04% of Carbon. It is an essential ingredient for carbohydrates, proteins, and lipids. When plants photosynthesize, carbon atoms become a part of the glucose and starch molecules in the plats. Some glucose is broken down by the process of respiration. The carbon in glucose becomes part of the carbon dioxide again and is released back into the air. Some of the carbon from plants is taken in by animals that respire. When the plants or animals die, decomposers feed on them, making carbon a part of their body. Due to the death and long-term subjection of plants and animals to high temperatures and pressures, they become fossil fuels. On the combustion of fossil fuels, carbon dioxide is released in the air. The combustion of fossil fuels releases carbon dioxide, thus its concentration increases. This traps infrared radiation and makes the atmosphere warmer. 

Figure (iii) Carbon Cycle, Credit:

Nitrogen Cycle

Figure (iv) Nitrogen Cycle, Credit: Khanacademy


Forests may be cut down for the manufacture of paper and for firewood. Wood is an excellent fuel and building material. Forests are also cleared for the purpose of agriculture and urbanization. There are many undesirable effects of deforestation, as mentioned below:

The roots will die so they do not bind the soil. The soil under the tree will be exposed to rain. It will get thin so it will be quickly washed away. Soil erosion is rapid.

The soil can be washed into rivers and cause silting of rivers, resulting in an increased risk of flooding. 

Soil erosion makes it difficult for forests to grow back again so no trees will be available to absorb the water. 

Deforestation leads to the loss of habitat for many organisms and the extinction of species. 

Fewer trees result in an increase in carbon dioxide concentration in the atmosphere.

Removed trees disrupt nutrient cycles. Transpiration rates reduce and the land is exposed to drying.        


The fertilizer enters the river causing algae bloom/growth. Algae blocks sunlight from entering the water so rooted plants are unable to photosynthesize. Thus, the plants die. Some bacteria decompose on these dead plants causing an increase in the bacterial population. The bacteria respire aerobically, using up the oxygen in the water. As a result, aquatic organisms suffocate, migrate, or die due to the lack of oxygen.

Effects of Non-Biodegradable Plastics on the Environment

Discarded plastic objects accumulate and increase rubbish heaps

Plastic waste thrown away in the sea can be mistaken for jellyfish by turtles and is consumed. It cannot be digested, resulting in suffocation. 

Plastics block the light required for photosynthesis.

May release oil-soluble toxins 

Large pieces of plastic may block the flow of water in a river, reducing aeration.

It destroys habitat so disrupts the food chain.

Trapped water acts as a breeding ground for mosquitoes.

It cannot decompose so it lasts a long time.

Impacts on Humans of the Introduction of Foreign Species in a Habitat:

Environmental harm.

Economic harm.

More competition for resources, thus loss of habitat for old species.

Old species die or migrate, due to a decrease in the resources available.

It alters the nutrient cycles in plants.

It alters the food chain, which affects the lives of many organisms.

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