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Since all living organisms must obtain energy and nutrients from the environment in order to remain alive, they are into feeding relationships. This makes an ecosystem a functional unit. All organisms fall into one of the three major groups of biotic community, namely: producers (autotrophs), consumer (heterotrophs) and decomposers

  1. The autotrophs provide food for other organism in the habitat. In terrestrial habitat, they include grasses, trees and shrubs while aquatic autotrophs include phytoplankton, seaweeds etc
  2. The heterotrophic include: (i) herbivores (called primary consumers) feeding on plants, carnivores (called secondary consumers) feeding on primary consumers and omnivores is other animals called (tertiary consumers) that feed on secondary consumer or on both. Terrestrial heterotrophs include cow, dog, lion, man etc while water fleas, tadpoles, larvae of insect and fishes are aquatic heterotrophs
  3. Decomposers like termites, larvae of housefly (maggot), bacteria and fungi break down dead organic matter to release simple chemical compounds which can be absorb and use again.

Of all the three biotic groups, consumers have better chance of survival than any other in an ecosystem.



  1. State two examples each of the following in both terrestrial and aquatic habitat Autotrophs B. heterotrophs C. Decomposers.
  2. How do the three groups of organisms in one above relate?



In an ecosystem, energy and nutrients are transferred step by step among organisms along a feeding path way. The feeding pathway in all ecosystems follows a similar pattern which is as


  1. It begins with a producer e.g green plant like grass.
  2. The producer is eaten by a primary consumer e.g Zebra , goat.
  3. The primary consumer is eaten by a secondary consumer e.g. lion.
  4. Decomposers convert the remains of dead producers and consumers into simple inorganic substance which return to the non –living environment. The difference in the feeding pathways within an ecosystem or between different ecosystems is the termination of the pathway. Some may end at the primary consumer step or may go on to secondary consumer, tertiary consumer etc.


Trophic (feeding) level: is each step along a feeding pathway. The order in which the trophic levels are arranged gives the path of energy (food) flow among the functional groups of organisms. The trophic levels are numbered in ascending order, starting from one to indicate the path of energy flow.


Trophic level 1 always consists of producers or autotrophs, trophic level 2 always consists of primary consumers. Above trophic level 2, consumers could be carnivores, parasitic organisms and scavengers. The final consumers eventually die and are fed upon by decomposers.



  1. Describe the pattern that feeding pathway follow in all ecosystems.
  2. What do you understand by feeding level?



This is the feeding relationship involving the transfer of energy through food from producers to consumers in a linear form.


Examples of food chain in terrestrial habitats are:

Grass                       zebra                                   lion

(Producer)      (Primary consumer)   (Secondary consumer)


Guinea grass                           grasshopper                     toad                    snake                      hawk                             (producer)                      (primary consumer)  (secondary consumer) (tertiary consumer)


In aquatic habitats are:

Spirogyra                             tadpoles                      crabs                             kingfish

(producer)            (primary consumer)    (secondary consumer)    (tertiary consumer)


Diatoms                         mosquito larva                        Tilapia fish                        whale

(producer)                 (primary consumer)       (secondary consumer)   (tertiary consumer)


Therefore, in a food chain, food energy is transferred from one organism to another in a linear form. Most food chains begin with producers but few of them start with dead plants or animals e.g

Humus                        earthworm                          domestic fowl                            man


Numerous food chains present in an ecosystem produce a food web.

FOOD WEB: –  is a complex feeding relationship among organisms in the same environment with two or more interrelated food chains. Food webs therefore contain more organisms than food chains. A single plant could be fed upon by more than one or two organisms

In an ecosystem, a consumer has a better chance of survival because it feeds on different types of plants or animals in a food web.


  1. What is a food chain?
  2. Draw a food chain involving four trophic levels in a A. marine B. terrestrial habitat.



Pyramids are diagrammatic representations used in ecology. They include pyramid of number, pyramid of energy and pyramid of biomass.

  1. PYRAMID OF NUMBER: this refers to the numbers of individual organisms at each trophic level, decreasing from the first to the last level in a food chain.

Pyramid of number has the following defects: The individual organisms is given the same status, though they varied greatly in size e. g. grass and trees grouped together as producer and is not drawn to scale.


  1. PYRAMID OF ENERGY: This is the amount of energy present in the living organisms at different trophic level of a food chain, which decreases from the first to the last i. e. from the base of the pyramid to the apex.
  2. PYRAMID OF BIOMASS: Biomass refers to the size and numbers of living organisms. It represents the total wet or dry mass of the organisms in each trophic level. This gives a more accurate picture of the relationship between the organisms at various trophic levels in a food chain than the pyramid of numbers.



Energy is the ability to do work. All living organisms obtain energy from the food they eat. The food is produced by green plants and other organisms depend on them. As energy is passed from one organism to another along the food chain, it is progressively lost due to respiration (energy used for various metabolic activities) and as heat. In most ecosystems, only about 1 – 10% of the solar energy may be available to photosynthetic producers.



  1. What is a pyramid? State three types of ecological pyramids.
  2. State the similarities between pyramid of numbers and pyramid of energy.



Thermodynamics simply means heat change. Heat as a form of energy that is subject to change in living organisms is governed by two laws

  1. First law of thermodynamics which states that energy can neither be created nor destroyed, although it can be changed from one form to another. E.g. chemical energy in food is converted to kinetic energy and heat energy in our muscles when we walk or run.
  2. Second law of thermodynamics which state that in any conversion of energy from one form to another, there is always a decrease in the amount of useful energy i. e. no transformation of energy from one state to another is ever 100 percent efficient.

During metabolic activities, some chemical energy is constantly lost as heat energy from the body of an organism. As energy is converted from one form to another, there will always be a loss.


  • Using the first law: in a food chain, energy from the sun is transferred from the producer to the final consumer and the sum total of the energy remains constant.
  • Using the second law: as the energy is transformed from one trophic level to another, part of it is converted into heat it is lost, among a progressive drop in energy in successive trophic levels.



(i)         Using the first law: The energy of the producers at the base of the pyramid is higher and it is gradually transformed to other trophic levels.

(ii)        Using the second law: As energy is transferred from one trophic level to another, part of the energy is converted to heat.



(i)         Using the first law: Energy flows from producers to 10 consumers, then to 20 consumers and finally to 30 consumers in a food chain. The energy flow in a food chain is therefore in one direction only.

(ii)        Using the Second law: Energy transfer between trophic levels is not 100%. Successive levels have less useful energy and so can only support fewer organisms. Produce (green plants) have the highest amount of energy. When herbivores feed on the plants, the energy level is reduced. When carnivores consume the herbivores, the energy level is reduced.



  1. State the laws of thermodynamics and explain.
  2. Explain pyramid of energy and food chain using the laws of thermodynamics.



  1. What is a food chain?
  2. Explain the role of the following in a food chain   (i) Producer   (ii) Consumer (iii) Decomposer.
  3. Draw a food chain involving four trophic levels in a marine habitat. B.Explain (i) the flow of energy through the food chain drawn in 3C                                    (ii) how energy in the chain is lost to the environment.



College Biology chapter 23, page 544 – 551



  1. The ultimate source of energy in nature is green plants    B. moon   C. star   D. sun
  2. The usable form of energy in animals is chemical energy    B. kinetic energy             C. potential energy     D. ATP
  3. Available to photosynthetic plants is ________ of the solar energy. 1-5%    B. 2-5% C. 5-15%     D. 1-10%
  4. The study of the relationship between heat energy and other forms of energy is called_____________. thermodynamics B. thermocouple C. thermosetting D. all of the above
  5. Energy can neither be created nor destroyed but can only be changed from one form to another This refers to the   first law of thermodynamics B. second law of thermodynamics C. third law of thermodynamics            D. none of the above



  1. State the first and second law of thermodynamics.
  2. In a tabular format, state four differences between food chain and food web.


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