Food chain and food web. 

Food chain and food web. 

v A linear sequence of energy flow from producers through a series of organisms  in which there is repeated eating and being eaten.

Two types exist i.e

(i) Grazing food chain    (ii) Detritus food chain (i).  Grazer food chain, 

  • starts with autotrophs (producers)/  green plants which convert carbon dioxide & water  into chemical  compounds. 
  • These are grazed upon by herbivores. 
  • Energy is further transferred to carnivores. It can be in grass land or water body (aquatic). E.g
  • (ii)  Detritus food chain
  • is the one  where  the  consumers obtain energy  from fragments of dead decaying  organic matter. 
  • exists in both aquatic and terrestrial  habiats. 
  • 1st trophic level is occupied by a decomposing organic matter 

 Food Web

is a complex nutritional interrelationship that illustrates alternative food sources and  predator for each organism. In a food web, there are several food chains.

 Examples of food webs in a grassland  

Food chain and food web. 


  1. Construct a food web using the following organisms: phytoplanktons, mosquito larvae, small fish, large fish, and crocodiles.
  2. (a) With reference to a named ecosystem, what is meant by the following terms;

(i) Energy flow   (ii) trophic levels   (iii) food web

(b). Discuss the interactions between the living and non-living components of such an ecosystem.

  • (a) What is an ecosystem?

NB. Techniques used in constructing food webs and food chains 

  • Direct observation of organisms as it feeds so as to establish the organisms prey. 
  • Examination of stomach content through dissecting the animals’ stomach 
  • Faecal method; observation of faecal materials egested by an animal. 
  • Use of radioactive tracers to label the environment from which organisms obtain their food and then trace them in the organisms gut. 

Assignment. State the advantages and limitations of the above methods

Food and nutrition as well as home economics or home management

Ecological Pyramids

These are histograms that provide information about feeding (trophic) levels in ecosystems.  Three types exist i.e (i).pyramid of numbers

(ii) pyramid of biomass (iii) pyramid of energy

NB. Length of a given bar  is proportional to the number, energy or biomass at a given trophic level in a given area.

Pyramid of numbers. It is a histogram representing the numbers of different organisms at each trophic level in an ecosystem at any one time. 

Food chain and food web. 

NB.  As a pyramid is ascended, the number of organisms decreases but the size of each individual increases.

In some cases, the consumers may be more than the producers e.g in a parasitic food chain, inverted pyramids B & C are obtained, because parasites progressively become smaller and many along a food chain.

Limitations of pyramid of number 

  • Drawing the pyramid accurately to scale may be difficult e.g where there a million plants. 
  • Pyramids may be inverted 
  • The trophic level of an organism may be difficult to ascertain. 
  • The young forms of species may have a different diet from adults.

pyramid of biomass; is a histogram showing the total dry mass of organisms present at each feeding level

Food chain and food web. 


  • Reduces the possibility of forming inverted pyramids because its construction depends n biomass of organisms

NB. Inverted pyramid of biomass is typical of an aquatic ecosystem, because diatoms (phytoplankton) have a lower biomass but with higher productive rate (caused by so rapid turnover rate), therefore capable of supporting a larger biomass of zooplanktons.

Disadvantages/limitations of pyramid of biomass

  • Does not allow for changes in biomass at different times of the year e,g deciduous trees have larger biomass in summer than in winter when they shed off leaves.
  • Does not take into account rate at which biomass accumulates e.g a mature tree has a large biomass which increases over  many years.
  • Impossible to measure exactly biomass of the organisms in an ecosystem, because the sample used may not true representation of the whole population.
  • Results may not be accurate,  e.g where killing is not allowed, the results are obtained by estimating the fresh mass.

(iii) Pyramid of energy flow   it is a histogram showing the total amount of energy present at each feeding level. 

  • Because only a proportion of energy is in a trophic level is transferred to the next, energy pyramids are never inverted nor do they have a central bulge.
  • More informative than than pyramids of numbers and biomass because it shows the amount of energy required to support each trophic level.
  • Energy values may be expressed variously as kJ/ m2 / yr1 or kCal /m2/ yr1.
  • explains why the earth can support more people if they eat at lower trophic level (by consuming grains, vegetables and fruits directly rather than passing such crops through another trophic level and eating grain eaters.
Food chain and food web. 

Material Cycling/Biogeochemical Cycling (Nutrient Cycling)  

  • process by which chemical compounds of a particular element that constitutes living matter are transferred between  living organisms (biotic phase) and non-living environment (abiotic phase). 
  • Are driven directly or indirectly by incoming solar energy and gravity

(a). Carbon cycle

  • Based on carbon dioxide gas, making up 0.036% of the volume of the troposphere and is also dissolved in water.
  • Carbon fixation involves the reduction of carbon dioxide to large organic molecules during photosynthesis and chemosynthesis.
  • During aerobic respiration by all organisms, carbon dioxide  is returned to the atmosphere or dissolves in water.
  • Over millions of years, buried deposits of dead plant debris and bacteria are compressed between layers of sediment to form the carbon-containing fossil fuels e.g. coal, oil and natural gas, which when burnt release carbon dioxide into air.
  • In aquatic ecosystems, carbon dioxide may (i) remain dissolved (ii) be utilised in photosynthesis (iii) react with water to form carbonate ions and bicarbonate ions . As water warms, more dissolved carbon dioxide returns to the atmosphere.
  • In marine ecosystems, some organisms take up dissolved carbon dioxide molecules, carbonate ions and bicarbonate ions and these ions react with calcium ions to form calcium carbonate (CaCO3) to build their shells and skeletons.
  • When the animals with calcium in shells and skeletons die and drift into deep bottom sediments of oceans, immense pressure causes limestone and chalk to form after a very long period of time. 
  • Weathering processes release a small percentage of carbon dioxide from limestone into the atmosphere.

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