Definition: When both population benefit and at least one of them is so dependent upon the other for some critical resource or function that it cannot survive in the given environment without the other species is referred to as mutualism or symbiosis. Symbiosis is a beneficial association and each member is called a symbiont.

Examples and features of organisms in Symbiotic Association

  1. Lichen: This result from mutualistic association between an alga and a fungus. The alga carries out photosynthesis through it tallus while the fungus absorbs rain water through its tangled network of mycelia which the alga uses to photosynthesize its food. The fungus gets ready-made food from the alga.
  2. Protozoa in the digestive tract of termites: The protozoa help the termite to digest the cellulose in the food while protozoa are protected by the termites.
  3. Nitrogen fixing bacteria in the root nodules of leguminous plants: A bacterium called Rhizobium leguminosarium living in the root nodules of leguminous plant grow and multiply or reproduce there. In return, the rhizobium (bacterium) fixes nitrogen directly into the plant from the atmosphere, thereby increasing the nitrogen requirement of leguminous plants.
  4. Micro-organisms in the intestinal tract of ruminants: Bacteria and other protozoa in the rumen of ruminant animal like cattle, sheep and goat help the ruminant to digest cellulose to sugars, synthesize amino acids and vitamins from other substances while the ruminant in turn provides food and shelter for the bacteria.
  5. Flower and insects: Insects obtain food from flowers in the form of pollen and nectars while in return, the insects bring about cross-pollination in the plant they visit, thus enabling plants to reproduce sexually.


Definition: Protocooperation is an association between organisms of different species in which both are mutually benefited but they can also survive individually of each other. Unlike mutualism, the association in protocooperation is not obligatory.

Example and features of organisms in protocooperative association

  1. Sea anemone and hermit crab: The sea anemone attaches itself to the shell of the hermit crab. The sea anemone provides camouflage protection to the hermit crab against predators while the hermit crab helps to transport the sea anemone to a feeding ground. It helps the sea anemone obtain food during its movement.
  2. Plover and crocodile: Plover (Pluvianus aegyptius), a crocodile bird, enters into the mouth of the crocodile to feed on parasitic leeches. As it obtains food from the mouth of the crocodile it also gets rid of the harmful leeches.
  3. Cattle and egret: The bird, egret, feeds on the parasite on the body of cattle. As the cattle benefits from the removal of parasites from it body, the egret benefits because it obtains food from the association.


Definition: Commensalism is an association between two organisms living together in which only one (commensal) benefits from the association while the other neither benefits nor is harmed.

Examples and features of organisms in Commensalism

  1. Shark and Remora fish: The remora fish attaches itself o the body of a shark, whereas the shark is neither harmed nor benefits as a result of the presence of the remora fish.
  2. Oyster and crabs: The habitation of a crab is in the oyster shell in which case the crab is not protected and no harm is done to the oyster.
  3. Man and intestinal bacteria: Some bacteria in the large intestine of man feed on digested food there. The bacteria receive food and protection from the man whereas the man neither gains nor suffer any disadvantages from the presence of the bacteria.


  1. What are symbiosis and commensalism?
  2. Explain two examples of symbiosis and commensalism.


Definition: Amensalism is an association between two organisms of different species where one species is inhibited or killed and the other is unaffected. In amensalism, one does not allow the organism to live or grow near it. This relationship is also called antibiosis. The species affected is called amensal while the species causing the effect is called the inhibitor. This inhibiton is done by the release of chemicals known as allochemics or allelopathic substances.

Examples of amensalism

  1. Penicillium notatum and bacteria: Penicillium releases antibiotics known as penicillin which inhibits the growth of bacteria.
  2. Streptomyces griseus and bacteria: Like Penicillium, Streptomyces also secretes chemicals that inhibit the growth of bacteria.


Definition: Parasitism is a close association between two organisms in which one, known as the parasite, lives in or on and feeds at the expense of the other organism which is known as the host. The parasite benefits from the association while the host usually suffers harm or may die.

 Examples and Features of Organisms in Parasitism.

  1. Man and tapeworm: The tapeworm is a parasite the lives in the small intestine of man where it derives the benefits of a habitat, protection and food. The tape worm attaches itself to the small intestine of man by means of hook and sucker. In the wall of the small intestine, absorption of digested food takes place from which the tapeworm benefits. The man who is the host suffers because he loses to the tapeworm part of the food he has eaten and digested.
  2. Mistletoe and flowering plant: The mistletoe is a plant parasite that lives on other larger flowering plants. The mistletoe benefits because it is raised up to a position from which it can receive sunlight. The parasite also absorbs water and mineral salts from the host while the host suffers harm by losing to the parasite part of the water and mineral salts that it has absorbed.


Definition: Predation is a type of association between two organisms in which the predator kills the other called the prey and directly feeds on it. The predator which is usually larger in size and always stronger than the prey is completely eliminated.

Examples of Predation

  1. The hawk and chicks of domestic fowls: The hawk is the predator that catches, kills and eats the prey (young chicks) of domestic fowls. The hawk benefits while the chicks are completely eliminated.
  2. The lion and goat: The lion is the predator that catches, kills and eats the prey (goat). The lion is stronger and bigger than the goat. The lion benefits while the goat is completely eliminated. 


  1. Define parasitism and predation.
  2. Give two examples of parasitism and predation.
  3. State the differences between parasitism and amensalism.
  4. Mention two growth hormones each in plants and animals.
  5. Discuss the stages involved in mitotic cell division.
  6. Outline four life processes involving mitosis.


  1. A ____ relationship is one in which one member of the association benefits A. parasitic B. symbiotic C. saprophytic D. mutualistic
  2. Tick feeds on A. blood B. water C. skin D. body
  3. The word commensal means A. sharing B. dividing C. scattering D. loving
  4. Chemicals secreted by inhibitors is called ____  A. biotics  B. inhibitors  C. amensal
  1. allochemics
  2. ____ consists of algae and fungi A. Lichen B. Bacteria C. Virus D. Fungi


  1. What is A. symbiosis B. commensalism C. predation?
  2. Mention three examples each of the following above.

See also






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