Note on Biotic Interactions

  • Note
  • Things to remember

The activity of living organisms which may cause the marked effect on the survival of others are called biotic interactions. The interactions may be food, shelter or substratum. Similarly, it may be intraspecific or interspecific. All the interactions are divided into three types they are;

  • Positive interactions
  • Negative interactions
  • Neutral interactions

Positive interactions

In positive interactions, the organisms help one another and either one or both the organisms are benefited but none is harmed.

  • Mutualism
  • Protocooperation
  • Commensalism
  • Social organization
  • Aggregation
source: fig: Mutualism
fig: Mutualism

The interactions between two different species in which both the species are benefited from each other is called mutualism. Such interaction is always obligatory because the existence of one partner is impossible without another. The organisms which involve in this association is called symbionts.

Mutualism between bacteria and plant species

Symbiotic nitrogen fixation by Rhizobium legumin osar um (bacteria) occurs in the nodules on the secondary roots of leguminous plants like the pea, gram etc. The bacteria obtain carbohydrates and shelter from leguminous plants while in return they fix nitrogen as nitrites and nitrates for the plant which is required for their growth.

Mutualism between animal and animal species

Termites (white ants) are not capable of digesting wood, which they ingest as food. A multi flagellate protozoan Trichonympha campanula, which lives in the intestine of white ant secretes 'cellulase' enzyme to digest the cellulose of wood. In return, the ant provides food and shelter to the protozoan.


It is the joint form of algae and fungi. The algae prepared its own food and supply it to fungi while the fungi provide support to algae.

Mutualism between animal and bacteria

Ruminococcus, a symbiotic bacteria, lives in the rumen part of the compound stomach in cud-chewing mammals like cattle, sheep, goat etc. which secrete cellulase enzyme and digest the cellulose of plant food. In turn, the bacteria obtain food and shelter by the ruminants.

Pollination and insects

Green alga Zoochlorella is a symbiont in the parenchyma of flatworm. Brown alga Zooxanthella lives as a symbiont with the mollusc-Tridacna. Pollination of flowers by insects is a mutual interaction.


It is a positive interspecific interaction in which both the partners are mutually benefitted and increase the chance of their survival. However, the interaction is not obligatory for their survival as both can live without this interaction. It is also called facultative mutualism.

Relationship between water snakes and large birds

The birds make their nest at the lower branches of the tree while the snakes gather around the base of the tree. These protect the bird from tree-climbing predators (racoons). In return, the snakes feed on fish dropped by birds.

Relationship between birds and cattle

The birds and cattle also form a protocooperation relationship. The birds feed on the ticks and other ectoparasite sticking to the skin of cattle. Later on, the cattle also get relieved from ectoparasites.

Relationship between sea anemone and hermit crab

Hermit crab (Eupagurus Prideaux) lives inside an empty gastropod shell and fixes a sea- anemone (Adams palliate) on the shell. The sea- anemone provide camouflage (protective colouration) and defence to the crab, while crab helps in the dispersal of the sea anemone and provide new feeding grounds.


source: fig; Commensalism
fig; Commensalism

It is a positive interspecific interaction, in which a smaller member called 'commensal', is benefitted, while the larger member called 'host', is neither benefitted nor harmed. Commensalism represents a beneficial relation. The organism which involved in this commensalism is commented.

Interaction between sucker fish and shark

Suckerfish (Echeneis) gets attached to the under the surface of sharks by its sucker. This provides easy transport for new feeding grounds and also food pieces falling from the shark's prey to Echeneis.


Epiphytes are small green plants found growing on other plants for space only. They absorb water and minerals from the atmosphere by their hygroscopic roots and prepare their own food. The plants are not harmed in any way. Examples: Orchids, lianas and Vanda hanging mosses are common epiphytes found on the tree of tropical rain forests.

Social organization

It is the grouping of individual of same species in which there is the division of labour for the welfare of society. Eg; ant, wasp, and bee.


It is the interaction that occurs among the same organism. It is the type of grouping of organisms of same species. A large number of independent individuals concentrates in an area. Aggregation is beneficial during the scarcity of food, space light and for better breeding. Eg; the swarm of locusts, grasshopper etc.

Negative interaction

In negative interaction, one partner must be harmed. It may be intraspecific or interspecific. Eg;

  • Predations
  • Parasitism
  • Competition
  • Amensalism

It is a negative, direct food related interspecific interaction between two species of animals in which larger species called predator attacks, kills and feeds on the smaller species called prey. Predator population adversely affect the growth and survival of smaller prey population and, therefore, predation is considered an antagonistic interaction. Predation keeps the prey population under check, so as to maintain an ecological balance. Weak and less efficient members in the prey population are removed. Eg; Tiger and deer, snake and frog, frog and insect, eagle and sparrow, cat and rat etc.

source: fig: Parasitism
fig: Parasitism

It is a type of antagonistic interspecific interaction in which smaller partner, called the parasite, derives food and shelter from in or on the body of larger partner, called host, which inhibits the survival of the host.The host can survive without the parasite, but the parasite cannot survive without the host. Parasitism can be rightly explained as weaker attacking the stronger. Parasitic interaction is generally found among protozoans, flat forms, nematodes, and arthropods.

It is an antagonistic interaction in which two or more members of same species (intraspecific), or two or more members of different species (interspecific) of same trophic level compete for the common resource like light, moisture, space, nutrients etc. which are in short supply in relation to the member of individuals. Competition is extremely important in determining the distribution of closely related species and habitat range and speciation. Eg: intraspecific and interspecific.
source: fig: Ammensalism
fig: Ammensalism

It is an antagonistic interspecific interaction in which one species is inhibited while other species is neither benefited nor harmed. In simple words, in amensalism, one organism does not allow other organisms to grow or live near it. It is also called antibiosis and the affected species is called a mental and the affecting species is called inhibitor. Such inhibition is achieved through the secretion of certain chemicals called alloo chemics or allelopathic substances. Eg; The roots of black walnut tree produce a chemical called vegetations to grow around it.

Neutral interaction

In neutral interactions, none of the organisms is negatively or positively affected. They may have the same shelter but do not have any effect on each other. Eg; birds and squirrel living in the same tree.

  • The interaction between two living organisms in which both of them are benefited and neither is harmed is known as mutualism. Eg; lichen thallus.
  • The association of two living organisms in which one partner is benefited whereas the other is neither benefited nor harmed is known as commensalisms. Eg; epiphytes.
  • The relationship in which the predators derive food by killing their prey is known as predation. This is the relationship between predators and prey.
  • The association in which the small parasites derive nutrition from big hosts without killing them but harming them is called as parasitism. Excessive parasitism may cause the death of the hosts. 

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