Biotic Interaction



  • In ecology, a biological interaction is the effect that a pair of organisms living together in a community have on each other
    • They can be either of the same species (intraspecific interactions), or of different species (interspecific interactions).
  • These effects may be short-term, like pollination and predation, or long-term; both often strongly influence the evolution of the species involved
  • Interactions can be indirect, through intermediaries such as shared resources or common enemies


Positive Interaction

  • This term is used for the types of interspecific relationships which are mutually beneficial and where one or both partners are benefited and no one is harmed
    • The benefit my be in terms of food, shelter, substratum or transport
  • This is mainly divided into three categories
    • Mutualism
      • It is mutually beneficial relationship between two organisms; here both species derive benefit
      • The relationship may be compulsory(obligatory) or facultative(optional)
      • Examples
        • Lichens
          • The term means ”living together”
          • This is an example where contact is close and permanent as well as obligatory
          • The fungus provides moisture as well as minerals and protection to algae; whereas alga manufactures food material
          • Neither of the two can grow alone independently



        • Crocodile and Bird association
          • Here the bird enters the mouth of crocodile and picks up leaches found between the teeth and feed on them
          • Thus, birds clean the teeth and crocodile is benefited; while the bird gets is nutritional requirement

    • Commensalism
      • In this inter-specific relationship, one of the partner is benefited and other partner is neither benefited nor harmed
      • Here, the partner getting the benefit is called commensal
      • Ecto commensalism
        • Here, the commensal lives on the body of the other partner, which may be called host
        • Example
        • Association between suckerfish (or Remora or Echeneis) and shark
          • The sucker fish has the dorsal fin modified as a sucker(or adhesive) disc, with the help of which it is attached to the body of shark so that the suckerfish gets free transportation
          • The sucker fish releases the attachment after sometime and swims in search of food


    • Endo Commensalism
      • Here the commensal lives within the body of host
      • Example
        • Association between saprophytic bacteria and fungi
          • A variety of microorganisms, saprophytic bacteria and fungi and protozoans live within the tissues or cavities of higher plants and animals
          • Many microorganisms like bacteria live as commensals in the digestive system of various animals. The microorganisms use undigested food for their nutritional requirements.
          • coli lives in the intestine of humans.
    • Protocooperation
      • Protocooperation is a form of mutualism, but the cooperating species do not depend on each other for survival
      • Example
        • Flowers and Insects
        • The flowers of plants that re pollinated by insects and birds benefit from protocooperation
        • The plants with colourful flowers, experience cross pollination because of insect activities
        • This is beneficial to the insect that has got its food supply as well


Negative Interaction

  • These interactions include association where one or both individuals are harmed.
  • The harm may be caused by eating other organism, competition for food, excretion of harmful wastes, etc
  • These have been sub-divided into
    • Exploitation
      • In this type of interaction, one species is benefited at the expense of another
      • Predation is an interaction between organisms in which one organism captures biomass from another
      • The term is used as a synonym for carnivory but in its widest definition includes all forms of one organism eating another, regardless of trophic level (eg., herbivory), closeness of association (e.g., parasitism) and harm done to prey (e.g., grazing)
      • Parasitism
        • This is an interaction between two organisms in which one (called parasite) derives synthesised food from another living organism (Called host).
        • A true parasite though obtains its food from the host, seldom kills it
        • Examples
          • Among the animals, ticks, mites and lice are external parasites or ectoparasites
          • The fungal parasites include Erysiphe (powdery mildew), Ustilago (smut), Puccinia (rust), etc; which cause diseases that result in serious losses of economically important crops
      • Predation
        • In contrast with a parasite which derives nourishment from its host without killing, a predator is free living which catches and kills another species for food or predator is a direct food relationship between two individuals in which an animal that remains free living (called predator) catches and kills another animal (called prey) for food
        • Examples
          • Tiger (predator) eating deer (prey), frog eating insects


    • Competition
      • Competition is an interaction between organisms or species in which both the organisms or species are harmed.
      • Limited supply of at least one resource (such as food. water and territory) used by both can be a factor
      • Species compete in the following ways
      1. By mechanism:
        • Biologists typically recognize two of types competition interference and exploitative competition
        • During interference competition, organisms interact directly by fighting for scarce resources. For example, large aphids defend feeding sites on cottonwood leaves by ejecting smaller aphids from better sites.
        • In contrast, during exploitative competition, organisms interact indirectly by consuming scarce resources. For example, plants consume nitrogen by absorbing it into their roots, making nitrogen unavailable to nearby plants
      2. By size asymmetry
        • Competition varies from complete symmetric (all individuals receive the same amount of resources, irrespective of their size) to perfectly size symmetric (all individuals exploit the same amount of resource per unit biomass) to absolutely size-asymmetric (the largest individuals exploit all the available resource)
        • The degree of size asymmetry has major effects on the structure and diversity of ecological communities,
        • Example, in plant communities size-asymmetric competition for light has stronger effects on diversity compared with competition for soil resources
      3. By taxonomic relationship
        • Competition can occur between individuals of the same species, called intraspecific competition, or between different species, called interspecific
        • Studies show that intraspecific competition can regulate population dynamics (changes in population size over time). This occurs because individuals become crowded as population grows.
        • Since individuals within a population require the same resources, crowding causes resources to become more limited
        • Some individuals (typically small juveniles) eventually do not acquire enough resources and die or do not reproduce.
          • This reduces population size and slows population growth
        • An example among animals could be the case of Cheetah and lions since both species feed on similar prey, they are negatively impacted by the presence of the other because they will have less food, however they still persist together, despite the prediction that under competition one will displace the other


    • Antibiosis
      • This is a complete or partial inhibition of one organism by another either by secreting some substance or by modifying its immediate environment
      • The substance or conditions produced by an organism are generally harmful for the other organism
      • This phenomenon is very common in micro-organisms which secrete substance called antibiotic
      • Example
        • Bacteria, actinomycetes and fungi produce a number of antimicrobial substances which are widespread in nature.
        • Lichens as well as large number of higher plants produce substances that inhibit molds and bacteria


Summarising table

Mutualism·         Both the species derive benefit. Association more or less obligatory, essential for survival of both.

·         e.g. pollination, Fruit and seed dissemination, Lichens

Commensalisms·         Only one species benefited, neither is harmed.

·         e.g. Hydroids on fish, Crab in the mantle cavity of oyster, Rhizosphere and micro-organisms.

Proto co-operation·         Both are benefited but is not obligatory


Parasitism·         Food derived from the host with or without causing its death
Predation·         Food-derived by killing the host

·         e.g. Browsing, Grazing, Seedling destruction, Plants as food, plants.

Competition·         Competition occurs when many organisms struggles for Same limited


Antibiosis·         One species produced a poisonous substance or a change in environmental conditions inhibitant to another species, none derives benefit