Population Interactions – Commensalism and Amensalism


It is an interaction between two species in which one species is benefitted and the other species is neither harmed nor benefitted. The species that benefits is the commensal and the other is the host.

  • Orchids grow as epiphytes on branches of mango.
  • Barnacles growing on whales back. Barnacles are arthropods and their larvae grow on the whale.
  • Cattle ergets and grazing cattle – when the cattle graze the grass it flushes out the insects.The ergets feed on these insects.
  • Clownfish living on the stinging tentacles of sea anemone protects itself from predators.

  • Jackal/hyena accompanies tiger/lion eating the leftovers.
  • Suckerfish & shark – dorsal fin of the fish acts as a holdfast by which it gets attached to the shark. It is protected from the predators & is carried to regions having plenty of food by the shark.
  • Lianes are woody climbers that coil around a support to grow towards the light.


It is an interaction between two organisms in which one inhibits the growth of the other by secreting chemicals called allochemics. E.g.,

  • Penicillium does not allow growth of Staphylococus.
  • Trichoderma does not allow growth of Asperigillus.
  • Smoother crops are Barley, Sorghum, and Sunflower as they do not allow the weeds to grow nearby.
  • Tagetes secrete chemicals toxic to soil nematodes.


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