Population Interactions – Parasitism


It is a relationship between two organisms in which one organism lives in or on the body of another organism.

The former one is referred as the parasite and the later as host.

Ø   Parasites are usually smaller in size and majority of them are host specific.


Ø   Both host and the parasite tend to co-evolve, i.e., if the host evolves a mechanism for resisting the parasite, the parasite must evolve mechanisms to counteract and neutralize them, to be successful with the specific host.

Ø   Parasites have evolved special adaptations like loss of unnecessary sense organs, the presence of adhesive organs or suckers, loss of digestive system and high reproductive capacity.

Ø   Parasites often involve one or two intermediate hosts or vectors to facilitate parasitism.

E.g., human liver fluke requires two intermediate hosts i.e., a snail and a fish to complete its lifecycle. The malarial parasite needs a vector i.e., a mosquito to spread.

Parasites reduce the survival, growth, and reproduction of the host and reduce its population density.

  • Temporary parasites–e.g., mosquito, leech, bedbug.
  • Hyperparasites – parasites growing over parasites. E.g., Bacteriophage, Pasteurella pestis is a parasite on rat flea, Cicinnobolus cestaii on powdery mildew (fungus).

·  Ectoparasites– e.g., lice on humans, ticks on dogs, copepods on marine fish. Cuscuta growing on hedge plants is without leaves.

·    Endoparasites – their lifecycle is more complex. E.g., Plasmodium, Taenia (gut parasite), Trypanosoma (body fluid parasite).

·     Brood parasitism – Parasitic bird lays its eggs in the nest of its host. They resemble the host’s egg in size and in color. E.g., cuckoo lays its egg in the crow’s nest.

·      Plant parasites – e.g., of stem parasites are Cuscuta, Viscum.

E.g., of Root parasites are Rafflesia, root nematode.



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