Symbiosis:  Parasitism

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Film Summary

Parasites are organisms which take nutrition from other living things. Discover the life cycle of parasites, from tapeworms to cuckoos.

Key Facts

  • Parasites live on, or inside, another species and gain nourishment from them.
  • Parasites have complex lifecycles, using host species in order to gain entry to another, e.g. the malaria parasite uses the mosquito to travel between human hosts.
  • Parasites can affect the health and behavior of their hosts.
  • Parasites must tread a careful balance - taking the nourishment they need without killing their hosts.


In every ecosystem, there are parasites, organisms that live on or inside another species, causing them harm.

Parasites: Gain nourishment from another species

Some parasites live on the surface of their host - like head lice that live and feed on the scalp.

And fleas that feed by sucking blood.

Others live inside their hosts. Tapeworms make the most of the free lunch inside another animal's gut, absorbing nutrients from their host.

Parasitic Lifecycles

Many parasites have complex lifecycles. They may infect more than one species, and often use another animal to get from one host to another.

For example, the malaria parasite uses a mosquito to transport it between human hosts.

Even some much larger animals show parasitic behavior, taking advantage of another species at their expense.

These cuckoos never make their own nests or raise their own chicks.

Instead they raid other birds' nests - like this reed warbler's nest - and lay their own eggs in place.

The unsuspecting reed warbler incubates the egg, and when the young cuckoo hatches it kicks out the remaining host's eggs.

The warbler is tricked into feeding and rearing the cuckoo - even when it looks comically big for the nest - and its host mother.

Effect on Host

Parasitism is a clever strategy for survival - and has important effects on the whole ecosystem.

The parasite can alter the growth and health of their host, and even affect their behavior.

But parasites must always tread a careful balance taking what they need without killing their hosts, whom they completely depend on to survive.

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