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Population Interaction: Parasitism, Commensalism, Practice Problems and FAQs

Population Interaction: Parasitism, Commensalism, Practice Problems and FAQs

No species can survive alone on this planet. Each species remains connected to at least two or three other species for one or the other reasons to survive. These species interactions can either affect both the species positively or negatively or might affect only one while the other remains unaffected. 

If you visit a paddy field where the cattles graze, you can find the true friendship between the cattles and the crane. Do you really think it is an unbreakable bond? Yes there is an unbreakable relationship because being close to the cattle helps the cranes to forage food. But not all relationships are beneficial like this. Sometimes the relationship between two species can end up harming one while benefiting the other. Can you think of an example? What about when we get lice infestation in the head? Who do you think gets benefited in such a relationship?

Keep reading if you want to know more about such relationships.

Table of Contents:


It is the relationship between organisms where one organism (parasite) lives and derives nutrition from another specific organism (host).


Parasites could be protozoans (Entamoeba histolytica), helminthes (Ascaris lumbricoides), fungi (Candida albicans) or even viruses (HIV).

some common parasites

Majority of the parasites harm the host. They can reduce the survival, growth and reproduction of the host. And thereby reduce its population density. They might even render the host more vulnerable to predation by making it physically weak.

Co-evolution of Parasites

Every parasite is host specific. They cannot complete their life cycle without their host.

Both the host and the parasite tend to co-evolve. Which means if the host evolves special mechanisms for getting rid of the parasite, then the parasite has to evolve mechanisms to override them. Then only they can be successful with the same host species.

Parasites have evolved special adaptations in accordance with their lifestyles such as -

  • Losing unnecessary sense organs
  • Developing adhesive organs or suckers for the purpose of clinging on to the host
  • Loss of the digestive system 
  • High reproductive capacity

Classification of Parasites

Parasites are mainly of two types:

types of parasites


Ectoparasites are those organisms that live and feed on the external surface of the host. The most familiar examples of ectoparasites are the ticks on dogs and lice on humans.


Many marine fishes are infested with ectoparasitic copepods. Copepods are crustaceans that live on the fish much like the lice on our body.

Ectoparasite on marine fish

Cuscuta is a parasitic plant. It can be commonly found growing on hedge plants. During the course of evolution it has lost its chlorophyll and leaves. Cuscuta can derive its nutrition from the host plant with the help of sucking roots known as haustoria.

Plant ectoparasites


Endoparasites live inside the host's body and derive nutrition from it. Due to the extreme specialisation of endoparasites, their life cycles are more complex. While their reproductive potential is highly developed, the morphological and anatomical features are greatly simplified. Liver fluke and Plasmodium are some common examples.


Facultative Parasite

A facultative parasite (meaning optional) is an organism that may resort to parasitism but does not completely rely on its host for its survival. 


Brain-eating amoeba is a microorganism which is typically found in warm freshwater bodies (warm water discharge from industrial or power plants, geothermal well water, poorly maintained swimming pools, water heaters etc). 

It normally eats bacteria. But amoeba uses the brain as a food source when it gets into humans. Hence it is a facultative parasite.

Brain eating amoeba

Symptoms of being infected by this amoeba may include headache, fever, nausea, vomiting, loss of appetite, etc. Later symptoms can include confusion, loss of balance, seizures, and hallucinations. This pathogen is quite fatal and causes death within two weeks of the appearance of the symptoms.

The Life Cycle of Parasites

The life cycles of parasites are often complex. They may even have more than one host as seen in the case of human liver fluke which infects the liver, bile duct and gallbladder in humans.

Eggs and adult human liver fluke

Life Cycle of Human Liver Fluke

The human liver fluke belongs to the class Trematoda. In the liver, the adult female liver fluke lays eggs. The eggs travel through the intestines and pass out with the faeces and come in contact with water where they hatch to produce immature larvae. The larvae are ingested by snails (first host) and mature inside them to produce swimming larvae which are ingested by fish, crabs or crayfish (second host). Fishes are ingested by humans ( third host). Inside the humans it matures to form adult liver flukes and cause infection. 

steps in the life cycle of human live fluke

Similarly, the Plasmodium parasite that causes malaria has two hosts, namely, the female Anopheles and humans.

Hosts of malaria causing plasmodium

Have you ever wondered why the female Anopheles mosquito is not considered a human parasite? It is because the female Anopheles mosquito does not need blood for its nutrition. Instead, it needs blood for reproduction. 

Brood Parasitism

Brood parasites are organisms that rely on other organisms, either from the same species or another species, to raise their young ones. 

In brood parasitism the parasitic bird lays its eggs in the nest of its host and relies on the host to incubate them. 

Over the years, the eggs of the parasitic bird have evolved to resemble the host’s egg in size and colour. This reduces the chances of the host bird detecting the foreign eggs and ejecting them from the nest.

Example: The cuckoo’s eggs have an uncanny resemblance to the crow’s (host) eggs.

black aarow indicates the eggs


The interaction in which one species benefits while the other species is neither harmed nor benefited is called commensalism.'


Epiphytes, also known as air plants, are plants that grow upon another plant or object merely for physical support. Epiphytes have no attachment to the ground. They do not have other obvious nutrient sources. And they are not parasitic on the supporting plants.

Example: Orchids growing as epiphytes on mango trees. Orchids get the physical support and the mango tree has no gain or no loss.

orchids growing on mango tree

Barnacles benefit from attaching to the whales as they get a stable place to live, access to plenty of food, and a free ride. The whale on the other hand remains unaffected by their presence.

barnaclemon whale

When cattles graze through fields to have their diet of tall grasses, the cattle egret, a stately white bird with long legs and graceful wings, feeds on the insects that emerge from the grass as the cattle move through them. The cattle in this case remain unbothered whereas the egret get access to food which would otherwise be difficult to arrange.

enret on castle

Another example is the clown fish living amongst sea anemone. The fish seeks refuge amongst the anemones to get protection from predators who do not approach to prevent being attacked by the stinging tentacles of the anemones. The sea anemones on the other hand remain unaffected by the presence of the fish.

cloenfish and sea enmone

Practice Problems of Population Interaction

Q1. How many among the following are incorrectly matched related to population interactions?

1). Endoparasitism - Lice on human hair

2). Ectoparasitism - Cuscuta growing on other plants

3). Commensalism - Barnacles growing on the back of a whale

  • 1
  • 2
  • 3
  • All are correct

Solution: Ectoparasites live on the external surface of the host organism. For example - lice on human hair, Cuscuta on other plants, etc.

In endoparasitism, the parasites live inside the body of the host organism, e.g, liver fluke parasite living within the human intestines.

Commensalism is a type of population interaction in which benefits one species while the other is neither harmed nor benefited, e.g, barnacles growing on the back of a whale.

Hence the correct option is a.

Q2. The two intermediate hosts in the lifecycle of the human liver fluke are ______A_______ and _____B______.

  • A - snail, B - fish
  • A - snail, B - mosquito
  • A - pig, B - mosquito
  • A - pig, B - housefly

Solution: Humans are one of the definitive hosts of the liver fluke as it harbors the adult parasite that reproduces sexually. Intermediate hosts are the ones that harbor the larval stage or the asexual forms of the parasite. Liver fluke requires two different intermediate hosts - snail and fish- in addition to a definitive host.

Hence the correct option is a.

Q3. Choose the incorrect statement related to the adaptations of endoparasites. 

  • Loss of unnecessary sense organs
  • Presence of adhesive organs or suckers
  • Loss of digestive system
  • Development of cryptically-coloured bodies

Solution: Camouflage or cryptic colouration, is a defence mechanism that organisms use to hide themselves from predators by blending in with their surroundings. As endoparasites live inside the body of the host, away from predators, cryptic colouration is absent in them. 

Hence the correct option is d.

Q4, Choose the correct option.

  • Commensalism - Species A (+), Species B (0)
  • Parasitism - Species A (+), Species B (+)
  • Parasitism - Species A (-), Species B (-)
  • Commensalism - Species A (+), Species B (-)

Solution: In a commensal relationship, one of the interacting species is benefited while the other remains unaffected. Taking two species, A and B, as examples, we can express the commensal relationship between them as A being positively affected (+) and B being unaffected (0).

In a parasitic relationship, the parasite is benefited (+) whereas the host is harmed (-).

Hence the correct option is.

FAQs of Population Interaction

Question 1.- Explain parasitism with an example.

Answer. Parasitism is the relationship between organisms where one organism (parasite) lives and derives nutrition from another specific organism (host). Just like blood-sucking lice living on humans for nutrition. 

Question 2.- What is brood parasitism?

Answer. In brood parasitism, organisms depend on other organisms of the same or different species for raising their young ones. The parasitic bird lays its eggs in the nest of its host and allows the host to incubate them. The eggs of the host and parasite being similar in size and colour are indistinguishable and hence the host is incapable of ejecting them.

Question 3.- Explain commensalism with an example.

Answer. Commensalism is an interaction between two species in which one is benefited while the other remains unaffected in any way. Orchids growing as epiphytes on mango trees is an example of commensalism. Orchids get the physical support and the mango tree has no gain or no loss.

Question 4.- What kind of relationship exists between a sea anemone and a clown fish?

Answer. Sea anemone has stinging tentacles over their body and the clown fish lives among the tentacles. In this relationship, sea anemone is neither harmed nor benefited but the clownfishes get a safe habitat which cannot be approached by predators. Hence it is a type of commensal interaction.

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