Parasitism, commensalism, amensalism and mutualism

Living together

A tick sits on a dog. An orchid grows on a mango tree. A fungus lives in a lichen. In each case, two species live in close contact. What does each gain or lose?

Parasitism

In parasitism, one species, the parasite, gets its food from the other, the host. The parasite gains (+), and the host is harmed (−).

• Ectoparasites live on the outside of the host. Lice on humans, ticks on dogs, and copepods on marine fishes are examples. Cuscuta is a plant parasite found on hedge plants. It has lost its leaves and its chlorophyll in the course of evolution.
• Endoparasites live inside the host, in the liver, the kidney, the lungs or the red blood cells. The liver fluke and the tapeworm are examples.

Parasites are usually specific to one host, and they have evolved adaptations.

• Loss of unnecessary sense organs.
• Adhesive organs or suckers to stay on or in the host.
• Loss of the digestive system.
• A very high capacity to reproduce.

Their life cycles are often complex, with one or more hosts. The human liver fluke needs two intermediate hosts, a snail and a fish, to complete its life cycle.

Parasites reduce the survival, the growth and the reproduction of the host, and so they reduce its density. They can make the host weak, and a weak host is easier prey.

Brood parasitism is a case in birds. The parasitic bird lays its eggs in the nest of its host, and the host hatches them. In the course of evolution, the eggs of the parasite have come to look like the eggs of the host in size and colour. The host then has less chance of finding the foreign eggs and throwing them out. The Asian koel, a cuckoo, lays its eggs in the nest of a crow.

Commensalism

In commensalism, one species gains (+), and the other is neither harmed nor helped (0). NCERT gives these examples.

• An orchid that grows on the branch of a mango tree. The orchid is an epiphyte, and the mango tree is not harmed.
• Barnacles that grow on the back of a whale. They gain a place to stay and food as the whale moves.
• The cattle egret and grazing cattle. The egret feeds near the cattle, because the cattle stir up the insects that the egret catches. The cattle do not gain or lose.
• The clown fish that lives among the stinging tentacles of a sea anemone. The fish is safe from its predators. NCERT says that the anemone does not seem to gain. Many modern books call this case mutualism, and this course does not use it in the papers.

Amensalism

In amensalism, one species is harmed (−), and the other is unaffected (0). NCERT gives no example in its text. A common example in other books is a mould that makes an antibiotic and kills the bacteria around it.

Mutualism

In mutualism, both species gain (+, +).

• Lichens are a partnership of a fungus and a photosynthesising alga or cyanobacterium.
• Mycorrhizae are partnerships of fungi with the roots of higher plants. The fungus helps the plant to take up nutrients from the soil. The plant gives the fungus energy-rich carbohydrates.
• Plants and animals: flowers give a reward, such as nectar or fruit, to the animals that pollinate them and spread their seeds.
• The fig and its wasp: each fig species is pollinated by its own wasp. The female wasp lays her eggs in the fruit, and the larvae feed on the developing seeds.
• The **Mediterranean orchid Ophrys: one petal looks like the female of a bee in size, colour and markings. The male bee tries to mate with the flower. It picks up pollen and carries it to the next flower. If the colour pattern of the female bee changes, the flower must change too, or it will not be pollinated. This is co-evolution**.

You should find that two pluses are mutualism and two minuses are competition. Opposite signs are predation or parasitism. A zero with a plus is commensalism, and a zero with a minus is amensalism.

Try it in Python

The program names four interactions from their signs.

Practise with new numbers

Common mistakes

• Mixing up predation and parasitism. Both are (+, −). A predator kills and eats its prey. A parasite lives on or in its host, usually for a long time.
• Mixing up commensalism and amensalism. Commensalism is (+, 0), and amensalism is (−, 0).
• Saying that the orchid harms the mango. The orchid is an epiphyte, and the tree is not harmed.
• Saying that every fungus partner is a parasite. In lichens and mycorrhizae, both partners gain.

Remember these

• Parasitism (+, −): ectoparasites (lice, ticks, Cuscuta) and endoparasites (liver fluke, tapeworm). Brood parasitism in birds.
• Adaptations of parasites: no sense organs, suckers, no digestive system, many offspring.
• Commensalism (+, 0): orchid on a mango, barnacles on a whale, cattle egret, clown fish and anemone (NCERT).
• Amensalism (−, 0).
• Mutualism (+, +): lichens, mycorrhizae, fig and wasp, Ophrys and the bee.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Name the interaction in each case: an orchid on a mango, a tick on a dog, a lichen.

2. Give two adaptations of a parasite.

3. What is brood parasitism?

4. What does the plant gain from a mycorrhiza, and what does the fungus gain?

5. How does Ophrys get pollinated?

References

• NCERT (2023). Biology, Textbook for Class XII, chapter 11, Organisms and Populations, section on population interactions.
• Campbell Biology, 12th edition (2021), Pearson, chapter 54, Community Ecology.
• OpenStax Biology 2e (2018), chapter 45, Population and Community Ecology.