Interactions Between Species: Quick Revision

A brief note on Interactions Between Species. Read it before a test. Try each question at the end before you open its answer.

The signs

• Mutualism: (+, +). Competition: (−, −). Predation: (+, −). Parasitism: (+, −).
• Commensalism: (+, 0). Amensalism: (−, 0).

Predation and competition

• Predators: carry energy, control the prey, keep species diversity (starfish Pisaster). Prickly pear in Australia was controlled by a cactus-feeding moth.
• Defences of prey: camouflage, poison with warning colours (monarch butterfly), thorns, chemicals (nicotine, caffeine, quinine, strychnine, opium, Calotropis).
• Competition: unrelated species can compete (flamingoes and fish, the Abingdon tortoise and goats). Interference competition.
• Gause: closely related species competing for a limiting resource cannot coexist for ever.
• Connell: Balanus excludes Chthamalus. Removal of the better competitor gives competitive release.
• MacArthur: five warblers share a tree by resource partitioning.

Parasitism, commensalism and mutualism

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

A worked example

In a survey, 20 of 80 pairs of species show mutualism. What per cent is that?

1. Divide the pairs of mutualism by all the pairs: $20 \div 80 = 0.25$.
2. As a per cent, it is 25 per cent.

Practice with new numbers

A program to try

The program names an interaction from the effect on each species.

Mistakes to avoid

• Mixing up commensalism and amensalism. Plus and zero against minus and zero.
• Saying that only related species compete. Flamingoes and fish are not related.
• Calling a parasite a predator. A parasite lives on or in its host, usually for a long time.
• Saying that the orchid harms the mango. It is only an epiphyte.

Check yourself

1. What are the signs of competition?

2. Which barnacle is the better competitor?

3. Name one ectoparasite and one endoparasite.

4. What does the plant gain from a mycorrhiza?

Sources

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