Predation and the defences of prey

No species lives alone

A lion needs a deer, a flower needs a bee and a cuckoo needs a crow. No species lives alone. The interactions between populations of two different species are called interspecific interactions.

We write the effect on each species with a sign. A plus means that the species gains. A minus means that it is harmed. A zero means that it is not affected. Six combinations are in NCERT.

The NTA text names four of these: mutualism, competition, predation and parasitism. Commensalism and amensalism are in the NCERT chapter too, so they can appear as options.

Predation

All animals depend on plants for their food, either directly or by eating other animals. When an animal eats another living organism, the interaction is predation. The eater is the predator, and the eaten is the prey. A herbivore that eats a plant also acts as a predator, in a broad sense.

Predators have three roles.

1. They carry energy from one level of the food chain to the next.
2. They keep the prey population under control. The prickly pear cactus was brought to Australia in the early 1920s. It spread over millions of hectares of range land. It was brought under control only when a cactus-feeding moth, a predator from its home, was introduced.
3. They keep species diversity. Predators reduce the competition among the prey species. In the rocky intertidal region of the Pacific coast of the United States, the starfish Pisaster is an important predator. In a field experiment, all the starfish were removed from an enclosed area. Within a year, more than ten species of invertebrates disappeared, because competition among them was no longer held in check.

Defences of prey

Prey are not helpless. They have evolved ways to avoid being eaten.

• Camouflage: some insects and frogs are coloured like their background, so they are hard to see.
• Poison and warning colours: the monarch butterfly is bright and tastes bad to birds. The caterpillar eats a poisonous weed, and the poison stays in the body of the butterfly.
• Thorns: Acacia and the cactus.
• Chemicals: many plants make chemicals that make the plant distasteful or poisonous. Examples are nicotine, caffeine, quinine, strychnine and opium. Calotropis makes poisonous cardiac glycosides, and cattle and goats do not graze on it.

The cycle of prey and predators

This part is not in the NEET syllabus, but A-Level and IB ask it. The Lotka-Volterra equations are a model of one predator and one prey. In a year, $x$ prey and $y$ predators change like this:

The prey grow by themselves at the rate $a$, and they are eaten at a rate that grows with the number of both. The predators are born at a rate that grows with the food, and they die at the rate $d$. The two populations rise and fall in cycles. The predators peak after the prey, because they need time to breed. At the balance point, the prey are $d/c$ and the predators are $a/b$.

The sim above uses the formulas of small swings around the balance point. The program below follows the full equations in small steps.

Try it in Python

The program steps the prey and the predators forward for ten years and reports when the predators reach their largest number.

Practise with new numbers

Common mistakes

• Saying that a predator is only a meat eater. A herbivore that eats a plant is also a predator in the broad sense.
• Mixing up the signs. In predation, the predator gains (+) and the prey is harmed (−).
• Thinking that predators only harm the prey population. They also keep it in check, and they keep other species alive.
• Putting the peak of the predators before the peak of the prey. In the model, the predators peak after the prey.

Remember these

• Interspecific interactions are shown with signs. Predation is (+, −).
• Roles of predators: energy transfer, control of prey, keeping species diversity (starfish Pisaster).
• Prickly pear in Australia was controlled by a cactus-feeding moth.
• Defences: camouflage, poison and warning colours (monarch butterfly), thorns, chemicals (nicotine, caffeine, quinine, strychnine, opium, Calotropis).
• Lotka-Volterra (not in the NEET syllabus): cycles in which the predators peak after the prey.

Check yourself

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

1. Give the signs of predation, and say who is the predator.

2. Name the three roles of predators.

3. What happened to the invertebrates when all the starfish were removed from the enclosed rocky area?

4. Why is the monarch butterfly safe from birds?

5. Name two chemicals that plants use as defence.

References

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