Population interactions: predation
In predation (+, −) a predator feeds on its prey. Predators pass energy up food chains, control prey numbers and keep species diversity, while prey evolve defences.
The six population interactions (+ benefits, − harmed, 0 unaffected)
| Interaction | Species A | Species B | Example |
|---|---|---|---|
| Mutualism | + | + | Lichen: fungus and alga |
| Competition | − | − | Flamingoes and fishes for zooplankton |
| Predation | + | − | Tiger and deer |
| Parasitism | + | − | Cuscuta on a hedge plant |
| Commensalism | + | 0 | Orchid on a mango branch |
| Amensalism | − | 0 | No textbook example; often quoted: Penicillium and bacteria |
What is predation?
In predation, one species, the predator, feeds on another, the prey. A tiger eating a deer and a sparrow eating a seed are both predation. In a broad sense, herbivores are predators of plants. Predation is nature's way of passing the energy fixed by plants to higher trophic levels, the feeding levels of a food chain.
Roles of predators
- Conduits for energy: they carry energy from one trophic level to the next.
- Keep prey numbers in check: without predators, prey could reach very high densities and make the ecosystem unstable.
- Control invasive species: an exotic species spreads fast where its natural predators are absent. The prickly pear cactus, introduced into Australia in the early 1920s, spread rapidly over millions of hectares of rangeland. It was controlled only after a cactus-feeding moth from its native habitat was introduced.
- Biological control of farm pests uses this same ability of predators to regulate prey.
- Maintain species diversity by reducing competition among prey species. When all the starfish Pisaster were removed from an enclosed intertidal area on the American Pacific coast, more than 10 invertebrate species became extinct within a year.
- Prudent predators: a predator that over-exploits its prey may drive it extinct, and then die out itself for lack of food. So predators in nature are 'prudent'.
Prey defences against predators
| Defence | How it works | Example |
|---|---|---|
| Camouflage (cryptic colouring) | The body colour blends with the surroundings, so the predator cannot spot it easily | Some insects and frogs |
| Being poisonous or distasteful | The predator learns to avoid it | Monarch butterfly: distasteful to birds because of a chemical it takes in as a caterpillar by feeding on a poisonous weed |
| Thorns (plants) | A physical barrier to grazing | Acacia, cactus |
| Chemical defences (plants) | Make the herbivore sick, stop feeding or digestion, disrupt reproduction or even kill it | Calotropis makes poisonous cardiac glycosides |
Why plants need strong defences
Nearly 25 per cent of all insects are phytophagous: they feed on plant sap and other plant parts. A plant cannot run away, so it has evolved many morphological (structural) and chemical defences. Many chemicals we extract from plants on a commercial scale, such as nicotine, caffeine, quinine, strychnine and opium, are made by the plants as defences against grazers and browsers.
Indian farmers use ladybird beetles to eat aphids on crops, a simple form of biological control. The caffeine in your tea and coffee is a chemical the plant makes to defend itself against the animals that feed on it.
2 marks: “Why are predators in nature said to be 'prudent'?” A predator that over-exploits its prey may make the prey extinct, and then die out itself for lack of food. So natural predators do not wipe out their prey.
2 marks: “How do plants defend themselves against herbivores? Give one example of each type of defence.” Morphological: thorns in Acacia and cactus. Chemical: Calotropis makes poisonous cardiac glycosides, so cattle and goats do not browse it.
3 marks: “Explain three ways in which predators are important in an ecosystem.” They transfer energy to higher trophic levels; they keep prey numbers in check (prickly pear controlled by a moth in Australia); they maintain species diversity by reducing competition among prey (Pisaster experiment).
Predators move Energy, check Numbers and keep Diversity: E-N-D.