Interspecific Relationships - Negative Interactions > Predation

Predation is an interspecific interaction in which one organism (the predator) captures, kills, and consumes another organism (the prey) - benefiting the predator (+) and lethally harming the prey (−).

Types of Predation:

1. Energy Transfer

Predation is the primary mechanism transferring energy fixed by producers upward through trophic levels.

2. Regulation of Prey Population

• Predators keep prey populations under check, preventing overpopulation and resource exhaustion.
• Without predators, prey can exceed the carrying capacity (K) of the habitat and crash.

3. Maintenance of Species Diversity

• Predators selectively consume dominant/competitive prey, preventing monopolisation of resources.
• This allows weaker competitors to survive, maintaining higher species diversity.

🔬 Pisaster Experiment (Robert Paine):

Removed starfish Pisaster from a rocky intertidal community

• Within one year: species diversity fell from 15 → 8 species 
• Mussels (dominant prey) monopolised the habitat without predation pressure

Conclusion: Pisaster is a keystone species - disproportionately large ecological influence relative to its abundance

4. Biological Control of Pests

The ecological principle of predation is applied to manage pest populations using natural predators instead of chemical pesticides.

Example: Gambusia affinis (mosquitofish) eats mosquito larvae → controls malaria vectors.

🔬 Prickly Pear–Cactoblastis Case (Australia):

• Opuntia cactus introduced into Australia → spread invasively over 60 million acres (no natural predators in new habitat)
• 1926: Moth Cactoblastis cactorum* introduced from Argentina → larvae fed on cactus pads and destroyed plants
• By 1933: most infested land had been cleared → greatest example of biological weed control via predation

1. Oscillating Populations:

Predator and prey populations do not remain constant - they oscillate in linked, cyclic patterns.

Pattern of oscillation

1. Prey population increases → food for predators becomes abundant
2. Predator population increases → more predation pressure
3. Prey population decreases → less food for predators
4. Predator population decreases → prey recovers
5. Cycle repeats

2. Prudence of Predators:

• Natural predators never overexploit their prey to the point of extinction, as that would eliminate their own food source.
• If a predator overexploits prey → prey population collapses → predator population starves and collapses too.
• This natural regulation maintains a stable predator–prey equilibrium in undisturbed ecosystems.

Animal Defences:

Plant Defences Against Herbivory:

• Predation (+/-) involves a predator killing and eating its prey, which acts as nature's main channel for transferring energy up the food chain.
• Predators naturally regulate prey numbers, preventing overpopulation - a principle used effectively in biological pest control.
• By consuming dominant prey species, predators prevent resource monopolisation and maintain overall species diversity.
• Predator and prey populations cycle together naturally; predators are "prudent" and do not hunt their food source to extinction.
• To survive, prey species continuously evolve physical defences (like camouflage or thorns) and chemical defences (like toxins).