Food Chains and Predator-Prey Relationships

Producers begin food chains

Organisms in a community depend on one another for food. A food chain is a simple representation of one route through these feeding relationships.

Every food chain begins with a producer. Producers are photosynthetic organisms, usually green plants or algae. They use light energy in photosynthesis to make glucose from carbon dioxide and water. Glucose can then be used to synthesise other molecules and build new living material, or biomass. Photosynthetic organisms therefore produce the biomass on which the rest of the food chain depends.

The Sun supplies the light energy for most food chains. Plants do not obtain this energy by eating, and they do not create energy; photosynthesis transfers light energy into chemical energy stores in molecules.

Reading the arrows

In a food chain, an arrow points from the organism being eaten to the organism that eats it. It therefore shows the direction in which food, biomass and its chemical energy are transferred.

For example:

grass → rabbit → fox

The grass is eaten by the rabbit, so the arrow points from grass to rabbit. The rabbit is then eaten by the fox, so the next arrow points from rabbit to fox. The arrows do not show which organism is chasing another.

Consumers and their feeding levels

An organism that eats another organism is a consumer. Consumers are named according to their position after the producer:

• the primary consumer eats the producer;
• the secondary consumer eats the primary consumer;
• a tertiary consumer, where present, eats the secondary consumer.

In grass → rabbit → fox, the rabbit is the primary consumer and the fox is the secondary consumer. In a longer chain such as alga → water flea → small fish → heron, the water flea is the primary consumer, the small fish is the secondary consumer and the heron is the tertiary consumer.

A food chain represents only one feeding route. In a real community, an organism may eat several species and may itself be eaten by several others, so many chains can be linked together.

Predators and prey

A predator is a consumer that kills and eats another animal. The animal it kills and eats is its prey. These words describe a relationship, rather than a permanent label: a small fish may be the predator of a water flea but the prey of a larger fish.

Predation connects the sizes of two populations. A population is all the members of one species living in a particular area at a particular time. If prey become more abundant, predators have more food. More predators may survive and reproduce, so the predator population can increase—but this response usually takes time.

Why predator and prey populations cycle

In a stable community, predator and prey numbers may rise and fall repeatedly while remaining fairly constant overall across a longer period. A typical cycle can be explained as a repeating sequence:

1. The prey population increases, providing more food for predators.
2. After a delay, more predators survive and reproduce, so the predator population increases.
3. Greater predation contributes to a fall in the prey population.
4. With less prey available, predators have less food, so their population falls after another delay.
5. Reduced predation can allow the prey population to recover, beginning the next cycle.

This is a feedback relationship: each population affects the other. The delay matters because reproduction, growth and death do not happen instantly.

Data for Illustrative predator–prey cycle

Interpreting a predator–prey graph

First identify the axes: time is usually on the horizontal axis and population size on the vertical axis. Then examine each line separately, locating its peaks and troughs. Finally, compare their timing.

In a typical model, the prey line rises first and the predator line follows. The predator peak therefore occurs after the prey peak. As predator numbers become high, prey numbers decline; predator numbers then decline after the prey have become scarce. The repeating shape indicates a cycle, not that either population stays at one fixed value.

Predation is not the only influence on population size. Water, temperature and sunlight are abiotic factors, while food supply, pathogens and the arrival of another predator are biotic factors. When interpreting real data, use the pattern provided and any contextual evidence rather than assuming that every change was caused solely by predation.