Community Interactions: Competition and Predation

Competition is an interaction in which organisms use the same limited resource, and predation is an interaction in which one organism, the predator, kills and eats another, the prey. In brief: two plants that compete for light both lose, while in predation the predator gains and the prey loses. Lions hunting zebras are predation, and two bird species fighting over the same nesting holes are competition.

These two interactions strongly influence which species live together and how many individuals there are. This guide defines competition and its types, explains predation and how predator and prey populations affect each other, works through a numerical example and points out common mistakes.

What are community interactions?

Organisms in a community are linked by many relationships. Ecologists classify them by their effect on the two species involved: positive, negative or neutral. Competition is a relationship in which both parties are usually harmed, and predation benefits one and harms the other. Symbiotic relationships, such as mutualism and parasitism, are considered separately.

What is competition?

Competition occurs when two or more organisms need the same resource that is in limited supply. Resources include food, water, light, nutrients, space, shelter and mates. When the resource is not enough for all, each organism gets less, and its growth, survival or reproduction decreases.

Competition is often strongest between organisms with the same needs. Individuals of the same species have the most similar needs, so competition within a species is usually intense.

What are the types of competition?

Intraspecific competition

This happens between members of the same species. Seedlings of the same tree compete for light, and male deer compete for mates. It helps to limit the size of a population and tends to favor the best-adapted individuals.

Interspecific competition

This happens between different species that use the same resources. Lions and hyenas both hunt zebra and wildebeest, and sparrows and starlings may compete for nest sites. The result may be that one species is forced out, as described by the competitive exclusion principle, or that the species adapt to use slightly different resources.

Competition can be direct, as when animals fight, or indirect, as when one species uses up a resource so that less remains for another. Plants compete mostly indirectly by shading or by absorbing water and minerals first.

What is predation?

In predation, a predator catches and kills prey for food. Predators include lions, owls, spiders, sharks and even plants such as the Venus flytrap. Herbivory and parasitism are related, but the plant or host usually survives.

Both sides have adaptations from long-term interaction. Predators have sharp senses, speed, claws, venom or cooperative hunting. Prey have camouflage, warning colors, speed, spines, chemical defenses or herd behavior. Mimicry is also common, as when a harmless hoverfly looks like a stinging wasp.

Predation can keep prey populations from growing until they run out of food, and it removes weak and sick individuals. Predators can also prevent one prey species from taking over, which helps maintain diversity.

How do predator and prey numbers change?

Predator and prey populations are linked. When prey are abundant, predators have plenty of food and increase. More predators eat more prey, so the prey population falls. With less food, predators decline, and the prey then recover. The cycle repeats, with the predator peak following the prey peak.

The classic data set is the number of pelts of Canada lynx and snowshoe hare bought by the Hudson’s Bay Company over many decades, which shows repeated cycles of about ten years. The causes are still discussed, because food supply for hares and other factors also matter. In many natural systems, cycles are weaker than in a simple model, because predators have alternative prey and prey can hide.

Competition and predation compared

Worked example: reading a predator-prey graph

The table below gives imagined yearly counts of hares and lynx in a forest. Use the numbers to describe the cycle.

1. Hares increase from 2000 in year 1 to a peak of 8000 in year 3. Food is plentiful and few lynx hunt them.
2. Lynx increase until year 4, when they peak at 350. Their peak comes one year after the hare peak, because lynx need time to breed.
3. In year 4, hares drop to 4000 because many are caught.
4. In year 5, hares fall to 1500, and with so little food the lynx population also falls, to 150.
5. In year 6, fewer predators allow the hares to recover to 2500, and a new cycle begins.

The data show a delay between the peaks of prey and predator, which is a typical feature. The numbers are invented for the exercise, not real measurements.

How were these interactions studied?

In the 1920s Alfred Lotka and Vito Volterra independently wrote mathematical equations for predator-prey cycles. In the 1930s Georgii Gause grew two species of Paramecium in test tubes. When grown together, one species survived and the other died out, which supported the competitive exclusion principle. Later field studies added a more complex picture.

Why do they matter in practice?

Biological control uses predators to reduce pests. Ladybugs are released to eat aphids, and this can reduce the use of chemical pesticides. Wildlife managers consider predator effects when they set hunting quotas or reintroduce species.

Competition is important in agriculture, where weeds compete with crops for light, water and nutrients. Farmers use crop rotation, spacing and weeding to reduce losses.

Which mistakes are common?

Mistake 1: Saying that competition always kills the loser

More often it reduces growth or reproduction. Death occurs only when the shortage is severe.

Mistake 2: Thinking that predators are harmful to ecosystems

Predators help regulate prey numbers and keep ecosystems balanced. Removing them can lead to overgrazing.

Mistake 3: Confusing the predator and prey peaks

The predator peak follows the prey peak, not the other way round. Prey numbers rise first.

Mistake 4: Treating cycles as a law

Predator-prey cycles appear only in some systems. Weather, disease and other food sources can break the pattern.