International A Level Biology / Ecology

Biotic factors: predation, competition and disease

Edexcel International A Level Biology (YBI11), chapter 15: Ecology, specification point 5.12. Free lesson.

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Biotic factors are influences from living organisms, including competition, predation, parasitism, disease and food supply. They change survival and reproduction, so they can change population size and produce cycles in predator and prey numbers.

The five biotic factors

Biotic factors are the living parts of an environment and the other organisms in it. They affect how many individuals survive and reproduce, which changes the size of a population. Competition happens when organisms need the same limited resource, such as food, light, water or space. Predation is when a predator kills and eats its prey, such as a lynx eating a snowshoe hare. Parasitism is when a parasite lives on or in a host and feeds from it, harming the host. Ticks on deer are an example. Disease is caused by pathogens. Myxomatosis is a viral disease that has killed many…

How the two types of competition differ

The names describe whether the organisms competing are from the same species or different species. Competition becomes more important as a population gets crowded, because more individuals need limited resources. Intraspecific competition is between members of the same species. Their needs are identical, so they compete for exactly the same resources. It limits population size as the population gets crowded. Interspecific competition is between members of different species. They compete for resources that their needs share. It can reduce the size of one species or remove it…

How to read a predator and prey graph

An idealised graph shows population size over time, often in thousands. Prey and predator numbers rise and fall in a repeating cycle. Real counts, such as hare and lynx records, are noisier but show the same general pattern. The prey peak comes first. The predator peak follows later, about a quarter of a cycle behind in the idealised model. The predator peak is lower because there are fewer predators than prey.

Why predator and prey numbers cycle

The cycle is driven by changes in food supply and predation. The lag is the delay before predator numbers respond to a change in prey numbers. Predators take time to breed and raise young, so their peak comes after the prey peak.

Density-dependent and density-independent factors

Population density means how crowded a population is. A density-dependent factor has a greater effect as the population becomes more crowded. A density-independent factor has the same effect whatever the population density. Density-dependent examples are competition, predation, parasitism and disease. Disease spreads faster in a crowded population because individuals are in closer contact. Density-independent examples include a flood, a hard frost and a drought. Density-dependent factors are usually biotic, while density-independent factors are usually abiotic.

Applying biotic factors to rabbits

To explain how a biotic factor reduces a population, name the factor and describe how it affects survival or reproduction. Predation by foxes reduces rabbit numbers because more rabbits are killed and eaten. Disease, such as myxomatosis, can kill rabbits and spread faster when they are crowded. Competition for grass can reduce rabbit numbers if less food is available. A drought is not a biotic factor. It is abiotic.

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