A density-dependent factor is one whose effect on a population depends on population density: the larger and more crowded the population, the greater the proportion of individuals affected. Most density-dependent factors are biotic (caused by living things).
Examples:
- Competition for food, water, space, light or mates: more individuals means less of the resource each.
- Predation: predators find prey more easily when prey are common.
- Disease and parasitism: pathogens and parasites spread faster between individuals living close together.
- Build-up of toxic waste in a confined space, for example in a culture of yeast.Because their effect increases as the population grows, density-dependent factors act by negative feedback. They raise the death rate and lower the birth rate when the population is large, and ease off when it is small. This keeps a population fluctuating around the carrying capacity, the maximum population size the environment can support.Worked example: in a pond, 100 tadpoles share a fixed amount of food and 20 of them (20%) die of starvation. When 400 tadpoles share the same food, 200 die.200 ÷ 400 × 100 = 50% The percentage dying rises with density, so food shortage is density dependent. If the percentage had stayed the same whatever the density, the factor would be density independent.
Examples:- Competition for food, water, space, light or mates: more individuals means less of the resource each.
- Predation: predators find prey more easily when prey are common.
- Disease and parasitism: pathogens and parasites spread faster between individuals living close together.
- Build-up of toxic waste in a confined space, for example in a culture of yeast.Because their effect increases as the population grows, density-dependent factors act by negative feedback. They raise the death rate and lower the birth rate when the population is large, and ease off when it is small. This keeps a population fluctuating around the carrying capacity, the maximum population size the environment can support.Worked example: in a pond, 100 tadpoles share a fixed amount of food and 20 of them (20%) die of starvation. When 400 tadpoles share the same food, 200 die.