A density-independent factor affects the same proportion of a population whatever its density. Most are abiotic (non-living) factors, such as:
- temperature, for example a severe frost or a heatwave
- rainfall, causing drought or flooding
- natural events such as fire, volcanic eruption or storms
- human activities such as pollution or habitat destruction
For example, a hard winter might kill 60% of a population of garden snails whether there are 50 of them or 5000. A forest fire destroys the same fraction of a population regardless of how crowded it is.Because their effect does not change with population size, density-independent factors cannot regulate a population around a set level. Instead they cause sudden, unpredictable crashes, and populations controlled mainly by them show large, irregular fluctuations. Many insects in temperate climates build up quickly in summer and are then cut back by the first frosts.In reality the two types of factor interact. A harsh winter kills more individuals if they are already weakened by competition for food, and a drought forces animals together around the remaining water holes, where disease spreads more easily. So weather can have density-dependent effects in some situations.Worked example: frost kills 150 of 500 snails in one garden and 30 of 100 in another.150 ÷ 500 × 100 = 30% and 30 ÷ 100 × 100 = 30% The percentage killed is the same at both densities, so frost is acting as a density-independent factor.
- temperature, for example a severe frost or a heatwave
- rainfall, causing drought or flooding
- natural events such as fire, volcanic eruption or storms
- human activities such as pollution or habitat destruction
For example, a hard winter might kill 60% of a population of garden snails whether there are 50 of them or 5000. A forest fire destroys the same fraction of a population regardless of how crowded it is.Because their effect does not change with population size, density-independent factors cannot regulate a population around a set level. Instead they cause sudden, unpredictable crashes, and populations controlled mainly by them show large, irregular fluctuations. Many insects in temperate climates build up quickly in summer and are then cut back by the first frosts.In reality the two types of factor interact. A harsh winter kills more individuals if they are already weakened by competition for food, and a drought forces animals together around the remaining water holes, where disease spreads more easily. So weather can have density-dependent effects in some situations.Worked example: frost kills 150 of 500 snails in one garden and 30 of 100 in another.