Density-independent factors
Any factor in the environment that does not depend on the number of members in the population per unit of area is a density-independent factor.
These factors are usually (but not always) abiotic. They include things like natural phenomena such as forest fires or weather events. Weather events that limit populations include drought or flooding, extreme heat or cold, tornadoes, and hurricanes.
Populations can be limited by the unintentional results of human activities. Such as pollution or alterations of a landscapes. Air, land, and water pollution (the results of human activities) can limit populations. Pollution reduces available resources by making some of the resources toxic.
For example, over the last 100 years, human activities on the Colorado River, such as building dams, water diversions, and water barriers, have significantly reduced the amount of water flow and changed the water temperature of the river. In addition, introduction of non-native fish species altered the biotic factors in the river. Because of changes in the river, the number of small fish called humpback chub was reduced. During the 1960s, the number of humpback chub dropped so low that they were in danger of disappearing from the Colorado River altogether.
