Density independent factors limit population sizes in an ecosystem due to events that occur regardless of the density of a certain population. Density independent factors include environmental stresses, weather, sudden climate changes, environmental pollutants and nutrition limitations. They are also called limiting factors because they cause a population of plants or animals to stop growing in a particular area.
Pertinent examples of density independent factors include volcanoes, earthquakes, floods, droughts, monsoons, wildfires and hurricanes. A volcano is a density independent factor in that the cataclysm destroys trees, which are a potential food source. Lava and soot can kill certain species and lower the population. Volcanic discharge can pollute local drinking water, forcing animals to find clean sources of water elsewhere.
Volcanic eruptions occur regardless of how many specimens live in close proximity to a volcano because geologic forces, not ecological ones, cause eruptions. This is a limiting factor because plants and animals die from this type of natural disaster if populations do not move far enough away from the mountain.
Density dependent factors are different in that the size of populations affect how certain factors increase or decrease numbers of specimens. Predation, disease and competition for resources affect populations based on their size. For example, certain diseases spread faster if large numbers of animals live closer together. These diseases can wipe out many specimens in a short period of time, but such an event may not have occurred so rapidly if the population was less dense.