The second group of limiting factors consists of density-independent limiting factors that affect per capita growth rate independent of how dense the population is.
Image of a forest fire with elk standing in a river for safety.
Image of a forest fire with elk standing in a river for safety.
Image credit: Elk bath by John McColgan, USDA, public domain
As an example, let's consider a wildfire that breaks out in a forest where deer live. The fire will kill any unlucky deer that are present, regardless of population size. An individual deer's chance of dying doesn't depend at all on how many other deer are around. Density-independent limiting factors often take the form of natural disasters, severe weather, and pollution.
Unlike density-dependent limiting factors, density-independent limiting factors alone can’t keep a population at constant levels. That’s because their strength doesn’t depend on the size of the population, so they don’t make a "correction" when the population size gets too large. Instead, they may lead to erratic, abrupt shifts in population size. Small populations may be at risk of getting wiped out by sporadic, density-independent events
