Every population in the logistic model is held near its carrying capacity by something. Ecologists sort those limits into two kinds, by one question: does crowding make it worse?
Key idea
Density-dependent factors hit harder as a population gets more crowded: competition for food, disease, predation, waste and stress. Density-independent factors hit no matter how crowded: floods, fires, hard frosts, droughts, hurricanes.
Why it works
In a crowded population, each animal has less food, meets sick neighbors more often, and is easier for predators to find. So the death rate per individual rises with density, which pulls the population back down: that is negative feedback, and it is what produces a carrying capacity. A hurricane, by contrast, kills about the same share of the trees whether the forest is thick or thin.
Worked example: Reading data
In years when a deer herd had 200 deer, 5% died over winter. In years with 600 deer, 20% died. Is the main cause of death density dependent?
- The death rate (the percent) rose as the herd grew.
- That is the mark of a density-dependent factor, such as food running short.
Answer: Yes, it looks density dependent.
Worked example: A second case
A frost kills about 30% of an insect population every time, whether the insects are few or many. Which kind of factor is the frost?
- The percent killed does not change with density.
Answer: Density independent.
Density-dependent factors work like a thermostat on population size: they push harder the farther the population rises. Density-independent factors are more like random shocks that knock a population down whatever its size. Real populations feel both, often in the same year.
Common mistake
People often call every limiting factor density dependent, because any limit "limits" the population. Ask a sharper question: does the effect on each individual grow with crowding? A hurricane does not care how dense the population is; a contagious disease does.