Density-dependent factors regulate population size by becoming stronger as population density increases. They create negative feedback, reducing population growth and tending to stabilize the population around its carrying capacity.
This is core SL and HL content in ESS subtopic 2.1, Individuals, populations, communities and ecosystems.
As a population grows, more individuals occupy the same area and demand the same limited resources. Competition intensifies, so birth rates may decrease and death rates may increase. Population growth therefore slows as population size approaches the carrying capacity, , which is the maximum population size supported by the available resources.
If population size falls, competition and disease transmission usually decline. Survival and reproduction may then increase, allowing the population to recover. This self-regulating process is negative feedback because the system's response counteracts the original change.
| Density-dependent factor | Regulatory effect at high population density |
|---|---|
| Competition | Reduced access to food, water, territory or nesting sites lowers survival and reproductive success. |
| Predation | Concentrated prey may be easier for predators to locate, increasing mortality. |
| Disease and parasitism | Close contact increases transmission between individuals. |
| Territorial behaviour | Limited territories prevent some individuals from breeding. |
| Accumulation of waste | Waste products may increase stress or reduce habitat quality. |
A named example is the Soay sheep population on Hirta in St Kilda, Scotland. When sheep density is high, competition for limited winter vegetation increases; malnutrition and severe weather can then produce high mortality, reducing the population.
A common misconception is that density-dependent factors always keep populations exactly at . In reality, carrying capacity can change, time lags occur, and populations may fluctuate above and below it.
Exam technique: For an “explain” question, identify a factor, show how its effect strengthens with density, and link it to birth rate, death rate and negative feedback. A labelled population-growth curve approaching can strengthen the response.