Which of the following groups of environmental factors are density-dependent?

Analyze the question You are asked which group contains only density-dependent factors, meaning factors whose effect on a population changes with the population's size or density.

Concept

• Density-dependent factors have a stronger effect when the population is crowded. Examples are competition for food, predation, disease and parasitism, and accumulation of toxic metabolites (waste products).
• Density-independent factors affect a population regardless of its size. These are mostly abiotic or climatic factors such as temperature, light, salinity, rainfall, floods, fire and pH.

Solution Check each option:

• A. Food, salinity, accumulation of metabolites, light. Salinity and light are density-independent, so A is wrong.
• B. Temperature, salinity, predation, disease. Temperature and salinity are density-independent, so B is wrong.
• C. Food, predation, disease, accumulation of metabolites. • Food: competition increases as density rises. • Predation: predators concentrate on dense prey populations. • Disease: it spreads faster in crowded populations. • Metabolites: wastes build up faster when many organisms are present.

All four are density-dependent, so C is correct.

• D. Temperature, food, disease, light. Temperature and light are density-independent, so D is wrong.

Answer: C

Exam tip Ask yourself, "Does crowding make this factor worse?" If yes, it is density-dependent. Biotic factors (food, predators, disease, wastes) are usually density-dependent. Physical or climatic factors (temperature, light, salinity) are usually density-independent. So if an option contains temperature, light or salinity, you can usually eliminate it quickly.