How does population density affect the factors that limit growth?
Topic 8.4 Effect of Density of Populations: distinguish density-dependent from density-independent factors and explain how each limits population size.
A focused answer to AP Biology Topic 8.4, covering density-dependent factors (competition, predation, disease) and density-independent factors (weather, disasters), how each regulates populations, and K-selected versus r-selected strategies, with a worked example.
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What this topic is asking
The College Board (Topic 8.4) wants you to distinguish density-dependent from density-independent limiting factors, explain how each regulates population size, and connect this to carrying capacity and life-history strategies.
Density-dependent factors
Density-independent factors
Life-history strategies
The two strategies represent different trade-offs in how an organism invests limited energy in reproduction, and they connect to the population-growth models: r-selected species often show rapid, exponential-style growth when resources are abundant, while K-selected species track the logistic model, staying near their carrying capacity.
Try this
Q1. Give one example each of a density-dependent and a density-independent limiting factor. [2 points]
- Cue. Density-dependent: competition, predation or disease. Density-independent: drought, flood, fire or another weather event or disaster.
Q2. Explain how density-dependent factors keep a population near its carrying capacity. [2 points]
- Cue. As the population grows toward K, these factors intensify, raising deaths and lowering births, which slows growth and pushes the population back toward K, acting like negative feedback.
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 2020 (style)4 marksSection II (long FRQ excerpt). A deer population grows until a harsh winter and a disease outbreak both reduce it. (a) Classify each factor (the harsh winter and the disease) as density-dependent or density-independent, and justify each. (b) Explain how density-dependent factors help keep a population near its carrying capacity.Show worked answer →
A 4-point classify-and-explain FRQ on limiting factors.
(a) Classify and justify (2 points): (1 point) the harsh winter is density-independent, because it affects the same proportion of deer regardless of how dense the population is; (1 point) the disease is density-dependent, because it spreads more easily and kills a greater proportion when the population is denser.
(b) Explain (2 points): (1 point) as a population grows toward K, density-dependent factors (competition, disease, predation) intensify, raising death rates and lowering birth rates; (1 point) this slows growth and pushes the population back toward K, acting like negative feedback.
Markers reward correctly classifying both factors and explaining density-dependent regulation as feedback toward K.
AP 2017 (style)1 marksSection I (multiple choice). Which of the following is a density-dependent limiting factor? (A) A drought. (B) A flood. (C) Competition for food. (D) A volcanic eruption.Show worked answer →
The correct answer is (C).
Competition for food intensifies as a population becomes denser, so it is density-dependent. Droughts, floods and eruptions (A, B, D) are density-independent: they affect the population regardless of its density.
Related dot points
- Topic 8.3 Population Ecology: explain exponential and logistic growth, carrying capacity, and the factors that regulate population size.
A focused answer to AP Biology Topic 8.3, covering exponential and logistic growth, carrying capacity, growth rate calculations, and the factors that shape population size, with a worked growth-rate calculation.
- Topic 8.5 Community Ecology: explain the types of interactions between species in a community and their effects on the species involved.
A focused answer to AP Biology Topic 8.5, covering competition, predation, the niche, symbiosis (mutualism, commensalism, parasitism), keystone species and trophic relationships, with a worked interaction example.
- Topic 4.4 Feedback: explain how negative feedback maintains homeostasis and how positive feedback amplifies a response, using examples from cellular and organismal systems.
A focused answer to AP Biology Topic 4.4, covering negative feedback and homeostasis, positive feedback and amplification, set points, and how feedback data are analyzed, with a worked chi-square example.
- Topic 8.2 Energy Flow Through Ecosystems: explain how energy flows through trophic levels and why energy is lost between levels.
A focused answer to AP Biology Topic 8.2, covering trophic levels, food chains and webs, the 10 percent rule, energy pyramids, productivity, and why energy decreases up the chain, with a worked energy-transfer calculation.
- Topic 8.7 Disruptions to Ecosystems: explain how natural and human-caused disruptions affect ecosystems and how ecosystems respond.
A focused answer to AP Biology Topic 8.7, covering natural and human disturbances, invasive species, habitat loss, climate change, ecological succession, and how ecosystems respond and recover, with a worked example.
Sources & how we know this
- AP Biology Course and Exam Description — College Board (2020)