Why do humans, songbirds and oysters each leave a differently shaped line on a survivorship graph?
Topic 3.2 Survivorship Curves: interpret Type I, II and III survivorship curves and link each shape to a species' reproductive and life-history strategy.
A focused answer to APES Topic 3.2, covering Type I, II and III survivorship curves, how each is read on a log scale, the species each describes, and how curve shape links to r- and K-selected strategies, with a worked curve-reading question.
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What this topic is asking
The College Board (Topic 3.2) wants you to read and interpret survivorship curves: graphs of how many individuals of a starting cohort are still alive at each age. You must know the three types, the species each describes, and how the shape reflects a species' reproductive strategy.
Reading the graph
The three types
- Type I species invest heavily in a few offspring, so most survive to old age.
- Type II species face a roughly even risk of death throughout life, regardless of age.
- Type III species "play the numbers": most offspring die early, but the survivors can live a long time.
Real species do not always fit one type perfectly; some shift shape as conditions change, and many plants and invertebrates lie between Type II and Type III. The exam, however, expects you to match the three idealized shapes to the strategies and example organisms above.
Linking shape to strategy
Survivorship curves connect directly to r- and K-selected strategies (Topic 3.3). Type III species are typically r-selected: many small offspring, no care, fast reproduction. Type I species are typically K-selected: few large offspring, much care, slow reproduction. Type II species fall in between. Recognizing the curve therefore tells you a great deal about how a species reproduces, how its population grows, and how it responds to environmental change. This is why the AP exam often gives you a curve and asks you to deduce the life history, or describes a life history and asks for the curve.
Try this
Q1. Identify the survivorship curve type with a constant death rate at all ages. [1 point]
- Cue. Type II, shown as a straight line on a log scale.
Q2. Explain why a Type III species produces so many offspring. [2 points]
- Cue. Most offspring die young because there is little parental care, so producing huge numbers ensures that at least a few survive to reproduce.
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 2022 (style)4 marksSection II (FRQ). A biologist plots survivorship curves for three species. (a) Describe the shape of a Type I survivorship curve and name one species that fits it. (b) Describe the shape of a Type III survivorship curve and name one species that fits it. (c) Explain why Type III species typically produce very large numbers of offspring. (d) Identify which curve type shows a roughly constant death rate across all ages.Show worked answer →
A 4-point FRQ on survivorship curves.
(a) Describe (1 point): a Type I curve stays high through early and middle life then drops steeply in old age (high survival when young, most deaths late); humans and large mammals fit it.
(b) Describe (1 point): a Type III curve drops steeply early then flattens (very high death rate among the young, the few survivors live long); oysters, many fish and most insects fit it.
(c) Explain (1 point): Type III species give little or no parental care, so most offspring die young; producing huge numbers ensures that at least a few survive to reproduce.
(d) Identify (1 point): the Type II curve, which is a straight line on a log scale, showing a constant death rate at every age (for example many songbirds and rodents).
Markers reward correct shape descriptions, valid example species, the link between high mortality and high offspring number, and naming Type II for constant mortality.
AP 2018 (style)1 marksSection I (multiple choice). A species produces millions of eggs, provides no parental care, and most offspring die before reaching adulthood. Which survivorship curve best describes it? (A) Type I (B) Type II (C) Type III (D) None of these. Justify your choice.Show worked answer →
A 1-point MCQ on survivorship curves. The answer is (C).
Very high early mortality with a few long-lived survivors is the signature of a Type III curve, which drops steeply at the start. Type I (A) has low early mortality and late deaths (large mammals); Type II (B) has constant mortality at all ages (many birds). The trap is matching "produces many offspring" to the wrong curve; high offspring number with no care and high early death is Type III.
Related dot points
- Topic 3.1 Generalist and Specialist Species: distinguish generalist from specialist species and explain how a changing or stable environment favors each.
A focused answer to APES Topic 3.1, covering the difference between generalist and specialist species, the role of niche breadth, and how stable versus changing environments favor each strategy, with a worked species-comparison question.
- Topic 3.3 Carrying Capacity: define carrying capacity, explain overshoot and dieback, and interpret population oscillations around the carrying capacity.
A focused answer to APES Topic 3.3, covering the definition of carrying capacity, limiting factors, overshoot and dieback, oscillation around K, and the difference between density-dependent and density-independent factors, with a worked overshoot calculation.
- Topic 3.4 Population Growth and Resource Availability: compare exponential (J-curve) and logistic (S-curve) growth, link them to r- and K-selected species, and calculate growth rate and doubling time.
A focused answer to APES Topic 3.4, covering exponential and logistic growth, r- and K-selected species, the role of resource availability, and quantitative growth-rate and rule-of-70 doubling-time calculations, with worked math.
- Topic 3.5 Age Structure Diagrams: interpret age structure diagrams (population pyramids) to predict population growth, stability or decline.
A focused answer to APES Topic 3.5, covering how to read age structure diagrams, the three pyramid shapes, the pre-reproductive, reproductive and post-reproductive cohorts, and how shape predicts future growth, with a worked pyramid-reading question.
- Topic 2.6 Adaptations: explain how natural selection produces adaptations and how environmental change shifts which traits are favored over time.
A focused answer to APES Topic 2.6, covering adaptations, natural selection, the role of genetic variation, structural, physiological and behavioral adaptations, specialists and generalists, and how environmental change drives evolution, with a worked selection question.
Sources & how we know this
- AP Environmental Science Course and Exam Description — College Board (2020)