AP Syllabus focus:
‘Survivorship curves can be Type I, Type II, or Type III, representing different patterns of survival across an organism’s lifespan.’
Survivorship patterns describe how the chance of surviving changes as organisms age. AP Environmental Science emphasises three classic curve types—Type I, II, and III—which reflect contrasting life-history risks and mortality timing.
Understanding survivorship patterns
Core idea: survival across the lifespan
A survivorship pattern is typically shown as a curve with age on the x-axis and proportion of the original cohort still alive on the y-axis.
Idealized Type I, Type II, and Type III survivorship curves plotted against age (with survivorship on a logarithmic scale). The figure makes the key contrast visually obvious: late-life mortality (Type I), age-independent mortality (Type II), and heavy early-life mortality followed by leveling (Type III). Source
The key difference among the three types is when most mortality occurs.
A compact reference plot showing the three general classes of survivorship curves on a logarithmic survivorship axis. This kind of idealized diagram is useful for quick identification: convex Type I, straight-line Type II, and concave Type III. Source
Why patterns differ
Survivorship patterns vary because species experience different:
Parental care levels and protection of young
Predation pressure at different life stages
Environmental stress (e.g., harsh seasons) that impacts certain ages more than others
Developmental vulnerability, such as fragile early life stages or risky juvenile dispersal
Type I survivorship (late-loss pattern)
What it looks like
Type I curves show high survival through early and middle life, followed by a steep decline in survival at older ages. Most individuals live to near their maximum lifespan, and mortality is concentrated late.
What it implies ecologically
Low juvenile mortality because young are often protected and well-provisioned
Mortality increases with age-related decline (senescence), disease accumulation, or reduced competitive ability
Population persistence depends strongly on adult survival because many individuals reach reproductive age
Common examples
Large mammals (e.g., humans, elephants) often approximate Type I in relatively stable, resource-supported conditions
Type II survivorship (constant-loss pattern)
What it looks like
Type II curves show an approximately constant probability of death across the lifespan. The curve declines at a fairly steady rate, meaning individuals are about as likely to die at one age as another.
What it implies ecologically
Risks are age-independent or only weakly age-dependent
Mortality may be driven by persistent hazards that affect all ages similarly, such as:
Ongoing predation
Accidents and exposure
Chronic food limitation or weather stress that does not target a particular life stage
Common examples
Many birds (e.g., songbirds) and some small mammals show Type II-like patterns in the wild, where risks remain relatively consistent year to year
Type III survivorship (early-loss pattern)
What it looks like
Type III curves show very high mortality early in life, followed by much higher survival among the individuals that make it past the vulnerable early stage. The curve drops sharply near the start, then levels off.
What it implies ecologically
Many offspring are produced, but most die young due to:
Predation on eggs/larvae/seedlings
High sensitivity to environmental conditions (desiccation, temperature swings, salinity changes)
Limited early-life defenses (small size, immobility, weak protective structures)
Survivors often have traits that increase later survival (size refuge, tougher tissues, better mobility), so mortality rates decrease after early selection filters out the most vulnerable individuals
Common examples
Many fishes, marine invertebrates (e.g., oysters), insects, and plants (many seedlings) commonly show Type III patterns
Comparing Type I, II, and III efficiently
Key contrasts to remember
Timing of mortality
Type I: mostly late-life deaths
Type II: deaths spread evenly across ages
Type III: mostly early-life deaths
Most vulnerable life stage
Type I: older individuals
Type II: no single dominant vulnerable stage
Type III: infants/juveniles/propagules (eggs, larvae, seeds, seedlings)
What “flattens” or “steepens” curves
More protection and care early tends to shift patterns toward Type I
Strong early predation or harsh early conditions tends to produce Type III
Persistent, age-neutral risks tend to yield Type II
Interpreting what a curve suggests (without overreaching)
Survivorship curves describe patterns, not exact causes. A Type III pattern strongly suggests intense early-life hazards, but identifying the specific driver (predation vs. climate vs. disease) requires additional evidence from field studies or demographic data.
FAQ
Captive populations often show higher early survival due to food, shelter, and veterinary care.
This can make curves appear more Type I than they would in the wild.
Real populations can have mixed mortality pressures.
For example, moderate juvenile mortality plus increasing old-age mortality can produce an “in-between” shape.
They may use cohort marking, repeated censuses, or life-stage proxies (e.g., size classes).
For tiny organisms, sampling designs and detectability strongly influence estimates.
Yes. Changes in predator communities, microclimate, or disturbance can shift which life stage is riskiest.
The same species may look more Type III in harsher sites and closer to Type II/I in protected sites.
Assuming “old plants rarely die.” Adult plant mortality can be substantial but less visible.
Type III mainly indicates that seeds/seedlings are disproportionately likely to die compared with established individuals.
Practice Questions
State one key feature of a Type I survivorship curve and one key feature of a Type III survivorship curve. (2 marks)
Type I: high survival through early/mid life with steep decline at old age (1)
Type III: very high mortality early in life with improved survival of those that reach later ages (1)
A species shows a survivorship curve with a roughly constant slope from birth to old age. Identify the curve type and explain what this pattern suggests about mortality risk across life stages. Include one plausible ecological reason for this pattern and one example organism group. (5 marks)
Correctly identifies Type II (1)
Explains mortality risk is approximately constant/age-independent across the lifespan (2)
Gives one plausible ecological reason (e.g., persistent predation/exposure/accidents affecting all ages similarly) (1)
Provides an appropriate example group (e.g., many birds) (1)
