Survivorship curves summarise when individuals die across a typical lifespan.

Idealized survivorship curves (Types I, II, and III) plotted as proportion surviving versus age, typically shown with a logarithmic y-axis. The contrasting shapes highlight whether mortality is concentrated late in life (Type I), roughly constant across ages (Type II), or strongly concentrated in early life (Type III). Source

Connecting curve type to r-selected and K-selected strategies helps predict which life stages experience the highest mortality and why different species invest in survival versus rapid reproduction.

Core link between survivorship and r/K strategies

Diagram showing the three classic survivorship-curve types (I, II, III) as survival versus age on a logarithmic scale. This figure emphasizes how curve shape encodes the timing of mortality risk across the life course, which is the key conceptual link to life-history trade-offs. Source

Survivorship patterns reflect trade-offs in life-history strategy: energy invested in growth, maintenance, defence, and reproduction. In AP Environmental Science, the key connection is:

• K-selected species tend to show Type I or Type II survivorship.

• r-selected species tend to show Type III survivorship.

This relationship is not about “better” survival overall; it identifies when mortality is most likely and how that aligns with reproductive investment and parental care.

K-selected species: why Type I and Type II are typical

Type I survivorship (common for many K-selected species)

In a Type I curve, most individuals survive through early and middle life, and mortality rises sharply in old age.

K-selected traits that support Type I patterns include:

• High parental care (feeding, protection, teaching), lowering juvenile mortality

• Large body size and stronger immune function, reducing death from predation and disease

• Stable access to resources and established territories, supporting consistent survival

• Slower development and longer time to maturity, made possible by high early survival

Ecological interpretation:

• If a species produces few offspring, each offspring is “high value,” so selection favours strategies that keep juveniles alive (shelter, guarding, provisioning).

• Mortality becomes concentrated later because senescence, cumulative disease risk, and competition effects increase with age.

Type II survivorship (also consistent with K-selection)

In a Type II curve, individuals have an approximately constant chance of dying at each age (a roughly straight decline).

K-selected species can fit Type II when:

• Predation, accidents, or disease risk remains fairly steady across life stages

• Parental care improves early survival, but hazards still affect juveniles and adults similarly

• Lifespan is moderate to long, with mortality spread more evenly than Type I

Ecological interpretation:

• Type II does not imply low mortality; it implies mortality is not strongly concentrated in either early or late life.

• Many K-selected organisms can still show Type II if their environment imposes consistent risks that do not disproportionately target juveniles.

r-selected species: why Type III is typical

Type III survivorship (typical for r-selected species)

In a Type III curve, mortality is extremely high early in life; those that survive the juvenile stage may live much longer afterward.

r-selected traits that generate Type III patterns include:

• Very high fecundity (many seeds, eggs, or larvae)

• Little to no parental care, so offspring survival depends on chance conditions

• Small offspring size and limited energy reserves, increasing vulnerability

• Early dispersal (e.g., planktonic larvae, wind-blown seeds), placing young into risky environments

Ecological interpretation:

• When offspring are numerous and inexpensive to produce, selection can favour “quantity over quality.”

• High juvenile mortality is “expected,” and population persistence relies on the small fraction of offspring that survive to reproduce.

• Survivors may experience lower mortality later because they have reached a less vulnerable size, secured habitat, or developed defences.

Using survivorship type to infer strategy (and vice versa)

The specification link is a practical inference tool:

• Seeing a Type I pattern suggests traits aligned with K-selection: fewer offspring, high investment, strong early survival.

• Seeing a Type II pattern can still indicate K-selection, especially when competition and stable habitats favour survival and repeated reproduction, but risks remain steady across ages.

• Seeing a Type III pattern strongly suggests r-selection: many offspring, low investment, high early mortality.

Key biological logic tying both frameworks together:

• Parental investment shifts mortality away from early life (supporting Type I/II and K-selection).

• Low investment and high fecundity concentrate mortality in early life (supporting Type III and r-selection).

• Survivorship curves describe mortality timing; r/K strategies describe reproductive investment and competitive context—the linkage works because both are driven by the same energy allocation trade-offs.