3.3 Survivorship Curves

Three types of survivorship curves

Survivorship curves graph the proportion of a population surviving at each age. Type I (K-strategists like humans, elephants) shows high survival early and mid-life, with mortality concentrated in old age. Type II (birds, small mammals) shows a constant, steady death rate across all ages. Type III (r-strategists like insects, fish, plants) shows massive early die-off, with few survivors reaching adulthood.

Linking curves to life history strategy

Type I species usually produce few offspring with high parental investment and care, ensuring most survive early life. Type III species produce many offspring with little to no parental care, relying on sheer numbers since most die young. Type II species fall in between, with intermediate offspring number and care.

Why survivorship curves matter

These curves help ecologists predict population growth patterns, identify vulnerable life stages for conservation, and understand trade-offs between offspring quantity and quality. Environmental changes (disease, habitat loss, predation) can shift where mortality occurs on the curve, affecting a species' long-term survival.

Key terms

Survivorship curve

A graph showing the proportion of individuals in a population surviving at each age.

Type I survivorship curve

High survival early and mid-life; mortality concentrated in old age (e.g., humans, elephants).

Type II survivorship curve

Constant mortality rate throughout life (e.g., birds, small mammals).

Type III survivorship curve

High mortality early in life, few survive to adulthood (e.g., insects, fish, plants).

Offspring number vs. parental care trade-off

Type I species have few offspring with high care; Type III species have many offspring with little to no care.

Try an AP-style question

Humans in developed countries show which survivorship curve?

1. Type I — high survival until old age
2. Type III — high early mortality
3. Type II — constant mortality
4. Type IV — random