01 / 21

Survivorship
Curves

Understand how population survival patterns vary across species, and learn to interpret the three classic curve types using logarithmic scales.

AP Environmental Science

What Is a Survivorship Curve?

A survivorship curve is a graph that shows the proportion of individuals in a population that survive to each age. The x-axis represents age (or percentage of maximum lifespan), while the y-axis shows the number of survivors, typically on a logarithmic scale.

Why Do They Matter?

Conservation
Identify age classes most vulnerable to decline and focus protection efforts.
Life History
Reveal whether a species invests in few offspring with high care or many offspring with low care.
Population Models
Serve as inputs for population projections and sustainable harvest calculations.

Key Vocabulary

1
Cohort: A group of individuals born at the same time whose survival is tracked.
2
Mortality rate: The proportion of a cohort that dies during a given age interval.
3
Life table: A data table recording survival and mortality at each age; the raw data behind every survivorship curve.
4
Logarithmic scale: A y-axis where each tick represents a tenfold change (e.g., 1000 → 100 → 10 → 1).

The Three Classic Types

1 10 100 1000 Age (% of maximum lifespan) Number surviving (log) 0 25 50 75 100 Type I Type II Type III

Type I: Late Loss

Type I organisms experience low mortality early in life and high mortality in old age. Most individuals survive to adulthood. These species typically produce few offspring and invest heavily in parental care.

Curve Shape
Convex: stays high, then drops steeply at the end of the lifespan.
Examples
Humans, elephants, whales, and other large mammals.
Strategy
K-selected: few offspring, long gestation, extensive parental care.

Type II: Constant Loss

Type II organisms experience a roughly constant mortality rate throughout their lives. The probability of dying is the same whether the individual is young or old.

Curve Shape
Straight diagonal line on a log-scale graph.
Examples
Many songbirds, squirrels, some lizards, and coral.
Strategy
Intermediate: moderate offspring number with some parental care.

Type III: Early Loss

Type III organisms experience extremely high mortality early in life. Only a tiny fraction survive to adulthood, but those that do may live a long time. These species produce vast numbers of offspring with little or no parental care.

Curve Shape
Concave: drops sharply early, then levels off for survivors.
Examples
Sea turtles, oak trees, oysters, many fish, and insects.
Strategy
r-selected: many offspring, little investment per individual.

r-Selection vs. K-Selection

r-selected K-selected
Offspring number Many Few
Parental care Little to none Extensive
Body size Typically small Typically large
Survivorship curve Type III Type I
Maturation Early / rapid Late / slow
Environment Unstable / variable Stable / predictable

Reading the Log Scale

Survivorship curves plot \(\log_{10}(\text{number surviving})\) on the y-axis. Each gridline represents a tenfold change. This transformation is critical because it makes constant mortality appear as a straight line.

1
Straight diagonal = equal proportion dying at every age (Type II).
2
Bulging outward (convex) = mortality increases with age (Type I).
3
Caving inward (concave) = mortality decreases with age after heavy early losses (Type III).

How to Identify a Curve

1
Check the y-axis: Confirm it uses a logarithmic scale (powers of 10).
2
Look at early life: Does the curve drop sharply right away? If yes, suspect Type III.
3
Look at the middle: Is the decline roughly constant (straight line)? If yes, it is Type II.
4
Look at late life: Does the curve stay high and then drop steeply near the end? That is Type I.
5
Match to life history: Connect the shape to offspring number, parental care, and body size.

Example: African Elephant

1 10 100 1000 Age (years) 0 15 30 45 60 Type I

Example: American Robin

1 10 100 1000 Age (% of max lifespan) 0 50 100 Type II

Example: Sea Turtles

1 10 100 1000 Age (% of max lifespan) 0 50 100 Type III

Common Mistakes

✕ Ignoring the Log Scale
Reading the y-axis as linear makes Type II look convex (curving steeply at first), not straight. Always check the axis labels first.
✕ Confusing Shape Names
Convex = bulging outward (Type I). Concave = curving inward (Type III). Mixing these up reverses the identification.
✕ Rigid Classification
Many species don't fit neatly into one type. Humans in developing countries may show a mix of Type I and Type III early mortality.

Summary Comparison

Type I Type II Type III
Shape (log scale) Convex Straight diagonal Concave
When most die Old age All ages equally Early life
Offspring number Few Moderate Many
Parental care High Moderate Low / None
Selection strategy K-selected Intermediate r-selected
Example Elephants Robins Sea turtles

Question 1 of 5

On a survivorship curve with a logarithmic y-axis, a species with a constant mortality rate at all ages would appear as:

Question 2 of 5

An oak tree produces thousands of acorns each year, but very few seedlings survive to maturity. Which survivorship curve type best describes this species?

Question 3 of 5

Which of the following is the primary reason survivorship curves use a logarithmic y-axis instead of a linear one?

Question 4 of 5

A population ecologist studies a species and finds that 90% of individuals survive to 75% of the maximum lifespan, but then mortality increases rapidly. This pattern is most consistent with:

Question 5 of 5

A conservation biologist wants to protect a species with a Type III survivorship curve. Which management strategy would likely be most effective?