## Learning Outcomes

- Define a **survivorship curve** and explain what it shows about survival by age
- Distinguish clearly between **Type I**, **Type II**, and **Type III** survivorship curves
- Link curve shape to **offspring number**, **parental care**, and **life-history strategy**
- Identify likely curve types from graphs, species examples, or verbal descriptions
- Explain how **cohorts** and **life tables** are used to build survivorship curves
- Avoid common exam confusions, especially survivorship curves versus age structure diagrams

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand survivorship curve types, with a focus on the following syllabus points:

- Know that a survivorship curve shows the pattern of survival in a population as individuals age
- Identify the three classic survivorship curve types: **Type I**, **Type II**, and **Type III**
- Explain the difference between late-loss, constant-loss, and early-loss mortality patterns
- Relate curve type to reproductive strategy, including parental care and number of offspring
- Recognise conservative species examples, such as humans and elephants for Type I, many birds for Type II, and oysters or many fish for Type III
- Understand that survivorship curves are based on age-specific survival data, often organised in a **life table**
- Distinguish a survivorship curve from an **age structure diagram**
- Understand that the three curve types are idealised patterns, and real species may fall between them

## Test Your Knowledge

_Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision._

1. What does a survivorship curve show about a population?
2. How does a Type I curve differ from a Type III curve?
3. Which curve type shows roughly constant mortality across the lifespan?
4. Name one species likely to show a Type II curve and one likely to show a Type III curve.
5. Why can a species with very high juvenile mortality still persist successfully?

## Introduction

## Introduction

A survivorship curve is one of the most tested population graphs in AP Environmental Science because it connects several ideas at once: mortality, reproduction, parental care, and population strategy. If you can read the shape of the curve, you can often infer how a species lives and reproduces.

This topic also creates common mistakes. Students often confuse survivorship curves with age structure diagrams, or they memorise examples without understanding why the curve has that shape. The exam usually rewards the second skill more. You need to know not just which type a species shows, but how the timing of death across the lifespan explains that pattern.

> **Key Term: survivorship curve**
> A graph showing the pattern of survival in a population as individuals age, usually by showing the number or proportion of a cohort still alive at each age.

### What a survivorship curve actually measures

A survivorship curve follows a group of organisms through time and asks a simple question: what fraction is still alive at each age or life stage? The key idea is **survival over time**, not just how many individuals exist right now.

> **Key Term: cohort**
> A group of individuals in a population born or starting at the same time and followed through time.

This matters because the graph is about age-specific survival. For example, if 1,000 fish eggs are produced, and only 100 survive to juvenile stage, then only 10% of that original cohort remains. If 80 of those 100 juveniles survive to adulthood, then survivorship has improved strongly after the early-life bottleneck.

A survivorship curve usually has:

- **x-axis:** age or life stage
- **y-axis:** number surviving, percent surviving, or survivorship

Some textbooks use a logarithmic y-axis. You do not need to panic about that. The exam is still testing the same pattern: when in life most deaths occur.

> **Key Term: mortality rate**
> The rate or risk of death in a population or age group during a given period.

A useful distinction is this:

- **Mortality** asks: how likely is death at this stage?
- **Survivorship** asks: how many are still alive by this stage?

Those are related, but not identical. If mortality is high early in life, survivorship drops steeply early on. If mortality stays low until old age, survivorship stays high for most of the graph and then falls sharply late in life.

Survivorship curves are often built from a **life table**, which organises how many individuals survive each age class.

> **Key Term: life table**
> A table that summarises survival, and often mortality, by age or age class in a population.

A simple version might begin with 100 individuals:

- Age 0: 100 alive
- Age 1: 95 alive
- Age 2: 90 alive
- Age 3: 40 alive
- Age 4: 5 alive

That pattern would suggest high survival through early and middle life, then a strong late-life drop: a Type I pattern.

> **Test Tip:** If a question gives raw survival numbers rather than naming a curve type, sketch the trend mentally. Ask: do deaths happen mostly late, steadily throughout life, or mostly early?

### Type I, Type II, and Type III: the three classic patterns

The three survivorship curve types are best understood by the **timing of mortality**.

#### Type I: late loss

A **Type I survivorship curve** shows high survivorship through early and middle life, followed by a sharp decline in old age. Most individuals survive long enough to reach adulthood and often old age.

> **Key Term: Type I survivorship curve**
> A survivorship pattern with low mortality in early and middle life, followed by a steep increase in mortality in old age.

Why does this happen? Species with Type I curves usually produce relatively few offspring and invest heavily in each one. Parents protect, feed, and often teach offspring. Because so much energy goes into each young individual, the chance of early survival is high.

Typical features include:

- few offspring
- high parental care
- larger body size is common
- long lifespan
- low juvenile mortality

Conservative examples include:

- humans
- elephants
- some large mammals such as whales

The logic is important. A human infant is not produced in large numbers, and it receives intense parental investment. If human populations had a Type III pattern, adults would need to produce huge numbers of offspring to compensate for early deaths. That does not fit human biology.

Type I is often associated with **K-selected** species, though you should not use that label instead of the actual curve type unless the question asks for strategy. AP scoring often wants the direct answer: **Type I**.

#### Type II: constant loss

A **Type II survivorship curve** shows a roughly constant death rate across the lifespan. The chance of dying is fairly similar at different ages.

> **Key Term: Type II survivorship curve**
> A survivorship pattern in which mortality remains relatively constant across the lifespan, so survivorship declines at a steady rate.

This is the easiest type to misread. Students sometimes think Type II means "medium mortality." It does not. It means **fairly even mortality** across ages.

If a graph shows a straight, steady decline, that suggests that organisms are about equally vulnerable throughout life. Predation is often a major factor here. If predators can take juveniles and adults at similar rates, the graph will not have a sharp early or late drop.

Typical examples include:

- many birds
- some small mammals such as squirrels
- some reptiles
- some perennial plants

A songbird is a common example because birds often face consistent risks from predation, weather, accidents, and food limitation across multiple life stages.

Type II sits between the extremes of heavy parental investment and minimal parental investment. It is less tightly tied to one population strategy than Type I and Type III are.

#### Type III: early loss

A **Type III survivorship curve** shows very high mortality early in life, but much higher survival for those individuals that make it past the early stages.

> **Key Term: Type III survivorship curve**
> A survivorship pattern with very high mortality in early life, followed by much higher survivorship for individuals that survive the early bottleneck.

This pattern is common in species that produce very large numbers of offspring and invest little or no parental care in each one.

Typical features include:

- many offspring
- little parental care
- high juvenile mortality
- survivors may live much longer
- often associated with unstable or risky early-life conditions

Classic examples include:

- oysters
- many fish
- many marine invertebrates
- many insects
- many plants and trees
- sea turtles

The key idea students miss is this: Type III does **not** mean every individual has a short life. It means most die young. Those that survive the dangerous early phase may live for a substantial time.

Imagine an oyster releasing huge numbers of larvae. Most are eaten, drift away, or fail to settle successfully. But the few that survive early vulnerability can persist much longer. That is exactly what the curve shows: a steep early drop, then a flatter tail.

Type III is commonly associated with **r-selected** species. Again, on the exam, the safest label for the curve itself is **Type III**.

> **Test Tip:** For Type III species, ask why producing many offspring still works. The answer is that even if survival is low, the starting number is so large that enough individuals still reach maturity.

### How to compare the curve types and avoid common mistakes

A good way to compare the three types is to think about one question: **When do most deaths happen?**

- **Type I:** mostly late
- **Type II:** fairly steadily throughout life
- **Type III:** mostly early

That gives you the memory shortcut:

- late loss
- constant loss
- early loss

Now connect each to life-history logic.

#### Offspring number and parental care

Species face a trade-off. Energy can be invested in:

- producing many offspring, or
- protecting and supporting fewer offspring

If a species produces many offspring with little care, early deaths are expected. That pushes the curve toward Type III.

If a species produces fewer offspring with a lot of care, early survival rises. That pushes the curve toward Type I.

Type II often reflects a more even risk pattern rather than one strong bottleneck.

#### Survivorship curve versus age structure diagram

This is one of the most common AP mistakes.

> **Key Term: age structure diagram**
> A graph showing the distribution of individuals among age groups in a population at one point in time.

A survivorship curve tracks one cohort through time.

An age structure diagram shows how many individuals are in each age group right now.

So:

- survivorship curve = survival pattern by age
- age structure diagram = age distribution in a population

A wide-based age pyramid does **not** automatically mean Type III survivorship. It may simply mean there are many young individuals in that population right now.

#### Real species are not always perfect textbook types

The three curve types are ideal models. Real populations may be intermediate. A species might look mostly Type II but with a steeper decline at one life stage. Another might appear Type I under one set of environmental conditions and less clearly so under another.

That is why exam questions usually use familiar, conservative examples. Stick to the safest associations unless a graph or description clearly tells you otherwise.

#### Worked reasoning example

Suppose an AP question says:

- a species produces thousands of offspring
- there is little parental care
- most young are eaten by predators
- survivors can live for many years

What curve type?

Step by step:

1. Thousands of offspring suggests low investment per individual.
2. Little parental care suggests high vulnerability early.
3. Most young die early confirms strong juvenile mortality.
4. Survivors live much longer means mortality falls after the early bottleneck.

Therefore, the best answer is **Type III**.

Now try another:

- offspring number is low
- juveniles are protected by parents
- most individuals survive into old age
- mortality rises sharply near lifespan limit

That is **Type I**.

And if the prompt says:

- individuals face similar risk of predation at all ages
- survivorship declines steadily through life

That is **Type II**.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- A survivorship curve shows how survival changes with age in a population
- It usually tracks a **cohort**, not the whole age distribution of a population at one moment
- A **Type I survivorship curve** shows low early and middle-life mortality with a steep late-life decline
- Type I species usually produce fewer offspring and invest more parental care
- A **Type II survivorship curve** shows roughly constant mortality across the lifespan
- Type II appears as a steady decline and is common in many birds and some small mammals
- A **Type III survivorship curve** shows very high early mortality but better survival after the juvenile stage
- Type III species usually produce many offspring with little parental care
- Survivorship curves are often built from **life tables** using age-specific survival data
- Survivorship is different from mortality: survivorship is the proportion still alive, while mortality is the risk of death
- Survivorship curves are different from **age structure diagrams**
- The three curve types are idealised patterns, and real species may be intermediate

## Key Terms and Concepts

- survivorship curve
- cohort
- mortality rate
- life table
- Type I survivorship curve
- Type II survivorship curve
- Type III survivorship curve
- age structure diagram


Survivorship Curves - Survivorship curve types