What is the primary difference between Type I and Type III survivorship curves regarding juvenile mortality?

Type I curves have very low juvenile mortality due to high parental investment, whereas Type III curves have extremely high juvenile mortality because they produce many offspring with little to no care.

How does the reproductive strategy of a K-selected species relate to its survivorship curve?

K-selected species typically follow a Type I (or sometimes Type II) curve because they invest heavily in a small number of offspring, increasing the probability that each individual survives to adulthood.

When comparing Type II curves to Type I and III, what is unique about their mortality rate?

Type II curves represent a constant mortality rate, meaning an individual has the same statistical chance of dying at any age, unlike Type I (late death) or Type III (early death).

What is a common mistake when interpreting the y-axis of a survivorship curve graph?

Students often fail to notice if the y-axis is logarithmic. On a log scale, a straight line represents a constant percentage of death (Type II), which would look very different on a linear scale.

Why is it incorrect to assume all individuals in a Type III population die young?

While the mortality rate is high for juveniles, the individuals that do survive the initial 'bottleneck' often have very high survival rates and can live for a long time as adults.

What error occurs if you classify a species solely by its curve without considering environmental context?

Survivorship is not just biological; environmental changes like the removal of a predator can change a species' curve from Type III toward Type II or I.

Define the term 'cohort' in the context of population ecology.

A cohort is a specific group of individuals within a population that were all born during the same time interval and are tracked throughout their lives for data collection.

What does the x-axis typically represent in a survivorship curve?

The x-axis represents age, often expressed as a percentage of the maximum lifespan or in relative time units from birth to death.

What is 'fecundity' and how does it relate to Type III curves?

Fecundity is the physical ability to reproduce and the number of offspring produced. Type III species have high fecundity to compensate for high early-life mortality.

Why do large mammals usually exhibit a Type I survivorship curve?

Large mammals tend to have long gestation periods, few offspring, and significant energy investment in nursing and protection, which minimizes early-life risks.

Survivorship Curves | College Board AP Environmental Science

Revision Notes

College Board

Environmental Science

Unit 3: Populations

Survivorship Curves

Summary

Survivorship curves are graphical representations that track the survival patterns of a population cohort over time. By plotting the number of individuals surviving at each age, ecologists can categorize species into three distinct life history strategies based on their mortality rates and reproductive investments.

1. Definition & Core Concepts

A survivorship curve is a graph showing the number or proportion of individuals surviving to each age for a given species or group. It provides a visual summary of the mortality risks faced by individuals at different stages of their life cycle.

The data is typically based on a cohort, which is a group of individuals born at the same time and followed from birth until the death of the last member. This allows researchers to calculate age-specific survival rates without the interference of fluctuating birth rates.

The y-axis of these graphs is often represented on a logarithmic scale to allow for the comparison of populations of different sizes and to highlight the rate of change in survival rather than just absolute numbers.

A graph showing three survivorship curves: Type I (convex), Type II (linear), and Type III (concave) on a log-linear plot.

2. Type I: High Late-Life Mortality

Type I curves are characterized by high survival rates during early and middle life, followed by a rapid decline in survival as the cohort approaches its maximum biological lifespan. This results in a convex curve shape.

Species exhibiting this pattern typically produce few offspring but invest heavily in parental care, which significantly reduces juvenile mortality. This strategy ensures that most individuals reach reproductive maturity.

Common examples include large mammals such as humans and elephants, which exist in stable environments where competition for resources is more significant than environmental volatility.

3. Type II: Constant Mortality

Type II curves represent a middle ground where the probability of dying remains relatively constant throughout the individual's entire life. This produces a straight, diagonal line on a semi-logarithmic plot.

In these populations, mortality is often driven by external factors like predation or disease that affect all age classes equally, rather than biological aging or lack of parental care.

Organisms such as songbirds, squirrels, and some lizards typically follow this pattern, as they face consistent environmental pressures from birth until death.

4. Type III: High Early Mortality

5. Key Distinctions

6. Exam Strategy & Tips

Survivorship Curves

Summary

1. Definition & Core Concepts

A graph showing three survivorship curves: Type I (convex), Type II (linear), and Type III (concave) on a log-linear plot.

2. Type I: High Late-Life Mortality

Common examples include large mammals such as humans and elephants, which exist in stable environments where competition for resources is more significant than environmental volatility.

3. Type II: Constant Mortality

In these populations, mortality is often driven by external factors like predation or disease that affect all age classes equally, rather than biological aging or lack of parental care.

Organisms such as songbirds, squirrels, and some lizards typically follow this pattern, as they face consistent environmental pressures from birth until death.

4. Type III: High Early Mortality

Type III curves describe species with extremely high mortality rates early in life, but significantly higher survival rates for the few individuals that manage to reach adulthood. This creates a concave curve shape.

These species typically produce a vast number of offspring but provide little to no parental care. The strategy relies on the sheer volume of offspring to ensure that at least a small percentage survives to reproduce.

This pattern is common among r-selected species such as many fish, marine invertebrates, and plants that release thousands of seeds or eggs into the environment.

5. Key Distinctions

The primary difference between the types lies in the timing of mortality and the level of energy invested in each individual offspring. Type I focuses on quality and survival of the few, while Type III focuses on quantity to overcome high loss.

Feature	Type I	Type II	Type III
Curve Shape	Convex	Linear	Concave
Mortality Timing	Late in life	Constant	Early in life
Offspring Number	Few	Moderate	Many
Parental Care	High	Moderate	Low/None

6. Exam Strategy & Tips

When analyzing a graph, always check the y-axis scale. If the axis is logarithmic, a Type II curve will appear as a straight line; if the axis is linear, the same data would appear as an exponential decay curve.

Be prepared to link survivorship curves to K-selection and r-selection strategies. Type I is almost always associated with K-selection (stability and care), while Type III is associated with r-selection (rapid growth and high fecundity).

Avoid the misconception that a species must fit perfectly into one category. Many real-world species show 'stair-step' patterns or characteristics of two different types depending on environmental conditions or life stages.