What are survivorship curves in AP Environmental Science?
A survivorship curve shows how survival rates of a cohort, a group of organisms born around the same time, change as they age from birth to the oldest age any member reaches. There are three types: Type I (most die when old), Type II (constant death rate at every age), and Type III (most die young). K-selected species usually follow Type I or Type II, while r-selected species follow Type III.
Why This Matters for the AP Environmental Science Exam
Survivorship curves connect directly to population dynamics, which is a graph-heavy part of AP Environmental Science. You need to read a curve, identify its type, and explain what it tells you about a species' survival pattern. This topic also links back to K-selected and r-selected species, so questions may ask you to match a reproductive strategy to a survivorship pattern. Building comfort reading these curves helps you analyze population data and explain trends, both skills the exam rewards.
Key Takeaways
- A survivorship curve tracks the relative survival of a cohort (same-age group) from birth to the maximum age reached.
- There are three curve types: Type I, Type II, and Type III.
- Type I: low mortality early and through middle age, high mortality in old age (think high parental care, few offspring).
- Type II: a roughly constant death rate at every age, so survival drops steadily.
- Type III: very high mortality early in life, with only a few surviving to old age (many offspring, little parental care).
- K-selected species usually show Type I or Type II; r-selected species usually show Type III.
Survivorship Curves Explained
A survivorship curve is a line that displays the relative survival rates of a cohort in a population, from birth to the maximum age any cohort member reaches. A cohort just means a group of individuals of the same age. By plotting how many survive at each stage of life, you can see the pattern of mortality for a species.
These curves connect to the reproductive strategies from K-selected and r-selected species. How many offspring a species produces and how much parental care it gives shapes what its survivorship curve looks like.
Type I
Most of the population survives through early and middle age and then dies off in old age. This pattern fits K-selected species: few offspring, high parental care, and individuals that tend to live long lives because of that investment.
Type II
Death happens at a roughly constant rate across all ages. Age does not strongly predict whether an individual lives or dies, so the curve drops at a steady pace. This pattern can also appear in K-selected species.
Type III
Most individuals die very early in life, and only a few survive to old age. This fits r-selected species: many offspring are produced, but little parental care means most do not survive their early stages.
Quick Comparison Table
| Survivability | Type I | Type II | Type III |
|---|---|---|---|
| Early Age | High | High | High |
| Middle Age | High | Medium | Low |
| Old Age | Low | Low | Low |
| Example | Humans | Song birds, bees | Frogs |
| K/r | K | K | r |
The example species in this table are illustrations of each pattern, not required AP content. Use them to remember the shapes, but focus your studying on what each curve type means.
How to Use This on the AP Environmental Science Exam
MCQ
Expect to be shown a survivorship curve and asked to identify the type or match it to a species type. A graph that stays high and then drops sharply at the end is Type I. A straight diagonal line (often on a log scale) is Type II. A line that drops fast at the start is Type III. Connect the shape back to K-selected or r-selected traits when the question asks.
Free Response
If a free-response prompt gives you population or survival data, be ready to describe the survival pattern and explain what it shows. Tie the pattern to reproductive strategy: high parental care and few offspring point toward Type I, while many offspring and low parental care point toward Type III. Explain trends in clear cause-and-effect language rather than just naming the curve.
Common Trap
Survivorship curves measure survival of a cohort over its lifespan, not population growth over time. Do not confuse them with population growth curves (like exponential or logistic curves). They answer different questions.
Common Misconceptions
- A survivorship curve does not show population size growing or shrinking over time. It shows how many of a starting cohort are still alive at each age.
- Type II does not mean half survive. It means the death rate stays about constant across ages, which creates a steady decline.
- Type I species are not the only ones with high early survival. All three types can start high; the difference is when the steep drop happens.
- K-selected species are not locked into one curve. They typically follow Type I or Type II, not just Type I.
- Producing many offspring (r-selected, Type III) does not mean a species is doing poorly. It is a strategy where survival of a few is enough to continue the population.
Related AP Environmental Science Guides
Vocabulary
The following words are mentioned explicitly in the College Board Course and Exam Description for this topic.Term | Definition |
|---|---|
cohort | A group of individuals of the same age in a population. |
K-selected species | Species that tend to be large, produce few offspring, invest significant energy in each offspring, mature slowly with extended parental care, have long lifespans, and reproduce multiple times in their lifetime, typically in stable environments with high resource competition. |
r-selected species | Species that tend to be small, produce many offspring, invest minimal energy in each offspring, mature early, have short lifespans, and may reproduce only once in their lifetime, typically in environments with low resource competition. |
survivorship curve | A line graph that displays the relative survival rates of a cohort from birth to the maximum age reached by any member of that cohort. |
Type I curve | A survivorship curve showing high survival rates throughout most of an organism's lifespan, with most deaths occurring at old age; typical of K-selected species. |
Type II curve | A survivorship curve showing a relatively constant death rate throughout an organism's lifespan; typical of some K-selected species. |
Type III curve | A survivorship curve showing high mortality rates early in life, with few individuals surviving to old age; typical of r-selected species. |
Frequently Asked Questions
What are survivorship curves in AP Environmental Science?
Survivorship curves show the relative survival of a cohort, or same-age group, from birth to the maximum age reached by any member of that group.
What is a Type I survivorship curve?
A Type I curve shows high survival through early and middle life, followed by a steep decline in old age. It is often associated with K-selected species and high parental care.
What is a Type II survivorship curve?
A Type II curve shows a roughly constant mortality rate across ages. Survival decreases at a steady pace instead of dropping mainly early or late.
What is a Type III survivorship curve?
A Type III curve shows very high mortality early in life, with relatively few individuals surviving to older ages. It is often associated with r-selected species and many offspring.
What is a common APES mistake with survivorship curves?
A common mistake is confusing survivorship curves with population growth curves. Survivorship curves track survival of a cohort by age, not total population size over time.