AP® Environmental Science review sheet from Aim for Five (aimforfive.com/enviro/units/3/3-3)
Unit 3 · Topic 3.3
3.3 Survivorship Curves
A survivorship curve shows how many members of a group born at the same time are still alive at each age. Type I species mostly survive to old age, Type II species die at a fairly steady rate, and Type III species lose most of their young early.
Key terms
- survivorship curve
- cohort
- Type I curve
- Type II curve
- Type III curve
What the graph shows
A survivorship curve tracks a cohort, which is a group of individuals born at the same time. It plots the number (or percent) still alive against age. Age is on the horizontal axis, usually as a percent of the maximum lifespan. Number surviving is on the vertical axis, usually on a logarithmic scale, where each step up is ten times the one below (1, 10, 100, 1,000).
Using a log scale matters. On it, a population that loses the same percentage each year plots as a straight line.
The three types
Type I species usually make few offspring and care for them, so most survive until old age. Type III species usually make huge numbers of offspring and give little care, so most die as eggs, seeds or larvae, but the few that reach adulthood may live a long time. Type II species face a similar risk of dying at every age, often from predators or accidents.
| Type | Shape (log scale) | When most deaths happen | Examples |
|---|---|---|---|
| Type I | Flat at first, then drops steeply near the end | Late in life | Humans, elephants, whales |
| Type II | Straight diagonal line | Steady rate at all ages | Many songbirds, squirrels, some lizards |
| Type III | Drops steeply at first, then levels off | Very early in life | Most fish, frogs, oysters, many plants |
Links to r and K selection
Survivorship curves line up with reproductive strategies. K-selected species tend toward Type I curves, and r-selected species tend toward Type III curves. Type II falls in between.
Survivorship can change with conditions. Human survivorship in many countries used to be closer to Type II, because many babies and children died of disease. Better nutrition, clean water, sanitation and vaccines shifted it strongly toward Type I. In countries with high infant mortality, the curve still drops more sharply at young ages.
Reading a curve
To identify a curve, look at where the steep part is. Steep at the end means Type I. Steep at the start means Type III. An even slope throughout means Type II. If the question gives a table instead of a graph, calculate the percent of the cohort lost in the first age interval: a huge early loss points to Type III, and very little early loss points to Type I.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Identifying a curve from data
Researchers tag 1,000 newly hatched turtles. Survivors: age 0: 1,000; age 1: 120; age 2: 60; age 3: 40; age 4: 25; age 5: 12. What percent of the cohort died in the first year, and which survivorship type does this suggest?
Show the solutionHide the solution
- Step 1: Deaths in year 1 = 1,000 − 120 = 880.
- Step 2: Percent died = 880 ÷ 1,000 × 100 = 88%.
- Step 3: Losing 88% in the first year, then much smaller losses later, is the pattern of a Type III curve.
Answer: 88% died in the first year, suggesting a Type III survivorship curve.
- Example 2
The straight-line trap
A survivorship curve graphed on a logarithmic y-axis is a straight line sloping downward from 1,000 at birth to 1 at maximum age. A student says this means the same number of individuals die each year. Is the student right? Which type is it?
Show the solutionHide the solution
- Step 1: On a log scale, equal vertical steps represent equal ratios, not equal numbers.
- Step 2: So a straight line means the same proportion dies each year. Example: if half die each year, the cohort goes 1,000 → 500 → 250 → 125, which plots as a straight line on a log scale even though the number dying shrinks each year.
- Step 3: A constant death rate at all ages is a Type II curve.
Answer: The student is wrong: a straight line on a log scale means a constant percentage dies each year. It's a Type II curve.
Common mistakes
- Mixing up Type I and Type III. Type I loses individuals late; Type III loses them early.
- Reading a straight line on a log scale as a constant number of deaths. It's a constant percentage.
- Assuming a species' curve can never change. Human survivorship shifted toward Type I as health care improved.
On the exam
- Expect to identify a curve type from a described graph or data table, then connect it to r- or K-selected traits.
- If asked to explain a curve, say when most deaths happen and why, such as little parental care or high predation on young.
Connected topics
Videos
Check yourself
4 questions on 3.3 Survivorship Curves. Pick an answer to see if you got it, and why.
| Age (% of maximum life span) | Species K survivors | Species L survivors | Species M survivors |
|---|---|---|---|
| 0 | 1,000 | 1,000 | 1,000 |
| 20 | 990 | 40 | 500 |
| 40 | 970 | 20 | 250 |
| 60 | 900 | 10 | 125 |
| 80 | 500 | 5 | 62 |
Hypothetical data for three cohorts of 1,000 individuals
Which statement best describes Species M?
Which organism is most likely to have a survivorship pattern like Species L?
Adult songbirds of a certain species have about the same chance of dying each year, no matter how old they are. If the number of survivors is plotted on a logarithmic scale against age, the curve would most likely be
Loggerhead sea turtles lay about 100 eggs per nest, and very few hatchlings survive their first years. Turtles that do reach a large size survive well, but they take about 20 years or more to start reproducing. Population models show that fishing nets kill many large juveniles and adults. Which strategy would most likely increase the population's growth the most?
0 of 4 answered