Survivorship Curves in APES- Analysis
What Survivorship Curves Actually Are
A survivorship curve shows how a population dies off over time. That's it. Plot the percentage of individuals surviving against age, and you get one of three basic shapes.
AP Environmental Science loves this concept because it connects directly to life history strategies—how organisms balance reproduction versus survival. You need to know the three types, what causes each pattern, and which species fit where.
The Three Types: Type I, II, and III
Type I: High Survival Until Old Age
These species keep most of their offspring alive, then die off rapidly at the end. The curve drops sharply at the end.
Humans, elephants, most large mammals follow this pattern. Parents invest heavily in few offspring. They protect them, feed them, teach them. The kids survive.
This works when parental care is high. If you've got one kid every few years and you stick around to raise it, survivorship stays flat until you're old.
Type II: Constant Death Rate
The line drops steadily, straight down. Same percentage dies at every age interval.
Many birds, some reptiles, rodents show this pattern. Parents provide some care but not indefinitely. Predation, disease, accidents hit equally across all age classes.
Think of a robin feeding its babies for two weeks, then kicking them out. After that, they're on their own. Same risk applies to juveniles and adults.
Type III: Most Death Happens Early
The curve crashes at the start, then flattens. Survivors who make it past the vulnerable juvenile stage tend to live long lives.
Most fish, invertebrates, plants, fungi follow this. They dump thousands of eggs or spores into the environment and hope a few survive. No parental investment. Just sheer numbers.
A single cod releases millions of eggs. Maybe 0.1% hatch. Of those, a few make it to adulthood. But once you're a grown cod, you've got decent odds.
Comparing the Three Curves
| Type | Shape | Who Uses It | Parental Investment | Number of Offspring |
|---|---|---|---|---|
| I | Flat, then drops steeply | Large mammals, humans, elephants | High | Low |
| II | Straight diagonal line | Birds, reptiles, rodents | Moderate | Moderate |
| III | Drops sharply, then flattens | Fish, invertebrates, plants | Low or none | High |
Why This Matters on the AP Exam
Survivorship curves show up in the population ecology section. You'll need to:
- Identify which type a given species follows based on its life history
- Connect the curve shape to reproductive strategy
- Use the concept in free-response questions about conservation or population management
The key insight: high parental investment = Type I. No parental investment = Type III. Type II sits in the middle.
Real Examples That Actually Stick
- Oak tree (Type III): Produces thousands of acorns. Most get eaten or rot. The few that germinate might get eaten by deer. But once an oak is 20 feet tall, it's not going anywhere.
- Robin (Type II): Lays 3-4 eggs, sits on them for two weeks, feeds the chicks for another two. After that, the kids are gone. Predation hits juveniles and adults roughly equally.
- Elephant (Type I): One calf every 4-5 years, gestates for 22 months, nurses for years, stays with the herd. Mothers actively protect calves from lions. Most elephants die of old age or poaching, not random predation.
How to Analyze a Survivorship Curve Question
When you see a curve on the exam:
- Check the steepness at the start. Does it drop fast or slow?
- Check the end behavior. Steep drop = old age deaths. Flat end = juveniles dying.
- Ask about parental care. Mammals with extended care = Type I. Broadcast spawners = Type III.
The r/K Selection Connection
Survivorship curves tie directly to r/K selection theory:
- r-strategists (high reproduction, low investment) show Type III curves. Think dandelions, oysters, most insects.
- K-strategists (low reproduction, high investment) show Type I curves. Think whales, elephants, primates.
- Type II falls between—species that don't fit neatly into r or K selection.
This matters for conservation. K-strategists recover slowly from population crashes because they produce few offspring. A Type I species hit by habitat loss takes decades to bounce back.
Quick Study Cheat Sheet
- If you see a species with few babies and long parental care → Type I
- If you see a species with moderate offspring and short parental care → Type II
- If you see a species with thousands of offspring and zero parental care → Type III
Draw the three curves from memory. Label the axes. Write one example species under each. Quiz yourself until this is automatic.
Survivorship curves aren't complicated. The exam just wants you to match life history patterns to curve shapes. Get that down, and the questions write themselves.