Survivorship & Reproductive Strategies
- Interpret Type I, II, and III survivorship curves
- Contrast r-selected and K-selected species by their life-history traits
- Connect a species’ reproductive strategy to its survivorship pattern
Survivorship curves
A survivorship curve plots the fraction of a population still alive against age (on a log scale). Three shapes recur. Type I curves are flat then drop steeply at the end: most individuals survive to old age, then die — typical of large mammals, including humans. Type II curves are straight lines: a roughly constant death rate at every age — typical of many birds and rodents. Type III curves drop steeply at the start then flatten: most offspring die young, but the few survivors live long — typical of oysters, fish, and most plants.
Two reproductive strategies
These curves reflect two strategies along a spectrum. r-selected species maximize the growth rate r: they produce many small offspring, reproduce early, provide little or no parental care, and mature fast. Most offspring die (a Type III curve), but the sheer numbers keep the population going — think insects, weeds, bacteria, and many fish. K-selected species are tuned to life near the carrying capacity K: they produce few, large offspring, reproduce later, invest heavy parental care, and live long (a Type I curve) — think elephants, whales, and humans.
Strategy shapes vulnerability
The two strategies fail in different ways. r-selected populations recover quickly from crashes because they reproduce so fast, which is exactly why many become invasive species or pests. K-selected species, with slow reproduction and few offspring, recover slowly and are far more likely to become threatened or extinct when hunted or when their habitat is disturbed. Neither strategy is "better" — each suits a different environment, with r-strategists favored in unstable or new habitats and K-strategists favored in stable, crowded ones.
A sea turtle lays about 100 eggs at a time, provides no parental care, and most hatchlings are eaten within days — but the rare survivors can live 50 years. Classify its reproductive strategy and survivorship curve.
- 1.Count the offspring and care: many eggs, no parental care — these are hallmarks of an r-selected strategy.
- 2.Look at the mortality pattern: most offspring die very young, but survivors live a long time.
- 3.That mortality pattern — steep early death, then a flat tail — is a Type III survivorship curve.
- 4.So the sea turtle is primarily r-selected with a Type III curve, even though the survivors are long-lived.
A survivorship curve that starts high and flat, then drops sharply only in old age, is characteristic of which type of species?
Don’t assume a Type III species has short-lived survivors. The curve describes when death is most likely, not the maximum lifespan. Sea turtles and many trees are Type III — nearly all offspring die young — yet the rare survivors are among the longest-lived organisms on Earth.
Two species suffer the same population crash. The r-selected species recovers within a few years while the K-selected species barely recovers in decades. What best explains the difference?
Link the concepts in one chain for the exam: r-selected → many offspring, little care → Type III → fast recovery / often invasive. K-selected → few offspring, much care → Type I → slow recovery / often threatened. Being able to run this chain both directions answers most population-strategy questions.
Answer the 2 checkpoints as you read.
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