AP® Environmental Science review sheet from Aim for Five (aimforfive.com/enviro/units/3/3-2)
Unit 3 · Topic 3.2
3.2 K-Selected r-Selected Species
r-selected species reproduce early and often, making many offspring with little care, so their populations boom and crash. K-selected species have few offspring, invest heavily in each and stay near carrying capacity, which makes them slow to recover and more likely to become endangered.
Key terms
- r-selected species
- K-selected species
- biotic potential
- parental care
- invasive species
Two reproductive strategies
The letters come from population growth equations: r stands for the maximum rate of growth, and K stands for carrying capacity. r-selected species are built to grow fast; K-selected species are built to compete well when a population is near its limit.
Biotic potential is the maximum rate at which a population could grow if it had unlimited resources. r-selected species have a very high biotic potential; K-selected species have a low one.
Each strategy fits a different kind of environment. In unpredictable places, where floods, fires or harsh winters regularly wipe out much of a population, making many offspring quickly pays off. In stable, crowded places where competition is intense, making a few well-cared-for offspring that can outcompete others pays off.
Comparing the strategies
Most species fall somewhere between the two extremes. For example, sea turtles live a long time like K-selected species but lay many eggs and give no care like r-selected species. Think of r and K as two ends of a spectrum.
| Trait | r-selected | K-selected |
|---|---|---|
| Number of offspring | Many per reproductive event | Few |
| Size of offspring | Small | Large |
| Parental care | Little or none | A lot |
| Age at first reproduction | Young | Older |
| Lifespan | Short | Long |
| Population pattern | Booms and crashes; often overshoots K | Stays near K |
| Times reproducing | Often just once | Many times over a long life |
| Competition for resources | Usually low | Usually high |
| Survivorship | Type III (most young die) | Type I or Type II |
| Examples | Insects, mice, many fish, frogs, weeds | Elephants, whales, great apes, humans, large trees |
Recovering from disturbances
After a disturbance, r-selected species recover fast. They reproduce young and often, so a few survivors can quickly rebuild a population. They're often the first to colonize disturbed areas.
K-selected species recover slowly. A female elephant has her first calf in her early teens and then one calf every four or five years. If a population is cut in half by poaching, it may take decades to rebuild. That low reproductive rate is why many endangered species are K-selected, including whales, great apes, rhinos and condors.
r-selection and invasive species
Many invasive species are r-selected generalists. They reproduce quickly, spread their offspring widely and can live on many resources. When introduced to a new area without their natural predators, they can multiply fast and outcompete native species. Zebra mussels, which release huge numbers of free-swimming larvae, and kudzu, a fast-growing vine, are good examples. Invasive species hurt K-selected natives the most, because slow-breeding natives can't replace their losses fast enough to keep up with a fast-breeding invader.
Because r-selected species can rebuild their numbers so fast, they're also hard to control. Spraying or trapping may kill most of a pest population, but the survivors can quickly reproduce, and the fast turnover of generations helps them evolve resistance.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Classifying a species
A species of fish reaches adulthood in one year, releases about 10,000 eggs each spawning season, and abandons the eggs after they're laid. Is it r-selected or K-selected? Predict its survivorship curve.
Show the solutionHide the solution
- Step 1: Look at the evidence: early maturity, a huge number of offspring and no parental care.
- Step 2: Each of these is an r-selected trait.
- Step 3: With no care, most eggs and young fish are eaten or die early, so survivorship is low at young ages: a Type III curve.
Answer: r-selected, with a Type III survivorship curve.
- Example 2
Which recovers faster?
A hurricane kills half of both a mouse population and a black bear population on a coastal plain. Predict which population returns to its original size first, and explain.
Show the solutionHide the solution
- Step 1: Mice are r-selected: they mature in weeks, have many litters per year and many young per litter.
- Step 2: Black bears are more K-selected: they mature in a few years and raise a few cubs every couple of years with lots of care.
- Step 3: Because mice can reproduce much faster, their population can rebuild within a season or two. The bears will take years.
Answer: The mouse population, because r-selected species mature early and produce many offspring quickly.
Common mistakes
- Mixing up the letters. r = rate (fast growth, many offspring); K = carrying capacity (few offspring, stays near K).
- Thinking r-selected species are less successful. They are very successful in unstable or disturbed environments.
- Forgetting that most species fall between the extremes. Use the specific traits in the question to decide.
On the exam
- Questions often give a list of traits and ask you to classify the species, then link it to survivorship curves or extinction risk.
- When explaining why a K-selected species is endangered, mention its low reproductive rate and slow population recovery.
Connected topics
Videos
Check yourself
4 questions on 3.2 K-Selected r-Selected Species. Pick an answer to see if you got it, and why.
Zebra mussels reached the Great Lakes in the 1980s. A female can release hundreds of thousands of eggs a year, the larvae drift freely, and the mussels mature within about a year. Within a decade they had spread through much of the region's waterways. These traits best match which kind of species?
| Trait | Species F | Species G |
|---|---|---|
| Offspring per reproductive event | About 2,000 | 1 |
| Age at first reproduction | 3 months | 12 years |
| Parental care | None | Several years |
| Typical life span | 1 year | 60 years |
Hypothetical data
A severe flood kills 90% of both populations. Which outcome is most likely?
Which survivorship curve would Species G most likely show?
Which set of traits would most likely help a newly introduced species become invasive?
0 of 4 answered