Reproductive Strategies

Essential Question

Why do some species have many offspring while others have only a few?

A single oyster can release millions of eggs, while an elephant may have one calf every few years. Both species keep their populations going. In this lesson, you’ll compare two reproductive strategies, use survivorship curves to see when individuals die, and explain how limited energy leads to trade-offs. By the end, you should be able to use data to explain how a species’ reproduction relates to its survival.

Two Reproductive Strategies

• Species differ in how many offspring they produce
• r-strategy species tend to have many small offspring and little parental care
• K-strategy species tend to have few large offspring and much parental care
• Most species fall between these two ends

Species differ a lot in how they reproduce. Species with an r-strategy (also called r-selected) tend to produce many small offspring, give little or no parental care, mature quickly, and live a short time. Species with a K-strategy (also called K-selected) tend to produce a few large offspring, give much more care to each one, mature slowly, and live a long time. These are the two ends of a range, not two separate groups. Many species fall somewhere in between, and a single species may show some traits from each end.

Survivorship Curves

• A survivorship curve shows how many individuals survive to each age
• Type I: most survive until old age, then many die
• Type II: individuals die at a steady pace across the life span
• Type III: most die when young, and a few live long

A survivorship curve shows how many individuals out of a starting group are still alive at each age. A Type I curve stays high for most of the life span and drops late. Many large mammals with much parental care show a pattern like this. A Type II curve falls steadily, so individuals die at a steady pace across the life span. A Type III curve drops quickly early in life and then levels off, because most offspring die when young. Many species that produce huge numbers of offspring, such as many fish and insects, show a pattern like this.

Trade-offs

• Energy is limited, so a parent cannot do everything
• More offspring means less energy for each one
• Fewer offspring means more energy for each one
• Both strategies can keep a population going

Every organism has a limited supply of energy. Energy spent making more offspring cannot also be spent on caring for each one, so there is a trade-off. A parent that makes thousands of tiny eggs has little energy left for each egg, and most of those offspring die. A parent that makes one large offspring can invest much more in it, and a larger fraction survives. Neither strategy is better in every environment. Each can keep a population going, and the number of offspring that survive to reproduce is what matters.

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Worked Examples

Try each problem on your own first, then open the solution to check your work.

Example 1: Eggs and Survivors

Species A lays 2,000 eggs and gives no parental care. Only 0.2% of the eggs survive to adulthood. Species B has 2 young and gives extensive parental care. About 85% of its young survive to adulthood. How many young survive to adulthood in each species? Which species has more offspring surviving?

Show the solution

1. Species A: 2,000 eggs × 0.002 = 4 survivors
2. Species B: 2 young × 0.85 = 1.7 survivors
3. Compare: Species A has 4 survivors and Species B has 1.7, so Species A has more survivors per breeding event.
4. Reasoning: Species A loses almost all of its offspring, but it starts with so many that some survive. Species B starts with few but protects them. Each pattern can produce survivors.
5. Answer: Species A: 4 survivors; Species B: 1.7 survivors.

Example 2: Reading Survivorship Curves

The graph shows survivorship for three species (sample data), starting with 1,000 individuals each. Find the number of each species still alive at 25% of its life span. Then match each species to a survivorship type: Type I, Type II, or Type III. Explain how you know.

Show the solution

1. At 25% of life span: X = 950, Y = 750, Z = 150.
2. Percent surviving at 25%: X = 95%, Y = 75%, Z = 15%.
3. Species X: Type I. Most individuals survive until late in life, and the curve drops steeply near the end.
4. Species Y: Type II. The number surviving falls by the same amount (250) in each 25% interval.
5. Species Z: Type III. It loses 850 of 1,000 individuals (85%) by 25% of the life span, so most die when young.
6. Answer: X = 950, Y = 750, Z = 150 alive at 25%; X is Type I, Y is Type II, Z is Type III.

Example 3: Predicting a Strategy

The table lists traits of two species. Use the r-strategy and K-strategy model to decide which trait pattern each species shows. Then predict which survivorship type you would expect for each, and explain your reasoning.

Trait	Species P	Species Q
Offspring per breeding	5,000	1 (every 3 years)
Parental care	None	Extensive
Time to mature	3 months	12 years
Life span	About 1 year	About 60 years

Show the solution

1. Species P: many offspring, no care, fast maturity, and a short life span. These traits tend toward the r-strategy end.
2. Species Q: one offspring at a time, extensive care, slow maturity, and a long life span. These traits tend toward the K-strategy end.
3. Survivorship: I would expect Species P to be Type III, because offspring with no care are likely to die young. I would expect Species Q to be Type I, because each offspring gets much care and most survive to old age.
4. Reasoning: the number of offspring and the amount of care show how a species spends its limited energy, which relates to when individuals tend to die.
5. Answer: P tends toward r-strategy (likely Type III); Q tends toward K-strategy (likely Type I).

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Practice

Compare reproductive strategies, calculate how many offspring survive, match survivorship curves to types, and explain trade-offs. Try it on your own first, then check your work with the answer key.

• Reproductive Strategies Practice Worksheet (PDF)
• Reproductive Strategies Practice Answer Key (PDF)

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Lesson Slides

• 2.5 Reproductive Strategies Slides (PDF)

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Big Ideas

• r-strategy species tend to have many small offspring and little care
• K-strategy species tend to have few large offspring and much care
• Survivorship curves show when individuals die: Type I (late), Type II (steady), Type III (early)
• Limited energy creates a trade-off between number of offspring and care for each
• Both strategies can keep a population going if enough offspring survive to reproduce

Back to the essential question: species differ in how many offspring they produce because energy is limited. Making many offspring leaves little energy for each one, and making few offspring allows much more care. Each strategy can work if enough offspring survive to reproduce. Next, you’ll see how living in groups affects survival.

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