Unit 3 · Topic 3.3 Beta

Survivorship Curves

A survivorship curve plots the number of survivors from a group born at the same time against age.

Practice 2: Visual RepresentationsPractice 5: Data Analysis

Part 1 · Hook

Why this matters

Follow 1,000 newborn humans in a wealthy country, 1,000 songbirds that just hatched, and 1,000 oysters that just settled on a reef. Count how many are still alive at each age. The humans mostly make it to old age. The songbirds die at a steady pace all through life. Nearly all the oysters are gone within weeks. Those three patterns are the three survivorship curves.

Part 2 · Before you start

What this builds on

Part 3 · Prerequisite check

Quick check before you start

1. Which traits describe a K-selected species?

  1. Few offspring, much parental care, long life
  2. Many offspring, no care, short life
  3. Many offspring, much care, short life
  4. Few offspring, no care, short life
Show the answer

K-selected species invest a lot in a few offspring and live long.

  • Correct: Few offspring, much parental care, long life:
  • Many offspring, no care, short life:
  • Many offspring, much care, short life:
  • Few offspring, no care, short life:

2. What does biotic potential describe?

  1. The fastest a population could grow with unlimited resources
  2. How long individuals live
  3. The number of species in an area
  4. How evenly species are spread
Show the answer

Biotic potential is the fastest possible population growth.

  • Correct: The fastest a population could grow with unlimited resources:
  • How long individuals live:
  • The number of species in an area:
  • How evenly species are spread:

Part 4 · See it

See it first

Three survivorship curves on a log scale, starting from 1,000 newborns. Type I stays near 1,000 until late in life, then drops steeply (most die old). Type II falls as a straight line on the log scale, so the same fraction die at every age. Type III drops to about 40 by 10% of the life span, then levels off (most die young).
Type I, II and III survivorship curves, each starting with 1,000 newborns, with survivors on a log scale. LevlPrep original diagram.

Part 5 · Step by step

How it works, step by step

  1. Scientists follow a group of individuals born at the same time and count survivors at each agethey get a survivorship curve: survivors (often on a log scale) plotted against age
  2. Most deaths happen late in life, as with parental care and good protection when youngthe curve stays high and then drops steeply at old age: Type I
  3. Deaths happen at a steady rate at every agethe curve falls as a straight line on a log scale: Type II
  4. Most deaths happen very early, as with many unprotected offspringthe curve drops steeply at first, then levels off for the few survivors: Type III

Part 6 · Key ideas

Key ideas

  • A survivorship curve shows how many individuals of a group born at the same time are still alive at each age. Survivors are usually plotted on a log scale, starting from 1,000.
  • Type I: high survival through most of life, with deaths concentrated in old age. Humans in wealthy countries, elephants, many large mammals.
  • Type II: a steady chance of dying at every age, so the curve is a straight line on a log scale. Many songbirds, lizards, some rodents.
  • Type III: very many deaths among the young, then high survival for the few that make it. Oysters, most fish, many insects and plants.

Part 7 · Misconception

A common mistake

The wrong idea: Survivorship curve and r- or K-selection are the same idea, so a Type III curve means the species is r-selected.

What actually happens: They are related but different. A survivorship curve shows when individuals die; r- and K-selection describe how they reproduce. K-selected species usually have Type I or II curves and r-selected species Type II or III, but the curve alone does not tell you the strategy.

Part 8 · Check yourself

Check yourself

Exam-style questions. Anything you miss goes into your review queue.

Model or diagram

Three survivorship curves

Three survivorship curves on a log scale, starting from 1,000 newborns. Type I stays near 1,000 until late in life, then drops steeply (most die old). Type II falls as a straight line on the log scale, so the same fraction die at every age. Type III drops to about 40 by 10% of the life span, then levels off (most die young).
LevlPrep original diagram.

1. Which curve in the figure shows a population in which most individuals die young?

  1. Type III
  2. Type I
  3. Type II
  4. Types I and II equally
Show the answer

A steep early drop means most of the young die.

  • Correct: Type III: Right: it drops from 1,000 to about 40 by 10% of the life span.
  • Type I: Type I stays near 1,000 until late in life.
  • Type II: Type II falls at a steady rate, not mostly at the start.
  • Types I and II equally: Neither drops steeply at the start.

2. Why does the Type II curve appear as a straight line on this graph?

  1. The same number die in each interval, so the line is straight
  2. The same fraction die in each age interval
  3. Few of them die until near the end of life
  4. They are counted just once, at the end of life
Show the answer

On a log scale, a constant proportion lost each interval gives a straight line.

  • The same number die in each interval, so the line is straight: With the same number dying each interval, the line would curve on a log scale.
  • Correct: The same fraction die in each age interval: Right: a constant share dying per interval gives a straight line when survivors are on a log scale.
  • Few of them die until near the end of life: That would be a flat line followed by a drop, like Type I.
  • They are counted just once, at the end of life: The curve shows counts at every age.

3. Which animal is most likely to have a Type I survivorship curve?

  1. An oyster, which releases millions of eggs into the water
  2. A songbird, which is about as likely to die at any age
  3. An elephant, which guards its calf for years
  4. A sea star, whose larvae drift unprotected in the ocean
Show the answer

Parental care and protection of the young push deaths toward old age.

  • An oyster, which releases millions of eggs into the water: Most oyster larvae die very young: Type III.
  • A songbird, which is about as likely to die at any age: A steady risk at every age is Type II.
  • Correct: An elephant, which guards its calf for years: Right: protection of the young means most survive until old age.
  • A sea star, whose larvae drift unprotected in the ocean: Unprotected larvae mostly die young: Type III.

4. A wealthy country improves care for newborns and young children. How would this most likely change the shape of its human survivorship curve?

  1. It would drop more steeply in the first years of life
  2. It would become a straight line on a log scale
  3. It would not change, as the curve shows old age alone
  4. It would stay high longer, pushing deaths to older ages
Show the answer

Lowering deaths at young ages keeps the curve high longer.

  • It would drop more steeply in the first years of life: Better care lowers early deaths, so the early drop shrinks.
  • It would become a straight line on a log scale: A straight line means the same risk at every age, which better child care does not create.
  • It would not change, as the curve shows old age alone: The curve shows deaths at every age, including childhood.
  • Correct: It would stay high longer, pushing deaths to older ages: Right: fewer deaths early in life keep the curve high, a stronger Type I.

Data

Survivors in two populations

Number alive at each age out of 1,000 individuals born in the same year, for two species (invented data for practice)
Age (years)Species M aliveSpecies N alive
01,0001,000
150500
230250
322125
41862
51531

5. What pattern does species N follow from age 0 to age 5?

  1. The same number of them die each year
  2. About half the survivors die each year
  3. Nearly the whole group dies in year one
  4. Hardly any die after the first year
Show the answer

A constant fraction lost each year is a Type II pattern.

  • The same number of them die each year: The number dying falls each year (500, 250, 125 ...); the fraction stays the same.
  • Correct: About half the survivors die each year: Right: 1,000, 500, 250, 125, 62, 31: halving each year, a Type II pattern.
  • Nearly the whole group dies in year one: Half survive the first year.
  • Hardly any die after the first year: Survivors keep falling every year.

6. What percent of the species N individuals alive at age 2 are still alive at age 4?

Type a number, then its unit (%). Scale words (2.5 million) and powers of ten (2.5 × 10^6) are read.

Show the answer

Survivors at 4 over survivors at 2, times 100: 62250×100=24.8%\frac{62}{250} \times 100 = 24.8\%, about a quarter, as two halvings predict.

  • Answer: 24.8 %

Part 9 · Summary

Summary

A survivorship curve plots the number of survivors from a group born at the same time against age. Type I curves stay high and drop in old age, Type II curves fall steadily (a straight line on a log scale), and Type III curves drop steeply early in life. Survivorship describes when deaths happen; it is related to, but not the same as, r- and K-selection, which describe how a species reproduces.

Part 10 · Up next

What comes next

Part 11 · Connections

Connections

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