Population Ecology

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Life Tables & Survivorship Curves

A life table is an age-specific summary of the survival pattern of a population.

It is best made by following the fate of a cohort, a group of individuals of the same age.

A survivorship curve is a graphic way of representing the data in a life table.

Slide 9

Survivorship curves for squirrels shows relatively constant death rate

Age (years)

2

0

4

8

6

10

10

1

1,000

100

Number of survivors (log scale)

Males

Females

Slide 10

Survivorship curves can be classified into three general types:

Type I: low death rates during early and middle life, then an increase among older age groups

Type II: the death rate is constant over the organism’s life span

Type III: high death rates for the young, then a slower death rate for survivors

Slide 11

Survivorship Curves

1,000

100

10

1

0

50

100

II

III

Percentage of maximum life span

Number of survivors (log scale)

I

Slide 12

Reproductive Rates

For species with sexual reproduction, demographers often concentrate on females in a population.

A reproductive table, or fertility schedule, is an age-specific summary of the reproductive rates in a population. It describes reproductive patterns of a population.

Slide 13

Life history traits are products of natural selection

An organism’s life history comprises the traits that affect its schedule of reproduction and survival:

The age at which reproduction begins

How often the organism reproduces

How many offspring are produced during each reproductive cycle

Life history traits are evolutionary outcomes reflected in the development, physiology, and behavior of an organism.

Slide 14

Evolution and Life History Diversity

Life histories are very diverse.

Species that exhibit semelparity, or big-bang reproduction, reproduce once and die.

Species that exhibit iteroparity, or repeated reproduction, produce offspring repeatedly.

Highly variable or unpredictable environments likely favor big-bang reproduction, while dependable environments may favor repeated reproduction.

Slide 15

“Trade-offs” and Life Histories

Organisms have finite resources, which may lead to trade-offs between survival and reproduction.