Population Ecology

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The logistic model of population growth produces a sigmoid (S-shaped) curve.

Slide 23

Logistic Growth model

2,000

1,500

1,000

500

0

0

5

10

15

Number of generations

Population size (N)

Exponential

growth

1.0N

=

dN

dt

1.0N

=

dN

dt

K = 1,500

Logistic growth

1,500 – N

1,500

Slide 24

The Logistic Model and Real Populations

The growth of laboratory populations of paramecia fits an S-shaped curve.

These organisms are grown in a constant environment lacking predators and competitors.

Some populations overshoot K before settling down to a relatively stable density.

Slide 25

The growth of laboratory populations fits an S-shaped curve which hovers around the Carrying Capacity of the area.

1,000

800

600

400

200

0

0

5

10

15

Time (days)

Number of Paramecium/mL

Number of Daphnia/50 mL

0

30

60

90

180

150

120

0

20

40

60

80

100

120

140

160

Time (days)

(b) A Daphnia population in the lab

(a) A Paramecium population in the lab

Slide 26

The Logistic Model and Life Histories

Life history traits favored by natural selection may vary with population density and environmental conditions.

K-selection = density-dependent selection, selects for life history traits that are sensitive to population density.

r-selection = or density-independent selection, selects for life history traits that maximize reproduction.

Slide 27

Many factors that regulate population growth are density dependent

There are two general questions about regulation of population growth:

What environmental factors stop a population from growing indefinitely?

Why do some populations show radical fluctuations in size over time, while others remain stable?

Slide 28

Population Change and Population Density

In density-independent populations, birth rate and death rate do not change with population density.

In density-dependent populations, birth rates fall and death rates rise with population density.

Slide 29

Density-Dependent Population Regulation

Density-dependent birth and death rates are an example of negative feedback that regulates population growth.