chapter 52 population ecology. copyright © 2005 pearson education, inc. publishing as benjamin...

Chapter 52Chapter 52

Population EcologyPopulation Ecology

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Population ecology• Study of populations in relation to environment

– Including environmental influences on

• Population density, distribution, demography

• Population growth models

• Regulation of populations

• Human population growth


Density and Dispersion

• Density - number of individuals within a boundary

Figure 52.2

Births and immigration add individuals to a population.

Births Immigration

PopuIationsize

Emigration

Deaths

Deaths and emigration remove individuals from a population.

Changed by:- births and deaths-immigration and emigration


Dispersion

• Pattern of spacing among individuals

• Influenced by environmental and social factors

Figure 52.17


Clumped dispersion

• Based on resource availability and behavior

Figure 52.3a

(a) Clumped. For many animals, such as these wolves, living in groups increases the effectiveness of hunting, spreads the work of protecting and caring for young, and helps exclude other individuals from their territory.


Uniform dispersion

• Social interactions, eg. territoriality, competition

Figure 52.3b

(b) Uniform. Birds nesting on small islands, such as these king penguins on South Georgia Island in the South Atlantic Ocean, often exhibit uniform spacing, maintained by aggressive interactions between neighbors.


Random dispersion

• Usually homogeneous habitat

– No attractions or repulsions between individuals

Figure 52.3c

(c) Random. Dandelions grow from windblown seeds that land at random and later germinate.


Demography

Describing a population by vital statistics:

- birth rate

- death rate

- sex ratio

- age structure


Demography

• Life table: age specific summary of survival patterns of a population


Survivorship Curves

• Graphic way of representing life table

– Belding’s ground squirrels - death rate ~constant

Figure 52.4

1000

100

10

1

Num

ber

of s

urvi

vors

(lo

g sc

ale)

0 2 4 6 8 10

Age (years)

Males

Females


Survivorship Curves

• Three general types:

– Type I, Type II, and Type III

Figure 52.5

I

II

III

50 10001

10

100

1,000

Percentage of maximum life span

Num

ber

of s

urvi

vors

(lo

g sc

ale)


Minutes

Months

1 year

4-5 years

5 years

5-11 years

many

years

Reproduction


Does human reproductive table follow this pattern?

Reproductive table


SEMELPARITY = breed only once (“big bang”)


ITEROPARITY = breed more than once

- frequency depends on:

environment – food, etc.

inherent biology

eg. elephants – 22 month gestation - long maternal care period


Why not do both?

• Organisms have finite resources

Figure 52.7

Researchers in the Netherlands studied the effects of parental caregiving in European kestrels over 5 years. The researchers transferred chicks among nests to produce reduced broods (three or four chicks), normal broods (five or six), and enlarged broods (seven or eight). They then measured the percentage of male and female parent birds that survived the following winter. (Both males and females provide care for chicks.)

EXPERIMENT

The lower survival rates of kestrels with larger broods indicate that caring for more offspring negatively affects survival of the parents.

CONCLUSION

100

80

60

40

20

0Reduced

brood sizeNormal brood

sizeEnlarged

brood size

Par

ents

sur

vivi

ng th

e fo

llow

ing

win

ter

(%)

MaleFemale

RESULTS


Fecundity = number of offspring per reproductive episode

Varies with species and environment

elephants and whales 1

squirrels 2-6

oak trees thousands of acorns

salmon thousands of eggs

many invertebrates hundreds of thousands of eggs


Plant parenting

• Many small seeds or fewer large seeds

(a) Most weedy plants, such as this dandelion, grow quickly and produce a large number of seeds.

(b) Some plants, such as this coconut palm, produce a moderate number of very large seeds.


Population growth models

• exponential growth requires an idealized environment

E. Coli growth curve


Exponential population growth

• J-shaped curve

• Resource limitations not a factor

Figure 52.9

0 5 10 150

500

1,000

1,500

2,000

Number of generations

Pop

ulat

ion

size

(N

)

dNdt

1.0N

dNdt

0.5N


J-shaped curve

• Is characteristic of some populations that are rebounding

Figure 52.10

1900 1920 1940 1960 1980

Year

0

2,000

4,000

6,000

8,000E

leph

ant p

opul

atio

n


“I think I may fairly make two postulata. First, that food is necessary to the existence of man. Secondly, that the passion between the sexes is necessary and will remain nearly in its present state. . . Population, when unchecked, increases in a geometrical ratio: Subsistence increases only in an arithmetical ratio. A slight acquaintance with numbers will show the immensity of the first power in comparison of the second.”

Thomas Malthus, 1798


Population

Food

Time

Ab

und

ance

or

De

nsity

Malthus (1798) was the first to recognize the potential for populations to grow beyond the resources that can support them.


Potential population growth

Resources

Time

Ab

und

ance

or

De

nsity

Actual population growth

Does it really happen?


Potential population growth

Resources

Time

Ab

und

ance

or

De

nsity

Darwin (Campbell, p. 435)

1. Species have potential to increase population size exponentially

2. Most do not; resources are limited.

3. More individuals are produced than the environment can support.

4. This leads to a struggle for existence, with only a fraction of offspring surviving each generation.

5. Individuals vary, variation is heritable, and survival and reproduction depend on hereditary constitution

6. Unequal ability to survive and reproduce leads to change in a population

Actual population growth

Population and evolutionary change


U.S. 295,995,954World 6,438,377,20214:44 GMT (EST+5) Apr 29, 2005

What about humans?


Logistic growth model

• Includes concept of carrying capacity (K)

– maximum population size the environment can support

• Exponential growth cannot be sustained

• This model limits growth by incorporating carrying capacity


Logistic model

• Produces a sigmoid (S-shaped) curve

Figure 52.12

dNdt

1.0N Exponential growth

Logistic growth

dNdt

1.0N1,500 N

1,500

K 1,500

0 5 10 150

500

1,000

1,500

2,000

Number of generations

Pop

ulat

ion

size

(N

)


Figure 52.13a

800

600

400

200

0

Time (days)0 5 10 15

(a) A Paramecium population in the lab. The growth of Paramecium aurelia in small cultures (black dots) closely approximates logistic growth (red curve) if the experimenter maintains a constant environment.

1,000

Nu

mb

er

of

Pa

ram

eci

um

/ml

Real Populations

• Laboratory populations of paramecia

– S-shaped curve


Real populations

• Some overshoot K then stabilize

Figure 52.13b

180

150

0

120

90

60

30

Time (days)

0 16014012080 100604020

Nu

mb

er

of

Da

ph

nia

/50

m

l

(b) A Daphnia population in the lab. The growth of a population of Daphnia in a small laboratory culture (black dots) does not correspond well to the logistic model (red curve). This population overshoots the carrying capacity of its artificial environment and then settles down to an approximately stable population size.


Some populations

– Fluctuate greatly around K. Why?

Figure 52.13c

0

80

60

40

20

1975 1980 1985 1990 1995 2000

Time (years)

Nu

mb

er

of

fem

ale

s

(c) A song sparrow population in its natural habitat. The population of female song sparrows nesting on Mandarte Island, British Columbia, is periodically reduced by severe winter weather, and population growth is not well described by the logistic model.


Life history traits

• K-selection, or density-dependent selection

– Selects for traits sensitive to population density

• r-selection, or density-independent selection

– Selects for traits that maximize reproduction


Populations regulated by:

• complex interaction of biotic and abiotic influences

• So –

– What environmental factors stop a population from growing?

– Why do some populations show radical fluctuations in size over time


Population Change

• 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


Population Regulation

• Density-dependent birth and death rates

– Are an example of negative feedback that regulates population growth

– Are affected by many different mechanisms


Competition for Resources

• Increasing population density

– Intensifies intraspecific competition

Figure 52.15a,b

100 100

100

0

1,000

10,000

Ave

rag

e n

um

be

r o

f se

ed

s p

er

rep

rod

uci

ng

ind

ivid

ua

l (lo

g s

cale

)

Ave

rag

e c

lutc

h s

ize

Seeds planted per m2 Density of females

0 7010 20 30 40 50 60 802.8

3.0

3.2

3.4

3.6

3.8

4.0

(a) Plantain. The number of seeds produced by plantain (Plantago major) decreases as density increases.

(b) Song sparrow. Clutch size in the song sparrow on Mandarte Island, British Columbia, decreases as density increases and food is in short supply.


Territoriality

• Cheetahs mark boundaries

Figure 52.16


Population density

• Also influenced by

– Disease

– Predation

– Toxic wastes

– Intrinsic factors eg. stress


Population Cycles

• Many populations

– Undergo regular boom-and-bust cycles

Figure 52.21 Year1850 1875 1900 1925

0

40

80

120

160

0

3

6

9

Lynx

pop

ulat

ion

siz

e (t

hous

and

s)

Har

e po

pula

tion

size

(t

hous

and

s)

Lynx

Snowshoe hare


Meadow Vole (Field Mouse)

Studies on Voles in Mission Valley, Western Montana (1999-2003)


Number of Voles on 29 Sites

0

2040

6080

100

1998 1999 2000 2001 2002 2003 2004

Year

Nu

mb

er



0

2040

6080

100

1998 1999 2000 2001 2002 2003 2004

Year

Nu

mb

er

N = 77

N = 44



0

200

400

600

1998 1999 2000 2001 2002 2003 2004

Year

Nu

mb

er

N = 533

N = 44



0

200

400

600

1998 1999 2000 2001 2002 2003 2004

Year

Nu

mb

er


Number of voles per site in 2001 (year of peak abundance)

0

10

20

30

40

50

60

Sites

Nu

mb

er

of

vo

les

/sit

e

Biological organisms are not evenly distributed in nature: Abundance varies in Time AND Space


0

200

400

600

1998 1999 2000 2001 2002 2003 2004

Year

Nu

mb

er

What are interesting questions?


What causes changes in numbers of biological organisms over time and space?

What are the mechanisms of change?

What are the biological consequences of changes in numbers (so what?)?

Why do patterns and mechanisms differ among species and areas?

Nutrients in rodent biomass, reduced plant growth, or impacts of grazing on plants

Predator response (individual and population)


Vole predators in western Montana grasslands

•Grizzly bears

•Coyotes

•Foxes

•Weasels

•Hawks (various)

•Owls (various)

•Skunks

•Ravens

•Gulls

•Snakes (various)

Love to eat them mousies, mousies what I love to eat.

Bite they little heads off, nibble on they tiny

feet



0

200

400

600

1998 1999 2000 2001 2002 2003 2004

Year

Nu

mb

er

A Predator’s View of Vole Populations


Vole predators in western Montana grasslands

•Grizzly bears

•Coyotes

•Foxes

•Weasels

•Hawks (various)

•Owls (various)

•Skunks*

•Ravens*

•Gulls*

•Snakes* (various)

*Significant nest predators when other prey unavailable


Prey (voles) abundant

Nest predation low

Bird reproductive success high

Predator populations grow

Predator populations remain highNest predation highBird reproductive success low


Human population growth

• - has slowed after centuries of exponential increase

• No population can grow indefinitely

– And humans are no exception


Global Human Population

• The human population

– Increased relatively slowly until about 1650 and then began to grow exponentially

Figure 52.22

8000 B.C.

4000 B.C.

3000 B.C.

2000 B.C.

1000 B.C.

1000 A.D.

0

The Plague Hum

an

pop

ulat

ion

(bill

ions

)

2000 A.D.

0

1

2

3

4

5

6


Still growing but….

• The rate of growth began to slow approximately 40 years ago

Figure 52.231950 1975 2000 2025 2050

Year

2003

Per

cent

incr

ease

2.2

2

1.6

1.4

1.2

1

0.8

0.6

0.4

0.2

0

1.8


Age structure• represented in pyramids

Figure 52.25

Rapid growth Afghanistan

Slow growth United States

Decrease Italy

Male Female Male Female Male FemaleAge Age

8 6 4 2 0 2 4 6 8 8 6 4 2 0 2 4 6 8 8 6 4 2 0 2 4 6 8Percent of population Percent of population Percent of population

80–8485

75–7970–7465–6960–6455–5950–5445–4940–4435–3930–34

20–2425–29

10–145–90–4

15–19

80–8485

75–7970–7465–6960–6455–5950–5445–4940–4435–3930–34

20–2425–29

10–145–90–4

15–19


Ecological Footprint

• The ecological footprint concept

– Summarizes the aggregate land and water area needed to sustain the people of a nation

– Is one measure of how close we are to the carrying capacity of Earth


Ecological footprints• Countries vary greatly in footprint size and available

ecological capacity

Figure 52.27

16

14

12

10

8

6

4

2

00 2 4 6 8 10 12 14 16

New Zealand

AustraliaCanada

Sweden

WorldChina

India

Available ecological capacity (ha per person)

SpainUK

Japan

GermanyNetherlands

Norway

USA

Eco

log

ica

l foo

tprin

t (h

a pe

r pe

rson

)

chapter 52 population ecology. copyright © 2005 pearson education, inc. publishing as benjamin...

Documents