Population Ecology

Chapter 52

Study of populations in relation to environment

– Environmental influences on:• population density and distribution• age structure• variations in population size

Definition of Population:

• Group of individuals of a single species living in the same general area

Density and Dispersion

• Density– Is the number of individuals per unit area or

volume

• Dispersion– Is the pattern of spacing among individuals

within the boundaries of the population

Density: A Dynamic Perspective

• Determining the density of natural populations is possible, but difficult to accomplish

• In most cases it is impractical or impossible to count all individuals in a population

– How do wildlife biologists approximate populations?

Estimating Wildlife Population Size

Defined Populations

Undefined Populations

• Density is the result of a dynamic interaction of processes that add individuals to a population and

those that remove individuals from it

Figure 52.2

Births and immigration add individuals to a population.

Births Immigration

PopuIationsize

Emigration

Deaths

Deaths and emigration remove individuals from a population.

How do these factors Contribute to Population Size??

• Births• Deaths• Immigration• Emigration

Patterns of Dispersion• Environmental and social factors influence the

spacing of individuals in a population

Clumped Dispersion

– Individuals aggregate in patches– May be influenced by resource availability

and behavior

Uniform Dispersion– Individuals are evenly distributed– May be influenced by social interactions

such as territoriality

Random Dispersion

• Position of each individual is independent of other individuals

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

Life history traits are products of natural selection

• Life history traits are evolutionary outcomes– Reflected in the development, physiology, and

behavior of an organism

Semelparity: Big Bang

– Reproduce a single time and die

Figure 52.6

Iteroparity – Repeated Reproduction

– Produce offspring repeatedly over time

“Trade-offs” and Life Histories• Organisms have finite resources

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

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win

ter

(%)

MaleFemale– Which may lead

to trade-offs between survival and reproduction

RESULTS

– Kestrels:

• Produce a few eggs?

– Can invest more into each, ensuring greater survival

• Produce many eggs?

– Costly but if all survive, fitness is better

More is Better?

• Some plants produce a large number of small seeds– Ensuring that at least some of them will

grow and eventually reproduce

Figure 52.8a

(a) Most weedy plants, such as this dandelion, grow quickly and produce a large number of seeds, ensuring that at least somewill grow into plants and eventually produce seeds themselves.

Fewer is Better?• Other types of plants produce a moderate

number of large seeds

– That provide a large store of energy that will help seedlings become established

Figure 52.8b

(b) Some plants, such as this coconut palm, produce a moderate number of very large seeds. The large endosperm provides nutrients for the embryo, an adaptation that helps ensure the success of a relatively large fraction of offspring.

Demography

• Study of the vital statistics of a population– And how they change over time

• Death rates and birth rates

• Zero population growth – Occurs when the birth rate equals the death

rate

Exponential Population Growth

Population increase under idealized conditionsNo limits on growth

• Under these conditions– The rate of reproduction is at its maximum,

called the intrinsic rate of increase

Example-understanding growth

Question: I offer you a job for 1 cent/day and your pay will double every day. You will be hired for 30 days. Will you take my job offer?

Answer: If you said YES, you will have made $~21 million dollars for 30 days of work.

How is this possible?????

1ST DAY OF WORK: 1 cent pay/day

30TH DAY OF WORK: ~10.2 million/day

How is this possible?????

Am

ou

nt

of

Pay/D

ay

# of Days

Exponential Growth Model*Idealized population in an unlimited

environment

*Very rapid doubling time; steep J curve

*r=N=(b-d)N tr=instrinsic rate of growth

dNdt rmaxN

Exponential Growth in the Real World

• Characteristic of some populations that are rebounding

1900 1920 1940 1960 1980Year

0

2,000

4,000

6,000

8,000

Ele

phan

t po

pula

tion

–Cannot be sustained for long in any population

Logistic Population Growth

• A more realistic population model– Limits growth by incorporating carrying capacity

Logistic Population Growth

• Carrying capacity (K)– Is the maximum population size the

environment can support

• In the logistic population growth model– The per capita rate of increase declines as

carrying capacity is reached

Logistic Growth Equation

– Includes K, the carrying capacity

dNdt

(K N)Krmax N

Logistic Population Growth

– Produces a sigmoid (S-shaped) curve

Figure 52.12

dN

dt 1.0N Exponential

growth

Logistic growth

dN

dt 1.0N

1,500 N1,500

K 1,500

0 5 10 150

500

1,000

1,500

2,000

Number of generations

Pop

ulat

ion

size

(N

)

dNdt

(K N)Krmax 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

The Logistic Model and Real Populations

• The growth of laboratory populations of paramecia

– Fits an S-shaped curve

Logistic Growth and The Real World

• Some populations overshoot K

– Before settling down to a relatively stable density

Figure 52.13b

180

150

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120

90

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30

Time (days)

0 16014012080 100604020

Nu

mb

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of

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/50

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(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.

Logistic Growth and the Real World• Some populations

– Fluctuate greatly around K

Figure 52.13c

0

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1975 1980 1985 1990 1995 2000

Time (years)

Nu

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(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.

The Logistic Model and Life Histories

• Life history traits favored by natural selection– May vary with population density and

environmental conditions

Life History and Logistic Growth

• K-selection, or density-dependent selection– Selects for life history traits that are sensitive to

population density• Reproduce slowly, small litters

• r-selection, or density-independent selection– Selects for life history traits that maximize

reproduction• Reproduce rapidly, large litters

Natural selection (diverse reproductive strategies)a) Relatively few, large offspring (K selected species)b) Many, small offspring (r selected species)

(r selected species)

(K selected species)

Human Populations• No population can grow indefinitely and humans are no

exception

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

Global Carrying Capacity

• Just how many humans can the biosphere support?

• Carrying capacity of earth is unknown….

http://www.youtube.com/watch?v=UUOEcNomakw&feature=rec-LGOUT-exp_fresh+div-1r-8-HMhttp://www.youtube.com/watch?v=4B2xOvKFFz4&feature=related

http://www.youtube.com/watch?v=9_9SutNmfFk

Populations Regulated Biotic and Abiotic Factors

Two general questions we can ask about regulation of population growth

1. What environmental factors stop a population from growing?

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

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

Density-Dependent 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

• In crowded populations, increasing population density

– Intensifies intraspecific competition for resources

Figure 52.15a,b

100 100

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10,000

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rag

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)

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Seeds planted per m2 Density of females

0 7010 20 30 40 50 60 802.8

3.0

3.2

3.4

3.6

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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

• In many vertebrates and some invertebrates– Territoriality may limit density

Territoriality Example: Cheetas• Cheetahs are highly territorial

– Using chemical communication to warn other cheetahs of their boundaries

Figure 52.16

Territoriality: Ocean birds

– Exhibit territoriality in nesting behavior

Figure 52.17

Health

• Population density– Can influence the health and survival of

organisms• In dense populations

– Pathogens can spread more rapidly

Predation

• As a prey population builds up– Predators may feed preferentially on that

species

Intrinsic Factors

• For some populations– Intrinsic (physiological) factors appear to

regulate population size

Population Dynamics

• The study of population dynamics– Focuses on the complex interactions

between biotic and abiotic factors that cause variation in population size

Fluctuations in Population Size• Extreme fluctuations in population size

– Are typically more common in invertebrates than in large mammals

Figure 52.19

1950 1960 1970 1980Year

1990

10,000

100,000

730,000C

omm

erci

al c

atch

(kg

) of

m

ale

crab

s (l

og s

cale

)

Metapopulations and Immigration

• Metapopulations– Groups of populations linked by immigration

and emigration

Immigration- Movement Into a Population

• High levels of immigration combined with higher survival can result in greater stability in populations

Figure 52.20

Mandarte island

Small islands

Nu

mb

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of

bre

ed

ing

fe

ma

les

1988 1989 1990 1991Year

0

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20

30

40

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Population Cycles• Many populations undergo regular boom-and-bust

cycles

Year1850 1875 1900 1925

0

40

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120

160

0

3

6

9

Lynx

pop

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siz

e (t

hous

and

s)

Har

e po

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size

(t

hous

and

s)

Lynx

Snowshoe hare

• Influenced by complex interactions between biotic and abiotic factors

The Global Human Population• The human population increased relatively

slowly until about 1650 and then began to grow exponentially

Regional Patterns of Population Change

• To maintain population stability– A regional human population can exist in

one of two configurations

• Zero population growth = High birth rates – High death rates

• Zero population growth = Low birth rates – Low death rates

Age Structure

• One important demographic factor in present and future growth trends– Is a country’s age structure, the relative

number of individuals at each age

• Age structure is commonly 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

Infant Mortality and Life Expectancy

• Infant mortality and life expectancy at birth

– Vary widely among developed and developing countries but do not capture the wide range of the human condition

Figure 52.26

Developed countries

Developing countries

Developed countries

Developing countries

Infa

nt

mo

rta

lity

(de

ath

s p

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1,0

00

birt

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Life

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yea

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