Chapter 52 (pgs. 1151- 1172) Population Ecology

AP minknow•How density, dispersion, and demographics can describe a population.•The differences between exponential and logistic models of population growth.•How density-dependent and density-independent factors can control population growth

• Characteristics of Populations• 1.Define the scope of population ecology • 2.Define and distinguish between density and dispersion. • 3.Explain how ecologists measure the density of a species. • 4.Describe conditions that may result in the clumped dispersion,

uniform dispersion, and random dispersion of populations. • 5.Describe the characteristics of populations that exhibit Type I,

Type II, and Type III survivorship curves.  • 6.Describe the characteristics of populations that exhibit Type I,

Type II, and Type III survivorship curves.

• Life History Traits• 7.Define and distinguish between semelparity and iteroparity. • 8.Explain how limited resources affect life histories. • 9.Give examples of the trade-off between reproduction and survival.

• Population Growth• 10.Compare the geometric model of population growth with the logistic

model.  • 11.Explain how an environment's carrying capacity affects the intrinsic rate

of increase of a population. • 12.Distinguish between r-selected populations and K-selected populations. • 13.Explain how a "stressful" environment may alter the standard r-selection

and K-selection characteristics.• Population-Limiting Factors• 14.Explain how density-dependent factors affect population growth. • 15.Explain how density-dependent and density-independent factors may

work together to control a population's growth. • 16.Explain how predation can affect life history through natural selection. • 17.Describe several boom-and-bust population cycles, noting possible

causes and consequences of the fluctuations. • Human Population Growth• 18.Describe the history of human population growth. • 19.Define the demographic transition. • 20.Compare the age structures of Italy, Kenya, and the United States.

Describe the possible consequences for each country. • 21.Describe the problems associated with estimating Earth's carrying

capacity.

• Population ecology is the study of populations in relation to environment– Including

environmental influences on population density and distribution, age structure, and variations in population size

52.1: Dynamic biological processes influence population density, dispersion, and demography

A population

• A population– Is a 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

• Density is the result of a dynamic interplay– Between processes that add

individuals to a population and those that remove individuals from it

Births and immigration add individuals to a population.

Births Immigration

PopuIationsize

Emigration

DeathsDeaths and emigration remove individuals from a population.

Patterns of Dispersion

• Environmental and social factors– Influence the spacing

of individuals in a population.

– There are three different Patterns of Dispersion

• Clumped Dispersion• Uniform Dispersion• Random Dispersion

• A clumped dispersion– Is one in which individuals aggregate in

patches– May be influenced by 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.

• A uniform dispersion– Is one in which individuals are evenly

distributed– May be influenced by social interactions

such as territoriality

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.

• A random dispersion– Is one in which the position of each individual

is independent of other individuals

Figure 52.3c(c) Random. Dandelions grow

from windblown seeds that land at random and later germinate.

Life Tables

• A life table– Is an age-specific summary of the survival pattern of a

population– Is best constructed by following the fate of a cohort

Survivorship Curves• A survivorship curve

– Is a graphic way of representing the data in a life table

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 can be classified into 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)

Many species fall somewhere between these basic types of survivorship curves.

Some invertebrates, such as crabs, show a “stair-stepped” curve, with increased mortality during molts.

52.2 Life histories are highly diverse, but they exhibit patterns in their variability.

• Life histories entail three basic variables:

– when reproduction begins– how often the organism reproduces– how many offspring are produced

during each reproductive episode.

• These histories are evolutionary outcomes reflected in the development, physiology, and behavior of an organism.

• Some organisms, such as the agave plant, exhibit semelparity. Big Bang Production. (then death)

• By contrast, some organisms exhibit iteroparity.

– They produce only a few offspring during repeated reproductive episodes.

• 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.

• 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.

• In other words, how much does an individual gain in reproductive success through one pattern versus the other?

• The critical factor is survival rate of the offspring.• When the survival of offspring is low, as in highly

variable or unpredictable environments, big-bang reproduction (semelparity) is favored.

• Repeated reproduction (iteroparity) is favored in dependable environments where competition for resources is intense.

• In such environments, a few, well-provisioned offspring have a better chance of surviving to reproductive age.

What factors contribute to the evolution of semelparity versus iteroparity?

Population Growth is measured byPer Capita Rate of Increase

• If immigration and emigration are ignored– A population’s growth rate (per capita

increase) equals birth rate minus death rate

Growth rate = rN

It can be found using the equation---

dN

dt rN

Exponential Population Growth

• Exponential population growth– Results in a J-

shaped curve

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

•Exponential population growthIs population increase under idealized conditions

•Under these conditions•The rate of reproduction is at its maximum, called the intrinsic rate of increase

dNdt rmaxN

The J-shaped curve of exponential growth

• Is characteristic of some populations that are rebounding

Figure 52.10

1900 1920 1940 1960 1980

Year

0

2,000

4,000

6,000

8,000

Ele

phan

t pop

ulat

ion

52.4: The logistic growth model includes the concept of carrying capacity

Exponential growth– Cannot be

sustained for long in any population

• A more realistic population model– Limits growth by

incorporating carrying capacity

• Carrying capacity (K)– Is the maximum

population size the environment can support

The Logistic Growth Model

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

carrying capacity is reached

We construct the logistic model by starting with the exponential model

And adding an expression that reduces the per capita rate of increase as N increases

Maximum

Positive

Negative

0N K

Population size (N)

Per

cap

ita r

ate

of

incr

ease

(r)

The logistic growth equation

Includes K, the carrying capacity

dNdt

(K N)Krmax N

Table 52.3

• The logistic model of 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

)

2. As N approaches K for a certain population, which of the following is predicted by the logistic equation? – The growth rate will not change. – The growth rate will approach zero. – The population will show an Allee effect. – The population will increase exponentially. – The carrying capacity of the environment will

increase.

800

600

400

200

0

Time (days)0 5 10 15

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

• Some populations overshoot K– Before settling down to

a relatively stable density

• Some populations– Fluctuate greatly

around K

180

150

0

120

90

6030

Time (days)0 16014012080 100604020

Nu

mb

er

of

Da

ph

nia

/50

ml

0

80

60

40

20

1975 1980 1985 1990 1995 2000

Time (years)

Nu

mb

er

of

fem

ale

s

The Logistic Model and Life Histories

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

environmental conditions

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

to population density• K-selection tends to maximize population size

and operates in populations living at a density near K.

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

reproduction• r-selection tends to maximize r, the rate of

increase, and occurs in environments in which population densities fluctuate well below K, or when individuals face little competition.

Controversy

52.5: Populations are regulated by a complex interaction of biotic and abiotic influences

• 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

• Determining equilibrium for population density

Figure 52.14a–c

Density-dependent birth rate

Density-dependent death rate

Equilibrium density

Density-dependent birth rate

Density-independent death rate

Equilibrium density

Density-independent birth rate

Density-dependent death rate

Equilibrium density

Population density Population density Population density

Birt

h or

dea

th r

ate

per

capi

ta

(a) Both birth rate and death rate change with population density.

(b) Birth rate changes with populationdensity while death rate is constant.

(c) Death rate changes with populationdensity while birht rate is constant.

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• Territoriality• Health (Disease/Parasites)• Predation• Toxic Wastes (think bacteria)

Competition for Resources• In crowded populations, increasing population

density– Intensifies intraspecific competition for resources

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 are highly territorial

– Using chemical communication to warn other cheetahs of their boundaries

Figure 52.16

Population Dynamics

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

biotic and abiotic factors that cause variation in population size

Stability and Fluctuation• Long-term

population studies– Have

challenged the hypothesis that populations of large mammals are relatively stable over time

Figure 52.18

The pattern of population dynamics observedin this isolated population indicates that various biotic and abiotic factors can result in dramatic fluctuations over time in a moose population.

Researchers regularly surveyed the population of moose on Isle Royale, Michigan, from 1960 to 2003. During that time, the lake never froze over, and so the moose population was isolated from the effects of immigration and emigration.

FIELD STUDY

Over 43 years, this population experiencedtwo significant increases and collapses, as well as several less severe fluctuations in size.

RESULTS

CONCLUSION

1960 1970 1980 1990 2000

Year

Moo

se p

opul

atio

n si

ze

0

500

1,000

1,500

2,000

2,500

Steady decline probably caused largely by wolf predation

Dramatic collapse caused by severe winter weather and food shortage, leading to starvation of more than 75% of the population

Extreme fluctuations in population sizeAre typically more common in invertebrates

than in large mammals

Figure 52.19

1950 1960 1970 1980

Year

1990

10,000

100,000

730,000

Com

mer

cial

cat

ch (

kg)

of m

ale

cra

bs

(log

sca

le)

Fluctuating Wind pushing eggs out to sea

Cannibalism

Metapopulations and Immigration

• Metapopulations – Are groups of

populations linked by immigration and emigration

• High levels of immigration combined with higher survival– Can result in greater

stability in populations– Are groups of

populations linked by immigration and emigration

Mandarte island

Small islands

Nu

mb

er

of

bre

ed

ing

fe

ma

les

1988 1989 1990 1991Year

0

10

20

30

40

50

60

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

Three main hypotheses have been proposed to explain the lynx/hare cycles.

•The cycles may be caused by food shortage during winter.

•The cycles may be due to predator-prey interactions.

•The cycles may be affected by a combination of food resource limitation and excessive predation.

Limiting Factors

Density Dependant Factors

Density Independent Factors

52.6: Human population growth has slowed after centuries of exponential increase

• No population can grow indefinitely– And humans are no exception

The Global Human Population

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

Global population Growth Rate• Though the global population is still growing

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

Figure 52.231950 1975 2000 2025 2050

Year

2003P

erce

nt in

crea

se

2.2

2

1.6

1.4

1.2

1

0.8

0.6

0.4

0.2

0

1.8

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– Usually presented 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

er

1,0

00

birt

hs)

Life

exp

ect

an

cy (

yea

rs)

60

50

40

30

20

10

0

80

60

40

20

0

Global Carrying Capacity

• Just how many humans can the biosphere support?

• It is complex and we just don’t know, but we have….

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

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

)

At more than 6 billion peopleThe world is already in ecological deficit