Population Ecology

Chapters 5 and 8

Ecology is studied at several levels

• Ecology and evolution are tightly intertwined

• Biosphere = the total living things on Earth and the areas they inhabit

• Ecosystem = communities and the nonliving material and forces they interact with

• Community = interacting species that live in the same area

Levels of ecological organization

• Population ecology = investigates the quantitative dynamics of how individuals within a species interact

• Community ecology = focuses on interactions among species

• Ecosystem ecology = studies living and nonliving components of systems to reveal patterns– Nutrient and energy flows

Some DefinitionsPopulation Ecology – The study of biological factors that affect the sizes of population

Demography – The study of human populations in numerical terms

Population – A set of potentially interbreeding individuals in a certain geographical location at a certain time

Some DefinitionsCensus – a head count of all the individuals living in a specified area.

Growth rate – The rate of change of population size

-birth rate – number of births per year divided by number of individuals in the population

-death rate - number of deaths per year divided by number of individuals in the population

Population Ecology

• Population- how to measure?

• Growth rates: J shaped, S shaped

• K, r, and reproductive strategies

• Human population

• Population density = number of individuals in a given area or volume

• count all the individuals in a population

• estimate by sampling

How are populations measured?

• mark-recapture method depends on likelihood of recapturing the same individual

Figure 35.2A

The dispersion pattern of a population refers to the way individuals are spaced in their area

(a) Clumped (elephants) (b) Uniform (creosote bush)

(c) Random (dandelions)

Figure 35.3A

0 5 10 15 20

0

500

1000

1500

Time (years)

Pop

ula

tion

size

r = 0.06

r = 0.02

r = 0

r = -0.05

high intrinsicrate of increase

low intrinsicrate of increase

zero populationgrowth

negative intrinsicrate of increase

Graph showing the growth of world’s human population

A street scene in Quito, Ecuador

A population tends to increase geometrically if its growth is unchecked

The available food supply increase only arithmetically

Since the population increases faster than the food supply, the increasing population causes human misery and poverty

MALTHUS' VIEW ON POPULATION (1798)

A new population shows rapid exponential growth at first

Its growth rate levels off when it approaches the carrying capacity of its environment, a phenomenon called logistic growth

MALTHUS' VIEW ON POPULATION (1798)

Limiting factors restrain growth

• declining birth rate or increasing death rate • Limiting factors = physical, chemical and

biological characteristics that restrain population growth– Water, space, food, predators, wastes and

disease• Environmental resistance = All limiting

factors taken together

How does population density affect population growth?• Density-Independent Factors

– Affect a population’s size regardless of its population density

– Floods, hurricanes, drought, weather, fire, habitat destruction, and pesticide spraying

• Density-Dependent Factors– As density of population increases, these

factors have a greater effect• Competition, predation, parasitism, and disease• Bubonic plague—urban hell batman…

Po

pu

lation

size (N

)

Time (t)

Figure 9-4Page 166

Carrying capacity (K)

Environmentalresistance

Bioticpotential

Exponentialgrowth

POPULATION SIZE

Growth factors(biotic potential)

Favorable lightFavorable temperatureFavorable chemical environment (optimal level of critical nutrients)

Abiotic

BioticHigh reproductive rate

Generalized niche

Adequate food supply

Suitable habitat

Ability to compete for resources

Ability to hide from or defend against predatorsAbility to resist diseases and parasitesAbility to migrate and live in other habitatsAbility to adapt to environmental change

Decrease factors(environmental resistance)

Too much or too little lightTemperature too high or too lowUnfavorable chemical environment (too much or too little of critical nutrients)

Abiotic

BioticLow reproductive rate

Specialized niche

Inadequate food supply

Unsuitable or destroyed habitat

Too many competitorsInsufficient ability to hide from or defend against predatorsInability to resist diseases and parasitesInability to migrate and live in other habitatsInability to adapt to environmental change

© 2004 Brooks/Cole – Thomson Learning

2. Logistic growth is slowed by density dependent limiting factors

K = Carrying capacity is the maximum population size that an environment can support

Figure 35.3B

Logistic Growth Equation

• logistic growth curve

– K = carrying capacity– The term

(1-N/K) accounts for the leveling off of the curve

Figure 35.3C

Exceeding K

• Some species do not make a smooth transition from exponential and logistic growth– Instead temporarily overshoots K because of

reproductive time lag (the period needed for birth rate to fall and death rate to rise in response to resource overconsumption)

– Dieback or population crash• Reindeer introduced onto island

• Technological, social, and other cultural changes have extended earth’s K for humans

2.0

1.5

1.0

.5

Nu

mb

er of sh

eep

(millio

ns)

1800 1825 1850 1875 1900 1925

Year

Overshoot

Carrying capacity

2,000

1,500Nu

mb

er of

reind

eer

1910 1920 1930 1940 1950

Year

1,000

500

Carryingcapacity

Populationovershootscarryingcapacity Population

crashes

Liebig's Law of the Minimum

• The limiting factor present in the smallest amount will function to limit population growth.

• If the most limiting factor is removed, the population will grow until it is limited by the next scarcest resource, and so on.

• In aquatic systems, nutrients are most limiting. If extra nutrients are added algal populations boom until light becomes limiting.

Taking Age into Account• The best answer is simple, but no simpler

Albert Einstein

• Seek simplicity, then distrust itLord Alfred North Whitehead

• The Exponential and Logistic Models Make Some Simplifying Assumptions– All individuals are equally likely to die (mortality)– All individuals are reproductively active (natality)

• Age structure models take age specific mortality and natality into account

Survivorship Curves• Type I

– Late loss; death usually due to old age– High survivorship to certain age—then high mortality– Mammals

• Type II– Environment causes death independent of age– Birds/reptiles

• Type III– Survivorship lowest in juvenile stages– Most common; insects, cane toads, bony fish, plants

Reproductive Curves

Lifespan

Life Tables

Age, years(x)

Probability of

surviving to age x (lx)

No. of female offspring born to a mother of age x (mx)

0 1.000 0.000

1 0.845 0.045

2 0.824 0.391

3 0.795 0.472

4 0.755 0.484

5 0.699 0.546

6 0.626 0.543

7 0.532 0.502

8 0.418 0.468

9 0.289 0.459

10 0.162 0.433

11 0.060 0.421 Source: http://www.gypsymoth.ento.vt.edu/~sharov/PopEcol/lec6/agedep.html

0 2 4 6 8 10 120

0.1

0.2

0.3

0.4

0.5

0.6

Reproduction

Age (Years)

Ag

e S

pe

cif i

c N

a ta

li ty

0 2 4 6 8 10 120.01

0.1

1

Survivorship

Age (Years)

% S

urv

ivin

g

Turkey Trouble!

Nx,t = number of organisms in age x at time t

sx = survival of organisms in age interval from x to x+1.

mx = number of offspring produced in the age interval from x to x+1

The Lewis-Leslie Matrix

Source: http://www.gypsymoth.ento.vt.edu/~sharov/PopEcol/lec7/leslie.html

Age Structure Diagram

Green - Pre-reproductive yearsDark Blue- Reproductive yearsLight blue - Post- reproductive years

Figure 9-9Page 169

Nu

mb

er of

ind

ividu

als

Time

Carrying capacity

K species;experienceK selection

r species;experiencer selection

K

G = rN (1-N/K) Life History Strategies

r-Selected Species

Cockroach

Dandelion

Many small offspring

Little or no parental care and protection of offspring

Early reproductive age

Most offspring die before reaching reproductive age

Small adults

Adapted to unstable climate and environmental conditions

High population growth rate (r)

Population size fluctuates wildly above and below carrying capacity (K)

Generalist niche

Low ability to compete

Early successional species

Fewer, larger offspring

High parental care and protection of offspring

Later reproductive age

Most offspring survive to reproductive age

Larger adults

Adapted to stable climate and environmental conditions

Lower population growth rate (r)

Population size fairly stable and usually close tocarrying capacity (K)

Specialist niche

High ability to compete

Late successional species

ElephantSaguaro

K-Selected Species

Human Survivorship Curves

Age Structure Diagram

Green - Pre-reproductive yearsDark Blue- Reproductive yearsLight blue - Post- reproductive years

Population Age Structures

• Even if global RLF level were magically lowered to 2.1, the population would continue to grow for at least 50 yrs because there are so many who have yet to reach child bearing years

• Population age structure diagrams help demographers understand future trends

• Any country with many people below age 15 has a powerful built in momentum to increase– In 2003, 30% of the people were aged 15 or less!!

•Baby Boom•From 1946 to 1964, the US pop increased by 79 million •Baby boomers now make up 50% of all adult Americans•Dominate demand for goods and services•Important political group

•Baby Bust Generation (GenX)•People born between 1965-1976•Retired baby boomers will likely use their political clout to force the GenXers to pay higher income, health care, and social security taxes

•Echo-Boom (born 1977 to 2003)

US Trends

The Population Debate

• Can the world provide an adequate standard of living for 3 billion more people without causing widespread environmental damage?

• Is the earth already overpopulated?– What measures should be taken to slow growth?

• Instead of asking what is the carrying capacity, some believe we should be asking what the optimum sustainable population of the earth might be

• Should people be allowed to have as many children as they want?

• What is your opinion on this issue?

Population growth affects the environment

• The IPAT model: I = P x A x T x S

– Our total impact (I) on the environment results from the interaction of population (P), affluence (A) and technology (T), with an added sensitivity (S) factor

– Population = individuals need space and resources

– Affluence = greater per capita resource use

– Technology = increased exploitation of resources

– Sensitivity = how sensitive an area is to human pressure

– Further model refinements include education, laws, ethics

Humanity uses 1/3 of all the Earth’s net primary production

Causes and consequences of population growth

Computer simulations predict the future

• Simulations project trends in population, food, pollution, and resource availability

• If the world does not change, population and production will suddenly decrease

• In a sustainable world, population levels off, production and resources stabilize, and pollution declines

How is Population Affected by Birth and Death Rates?

• Pop Change = (B + I)- (D + E)• Demographers use

– crude birth rate (# of live births per 1000 people per year) and

– crude death rate (# of deaths per 1000 per year)

• Birth and death rates are coming down worldwide but death rates have fallen more sharply than birth rates– 216K people added every day (mostly where?)

Annual Population Growth Rate

<1%

1-1.9%

2-2.9%

3+%Data notavailable

Annual worldpopulation growth

Ave Crude Birth and Death Rates Average crude birth rate Average crude death rate

World

All developedcountries

All developingcountries

Developingcountries

(w/o China)

22

9

11

10

25

9

29

9

Developed Countries50

40

30

20

10

017751800

18501900

19502000

2050

Rate p

er 1,000 p

eop

le

Year

Rate ofnatural increase

Crudebirth rate

Crudedeath rate

Developed Countries50

40

30

20

10

017751800

18501900

19502000

2050

Rate p

er 1,000 p

eop

le

Crudebirth rate

Rate ofnaturalincrease Crude

death rate

Year

Rate of natural increase = (crude birth rate - crude death rate) 10

Developing

Comparing 3 Countries

0- 4

5- 9

10-14

15-19

20-24

25-29

30-34

35-39

40-44

45-49

50-54

55-59

60-64

65-69

70-74

75-79

80-84

85+

0

50

100

150

200

250

0

10

20

30

40

50

60

70

80

90

100

Fertility and Survivorship

Pakistan Fert Ecuador Fert USA Fert Pakistan Surv Ecuador Surv USA Surv

Age Class

Ag

e S

pec

i fi c

Fe r

t il i

ty R

a te

(pe r

10 0

0)

Per

c en

t S

ur v

i vi n

g

Changes in Global Fertility Rates

• Replacement Level Fertility (RLF)– # of children a couple must bear to replace

themselves– Slightly higher than 2 per couple (2.1 in developed

and ~2.5 in developing) WHY?– Does reaching RLF mean an immediate halt in

pop growth?• No b/c so many future parents are alive

• Total Fertility Rate (TFR)– An estimate of the average # of children a woman

will have during child bearing years if between the ages of 15 and 49 she bears children at the same rate as women did this year

What factors affect TFR?

• Importance of children in labor force• Cost of raising and educating children• Availability of public/private pension • Urbanization (access to birth control)• Educational/employment opportunities for

women• Infant mortality rate• Ave age at which women start having children• Availability of birth control and legal abortions

Fertility Rates and Poverty

• 97% of the future population growth is expected to take place in developing countries– Acute poverty is a way of life for 1.4 B– Between 2003 and 2050, the population of

developing countries is projected to increase to 8 billion from 5.2 billion

– Why would poor women have more children???

Fertility Rates

• In 2003:– Ave global TFR was 2.8 per woman

• 1.5 in developed (down from 2.5 in 1950)• 3.1 in developing (down from 6.5 in 1950)

– Still far above global replacement level!

• UN population projections to 2050 vary depending upon world’s projected average TFR

Decline in Total Fertility RatesWorld

Developedcountries

Developingcountries

Africa

LatinAmerica

Asia

Oceania

NorthAmerica

Europe

5 children per women2.9

2.51.5

6.53.2

6.65.3

5.92.8

5.92.8

3.82.4

3.52.0

2.61.4

1950 2000

Fig 11-5

TFR

Fig. 11.8, p. 242

Births per woman

< 2

2-2.9

3-3.9

4-4.9

5+

Data notavailable

Falling growth rates do not mean fewer people

Falling rates of growth do not mean a decreasing population, but only that rates of increase are slowing

Projected Population as of 2050

Fig. 11.6, p. 225

High

Medium

Low

12

11

10

9

8

7

6

5

4

3

21950 1960 1970 1980 1990 2000 2010 2020 2030 2040 2050

High10.7

Medium8.9

Low7.3

Year

Po

pu

lation

(b

illion

)

TFR

High=2.6

Med=2.1

Low=1.7

What factors affect death rates?

• Rapid increase in world’s pop due to decline in crude death rates (not births)

• More people started living longer b/c:– Increased food supplies and distribution– Better nutrition– Improved public heath (immunizations etc)– Improved sanitation and hygiene)– Safer water supplies

Two Indicators of Overall Health of People in a Country

• Life Expectancy– Ave # of years an infant can expect to live– Global LE increased from 48 to 67 (76 in

developed; 65 in developing) 1955-2003– In world’s poorest =55 yrs or less

• Infant Mortality Rate– # of babies out of 1000 that die before 1yr– Usually indicates lack of food, poor nutrition, poor

health care, and high incidence of disease– From 1965 to 2003, IMR dropped from 20 to 7 in

developed; and 118 to 61 in developing– Still means 8M infants die of preventable causes

each year (=22,000 per day)

Human Life Expectancy

(1999)

Demographic Equations

Crude Birth Rate (births/1000 people) = Live Births X 1000 Popl * Yr

Crude Death Rate (deaths/1000 people) = Deaths X 1000 Popl * Yr

Crude Growth Rate (new people/1000 people) = CBR - CDR

Annual Growth Rate % = CGR 10

Doubling Time: %R*T2 = 70

dN/dt = rN Nt = Noert

Mathematical Population Relationships Converting Rates

r1=

%R100

=CGR1000

Per Capita Percent Crude (per individual) (per Hundred) (per thousand)

Use with Use with Use with Nt = Noert %R*T2 = 70 Demographic Data

Ave Crude Birth and Death Rates Average crude birth rate Average crude death rate

World

All developedcountries

All developingcountries

Developingcountries

(w/o China)

22

9

11

10

25

9

29

9

Demographic Transition Model

• DTM is a hypothesis involving population changes over time

• As countries become more industrialized, first their death rates and then their birth rates decline

• According to the hypothesis, this transition occurs over 4 phases

Demographic Transition

Fig. 11.18, p. 233

Low

High

Rela

tive p

op

ula

tion

size

Birth

rate an

d d

eath rate

(nu

mb

er per 1,000 p

er

year)

80

70

60

50

40

30

20

10

0

Stage 1Preindustrial

Stage 2Transitional

Stage 3Industrial

Stage 4Postindustrial

Lowgrowth rate

Increasing Growthgrowth rate

Very highgrowth rate

Decreasinggrowth rate

Lowgrowth rate

Zerogrowth rate

Negativegrowth rate

Birth rate

Total population

Death rate

Time

Demographic Transition

• 1st:– Preindustrial Phase

• Little pop growth b/c harsh living conditions lead to high birth and high death rate

• 2nd:– Transitional Phase

• Industrialization begins, food supply increases, and health care improves

• Death rate drops and birth rate stays high

• Pop grows dramatically

• 3rd:– Industrial Stage

• Birth rate drops and approaches death rate

• Industrialization and modernization become widespread

• Pop growth slows

• 4th: Postindustrial – BR=DR (ZPG)– 38 countries accounting for

13% are in this stage

Factors Affecting Birth and Death Rates in the Demographic Transition

• Death Rates Decrease– Improved Medicine

• Maternity Care

– Improved Sanitation– Improved Hygiene– Improved Water supply– Improved Food/Nutrition

• Agriculture

• Food preservation

– Improved Transportation– Cessation of Military

Conflict

• Birth Rates Remain High – Compensate for high infant

mortality– Assure care for elders– Provide labor– Cultural/Religious practices

• Prohibit Birth Control

• Favor large families

– Lack of contraceptives– Lack of education @ family

planning– Lack of women’s rights

Why is the Birth Rate Slow to Decrease?

• Cultural or Religious practices take time to change

• Immigration of women of child-bearing age• Slow acceptance of changes in women’s status• Educational and employment opportunities for

women slow to appear• Slow advances in the production and distribution

of birth control• Government slow to provide support for elderly

Role of Family Planning

• FP has been responsible for at least half of the drop in TFR’s in developing countries

• Reduces the number of legal and illegal abortions each year

• Decreased risk of death from pregnancy• Dev’ing: 10% in 1960s to 51% use• But, still 250-350M women want access but

don’t yet have it– UN says it would cost $17B/yr (8 days of military

expenditures) (about $5 per person) to do this!

Control of Pregnancy• Behavioral methods

– Abstinence– Coitus interruptus– Rhythm method

• Barrier methods– Condom

• Male and female– Diaphragm– Cervical cap– Spermicidal agents

• Lactation• Chemical methods

– Oral contraceptives– Injections as Depo-

Provera– Implants– Morning-after pills

• Surgical methods– Vasectomy– Tubal ligation– Abortions

Role of the Status of Women

• Studies show that women tend to have fewer and healthier children and live longer when they have access to education and to paying jobs and live in societies where they are not oppressed

• Make up 50% of population but…– Do almost all domestic and child rearing – 60-80% of work growing food, getting H2O– 66% of all hours worked; 10% of world’s income– Own less than 2% of world’s land

• Many do not have right to own land, inherit estates, or borrow money

Education of women reduces the average number of children per family

Economic Rewards and Penalties

• Some believe we need to go beyond family planning and offer economic rewards and penalties to help slow population growth– $ for those who are sterilized or use

contraceptives• Usually only those done having a family will do

this!

– China penalizes couples who have more than one or two via taxes, fees, etc

A Poster Promoting Birth Control in China

CHINA’S POPULATION CONTROL

•The Chinese government has used several methods to control population growth

•In 1979, China started the "one child per family policy"

•This policy stated that citizens must obtain a birth certificate before the birth of their children

CHINA”S POPULATION CONTROL

•The citizens would be offered special benefits if they agreed to have only one child

•Citizens who did have more than one child would either be taxed an amount up to fifty percent of their income, or punished by loss of employment or other benefits

India's population has passed the one billion mark, according to the country's census commission

Reference: http://news.bbc.co.uk/1/hi/world/south_asia/744507.stm

India

Cultural factors like pressure to have male children, the economic importance of child labor and religious restrictions on sexual education are just some of the factors influencing this high population growth rate.

Reference: (http://www.asiasource.org/asip/ngos_health2.cfm)

Role of Predators in Controlling Population

Po

pu

latio

n s

ize

Po

pu

latio

n s

ize

(tho

us

an

ds

)(th

ou

sa

nd

s)

160160

140140

120120

100100

8080

6060

4040

2020

0018451845 18551855 18651865 18751875 18851885 18951895 19051905 19151915 19251925 19351935

YearYear

Hare

Lynx

Predator-prey cycles

Role of Predators in Controlling Population

• Cyclic increases followed by crashes• Snowshoe hare and lynx exemplify argument:• Top Down Control

– Lynx preying on hares reduce population– Shortage of hares reduces lynx population– Allows hare population to build up again– Other examples inc. wolves/deer; sharks/fish

• Bottom Up Control– Hares consuming plants is real issue– Changing hare population effects lynx population

Mathematics of population growth(practice worksheet)

•Births per year = bN (where b = per capita birth rate, N = Population Size)

•Deaths per year = dN (where d = per capita death rate , N = Population Size)

•dN/dt = bN - dN = (b-d) N = r N (where r = b-d, dN/dt = Change per year)

•r is called Biotic Potential or intrinsic rate of natural increase. •If immigration and emigration also occur then

r = (b + i) – (d + m)•dN/dt = rN Nt = Noert

•Annual rate of change =Birth rate-Death rate x 100 1,000 persons

OR (Birth rate-Death rate)/10

•Doubling Time = 70/Annual Rate

Crude Growth Rate

(CGR)

Percent Rate(R)

Decimal Rate(r)

Per 1000 Per 100 Per Capita

CBR- CDR CGR/10 R/100

R * 10 r * 100 r

Demographic Data

Use with

T2 = 70/R

Use with

Nt = N0ert

Mathematical Population Relationships