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