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Population Biology
Population: all members of a single species that live together in a specific
area
Population size: produces baseline description of population, then used to
describe changes over time
Compare changes
Populations
Change in size (total number of individuals)
Change in density (number of individuals in a certain space)
Change in age distribution (proportion of individuals in each age group)
Change in habitat distribution
Distribution
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Dispersion
Clumps
Uniform
Random
Most species live in clumps or groups
Clumps
Resources may vary greatly from place to place
Better protection against predators
Some predator species increase chances of securing a meal—wolf pack
Temporary clumps for mating and care for young
Population Measurement
Count directly
Mark the presence of individuals in a representative area and extrapolate to a
larger area
Estimated based on sign of presence such as droppings, birdcalls, etc.
Change in Population Size
Increases are exponential, not arithmetic
Increase is proportional to number of individuals already present
J-shaped growth
Exponential growth: slow at first, followed by increasingly faster rates of
growth, also called
Geometric growth: growth follows a geometric pattern of increase (2,4,8,16)
Rule of 70: divide 70 by annual percentage yields approximate doubling time
in years
S-shaped Growth
Growth model starts off like J-shaped form, but rate of growth slows and
ceases altogether
Stabilizes at certain level, denoted as K
Environmental resistance limits growth in S-shaped growth
Logistic growth
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S-shaped Growth
Growth model starts off like J-shaped form, but rate of growth slows and
ceases altogether
Stabilizes at certain level, denoted as K
Environmental resistance limits growth in S-shaped growth
Logistic growth
Biotic Potential
Capacity for growth
“r”=intrinsic rate of increase if the population has unlimited resources
Most populations grow at rate less than this
Real World
 Population growth varies in complex ways
 May show J- or S-shaped growth at various times
 Growth rates may hover around K
K
 Carrying capacity not fixed
 Stable in stable environments, population can hover above K or below K
depending on environmental variables
Patterns
 Rate of population decrease is faster than increase=population crash or
dieback
 Population explosion followed by dieback is called irruptive of Malthusian
growth
Growth Rate
 Difference between birth and death rate over a period of time
 birthrate=# of individuals born in a given time expressed as a proportion of
the total population=r
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Growth Rate
Death rate=# of individuals who die in a given time
Growth rate (r)=birth rate-death rate
r<0: population shrinks
r>0: population grows
r=0: population at zero population growth
Reproductive Strategies
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r-selected or Malthusian:
Opportunistic species in highly variable environments
High growth rates—take advantage of infrequent favorable conditions
Reproduction is rapid, with little care of offspring
r-selected
High growth rates—take advantage of infrequent favorable conditions
Pressures are density-independent: physical (abiotic) forces and predation
limit growth
Smaller size
r-selected
Reproduction is rapid, with little care of offspring, many offspring
Sheer numbers and effective dispersal
Reproductive Strategies
 K-selected or Logistic: relatively stable environment, numbers fluctuate within
narrow range around K
 Population size limited by carrying capacity
 Reproduction slower, considerable care of offspring
K-selected
 Larger size
 Pressures are density-dependent: as population increases, food and living
space limit growth
 Live longer, mature slowly, fewer offspring/generation
Examples
 r-selected: rodents, many insects, marine invertebrates, weedy annual plants
 K-selected: whales, wolves, elephants, primates
 What about humans?
Factors that Increase or Decrease Population Size
 Natality—production of new individuals, by whatever means, main source of
addition
 Sensitive to environment
 Wide variation among species
 Variation within species depending on conditions
+/- Population Size
 Fecundity—physical ability to reproduce
 Fertility—measure of actual number of offspring produced
 Not all fecund individuals find mate or successfully produce offspring
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+/- Population Size
Immigration—movement into new ecosystem
Fly or float
Hitchhike
Rafts
Fly, swim, walk
+/- Population Size
Emigration
 Movement out of ecosystem
+/- Population Size
Mortality—death rate
Number of organisms that die in a period÷number alive at the beginning of
the period
Survivorship—percentage that survives to certain age
Life expectancy—probable # of years survival for individual at given age
+/- Population Size
Life span—longest period of life reached by organism
Wide variation—minutes (bacteria) to thousands of years (bristle cone pines in
CA White Mountains)
Survivorship
Survivorship curves—depict time in lifespan when death occurs
Late loss—(K-selected), most lives lost late in life because most survive into
old age
Constant loss—nearly constant death rate throughout potential life span:
birds
Early loss—highest death rates in early life, insects, marine inverts., trees
+/- Population Size
 Emigration—movement of individuals out of population
 Dispersal
 Take genes with them—protect species in case of catastrophe
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Factors that Regulate Growth
Intrinsic—within organism
Extrinsic—outside the population
Biotic
Abiotic
Density-dependent
Density-independent
Density-independent
 Abiotic-weather, climate
 Extremes at wrong time of year
 Without regard to the number of organisms present
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Density-dependent
Decrease natality, increase mortality
Interactions between populations
Some within population
Interspecific—predator-prey population levels, peaks and valleys slightly offset
Cyclical
Density-dependent
Intraspecific—compete for resources
As carrying capacity is reached, resources are limited
First, quickest, clever, lucky
Territoriality
Fighting equipment
Attractiveness-song and dance
Density-dependent
Stress—”stress shock” causes physical and psychological and behavioral
changes
Combinations—OK food and water, space is lacking, lab animals, humans,
chickens
Humane livestock practices—not crowding chickens, livestock, “slow food”
movement
Human Populations
We don’t have a separate set of rules
We play within them, with variation
Over 6 billion, with a b, humans
Concentrated population centers
Environmental impacts
Goal: sustain, or improve, conditions
U. S. Population
11:30 a. m., Feb. 18, 08: 303,462,880
1 birth every 7 seconds
1 death every 13 seconds
1 international migrant net every 26 seconds
Net gain of 1 person every 10 seconds
World Populations
As of 11:30 a.m.: 6,651,418,102
18 February 2008
Human Population
 Focus only on numbers?
 Focus on wealth distribution?
 Focus on power?
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Focus on type of government?
Focus on women’s role?
?????—no easy approach or answer results in solving dilemma
Human Population
 Resource depletion and degradation?
 Human talents—deal with it
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Grow more food
Pollution control
Energy sources
Most Populous Countries 2007
Rank Country Population
1.China 1,321,851,888
2.India 1,129,866,154
3.United States 301,139,947
4.Indonesia 234,693,997
5.Brazil 190,010,647
6.Pakistan 169,270,617
7.Bangladesh 150,448,339
8.Russia 141,377,752
9.Nigeria 135,031,164
10.Japan 127,467,972
11.Mexico 108,700,891
12.Philippines 91,077,287
13.Vietnam 85,262,3561
14.Germany 82,400,996
15.Egypt 80,264,543
Most populous country in 50 years
Uneven Distribution of Population
Human Population Trends
J-shaped curve
300 million at beginning of common era
Disease and starvation major controls
Black Death—plague, periodically reduced number
1700—600 million
Human Population Trends
1 billion—1804
3 billion—1960
6,651,418,102: 2/28/08
Pre-industrial humans: 90% of time
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Today: 90% of all humans are alive today
Not just numbers, but the rate
Overall—global annual growth=1.25% (2004) (2.25% in 1963)
Doubling Time
 Rule of 70: 70 divided by the percentage growth rate
 As percentage increases, doubling time decreases
 Resources delivered and consumed at faster and faster rates
Fertility rates in U.S.
Demography
 Study of size, composition, distribution of human population and causes and
consequences of characteristics
 Changes in size, composition and distribution have social, economic, health
effects
Demographic Transition
 Change in patterns of growth and death rates
 High birth and death rates to lower birth and death rates
Demographic Transition
Demographic Transition
 Used to link economic development and population growth
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Growth Rates
Developed countries—0.1%
Developing—1.5%
Developed—1.2 billion—little change
Developing—5.2 billion (2004) to 8 billion in 2050
Birth Rates
 Fertility--# births per individual woman (or in population)
 Three types:
Crude --# of births/1000 persons
 Replacement-level--# of children/couple to replace themselves (2.1-2.5)
 Total fertility—average # of children a woman has during reproductive
years
BR and FR Factors
Importance of children as members of workforce
Cost of raising and educating children
Availability of private and public pension systems
Educational and employment opportunities for women
Total Fertility Rate
Dropped since 1950
Global: 2.8 children/women
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Developed: 1.5
Developing: 3.1
Highest: 5.2 parts of Africa
BR and DR Factors
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Infant mortality rate
Average age at marriage
Availability of legal abortions
Religious beliefs, traditions and cultural norms
Availability of reliable birth control methods
Fertility Control
Traditional—breast feeding, sex avoidance
Modern:
 Hormonal—pills, patches, rings, injections
 Physical—IUD, diaphragm, condoms
 Chemical—foams, films, creams
 Abortions
Fertility Rates
Vary
US—Depression, World War II, baby boom, echo baby boom
Also: famine, epidemics, war, genocide
Zero Population Growth
 Births + immigration=deaths + emigration
 Several generations of replacement-level
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Poorest and Least Developed
Life expectancy = ≤ 49
AIDS, genocide, war
Infant mortality=excellent indicator of quality of life
Level of nutrition
Level of health care
Level of infrastructure (water delivery and sewage treatment)
Infant Mortality
Undernutrition
Malnutrition
Infectious disease
AIDS
Age Structure
Number of people in young, middle and older age groups determine growth
rate (+ or -)
% males, females in 3 categories
 Prereproductive (0-14)
 Reproductive (15-44)
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Postreproductive (45 +)
Age Structure
 #of individuals under 15—major factor
 “built in” momentum to increase
 More reproducers, even if 1-2 children/female
 Important social implications
 Providing services
Education-which types
Medical care-pediatric, middle age, geriatric
 Younger members supporting older
 Older supporting “young” concerns
 Education
 recreation
Age Structure
Political implications—right of return for Palestinians
Loss of older members
 Genocide, AIDS
 Loss of collective identity
 No experience in new generations
 No babysitters
Poorest and Least Developed
Life expectancy = ≤ 49
AIDS, genocide, war
Infant mortality=excellent indicator of quality of life
Level of nutrition
Level of health care
Level of infrastructure (water delivery and sewage treatment)
Infant Mortality
Undernutrition
Malnutrition
Infectious disease
AIDS
Age Structure
#of individuals under 15—major factor
“built in” momentum to increase
More reproducers, even if 1-2 children/female
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Age Structure
Age Structure
Old Populations
 Are there enough young to support old
 Old people can be expensive
 Not forward looking
 www.iiasa.ac.at/Research/LUC/ChinaFood/data/anim/pop_ani.htm
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3 Major Impacts of Humans on Environment
Hunter-gatherers had little impact
Some changes have allowed us to live with J-shaped curve
3 major cultural changes
 Agricultural revolution (10,000 to 12,000 years ago)
 Industrial and medical revolution (275 yeas ago)
 Information and globalization revolution (began 50 years ago)
Controversies
State mandated controls
 1 child policies
 Subsidizing additional children
 Forced birth control
 Abortion control
Controversies
Which group has more negative impact?
Developed countries with high consumption and pollution rates
Developing countries with high growth rates and environmental demands
Both present problems with deveolping sustainability
Controversies
Lifeboat ethics
Social justice—fair share for all
Emigration control—Sierra Club