PopulPopul

ationation

Chapter 8

Principles of Population Principles of Population EcologyEcology

i. Population Ecology = deals with species populations in an area and how/why those numbers fluctuate over time.– Study how a population responds to its

environment. – Populations cannot increase indefinitely

– environmental pressures called “limiting factors”

ii. Population Density = # of individuals of a species /area or volume at a given time

iii.Age Structure: # of males and females at each age from birth to death.

Competition, predation, disease

World Map

… by Population

… by Gross Domestic Product

… by Child Mortality

… by People living with HIV/AIDS

Current World PopulationCurrent World Population

Global population was 6,904,947,450On March 10, 2011

• The global population grows by: – Nearly 2.3 persons per seconds– Nearly 8,343 persons per hour– Over 200,234 persons per day – Over 73 million persons per year

Human Population Human Population DynamicsDynamics

There are just three sources of change in population size 1.Fertility2.Mortality

–"natural decrease" refers to population decline resulting from more deaths than births

3.Migration–Net migration is the number of immigrants minus emigrants

How do Populations Change in How do Populations Change in Size?Size?

• Birth Rate (b) = # births per 1000 people / year

• Death Rate (d) = # deaths per 1000 people / year

• Growth Rate (r) = natural increase of a population, ( % / year ). Called “natural increase” in human pops.

• Dispersal:– Immigration (i) = entering a pop.– Emigration (e) = exiting a pop.

• Example 1:Total Population =

10,000200 Births/year

(20 births/1000 people)100 Deaths/year

(10 deaths/1000 people)

r = b – d r = 20/1000 – 10/1000r = 0.02 – 0.01r = 1% per year

• Example 2:Total Population = 10,000 100 Births/year50 Deaths/year10 Immigrants100 Emigrants

r = ( b – d ) + ( i – e )r = (10/1000 – 5/1000) +

(1/1000 – 10/1000)r = (.01 – .005) + (.001

– .01) r = -0.004 …or -0.4%

Doubling Time Doubling Time

• Time it takes for a population to double itself, assuming that the current growth rate does not change.

DT=70/AGR%

• You may be given the doubling time and asked to calculate the AGR% (EASY MATH!)

World population is growing at a rate of about 1.7%

IF this rate continues, the population will double in ~ 42 years ( 70/1.7 = 42)

Unabated, such a rate would lead to a point about 2000 years when the mass of humanity would weigh more, and be larger, than the Earth.

Luckily, the global population rate is decreasing...

AGE STRUCTURE DIAGRAMSAGE STRUCTURE DIAGRAMSDiscuss with a neighbor WHY

these are expanding, stable, and declining.

Age Structure DiagramsAge Structure Diagrams

LARGEST POPULATION IN

PRE-REPRODUCTIVE

YEARS

POSITIVE POPULATION

GROWTH MOMENTUM!!

PRE-REPRODUCTIVE & REPRODUCTIVE AGE

GROUPS ARE NEARLY EQUAL

FEWER PEOPLE WILL BE PARENTS OF THE NEXT GENERATION

PRE-REPRODUCTIVE

GROUP IS SMALLER

POP WILL FALL

WORLD - 1999MORE DEVELOPED – 1999 & 2050

LESS DEVELOPED – 1999 & 2050 LEAST DEVELOPED – 1999 & 2050

Source: http://www.un.org/esa/population/publications/aging99/a99cht3.htm

Tracking the baby-boom generation Tracking the baby-boom generation

in the United Statesin the United States

The Power of the Pyramids!• Put your name on your paper.• Look at the back of your page…

– Find 2 other students who have DIFFERENT countries as you.

• You’re making Age-Sex Histograms today.• Make ‘em colorful and fun! Don’t spend

too much time “prettying it up” – you have analysis questions to do!

• Calculator time-saver: – If you’re using a TI-83, store the # you’re

dividing by as a letter. It’ll save tons of time!

Maximum Population Maximum Population GrowthGrowth

• Biotic Potential or Intrinsic Rate of Increase– Max. rate at which a pop could increase

under ideal conditions.

• Life History Characteristics affect biotic potential.– Age at which reproduction begins– Fraction of life when reproduction is possible– # reproductive periods per lifetime– # offspring produced during each reproductive

period

• Generally…– Smaller organisms have high biotic potential.– Larger organisms have low biotic potential.

World Pop 1800-2100

Based on UN 2004

Projections (red, orange,

green) & US

Census Bureau

historical estimates (black).

10.910.9

9.3 9.3

7.9 7.9

EXPONENTIAL GROWTHEXPONENTIAL GROWTHALL organisms show Exponential

Growth… …IF they’re growing at their biotic potential (time may vary)

J-shaped

curve

Increase or decrease

competition?

Is crowding a problem?

YES!

EXPONENTIAL EXPONENTIAL MODELMODEL

CARRYING CAPACITY “K”CARRYING CAPACITY “K”Carrying Capacity (K)= # organisms that the environment can support over a given time.•Fluctuates due to environmental changes or limiting factors! ENVIRONMENTAL RESISTANCE

– abiotic (light, temp, nutrients, weather, etc) – biotic (food, prey, disease, adaptable, etc)

Forms an S- shape graph Limits population growth

Feedback loop… + or – ?

LOGISTIC LOGISTIC MODELMODEL

LOGISTIC Curves:

show fluctuations!show fluctuations!

LOGISTIC Curves:

show fluctuations!show fluctuations!

S-shaped Curves: a CRASH CRASH could occur!!

WHY would this happen?

Resources are

exhausted!

Why is estimating carrying capacity for an individual country misleading, based on what you

know about human civilization & resource use?

IMMIGRATION

IMPORTS

TRANSPORTATION

EXCHANGING SERVICES

TECHNOLOGY

ENVIRONMENT ALTERATION

MOTHER NATURE

EXPORTS

EMIGRATION

Reproductive StrategiesReproductive Strategiesr strategists

– Recall: r = growth rate!– r-selected species have a

high growth rate– Characteristics:

• Small body size• Early maturity• Short life span• Large broods• Little or no parental care• Usually in unpredictable

environments (long-term survival low)

• Low ability to compete

Mosquitoes & Weeds

K strategists– Recall: K = carrying cap.– Maximize survival when

the pop. is near K.– Characteristics:

• Few offspring• Long life spans• Slow reproduction• Late reproduction• Parental care• Large body size• Usually in stable

environments

Humans, Owls, Trees,Whales

SurvivorshipSurvivorship• Probability that a given individual in a

population will survive to a particular age.– Type I = prob. of death greatest when old.– Type III = prob. of death greatest when young.

• If early death is avoided, survival chance is high

– Type II = prob. of death equal through life.• Death events have little age bias.

THESE ARE GENERALIZATIONS!! (few species fit 1 curve)

Survivorship curves changethroughout life for most species.

Factors that Affect Population Size

• Density-Dependent Factors– Effects of the factor are changed with a changing

population size– Examples:

• Disease• Competition for resources (food, space, etc)• Predator-prey relationships

• Density-Independent Factors– Effects of the factor are unchanged with a

changing population size– Examples:

• Typically abiotic factors• Random weather events

Predator-Prey CurvePredator-Prey Curve

Case-in-Point: Predator-Prey Dynamics

on Isle Royale• Moose wandered to this island by crossing the frozen

Lake Superior – Population Boomed!• Wolves wandered there later – Population Boomed!

Canine Parvovirus

Moose inc. due to few predators

Wolves inc. due to more food

Moose over K Moose dec. due to being

over K –

Lack of Food!

DD or DI?

Wolf dec. due to less food

Boom – Or – Bust CycleBoom – Or – Bust CycleLemming Population

(1)population is increasing, and

density-dependent factors are

increasingly severe.

(2) Population is declining, and

density-dependent factors are increasingly

relaxed.

Current Human PopulationCurrent Human Population• Between 2001 and 2002 – World

Population increased by 78M.– Not due to increased birth rate – it declined!– It’s due to a decreased death rate!

• Why?– Better medicine– Better technology– Lots of reasons!

Human Population (1800 – present)

Projecting Future Projecting Future PopulationPopulation

• Population continues to increase• Growth rate continues to decrease• Prediction:

– Growth rate will continue to decrease slowly until zero population growth is attained (b = d)

b and r in Mexico

(1900 – 2000)MexicanRevolutio

n

Types of Fertility Rates that Types of Fertility Rates that

Affect Human Population GrowthAffect Human Population Growth

• Total fertility rate (TFR)– The average number of children born to a woman– Average in developed countries = 1.5– Average in developing countries = 3.8– Worldwide 1990: 3.1 now: 2.8

• Replacement fertility rate (RFR)– The number of children a couple must have to

replace themselves– A RFR of 2.1 for developed countries with low

infant and child mortality rates– Africa RFR = 2.5

Projecting Future PopulationThis graph is based on

different fertility rates.2002: 2.8 children/woman

– If it declines to 1.5, population will be 7.9 B

– If it declines to 2.5, population will be 10.9 B

small changes make a

BIG difference!

??? What is Earth’s K ???– 4 B – 16 B??– Based on standards of living,

resource consumption, technology, and waste generation.

2000 UN EstimatesWorld

population will be

between…

10.9 B

7.9 B

9.3 B = “most likely”

Projections use estimates of future b, d, and migration.

DemographicsDemographics

HIGHLY DEVELOPED / DEVELOPED

– Low r, low b– highly industrialized– low infant mortality (#

deaths age 0-1 per 1000 live births)

– Longer life expectancy– High avg. per capita

GNI PPP (amt. goods & services the average citizen could buy)

DEVELOPING– High b, high r– Less industrialized

overall– Shorter life expectancy– Lowest avg. GNI PPP

• Can be moderately developed or less developed.

• Moderately developed is just that… moderate!

The applied branch of sociology that deals with population statistics

Doubling Time & Doubling Time & Replacement-Level FertilityReplacement-Level Fertility

Doubling Time = amount of time it would take for the population to double in size.– Assumes the r won’t change.

td = 70 / r– Can identify a country as highly, moderately, or less

developed. (Shorter doubling time = less developed)

Replacement-Level Fertility = # children a couple must produce to “replace” themselves.– 2.1 children – It’s not 2.0 because some infants and children die

before reaching reproductive age– Currently, total fertility rate worldwide = 2.8

Demographic StagesDemographic Stages(1) Preindustrial• High b, d, & infant mortality• Pop grows slowly or declines

slightly• War, plague, or famine increases

d

(2) Transitional• Lowered d, but b remains high• Population grows rapidly

(3) Industrial• Industrialization occurs• Decline in b, relatively low d• Population growth rate slows.

(4) Postindustrial• Low b and d• People desire smaller families

(better education, more $$)• Population grows slowly or not at

all

Population and Resource Population and Resource USEUSE