Population Ecology

Population =

Demography =

Ways of Expressing Population Growth

• Net birth rate = Births per unit time • Net death rate = Death per unit time• Net population increase = Net Birth rate – Net

Death rate• World Population mid-year 2007• 6.625 billion people

– Net birth rate/ year = 139, 125,000

– Net death rate/year = 59,625,000

– Net Increase/year = 78,500,000– USA Population mid-year 2007 = 302,200,000

• World Increase = USA Population - 3.85 years

Expressing Population Growth

• Births/1,000 =

• (Births per year/ mid-year population size) X 1,000

• Deaths/1,000 =

• (Death per year /mid-year population size) X 1,000

• Natural Rate of Increase =

• Births/,1000 – Death/1,000

World Population mid-year 2007

• Birth rate = 21

• Death rate = 9

• Rate of Natural Increase = 12

• Percent Growth = – 1. Rate of Natural Increase/10– 2. 12/10 = 1.2%

• Approximate doubling time =

• 70 years/ % Growth

• World Doubling Time =70 yrs/1.2 % = 58 yrs

Comparison of Ways to Express Population Growth

A. Population size = 1,000 B. Population size = 1,000,000

1. Net birth = 10 Net births = 100

2. Net deaths = 5 Net deaths = 50

3. Net increase = 5 Net increase = 50

1. Birth /1000 = 10 = 0.10

2. Death/1000 = 5 = 0.05

3. Rate of natural increase = 5 = 0.05

4. % growth = 0.5% = 0.005%

Rate of Natural Increase and % Growth do not include net immigration

• United States

• Birth per 1000 = 14

• Deaths per 1000 = 8

• Net Immigration per 1000= 4

• % Growth with Immigration = 18-8 = 1.0%

• % Growth w/o immigration = 14-8 = 0.6%

Population Growth(Basic Growth Curves)

• “J-shaped” growth or exponential growth

• Sigmoid or “S-shaped” growth

J-shaped or exponential growth curve

PopulationSize

TIME

LAG

BEND

EXPONENTIAL

GROWTH

EXPONENTIAL GROWTH

• Population increase – Fixed percentage of population size at the

beginning of the period

• Example of exponential growth is geometric growth - – Growth by doubling – Fixed percentage is 200 percent

Geometric Growth of a Sheet of Paper• Number of doubles Thickness

• 1 2/254 inch

• 2 4/254

• 3 8/254

• 4 16/254

• 5 32/254

• 6 64/254

• 7 128/254

• 8 256/254

Geometric Growth of a Sheet of Paper

• Number of doubles Thickness

• 12 1.25 foot

• 20 340 feet

• 35 3,000 miles

• 42 384,000 miles (reaches the moon)

• 50 93,000,000 miles (reaches the sun)

Exponential Growth

• Initially, no limit placed on growth• Population grows at max possible rate

– Intrinsic growth rate (biotic potential) • r = biotic potential• Maximum rate/individual

• Growth is halted abruptly:– Depleted resource (food or space)– Epidemic disease

• Environmental resistance – Applied all at once

J-shaped Growth Curve

Environmental

resistance – applied

abruptly

Time

Carrying Capacity

PO

PU

LA

TIO

N S

IZE

J-shaped growth curves are characteristic of populations in environments of

• Low diversity

• Few predators or competitors (few ecological controls)

• Simple food webs

Sigmoid or S-shaped Growth Curve

Time

PopulationSize

Carrying Capacity

Biotic Potential

Sigmoid Curve

• Detrimental factors (environmental resistance) – Increases in proportion to population

increase

• Characteristic of environment– High diversity – Predators, competitors exert biological

control

AGE STRUCTURE DIAGRAMS

• Graphically – % of individuals within various age categories

• Three age categories

• Prereproductive (Age 0-14 years)

• Reproductive (Age 15-44 years)

• Postreproductive ( Age 45+)

• Plot males on left and females on right

Age Structure DiagramsAge

% males in theage group

% females in theage group

Post-reproductive

Reproductive

Pre-reproductive You

nger

to o

lder

What Does the Age Structure Diagram Indicate?

• Growth Patterns

• Proportional Distribution in Age Categories

Three general types of age structure diagrams

• Expanding

• Stabilizing

• Diminishing

Pre-reproductive

Reproductive

Post-reproductive

Expanding Growth 2.7%

Stabilizing 0.6%

Diminishing –0.2%

Changes in USA Age Structure

• Last century USA – Expanding population to a stabilizing population– Total fertility rate (2007 USA 2.1)

• Post war baby boom (1946 and 1964) – Peaked 1955-1959– 75 million bulge in population – Large affect on social and economic structure

Effect of Post War Baby Boom

• 1960-1975 – expansion of schools

• Late 1970’s – 1980’s high unemployment

• 2,005-2025 – Dominance of middle age (Pension cost begin to rise)

• 2,025 – 2,040 – Period of Senior Citizens

Changes -USA Birth Rates and Death Rates

• Year BR DR RNI %Growth • 1947 26.6 15.0 11.6 1.16• 1977 14.7 9.0 5.7 0.57• 1987 16.0 9.0 7.0 0.70• 1996 14.6 8.8 5.8 0.58• 2000 15.0 9.0 6.0 0.60• 2004 14.0 8.0 6.0 0.60• 2007 14.0 8.0 6.0 0.60• Over the last 50 years births and death rates have declined • BR = Birth Rate, DR = Rate, RNI = Rate of Natural Increase

Implications of Death Rates and Birth Declining

• Fewer Births = fewer young people

• Fewer Deaths = More Older People

• Birth rates declined more rapidly than death rates = fewer young people more older people

Results of declining growth in USA • USA has an aging population:

– Proportionately fewer young people and more older people

• Median Age of the USA Population

• 1970 -- 29 years

• 1990 -- 33

• 2,000 -- 36

• 2010 -- 39

United States’ Aging Population

• Population 1950 1985 2020• Ages 65-84 (Millions) 11.7 25.8 44.3• 85 and over (Millions) 0.6 2.7 7.1• 65 and older % total 7.7% 12% 17.3%

• LIFE EXPECTANCY

• Total Pop. (years) 68.2 74.7 78.1• Federal Spending• Pension & Health-care

As a % of GNP 1.6% 9.3% 11.8%

Social Security

• Continued adjustments in Social Security Social Security – Not a pay as you go system.

– Pay while you work get benefits later

• As the populations ages:– Future retirees will have few workers

supporting their retirement than current retirees or retirees in the past

Ratio: retirees/worker

• Date Retiree/workers

• 1950 1/16

• 1960 1/5

• 1965 1/4

• 1985 1/3

• 2025 1/2

Social Security: Current Status

• 2018 – More expenditures than income

• 2042 –Trust Fund Depleted– Pay out all in coming funds– 75 % of current benefits

• Reality Trust fund is not fully funded

Suggested Solutions

• Raise FICA Tax (Federal Insurance Contributions Act) – Current (7.65%)

• Tax income over $90,000 (current cap)

• Tax one-half of Social Security Income over $32,000

• What changes have occurred?

Birth Year & Age for Full SS Retirement Benefits

Birth Year Age for full benefits

1937 or earlier 65

1939 65 and 4 months

1941 65 and 8 months

1943-1954 66

1955 66 and 2 months

1957 66 and 6 months

1959 66 and 10 months

1960 and later 67

Cunningham, Cunningham and Saigo, “Environmental Science, 8th ed.” McGraw Hill, Fig. 7.9

Population Structures by Age and Sex, 2005 Millions

300 100 100 300300 200 100 0 100 200 300

Less Developed Regions

More Developed Regions

Male Female Male Female

80+ 75-79 70-74 65-69 60-64 55-59 50-54 45-49 40-44 35-39 30-34 25-29 20-24 17-19 10-16

5-90-4

Age

Source: United Nations, World Population Prospects: The 2002 Revision (medium scenario), 2003.

Age Distribution of the World’s Population

Provided online by the Population Reference Bureau Graphics Bank, The Graphics Bank was prepared by Allison Tarmann, senior editor, and Theresa Kilcourse,senior graphics designer. Please visit www.prb.org.

Examples of Age Structure Diagrams

• Rapid Growth: Kenya, Nigeria, Saudi Arabia (doubling times 20-35 years)

• Slow Growth: United States, Australia, Canada (doubling times 88-175)

• Zero Growth: Denmark, Austria, Italy• Negative Growth: Germany, Bulgaria,

Hungary