Human Population Growth
April 19 and 21, 2004

Readings: Ch 39, p 670-687

On October 12, 1999, a baby (was) born somewhere on Earth. With that birth, the world's population reached a milestone of 6 billion humans on the planet.
M.Vilar (Science World)

I. Characteristics of populations : Each population has its own group of individuals of the same species in a given area :

1. Population size: number of individuals in the gene pool
2. Population density: number of individuals in a given area
3. Population distribution: pattern of distribution (uniform, random, clumped)

4. Age structure: pre-reproductive, reproductive, post-reproductive
5. Reproductive base: those individuals in the pre-reproductive or reproductive stage

II. Population Size and Exponential Growth

Growth of populations over time:

For a small population, as long as the birth rate is slightly above the death rate, a population grows exponentially with a characteristic J-curve:

Example: population of 2000 mice; 200 die each month, but 1000 are born

Birth rate: 1000 born / 2000 = 0.5 (50%)
Death rate: 200 die / 2000 = 0.1 (10%)
Net change: 0.5 - 0.1 = 0.4 (net growth rate) (40%)

If you plot number of mice per time: J-curve

Why is this exponential (or geometric - a curved line) rather than linear (or arithmetic - a straight line)? Every month, the reproductive base gets larger!

III. Limits to Population Growth

Two major themes govern the growth of populations over time:

1. The resources of an environment control the growth and continued existence of populations
2. The resources of an environment are limited

Few populations can live up to their biotic potential = where conditions are ideal, every member has food, shelter, no predators or pathogens in the environment, maximum reproduction. Limiting factors in the environment on food, water, minerals, predators, shelter, buildup of waste materials usually prevent organisms from reaching their biotic potential

Carrying capacity: maximum number of individuals an environment's resources can sustain

Typically, after a period of exponential growth, growth levels off or even stops when the carrying capacity is reached

Density-dependent factors that are related to carrying capacity:

Sometimes, populations grow too large and "overshoot" their carrying capacity
Overcrowding can cause diseases, pathogens, and parasites to spread quickly
Examples: 1910 - Reindeer on St. Matthew Island, Alaska :

1910: 4 male, 22 female deer
1930: 250 deer
1940: 2,000 deer
1950: CRASH ! = 8 deer left!

Bubonic plague in Europe, 14th century = 25 million humans died in 5 years, over 100 million died in the decades following....Effects of Yersinia pestis on the human population

A population can also "crash" due to factors like depletion of resources, buildup of wastes and pollutants.

Density-independent factors:

Natural or man-made disasters can reduce population size, regardless of whether carrying capacity has been reached.

IV. Human Population Growth

1. The history of human population growth: How we began sidestepping controls over our growth rate:

FIRST, Humans developed the capacity to expand into new habitats

SECOND, Humans increased our carrying capacity in existing habitats

THIRD, Humans sidestepped limiting factors over our growth rate

2. Growth curve for the human population: YIKES!!!!!

3. Human Population Growth Adding a billion new people has taken LESS AND LESS time.... (PS. Fill in the numbers of the left column!!!)

 Elapsed time


 Human Population

 ~2,000,000 years

 ~10,000 BC

 5 - 10 million

 10,000 years

 1 A.D.

 170 million

 1,800 years














 ??? 8,500,000,000 ???

On October 12, 1999 the population of the Earth reached 6,000,000,000 - "Y6B"

4. Births and Deaths Per Time Unit: 2004 (year shown below is 1999)

 Time unit  Births  Deaths  Natural increase
 Year  131,468,233  54,147,021  77,321,212
 Month  10,955,686  4,512,252  6,443,434
 Day  360,187  148,348  211,839
 Hour  15,008  6,181  8,827
 Minute  250  103  147
 Second  4.2  1.7  2.5

"Someone make it stop!"

5. What is the current world and US population? U.S. Census Bureau:

   US population  World population
Friday, April 9th, 1999 272,252,017 5,978,619,205
Monday, April 17, 2000 274,623, 248 6,064,415,203 (85 M more!)
Friday, April 24, 2001 284,036,458 6,142,298,012 (77 M more!)
Friday, March 1, 2002 286,538,112 6,208,604,220 (66 M more!)
Friday, April 17, 2004 293,034,631 6,360,788,963 (152 M more in 2 years)

Check back next year for ~75 million more!

6. Total population and population density for a select few cities: Feeling a little crowded yet?

 Total population  People / sq. mi  City
  108,978   276.3  Bloomington, IN
6,069,974  3,221.4  Chicago, IL
  363,811   268.4  Fort Wayne, IN
1,249,822   407.0  Indianapolis, IN
  96,946   175.2  Kokomo, IN
8,063,164  2,183.1  Los Angeles, CA
8,546,846  4,847.6  New York, NY

Slightly different values (people / sq. km), worldwide population density of the 20 most populous "urban areas": OK, now we're talking crowded!!!

Top 100 countries ranked by population. Feeling a little less crowded now?

You decide: which country is the most overpopulated in terms of resource consumption and creation of environmental damage? See more at Population and the Environment




Production / consumption of goods & services  21%   1%
Use of world's non-renewable resources  25%   3%
Creation of world's trash and pollution  25%   3%
Percent of world's population   5%

~280 million


~1.10 billion

7. Sustaining the masses:

Our staggering population growth continues even though worldwide 2,000,000,000 people live in abject poverty (malnourished or starving, without clean water, sewage treatment, medical treatment, or shelter)

Every day, ~24,000 - 40,000 people die from starvation. WHY? - when there is enough food already on the planet to feed everyone? ...what are some of the real issues behind hunger? [Image]

Oxfam America
Feeding a Growing World Population: Toward a "Greener" Revolution
America's Second Harvest: Who are the hungry in America?

V. Population Control

Many governments are trying to lower birth rates through family planning programs.

A country's age structure tracks the number of prople in the pre-reproductive, reproductive, and post-reproductive years. Age structures for a few countries:

Slowing the birth rate - some trends:

VI. Population growth and economics

The Demographic Transition - Differences in a country's population growth correlate with economic development