Human Population Growth
April 19 and 21, 2004
Readings: Ch 39, p 670-687
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%)
- After one month: 2000 x 0.4 = 2000 + 800 = 2800 mice!
- After 2 months: 2800 x 0.4 = 2800 + 1120 = 3920 mice!
- After 3 months: 3920 x 0.4 = 3920 + 1568 = 5488 mice!
- After 1 year: 158,726 mice!
- After 18 months: 1,195,134 mice!...and so on
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.
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
- 2,000,000 YPB (years before the present): Early humans were vegetarians, but ate meat when they could get it!
- 200,000 YBP: Humans organized into bands of hunter/gatherers
- 40,000 YBP: Hunter / gatherers spread throughout the world, using knowledge passed on to generations to live in diverse habitats by building fires, building shelter, making clothing and tools.
- Knowledge and language development allowed humans to survive in habitats throughout the world.
SECOND, Humans increased our carrying capacity in existing habitats
- 12,000 YBP: Humans shifted to farming - agriculture, domestication of animals and wild grasses (rice, corn, wheat, rye), irrigation, to meet specific human needs. (Human population in 10,000 BC: 5-10 million)
- Agriculture increased the carrying capacity for the human population.
THIRD, Humans sidestepped limiting factors over our growth rate
- ~200 YBP: (1802) Knowledge of disease processes, increased medical care, improved sanitary conditions, use of fossil fuels for heat, warmth = dropped death rate sharply. (Human population in 1800: 1,000,000,000)
- ~100 YBP: (1902) Development of antibiotics, surgical procedures, vaccines, machines, allowed support of larger population. (Human population in 1930: 2,000,000,000)
- Knowledge about disease and treatments allowed humans to sidestep limiting factors to growth.
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!!!)
5 - 10 million
??? 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%
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]
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.
- Average "replacement rate" of ~2 children per couple
- But, even if each couple on the planet decided TODAY to have only 2 kids, the human population would keep growing for another 60 years because of all the young people around today who will one day reach reproductive age! (Currently, more than 33% of the world's population is in the pre-reproductive state!)
- Today: total fertility rate in the US - 6.5 in 1950 and 3.1 in 1995
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:
- Delayed reproduction: until '30s (rather than teens or 20s)
- Use of natural or artificial means of birth control
- Nature's most important check on human fertility - breastfeeding!
- Breastfeeding prolongs the return of menstruation after childbirth, and inhibits ovulation. Note: don't count on it :-)
- The erosion (in the 1930s - 60s) of breastfeeding and, with it, the early return of fertility has fueled the exponential growth of the human population! (80s and 90s - big comeback!)
VI. Population growth and economics
The Demographic Transition - Differences in a country's population growth correlate with economic development
- Pre-industrial: Birth rates high, but death rates high too. Population growth is LOW
- Transitional: Industrial society begins. More foor, more medical care. Birth rates high, death rates lowered. Population growth is HIGH
- Industrial: Population is "well off" and realtively wealthy. Birth and death rates are low, but population size grows rapidly!
- Post-industrial: ZPG: Birth rate very low, even less that death rate, but population size is still high