Population Ecology
April 17, 2000
Population Ecology
April 17, 2000
Readings - Starr Text: Ch 39 cover page, 39.1, 39.3-39.4 , 39.7 - 35.11
"According to population estimates released by the United Nations, Y6B (the year humans reached the 6 billion mark) was reached on October 12 1999.
- ZPG (Zero Population Growth)
Outline:
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What is population Ecology?
Population: group of individuals of the same species in a given area (habitat)
Ecology: the study of how organisms interact with each other and their environment
I. Characteristics of populations : Each population has its own:
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
- Births and immigration increase population size,
- Deaths and emigration decrease population size
- Zero Population Growth (ZPG) = no net increase or decrease
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: (Figure 39.4)
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: limits 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:
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 (Fig 39.7):
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
A population can also "crash" due to factors like depletion of resources, buildup of wastes and pollutants.
Effects of Yersinia pestis on the human population
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
- 2,000,000 YPB: 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. Agriculture increased the carrying capacity for the human population. (Human population in 10,000 BC: 5-10 million)
THIRD, Humans sidestepped limiting factors over our growth rate
- ~200 YBP: Knowledge of disease processes, increased medical care, improved sanitary conditions, use of fossil fuels for heat, warmth = dropped death rate sharply. Humans sidestepped limiting factors to growth. (Human population in 1800: 1,000,000,000)
- ~100 YBP: Development of antibiotics, surgical procedures, vaccines, machines, allowed support of larger population. (Human population in 1930: 2,000,000,000)
2. Growth curve for the human population: YIKES!!!!!
3. Doubling times have decreased rapidly since then: (in increases of 1 billion humans)
Elapsed time
Year
Human Population
~2,000,000
10,000,000 BC 5 - 10 million
10,000
1 A.D. 170 million
1,800
1800 1,000,000,000
1930 2,000,000,000
1960 3,000,000,000
1975 4,000,000,000
1987 5,000,000,000
1999 6,000,000,000
2050 ??? 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: 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
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 Wednesday, April 19, 2000 Stay tuned... 6. Total population and population density for a select few cities:
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":
Top 100 countries ranked by population:
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, ~40,000 people die from starvation.
Feeding a Growing World Population: Toward a "Greener" Revolution
US Foreign Aid policy:
1. Individuals of every nation have the right to bear children, even if unrestricted reproduction ruins the environment that might sustain them
2. Because human life is precious above all else, the wealthiest nations have an absolute moral obligation to save lives everywhere
V. Control through family planning
Many governments are trying to lower birth rates through family planning programs.
The goal: achieving Zero Population Growth - ZPG
- 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
- Still above the replacement rate, but getting better!
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:
- 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!)
China - world's most extensive family planning program:
- Actively discourages premarital sex
- Encourages delayed reproduction
- Limits family size to 1 child - receive more food, free medical care and contraceptives, better housing, salary bonuses.
- Child receives free tuition, better jobs / social status
- If couple has more than one child (exception: twins or other multiples), pay higher taxes and forego all above benefits
Since 1972, China's fertility rate has dropped from 5.7 to 1.9, but population will contunue to increase as 340 million young women are entering reproductive years! (1.22 billion now, 1.5 billion in 2025)
Why was this program started? Between 1958 and 1962, 30 million Chinese people died of starvation.
Human rights violations - baby girls are less "valued"
What about family planning in the US?
October 1998 - The good news! - US teen pregnancy statistics
- Teen pregnancy rate reached lowest rate since 1973
- 117 pregnancies / 1000 teens (1990)
- 91 pregnancies / 1000 teens (1998)
- Fewer teens are having sex; sexually active teens are using contraceptives.
- Greater emphasis on postponing sexual activity
- Fear of STDs especially AIDS
Note: Both family planning and abstinence-only proponents are calling the report a victory for their respective sides!
For more information, see these websites (only if you are personally interested in this topic; will not be on the test)
National Family Planning and Reproductive Health Association
You decide: which country is the most overpopulated in terms of resource consumption and creation of environmental damage?
U.S.A.
India
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%
~270 million
16%
~980 million
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
A final cheery note from your text:
"It's up to us to make a global effort to limit population growth according to the environmental carrying capacity - or we can wait until the environment does it for us."
Objectives: 1. Distinguish between population size, population density, population distribution, age structure, and reproductive base. 2. Know how to calculate the net growth rate for a population, and how to graph the resulting numbers to produce a J-curve 3. Explain the factors that limit the biotic potential and carrying capacity of a population, including density-dependent, and density-independent limiting factors. 4. Know how to interpret how a country's age structure affects its growth rate |
