Population EcologyThe factors and interactions that affect
population size
How do we study populations? Key Terms
• Demography- study of human populations
• Density- number of individuals per unit area
• Dispersion- relative arrangement of individuals within a given amount of space
Population Density of U.S.
Dispersion• There are 3 types:
Dispersion
How do we study populations?Growth Rates
• Growth Rate: change in population over time• Positive (+): birth rate > death rate• Zero (0): birth rate = death rate• Negative (-) = birth rate < death rate
• Fertility Rate: The average number of offspring a female has in its lifetime
• Replacement Rate: each mating couple has two offspring that survive to reproduce, replacing their parents.
Reproductive Potential
• Maximum number of offspring each member of a population can produce
• Increases when members reproduce often and earlier in their reproductive life
• Generation time- time it takes for an average member of a population to reproduce. • Human Generation time: 20 years• E.Coli Generation time: 17 minutes
Population Growth Curves
Exponential Growth
• Increase in population size due to a constant growth rate
• Unlimited resources
• Ideal conditions
• “J” curve
Logistic Growth
• Population growth slows or stops following a period of exponential growth
• Birth rate decreases, Death rate increases OR Both
• “S curve”
Carrying Capacity (K)
• The maximum amount of individuals an environment can support
• What happens to growth rate as it approaches K?
Reaching Carrying Capacity
www.algebralab.org
Logistic Growth- “Overshoot”
rewild.info
Carrying Capacity is
often diminished
due to overshoot
Population grows beyond carrying
capacity and collapses
Limiting Factors
Pair and Share (2 min):
• What factors would cause a population’s size to increase?
• What factors would cause a population’s size to decrease?
Limiting Factors
• Food
• Water
• Sunlight (producers)
• Space
• Disease
• Competition
• Predation
Limiting Populations
Density-Dependent
• Deaths occur more quickly in a more crowded environment
• Limited resources
• Disease
Density-Independent
• Size of population does not matter;
• Deaths occur independently of population density
• Ex: natural disasters
If humans have no natural predators,
how is our population
controlled?
Human Carrying Capacity?
• Current human population: approx 7.12 billion people
• Ranges estimated from 4-16 billion people
• Hard to estimate how many people this world can hold• Technological innovations• Medical breakthroughs
• http://www.census.gov/popclock/
What caused this
rapid growth?
What sparked our growth?
• Industrial Revolution (~1750)
• Modern medicine (20th century)• Death rates DROPPED due to better care
• Agricultural Advances
• Transportation Advances
Human Population Growth
Population (in billions) 1 2 3 4 5 6 7 8 9
Year 1804
1927
1960
1974
1987
1999
2012 2027 2046
Years elapsed between milestones
-- 123 33 14 13 12 13 16 19
Source: U.S. Census Bureau- World POPclock Projection
What takes us out?
Pair and Share (2 min):
Humans are at the top of any food web resulting in no natural predators to keep our populations in check…
1. What are the limiting factors of the human population?
2. How are we different than other species in regards to population control?
What’s our limiting factor?
• Disease: Bubonic Plague, AIDS, Flu, Malaria
• Access to food
• Access to clean water
• Competition (a.k.a. War)
Event Casualties
American War Deaths, all-time
600,000
India Famine (1769-70) 3,000,000
AIDS deaths in 2011 1,700,000
Influenza Epidemic (1918) 21,000,000
Indonesian Tsunami/Earthquake
(2004)
230,000
Bubonic Plague (1347-51) 75,000,000