Population Change
Chapter 8
Principles of Population Ecology
Population ecologists ask:
1) How many are in the population?
2) Are its numbers increasing / decreasing?
3) What is its pattern of reproduction?
4) What is its pattern of mortality?
Principles of Population EcologyPopulation Density (because you can’t count them all!):• Not enough to know just how many.• Need to know per unit area.• Which environmental factors may influence population
density?
20 / 100 m220 / 25 m2
Principles of Population Ecology
How Do Populations Change in Size?
On a global scale (closed system):
Principles of Population Ecology
How Do Populations Change in Size?
On a local scale (open system):
Principles of Population EcologyHow Do Populations Change in Size?
Global scale:
Births and deaths per 1,000 people per year
r = b/1,000 – d/1,000
Growth rate
Birth rate
Death rate
Principles of Population Ecology
How Do Populations Change in Size?
Local scale (all per 1,000 people):
r = (b – d) + (i – e)
Growth rate
Birth rate
Death rate
Immigration rate
Emigration rate
What would a positive r or a negative r tell you about the population?
Principles of Population Ecology• Maximum Population Growth- under ideal
conditions = intrinsic rate of increase (biotic potential)
• Which factors could influence the intrinsic rate of increase?
J-shaped curve (exponential growth)
All species follow J-curve based on intrinsic rate.
Major variable is pop. size and time!
Principles of Population EcologyEnvironmental Resistance-• Exponential growth at intrinsic cannot occur forever.• What factors prevent it from doing so?• Crowding makes a pop. more susceptible to parasites,
viruses, predators, waste, food / space shortage
S-shaped curve (logistic growth)
• Environment influences K (ex. drought)
• How could one environmental influence have a “domino” effect on many species?
• Rate of population growth is proportional to the amount of resources
Principles of Population EcologyOvershooting the carrying capacity can lead
to a population crash.
• Abrupt decline from high to low population density
• Over consumption of resources because of population size
Reproductive StrategiesLife History Strategies - Trade offs!
r-selected K-selectedvs.
high growth rate slow growth rate
small body size large body size
early maturity late maturity
short life span long life span
large broods small broods
little / no parental care high parental care
“perfect”: continual reproduction at intrinsic rate of increase with all offspring surviving to reproduce
But… addt’l energy needed for their own survival
Reproductive StrategiesSurvivorship: proportion of individuals alive at a
certain age (insurance companies used these!)
Factors that Affect Population SizeDensity-Dependent Factors-
• Factors that have a greater influence on population growth when density is high.
• Predation, competition, disease.
• Living space, food, cover, water, minerals, sunlight in high demand
• Large population: parasites and predators have more hosts / prey– survive longer / reproduce more
• In lab: control all factors except one
Factors that Affect Population SizeDensity-Dependence and Boom-or-Bust
Population Cycles • More prey - predator population increases
•Prey population decreases
•OR overwhelm food supply (plants=prey)
Factors that Affect Population Size
Case-in-Point: Predatory Prey Dynamics on Isle Royale
Canine parvovirus outbreak
Tick outbreak
Factors that Affect Population Size
Density-Independent Factors-
• Abiotic
• Regardless of population density, influences population growth.
• Frost, severe weather, fire.
• Example: Adult mosquitoes wiped out in winter. Species survives from hibernating larvae
The Human Population: Demography
Human population size• Would not have occurred if not for:
•Food production technology
• Medical advancements
•Water quality
•Decline in birth rate and death rate
The Human Population
Current Population Numbers
In 2004 = ~6.4 billion
Check out:
http://www.census.gov/main/www/popclock.html
The Human PopulationCurrent Population Numbers
Rapid growth primarily due to drop in death rates
The Human PopulationProjecting Future Population Numbers
When will zero population growth occur?
•K for Earth: 7.7 billion (most recent analysis)
•Past analyses: 4 - 11 billion
•Assumptions: standard of living, consumption, tech. advancements, waste generation
Demographics of Countries
Most Populous Countries Insert Table 8.1
Demographics of CountriesDeveloped vs. Developing Countries
Demographics (includes migration)
Demographics of Countries
Developed vs. Developing Countries Demographics
Demographics of Countries
Demographic Stages
Demographics of CountriesAge Structure of Countries
Generalized Age Structure: Factors are age distribution and male-female ratio
Next Generation is larger
Next Generation is almost same size
Next Generation is smaller
Demographics of Countries
Examples:
Demographics of Countries
Examples:
Demographics of Countries
Examples:
Demographics of CountriesPopulation Under Age 15 (relative size of next
generation):
Demographics of the USCase-in-Point: US Immigration
• Birth rate declining
• Increasing rate of immigration
• Consumption overpopulation
• Estimates 300,000 illegal per year, 1 million legal
Demographics of the USCase-in-Point: US Immigration• Immigrate because of:
•Persecution
• High growth rate in developing countries reduces resources / jobs available
• Deteriorating environmental conditions in developing countries
• Top 5 immigrants: Mexico, Philippines, Vietnam, Dominican Republic, China
•70% poor with few skills, 30% college graduates
•Should the US increase or decrease immigrants?