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?