Ch. 46 - Part 2 - Regulation of Population Size

Regulation of Population Size• The study of population dynamics– Focuses on the complex interactions

between biotic and abiotic factors that cause variation in population size

Population Change & Population Density

• In density-independent populations– Birth rate and death rate do not

change with population density• In density-dependent populations– Birth rates fall and death rates rise

with population density

Density-independent factors

• Population limiting factors whose occurrence is not affected by population density

• Abiotic factors such as climate and weather, fire, physical disruption of habitat

• Limit population size well before resources or other density-dependent factors become important

• Can cause rapid crashes in populations

Density-independent Effect

Impact of flash flood not affected by density:

3/5 die in low-density area = 60%

12/20 die in high-density area = 60%

Density-dependent factors• Population limiting factors whose

effects depend on population density• As the number of individuals increases,

so does the percentage of individuals affected

• Generally are biotic factors like: - Competition

- Predation - Parasitism

Competition:• Intraspecific competition is competition

between members of the same species for resources that are limited in supply (food, shelter, mates)

- Populations usually compete for resources unless population is

held below carrying capacity by other factors

• Interspecific competition is competition between members of different species

Density-dependent Effects - Competition

When density is low (a) every bird has access to nesting hole

When density is high (b) there is competition & not all birds gain a nest hole

Competition for Resources

• In crowded populations, increasing population density intensifies intraspecific competition for resources

100 100

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Seeds planted per m2 Density of females

0 7010 20 30 40 50 60 802.8

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(a) Plantain. The number of seeds produced by plantain (Plantago major) decreases as density increases.

(b) Song sparrow. Clutch size in the song sparrow on Mandarte Island, British Columbia, decreases as density increases and food is in short supply.

Density-dependent Effect on Reindeer

Individuals in a large population have a

smaller share of the limited food supply.

Reindeer on St. Paul Island, Alaska

underwent a sharp decline as a result of

overgrazing of the land

Territoriality

• In many vertebrates and some invertebrates– Territoriality may limit density– Cheetahs mark their territories with chemicals

• Oceanic birds– Exhibit territoriality in nesting behavior

Figure 52.17

Health• Population density can influence the

health and survival of organisms• In dense populations pathogens and

disease can spread more rapidly

Predation• As a prey population builds up predators

may feed preferentially on that species• The major limiting factor for predator

populations is the availability of its prey• Predation is probably a significant factor

for the prey population

Density-dependent Effects - Predation

Harder to find prey at low densities Easier to find prey at high densities

• Most populations are probably regulated by a mixture of density-independent and density dependent factors

• Many populations are fairly stable and near carrying capacity that and are regulated by density-dependent factors

• Many others show short-term fluctuations due to density-independent factors

Life History PatternsPopulations vary on factors such as:• number of births per reproduction• age of reproduction• life span of individuals• probability of living entire life span• These factors are part of a species life

history. - There are trade-offs to be made

regarding how much energy to use for which purposes

• Natural selection shapes the life histories of species

Two Types of Life History Patterns:• K-selection, or density-dependent

selection– Selects for life history traits that are

sensitive to population density• r-selection, or density-independent

selection– Selects for life history traits that

maximize reproduction

Opportunistic life history (r-strategists)• Small bodied species, reproduce when

young, produce many offspring, populations tend to grow exponentially when conditions are favorable

• Such populations typically live in unpredictable environments and are controlled by density-independent factors

• Emphasis on quantity of reproduction rather than on individual survivorship or quality

• Exhibit type III survivorship curves• Use a tactic called r-selection• Examples: bacteria, insects, rodents, annual

plants

Equilibrium life history (K-strategists)

• Larger bodied species, produce few slowly maturing offspring but provide care for their young

• Population size stable, near carrying capacity, held there by density-dependent factors

• Natural selection resulted in production of better-adapted offspring

• Exhibit type I survivorship curve• Use tactic called K-selection• Examples: Saguaro cacti, oaks, pines,

birds of prey, large mammals, humans

Life History Strategies

Human Population Growth• Human population has gone through a

period of rapid exponential growth• Went around “the bend” of the J-shaped

curve around 1650. - It had taken millions of years to reach

that point. • Once around the bend, the time it takes

to double the population (doubling time) shrinks rapidly. Now about 53 years.

• No population can grow indefinitely– And humans are no exception

The Global Human Population

8000 B.C.

4000 B.C.

3000 B.C.

2000 B.C.

1000 B.C.

1000 A.D.

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Though the global population is still growing– The rate of growth began to slow approximately

40 years ago

Figure 52.23

1950 1975 2000 2025 2050Year

2003

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Human Population MilestonesPop. Size (Billions)

Years Time Elapsed since previous billion

1 1804 ~ 4 million years

2 1927 123

3 1959 32

4 1974 14 years, 9 months

5 1987 13 years, 3 months

6 1999 12 years, 3 months

Predicted 7 2012 12 years, 4 months

Predicted 8 2025 13 years

Predicted 9 2040 15 years

World Population Growth

Country Development:• More-Developed Countries (MDCs)– Slow population growth (0.1% overall) (0.6% in

United States due to high immigration levels)– High standard of living– North America, Europe,Japan, Australia– Doubled populations between 1850 & 1950.

Due to:

1. Decline in death rate due to modern medicine & improved socioeconomic

conditions.

2. Decline in birthrates shortly thereafter.

This is sequence of events is called the demographic transition.

Country Development;• Less-Developed Countries (LDCs)– Rapid population growth; current rate is

1.6%– Low standard of living– Latin America, Africa and Asia– Continuing increases in population due to:

1. Steep decline in death rate after WWII due to importation of modern

medicine

2. Demographic transition occurred later

- 82% of world’s population lives in Africa, Asia & Latin America

Infant Mortality and Life Expectancy

• Infant mortality and life expectancy at birth

– Vary widely among developed and developing countries but do not capture the wide range of the human condition

Figure 52.26

Developed countries

Developing countries

Developed countries

Developing countries

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Age Distributions• A country’s age structure can be used to predict

future population growth.• LDC’s have larger population growth because

they have more women entering the reproductive years than older women leaving them.

• Zero Population Growth: If each couple has two children to replace

themselves, eventually the human population will level off but NOT immediately. Why?

• Replacement Reproduction: The greater number of women entering their

reproductive years compared to older women leaving them, will cause most countries to continue to grow for at least one or two generations.

Age Structure Diagrams (2002)

Age Structure Pyramids

Figure 52.25

Rapid growth AfghanistanSlow growth United States

Decrease Italy

Male Female Male Female Male FemaleAge Age

8 6 4 2 0 2 4 6 8 8 6 4 2 0 2 4 6 8 8 6 4 2 0 2 4 6 8Percent of population Percent of population Percent of population

80–8485

75–7970–7465–6960–6455–5950–5445–4940–4435–3930–34

20–2425–29

10–145–90–4

15–19

80–8485

75–7970–7465–6960–6455–5950–5445–4940–4435–3930–34

20–2425–29

10–145–90–4

15–19

Environmental Impact• Environmental impact of a population is

measured in terms of: 1. Population size

2. Resource consumption per capita 3. Resultant pollution caused by

population size and increased resource consumption

Thus, there are two types of overpopulation:1. Due to population growth (obvious in

LDCs)2. Due to resource consumption (obvious in

MDCs) Average person in US = 30 in India

• Population growth is putting extreme pressure on each country’s social organization, the Earth’s resources, and the biosphere

• MDCs consume a much larger proportion of the Earth’s resources than do LDCs

Environmental Impact Caused by MDCs & LDCs

U.S.Population Statistics• 281,000,000 (April 2000); • 309,000,000 (March 2010)– Most people are Caucasian (66%)– Hispanic (15%) - 60% from Mexico– Black (12%)– Asian (4%) - ~36% live in CA– Mixed (2%)– Native Americans (1%)

• 40% of population under 18 belong to a minority group - minorities increasing

Average Density of People in U.S. & Diamond Bar, CA

• U.S. Density = 73 people/mi2

• Diamond Bar area = 14.9 mi2 or 38.8 km2

• Population = 58,763 people• D.B. Density = 3,944 people/mi2 or

1,515 people/km2

Mumbai, India - World’s Most Densely Populated City

• 14.35 million people/484 km2

• Density Mumbai = 29,650 people/km2

• Let’s get the Diamond Bar equivalent: 29,650 people/km2 (density-Mumbai) X 38.8 km2

(D.B.) would be 1,150,420 people in DB

• Others:– Shenzhen, China (5th) - 466 km2 / 8 million

people (17,150 people/km2)– Seoul, South Korea - 16,700 people/km2

– Taipei, Taiwan - 15,200 people/km2

Mumbai, India