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UNIT 2: ECOLOGYPOPULATIONS

1. How Populations Grow 5-1

2. Limits to Growth 5-2

3. Human Population Growth 5-3

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1. How Populations Grow

Three important characteristics of a population are:Geographic DistributionDensityGrowth Rate

Characteristics of Populations

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Geographic Distribution (Range) = the area inhabited by a population

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Population Density = the number of individuals per unit area

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Population Growth

Populations can stay the same (=), grow (+), or decrease (-) in size from year to year

Three factors can affect population size:The number of birthsThe number of deaths

The number of individuals that enter or leave the population

Population Size = # of Births - # of Deaths

Births (olive)Deaths (red)

Population size (blue)

?

?

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Characteristics of a PopulationCharacteristics of a Population

• Population - individuals inhabiting the same area at the same time

• Population Dynamics: Population change due to– Population Size - number of individuals– Population Density - population size in a certain

space at a given time– Population Dispersion - spatial pattern in habitat– Age Structure - proportion of individuals in each age

group in population7

Population SizePopulation Size• Natality

– Number of individuals added through reproduction– Crude Birth Rate - Births per 1000– Total Fertility Rate – Average number of children

born alive per woman in her lifetime

• Mortality– Number of individuals removed through death– Crude Death Rate Deaths per 1000

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Population DensityPopulation Density

• Population Density (or ecological population density) is the amount of individuals in a population per unit habitat area– Some species exist in high densities - Mice– Some species exist in low densities - Mountain lions

• Density depends upon– social/population structure– mating relationships– time of year

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Immigration = the movement of individuals into an area

Emigration = the movement of individuals out of an area

Which causes an increase in population size? Decrease?

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Exponential Growth

Exponential Growth = occurs when the individuals in a population reproduce at a constant rate

“J-shaped curve “

Under ideal conditions with unlimited resources

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E. coli

Reproduce once/20 minutes

20 minutes = 240 minutes = 4

60 minutes = 8

80 minutes = 16100 minutes = 32120 minutes = 64140 minutes = 128160 minutes = 256180 minutes = 512200 minutes = 1024220 minutes = 2048240 minutes = 4096

24 hours? 5 hours = 32,7686 hours = 262,1448 hours = 2,097,152

2n

n = # times reproduced

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4 hours?

What if “Unchecked? 12

Logistic GrowthLogistic Growth = occurs when a populations growth slows or stops following a period of exponential growth

“S-shaped” Curve

What factors decrease?What factors increase?

Birthrate and ImmigrationDeath rate and Emigration

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Carrying Capacity (K)Carrying Capacity (K)

• Exponential curve is not realistic due to carrying capacity of area

• Carrying capacity is maximum number of individuals a habitat can support over a given period of time due to environmental resistance (sustainability)

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Carrying Capacity = the largest number of individuals (within a species) that a given environment can support

More Natural

Growth rate equals zero

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Age StructureAge Structure• The age structure of a population is usually

shown graphically

• The age structure of a population dictates whether it will grow, shrink, or stay the same size

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Age Structure DiagramsAge Structure DiagramsPositive Growth Zero Growth Negative Growth (ZPG)Pyramid Shape Vertical Edges Inverted Pyramid

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2. Limits to Growth

Primary ProductivityLimiting Nutrients

Limiting Factors Limiting Factor = a factor that causes population growth to decrease

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Density-Dependent Factors

Density-Dependent Limiting Factor = a limiting factor that depend on population size

These factors operate most strongly when a population is large and dense

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Competition

When populations become crowded, organisms compete with one another for food, water, space, sunlight, and other essentials

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• Biotic Potential– factors allow a population to increase

under ideal conditions, potentially leading to exponential growth

• Environmental Resistance – affect the young more than the elderly

in a population, thereby affecting recruitment (survival to reproductive age)

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Exponential and Logistic Population Growth: J-Curves and S-Curves

Populations grow rapidly with ample resources, but as resources become limited, its growth rate slows and levels off.

Figure 8-4Figure 8-422

Exponential and Logistic Population Growth: J-Curves and S-Curves

As a population levels off, it often fluctuates slightly above and below the carrying capacity.

Figure 8-4Figure 8-423

Predator- Prey Relationship

“Inverse” relationship

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Parasitism

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Density-Independent Factors

Density-Independent Limiting Factors = a limiting factor that does not depend on the density of a population

They affect all populations in similar ways, regardless of the population size

Pop <then >

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Natural DisastersHuman Disturbance

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3. HUMAN POPULATION GROWTH

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Population GrowthPopulation Growth• Population growth depends upon

– birth rates– death rates– immigration rates (into area)– emigration rates (exit area)

Pop = Pop0 + (b + i) - (d + e)

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5-3 Human Population Growth

•Demography- the study of human populations.

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Like the populations of many other living organisms, the size of the human population tends to increase with time

Life was harsh, and limiting factors kept population sizes low

For most of human existence, the population grew slowly

Food was scarce and diseases were rampant

Until fairly recently, only half the children in the world survived to adulthood

Families would have many children to offset the death rate

About 500 year ago, the human population began growing more rapidly

Why?

Historical Overview

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Case Study: Fertility and Birth Rates in the United States

In 2006, the total fertility rate in the United States was slightly > 2.0

Figure 9-5Figure 9-533

Fig. 9-5, p. 175

ReplacementLevel

Year

Bir

ths

per

wo

man

Baby boom(1946–64)

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Case Study: Fertility and Birth Rates in the United States

The baby bust that followed the baby boom was largely due to delayed marriage, contraception, and abortion.

Figure 9-6Figure 9-635

Agriculture and industry Improved sanitation, medicine, and health

Death Rate?Birth Rate?

Experienced exponential growth

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1650 = ~ 500 million1850 (200 years) = doubled 1 billion1930 (80 years) = doubled 2 billion1975 (45 yeas) = doubled 4 billionToday = + 6 billion

Increasing ~80 million/year214,000 people/day = Amarillo populationWeekly = + San Antonio3 years = + United States2025 = 7.8 billion

Indefinite Growth?

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Limited Resources

The question is, when and how will our population growth slow? 38

people per km²

Demography = the scientific study of human populations

Examines the characteristics of human populations and attempts to explain how those populations will change over time

Examine: birthrates, death rates, and the age structure of a population

Patterns of Population Growth

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Over the past century, population growth in the U.S., Japan, and much of Europe has slowed dramatically (Developed Countries)

Hypothesis: These countries have completed the demographic transition

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Throughout most of history, human societies have had high death rates and equally high birthrates

With advances in nutrition, sanitation, and medicine, more children survive to adulthood and more adults live to old age

These changes lower death rates and begin the demographic transition

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5-3

• The shift from high birth and death rates to low birth and death rates is called the demographic transition.

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The demographic transition has three stages.

Write this in off to the side.

• In stage 1, there are high death rates and high birthrates.

• In stage 2, the death rate drops, while the birthrate remains high. The population increases rapidly.

• In stage 3, the birthrate decreases, causing population growth to slow.

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Demographic transition graph

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Demographic Transition = a dramatic change in birth and death rates

BR = DR

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5-3

Stage III• Japan• Most European countries• United States

Stage II• African countries• South and Central American countries• Many Asian countries

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Much of the population growth today is contributed by only 10 countries, with India and China in the lead, where birthrates remain high 47

Age Structure:

Age-Structure diagrams (Population Profiles) = graph the numbers of people in different age groups in the population

Used to predict future growth

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Future Population Growth Current projections suggest that by 2050, the world’s population may reach more than 9 billion people

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Most ecologists suggest that if this growth does not slow down even more, there could be serious damage to the environment as well as to the global economy

On the other hand, many economists assert that science, technology, and changes in society will control those negative impacts on the environment and economy

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