Chapter 5PopulationsChapter 5

Populations5-1

How Populations Grow

How Populations GrowHow Populations Grow

Characteristics of Populations–4 important characteristics of a populationGeographic distributionDensityGrowth rate

Age structure

Geographic DistributionGeographic Distribution

Geographic distribution, or range, is a term that describes the area inhabited by a population.Ranges can very enormously in size

Population DensityPopulation Density

Population density is the number of individuals per unit area.

The population of saguaro cactus in the desert plant community has a low density, whereas other plants in that community have a relatively high density.

Populations GrowthPopulations GrowthThree factors can affect population

size:number of birthsthe number of deathsthe number of individuals that enter or leave the population. * Simply put, a population will increase or decrease

in size depending on how many individuals are added to it or removed from it

Immigration & EmigrationImmigration & EmigrationImmigration

the movement of individuals into an area, is another factor that can cause a population to grow.

Emigration the movement of individuals out of an area, can cause a population to decrease in size.

Word OriginWord Origin Immigration is formed from the Latin

prefix in-, meaning “in,” and migrare, meaning “to move from one place to another.

If the Latin prefix e- means “out,” then which of the following means “migration out”?

A.) emigrationB.) migrationC.) Immigration

Age StructureAge Structure

The number of males and females of each age a population contains.

Age structure greatly effects reproduction

Exponential GrowthExponential GrowthWhen the size of each generations

offspring is larger than the generation before it this is exponential growth.

If a population has abundant space and food, and is protected from predators and disease, then organisms in that population will multiply and the population size will increase.

Under ideal conditions where resources with unlimited resources.

Exponential GrowthExponential Growth

Exponential growth occurs when the individuals in a population reproduce at a constant rate. Under ideal conditions with unlimited resources, a population will grow exponentially.

CheckpointCheckpoint

Which of the following is NOT a condition for a population to reach exponential growth?A.) presence of unlimited

resourcesB.) Absence of predation and

diseaseC.) movement of individuals

out of the population

Logistic GrowthLogistic GrowthLogistic growth occurs when a

population's growth slows or stops following a period of exponential growth.

  As resources become less

available, the growth of a population slows or stops. The general, S-shaped curve of this growth pattern, is called logistic growth.

Carrying CapacityCarrying Capacity

The maximum number of individuals of a particular species that a given environment can support.

List four characteristics that are used to describe a population.

What factors can change a population's size?

What is the difference between exponential growth and logistic growth?

Limits to GrowthLimits to Growth

Limiting factor – a factor that controls the growth of a population.

Density dependentDensity independent

Limiting factors determine the carrying capacity of an environment for a species.

Density- Dependent Factors

Density- Dependent Factors

Limit a population when the number of individuals reach a certain level

Competition for food, water, space, sunlight, etc.

Predation and Herbivory – populations cycle up and down

Wolf/Moose graphWolf/Moose graph

Moose Wolves

Density- Dependent FactorsContinued

Density- Dependent FactorsContinued

Parasitism + disease – the denser the population the easier it spreads

Stress from overcrowding – can lower birth rates, higher death rates, can cause mothers to neglect young, even lead to emigration

Density -Independent Factors

Density -Independent Factors

Affect all populations in similar ways regardless of size and density

Weather, natural disasters, seasonal change, human interventions (dams, logging, housing developments)

Populations usually decrease dramatically after such an event

Can be permanent + cause extinction

Historical OverviewHistorical OverviewFor most of human existence, the

population grew slowly because life was harsh. Food was hard to find. Predators and diseases were common and life-threatening.

Historical OverviewHistorical OverviewThese limiting factors kept human death

rates very high. Until fairly recently, only half the children in the world survived to adulthood.

Because death rates were so high, families had many children, just to make sure that some would survive.

Exponential Human Population Growth Exponential Human Population Growth

As civilization advanced, life became easier, and the human population began to grow more rapidly. That trend continued through the Industrial Revolution in the 1800s.

The Predictions of Malthus The Predictions of Malthus This kind of exponential growth could not continue

forever.

Two centuries ago, English economist Thomas Malthus suggested that only war, famine, and disease could limit human population growth.

Malthus thought that human populations would be regulated by competition (war), limiting resources (famine), parasitism (disease), and other density-dependent factors.

Malthus’s work was vitally important to the thinking of Charles Darwin.

Patterns of Human Population GrowthPatterns of Human Population Growth

Birthrates, death rates, and the age structure of a population help predict why some countries have high growth rates while other countries grow more slowly.

The scientific study of human populations is called demography. Demography examines characteristics of human populations and attempts to explain how those populations will change over time.

The Demographic Transition The Demographic Transition

To explain this shift, demographers hypothesize that these countries have completed the demographic transition, a dramatic change from high birthrates and death rates to low birthrates and death rates.

The Demographic Transition The Demographic Transition

Three stages

Stage I, birthrates and death rates are high for most of history.

The Demographic Transition The Demographic Transition

In Stage II, advances in nutrition, sanitation, and medicine lead to lower death rates.

Birthrates remain high for a time, so births greatly exceed deaths and the population increases exponentially.

The Demographic Transition The Demographic Transition

During Stage III, as the level of education and living standards rise, families have fewer children and the birthrate falls; population growth slows.

The demographic transition is complete when the birthrate meets the death rate, and population growth stops.

The Demographic Transition The Demographic Transition

So far, the United States, Japan, and Europe have completed the demographic transition.

Parts of South America, Africa, and Asia are passing through Stage II.

A large part of ongoing human population growth is happening in only ten countries, with India and China in the lead.

The Demographic Transition The Demographic Transition

Globally, human population is still growing rapidly, but the rate of growth is slowing down.

Our J-shaped growth curve may be changing into a logistic growth curve.

Age Structure and Population Growth Age Structure and Population Growth

To understand population growth in different countries, we turn to age-structure diagrams.

These diagrams compare the age structure of the U.S. population with that of Guatemala.

Age Structure and Population Growth Age Structure and Population Growth

In the United States, there are nearly equal numbers of people in each age group.

This age structure predicts a slow but steady growth rate for the near future.

Age Structure and Population Growth Age Structure and Population Growth

In Guatemala, on the other hand, there are many more young children than teenagers, and many more teenagers than adults.

This age structure predicts a population that will double in about 30 years.

Age diagramAge diagram

US – slow but steady growth rate predicted

Guatemala – predicts population that will double in about 30 years.

Future Population Growth Future Population Growth

To predict how the world’s human population will grow, demographers consider many factors, including the age structure of each country and the effects of diseases on death rates—especially AIDS in Africa and parts of Asia.

Future Population Growth Future Population Growth Current data

suggest that global human population will grow more slowly over the next 50 years than it grew over the last 50 years.

Future Population GrowthFuture Population Growth

9 billion predicted by 2050

What will stop/slow the growth?