Chapter 20Chapter 20

POPULATIONSPOPULATIONS

20.1 Understanding 20.1 Understanding PopulationsPopulations

• As the human As the human population grows, population grows, the population of the population of other species other species declinesdeclines

• Population:Population: a group a group of organisms that of organisms that belong to the same belong to the same species and live in a species and live in a particular place at particular place at the same timethe same time

Population SizePopulation Size

• The actual population size can be The actual population size can be difficult to measuredifficult to measure

• Individuals can be too abundant, too Individuals can be too abundant, too widespread, or too mobile to be widespread, or too mobile to be easily countedeasily counted

Sampling a PopulationSampling a Population

• When it isn’t When it isn’t possible to count possible to count every member of every member of the population, a the population, a sample is takensample is taken

• From that sample, From that sample, the population can the population can be estimatedbe estimated

Population DensityPopulation Density

• Measures how crowded a population Measures how crowded a population isis

• Expressed as the number of Expressed as the number of individuals per unit of area or volumeindividuals per unit of area or volume

• Estimates are calculated for the total Estimates are calculated for the total land arealand area

DispersionDispersion

• The spatial distribution The spatial distribution of individuals within a of individuals within a populationpopulation

• Clumped:Clumped: clustered clustered togethertogether

• Even:Even: separated by the separated by the same amount of spacesame amount of space

• Random:Random: no pattern no pattern

Clumped DistributionClumped Distribution

• Food or living space may be clumpedFood or living space may be clumped• Social behavior (herds)Social behavior (herds)• Flocking behaviorFlocking behavior

Even DistributionEven Distribution

• Social interactionsSocial interactions• Colonial nesting behaviorColonial nesting behavior• V-pattern in flight (waterfowl V-pattern in flight (waterfowl

migration)migration)

Random DistributionRandom Distribution

• Results from seed dispersal by wind Results from seed dispersal by wind or by birdsor by birds

• Example: forests or a field of Example: forests or a field of wildflowerswildflowers

Population DynamicsPopulation Dynamics

• Birth rate:Birth rate: the the number of births number of births occurring over a occurring over a certain period of certain period of timetime

• Death rate/mortality Death rate/mortality rate:rate: the number of the number of deaths over a deaths over a period of timeperiod of time

• Life expectancy:Life expectancy: how long an how long an individual is individual is expected to live expected to live (the average life (the average life span)span)

Age StructureAge Structure

• Shows the distribution of individuals Shows the distribution of individuals in different age groups in a in different age groups in a populationpopulation

• Populations with a higher Populations with a higher percentage of younger individuals percentage of younger individuals will tend to have rapid growthwill tend to have rapid growth

Patterns of MortalityPatterns of Mortality

• Survivorship Survivorship curves:curves: show the show the likelihood of likelihood of survival at survival at different ages different ages throughout the life throughout the life of an organismof an organism

• Type I, II, or IIIType I, II, or III

Types of Survivorship Types of Survivorship CurvesCurves

• Type I:Type I: the chance of dying is small the chance of dying is small until late in lifeuntil late in life

• Type II:Type II: the chance of dying is the the chance of dying is the same regardless of agesame regardless of age

• Type III:Type III: the chance of dying while the chance of dying while young is greatyoung is great

20.2 Measuring 20.2 Measuring PopulationsPopulations

Growth Rate:Growth Rate: the amount by which a the amount by which a population changes size in a given population changes size in a given amount of timeamount of time

Depends on birth, death, emigration Depends on birth, death, emigration and immigrationand immigration

Birth rate – death rate = growth rateBirth rate – death rate = growth rate

Immigration vs. Immigration vs. EmigrationEmigration

• Immigration:Immigration: movement INTO a movement INTO a populationpopulation

• Emigration:Emigration: movement OUT OF movement OUT OF a populationa population

The Exponential ModelThe Exponential Model

• Describes a population that increases Describes a population that increases rapidly after only a few generationsrapidly after only a few generations

• The larger the population gets, the faster it The larger the population gets, the faster it growsgrows

• J-shaped curveJ-shaped curve

• Will become limited by not Will become limited by not enough food and too much enough food and too much wastes wastes (limiting factors)(limiting factors)

The Logistic ModelThe Logistic Model

• Describes a population that becomes Describes a population that becomes stable at the carrying capacity stable at the carrying capacity

• Carrying capacity:Carrying capacity: the number of the number of individuals the environment can individuals the environment can support over a long period of timesupport over a long period of time

• S-shaped curveS-shaped curve

Population RegulationPopulation Regulation

• Density-independent Density-independent factors:factors: reduce a reduce a population regardless of population regardless of the population size the population size (weather, natural (weather, natural disasters, fire)disasters, fire)

• Density-dependent Density-dependent factors:factors: the higher the the higher the population, the more population, the more organisms that are organisms that are affected (food, territory, affected (food, territory, nest sites)nest sites)

Population FluctuationPopulation Fluctuation

• Populations can fluctuate due to Populations can fluctuate due to environmental changesenvironmental changes

• Example: population cycle of the Example: population cycle of the snowshoe hare and the lynxsnowshoe hare and the lynx

Perils of Small Perils of Small PopulationsPopulations

• Small populations are at Small populations are at risk of extinctionrisk of extinction

• Vulnerable to Vulnerable to inbreeding, disease, inbreeding, disease, environmental environmental disturbances, etc. disturbances, etc.

• Inbred offspring have Inbred offspring have shorter life spans, shorter life spans, decreased genetic decreased genetic variability, and are more variability, and are more susceptible to diseasessusceptible to diseases

20.3 Human Population 20.3 Human Population GrowthGrowth

• Early human populations lived in Early human populations lived in small nomadic groups and obtained small nomadic groups and obtained food by hunting and gatheringfood by hunting and gathering

• Had a high mortality rate, especially Had a high mortality rate, especially among infants and childrenamong infants and children

The Agricultural The Agricultural RevolutionRevolution

• 10,000 years ago, 10,000 years ago, humans humans domesticated domesticated animals and animals and cultivated plantscultivated plants

• Agriculture Agriculture increased the increased the available food supplyavailable food supply

• Human population Human population began to grow fasterbegan to grow faster

The Industrial The Industrial RevolutionRevolution

• After the Middle Ages, 25% of the human After the Middle Ages, 25% of the human

population in Europe died due to the population in Europe died due to the bubonic plaguebubonic plague

• The industrial revolution began around 1780 The industrial revolution began around 1780 when there was a shift to using machinery when there was a shift to using machinery and burning fossil fuelsand burning fossil fuels

• Improvements in hygiene and in Improvements in hygiene and in

health care led to population growthhealth care led to population growth

Developed CountriesDeveloped Countries

• Developed countries:Developed countries: modern, modern, industrialized areas industrialized areas with better health with better health care, longer life care, longer life expectancies, but expectancies, but lower population lower population growthgrowth

• Examples: U.S., Examples: U.S., Japan, Canada, Japan, Canada, Russia, Germany, Russia, Germany, AustraliaAustralia

• Includes only 20% Includes only 20% of the world’s of the world’s population, but population, but uses 75% of all uses 75% of all fossil fuels!fossil fuels!

Developing CountriesDeveloping Countries

• Developing Developing countries:countries: poorer poorer areas, lower life areas, lower life expectancy, but expectancy, but higher population higher population growthgrowth

• Examples: Central Examples: Central America, South America, South America, Mexico, America, Mexico, AfricaAfrica

• Includes 80% of Includes 80% of the world’s the world’s population, but population, but uses only 25% of uses only 25% of the fossil fuels the fossil fuels globallyglobally

Human Population Human Population ExplosionExplosion

• By 2020, the human population By 2020, the human population could be as high as 12.5 billioncould be as high as 12.5 billion

• Will Earth be able to sustain that Will Earth be able to sustain that

amount of people or will the amount of people or will the

human population reach its human population reach its

carrying capacity?carrying capacity?