population growth apbio

CH. 26CH. 26

Population Growth Population Growth and Regulationand Regulation

Ch. 26 2 The Study of Ecology The Study of Ecology

EcologyEcology: the study of interrelationships : the study of interrelationships between living things and their nonliving between living things and their nonliving environmentenvironment

The environment consists of two The environment consists of two componentscomponents• AbioticAbiotic component: nonliving, such as soil component: nonliving, such as soil

and weatherand weather• BioticBiotic component: all living forms of life component: all living forms of life

Ch. 26 3 The Study of Ecology The Study of Ecology

Ecology can be studied at several Ecology can be studied at several organizational levelsorganizational levels• PopulationsPopulations: all members of a single species : all members of a single species

living in a given time and place and actually or living in a given time and place and actually or potentially interbreedingpotentially interbreeding

• EcosystemEcosystem: all the interacting populations in a : all the interacting populations in a given time and placegiven time and place

• CommunitiesCommunities: all the organisms and their : all the organisms and their nonliving environment in a defined area nonliving environment in a defined area

• BiosphereBiosphere: all life on Earth: all life on Earth

Ch. 26 4

How Does Population Size Change?How Does Population Size Change?

Several processes can change the size of Several processes can change the size of populationspopulations• BirthBirth and and immigrationimmigration add individuals to a add individuals to a

population population • DeathDeath and and emigrationemigration remove individuals from remove individuals from

the populationthe population

Change in population sizeChange in population size = (births – deaths) + (immigrants – emigrants)= (births – deaths) + (immigrants – emigrants)

Ch. 26 5

How Does Population Size Change?How Does Population Size Change?

Ignoring migration, population size is Ignoring migration, population size is determined by two opposing forcesdetermined by two opposing forces• Biotic potentialBiotic potential: the maximum rate at which a : the maximum rate at which a

population could increase when birth rate is population could increase when birth rate is maximal and death rate minimalmaximal and death rate minimal

• Environmental resistanceEnvironmental resistance: limits set by the : limits set by the living and nonliving environment that living and nonliving environment that decrease birth rates and/or increase death decrease birth rates and/or increase death rates (examples: food, space, and predation)rates (examples: food, space, and predation)

Ch. 26 6 Population GrowthPopulation Growth

The The growth rategrowth rate ( (rr) of a population is the ) of a population is the change in the population size change in the population size per per individualindividual over some time interval over some time interval

Determined byDetermined byGrowth rate (Growth rate (rr) = birth rate () = birth rate (bb) – death rate () – death rate (dd))


Birth rateBirth rate ( (bb) is the average number of births ) is the average number of births per individual per unit timeper individual per unit time• Example: if there are 5 births among 10 Example: if there are 5 births among 10

individuals, individuals, bb = 5/10 = = 5/10 = 0.50.5


Death rateDeath rate ( (dd) is the proportion of individuals ) is the proportion of individuals dying per unit timedying per unit time• Example: if 4 of 10 individuals die, Example: if 4 of 10 individuals die, dd = 4/10 = 4/10

= = 0.40.4• Thus, Thus, rr = = bb – – dd

= 0.5 – 0.4 = 0.5 – 0.4 = = 0.10.1


Population growth per unit of time can be Population growth per unit of time can be calculated by multiplying growth rate (calculated by multiplying growth rate (rr) by ) by the original population size (the original population size (NN))

Population growth (G) = Population growth (G) = rNrN In the previous example, population growth In the previous example, population growth

= = rNrN = 0.1(10) = = 0.1(10) = 11, so the population has , so the population has grown by one individualgrown by one individual


To determine the size of the population at To determine the size of the population at the end of the time period, add the the end of the time period, add the population growth (population growth (rNrN) to the initial ) to the initial population size (population size (NN))

= = NN + + rNrN = 10 + 0.1(10) = 10 + 0.1(10) = 10 + 1 = 10 + 1 = = 1111

Ch. 26 11 Exponential GrowthExponential Growth

Exponential growthExponential growth occurs when a population occurs when a population continuously grows at a fixed percentage continuously grows at a fixed percentage of its size at the beginning of each time of its size at the beginning of each time periodperiod• This results in a This results in a J-shaped growth curveJ-shaped growth curve


Doubling timeDoubling time describes the amount of time it describes the amount of time it takes to double its population at its current takes to double its population at its current state of growthstate of growth

Doubling time can be calculated as 0.7 Doubling time can be calculated as 0.7 divided by divided by rr • In our previous example, 0.7/0.1 = In our previous example, 0.7/0.1 = 7 time 7 time

intervalsintervals

Ch. 26 13 Biotic PotentialBiotic Potential

Biotic potential is influenced by several Biotic potential is influenced by several factorsfactors

The age at which the organism first The age at which the organism first reproduces reproduces

• Populations that have their offspring earlier Populations that have their offspring earlier in life tend to grow at a faster ratein life tend to grow at a faster rate

(2) The frequency at which reproduction (2) The frequency at which reproduction occursoccurs

Ch. 26 14

ExponentialExponentialGrowth CurvesGrowth Curves

Ch. 26 15

ExponentialExponentialGrowth CurvesGrowth Curves

Ch. 26 16 Biotic PotentialBiotic Potential

(3) The average number of offspring (3) The average number of offspring produced each timeproduced each time

(4) The length of the organism's (4) The length of the organism's reproductive life spanreproductive life span

(5) The death rate of individuals(5) The death rate of individuals• Increased death rates can slow the rate of Increased death rates can slow the rate of

population growth significantlypopulation growth significantly

Ch. 26 17

bacteria

Effect of Death Rates onEffect of Death Rates onPopulation GrowthPopulation Growth

Nodeaths

10% diebetween

doublings

25% diebetween

doublings


Exponential growth cannot continue Exponential growth cannot continue indefinitelyindefinitely

All populations that exhibit exponential All populations that exhibit exponential growth must eventually stabilize or crashgrowth must eventually stabilize or crash

Exponential growth can be observed in Exponential growth can be observed in populations that undergo populations that undergo boom-and-bust boom-and-bust cyclescycles• Periods of rapid growth followed by a Periods of rapid growth followed by a

sudden massive die-offsudden massive die-off


Boom-and-bust cycles can be seen in short Boom-and-bust cycles can be seen in short lived, rapidly reproducing specieslived, rapidly reproducing species• Ideal conditions encourage rapid growthIdeal conditions encourage rapid growth• Deteriorating conditions encourage massive Deteriorating conditions encourage massive

die-offdie-off

ExampleExample• Each year cyanobacteria in a lake may exhibit Each year cyanobacteria in a lake may exhibit

exponential growth when conditions are ideal, exponential growth when conditions are ideal, but crash when they have depleted their but crash when they have depleted their nutrient supplynutrient supply

Ch. 26 20

A Boom-and-bustA Boom-and-bustPopulation CyclePopulation Cycle

Conditions good, Boom; Conditions bad, BustConditions good, Boom; Conditions bad, Bust


ExampleExample• Lemming cycles are more complex and Lemming cycles are more complex and

involve overgrazing of food supply, large involve overgrazing of food supply, large migrations, and massive mortality caused by migrations, and massive mortality caused by predators and starvation predators and starvation

Ch. 26 22

Lemming Population CyclesLemming Population Cycles

Ch. 26 23

Environmental ResistanceEnvironmental ResistanceLimits Population GrowthLimits Population Growth

Decreases birthrate, increases death rateDecreases birthrate, increases death rateDensity-dependent factorsDensity-dependent factors

• PredationPredation• ParasitismParasitism• Competition (inter- and intraspecific)Competition (inter- and intraspecific)

Density-independent factorsDensity-independent factors• Weather, pesticides, pollutantsWeather, pesticides, pollutants

Causes populations to stabilize at or below carrying Causes populations to stabilize at or below carrying capacity capacity

• Maximum population size an area can support long term Maximum population size an area can support long term • Limitations on population growth necessary Limitations on population growth necessary

Ch. 26 24

The S-Curve of Population GrowthThe S-Curve of Population Growth

(biotic(bioticpotential)potential)

Carrying CapacityCarrying Capacity

Num

ber o

f Ind

ivid

uals

Num

ber o

f Ind

ivid

uals

TimeTime

ExponentialExponentialGrowthGrowth

EquilibriumEquilibrium

(environmental resistance)(environmental resistance)

Ch. 26 25 Environmental ResistanceEnvironmental Resistance

Carrying capacityCarrying capacity ( (KK) is the maximum ) is the maximum population size that can be sustained by population size that can be sustained by an ecosystem for an extended time without an ecosystem for an extended time without damage to the ecosystem damage to the ecosystem

Ch. 26 26

Ch. 26 27

Ch. 26 28

Environmental ResistanceEnvironmental Resistance

Logistic population growth can occur in nature Logistic population growth can occur in nature when a species moves into a new habitat, e.g. when a species moves into a new habitat, e.g. barnacles colonizing bare rock along a rocky barnacles colonizing bare rock along a rocky ocean shorelineocean shoreline

Initially, new settlers may find ideal conditions Initially, new settlers may find ideal conditions that allow their population to grow almost that allow their population to grow almost exponentiallyexponentially

As population density increases, individuals As population density increases, individuals compete for space, energy, and nutrientscompete for space, energy, and nutrients

Ch. 26 29

The Effects of ExceedingThe Effects of ExceedingCarrying CapacityCarrying Capacity

Ch. 26 30 Environmental ResistanceEnvironmental Resistance

Environmental resistance can be classified Environmental resistance can be classified into two broad categoriesinto two broad categories• Density-independent factorsDensity-independent factors• Density-dependent factorsDensity-dependent factors

Ch. 26 31 Density-Independent FactorsDensity-Independent Factors

Density-independent factorsDensity-independent factors limit populations limit populations regardless of their densityregardless of their density• Examples: climate, weather, floods, fires, Examples: climate, weather, floods, fires,

pesticide use, pollutant release, and pesticide use, pollutant release, and overhuntingoverhunting

• Some species have evolved means of limiting Some species have evolved means of limiting their lossestheir losses

• Examples: seasonally migrating to a better Examples: seasonally migrating to a better climate or entering a period of dormancy when climate or entering a period of dormancy when conditions deteriorateconditions deteriorate

Ch. 26 32 Density-Dependent FactorsDensity-Dependent Factors

Density-dependent factorsDensity-dependent factors become more become more effective as population density increaseseffective as population density increases

Exert negative feedback effect on Exert negative feedback effect on population sizepopulation size

Density-dependent factorsDensity-dependent factors can cause birth can cause birth rates to drop and/or death rates to increaserates to drop and/or death rates to increase• Population growth slows resulting in an S-Population growth slows resulting in an S-

shaped growth curve (or shaped growth curve (or S-curveS-curve))


At carrying capacity, each individual's share At carrying capacity, each individual's share of resources is just enough to allow it to of resources is just enough to allow it to replace itself in the next generation replace itself in the next generation

At carrying capacity birth rate (At carrying capacity birth rate (bb) = death ) = death rate (rate (dd))


Carrying capacity is determined by the Carrying capacity is determined by the continuous availability of resourcescontinuous availability of resources

Include community interactionsInclude community interactions• PredationPredation• ParasitismParasitism• CompetitionCompetition

Ch. 26 35 PredationPredation

PredationPredation involves a involves a predatorpredator killing a killing a preyprey organism in order to eat itorganism in order to eat it• Example: a pack of grey wolves hunting an Example: a pack of grey wolves hunting an

elkelkPredators exert density-dependent controls Predators exert density-dependent controls

on a populationon a population• Increased prey availability can increase birth Increased prey availability can increase birth

rates and/or decrease death rates of rates and/or decrease death rates of predatorspredators– Prey population losses will increasePrey population losses will increase

Ch. 26 36 PredationPredation

There is often a lag between prey availability There is often a lag between prey availability and changes in predator numbers and changes in predator numbers • Overshoots in predator numbers may cause Overshoots in predator numbers may cause

predator-prey predator-prey population cyclespopulation cycles• Predator and prey population numbers Predator and prey population numbers

alternate cycles of growth and declinealternate cycles of growth and declinePredation may maintain prey populations Predation may maintain prey populations

near carrying capacity near carrying capacity • ““Surplus" animals are weakened or more Surplus" animals are weakened or more

exposedexposed

Ch. 26 37

Population Cycles inPopulation Cycles inPredators and PreyPredators and Prey

Ch. 26 38

Experimental Predator–Experimental Predator–prey Cyclesprey Cycles

Ch. 26 39 ParasitismParasitism

ParasitismParasitism involves a involves a parasiteparasite living on or in a living on or in a hosthost organism, feeding on it but not generally organism, feeding on it but not generally killing itkilling it• Examples: bacterium causing Lyme disease, Examples: bacterium causing Lyme disease,

some fungi, intestinal worms, ticks, and some some fungi, intestinal worms, ticks, and some protists protists

While parasites seldom directly kill their hosts, While parasites seldom directly kill their hosts, they may weaken them enough that death due they may weaken them enough that death due to other causes is more likelyto other causes is more likely

Parasites spread more readily in large Parasites spread more readily in large populationspopulations

Ch. 26 40 Spatial Distributions Spatial Distributions

The spatial pattern in which individuals are The spatial pattern in which individuals are dispersed within a given area is that dispersed within a given area is that population’s population’s distribution, distribution, which may vary which may vary with timewith time

There are three major types of spatial There are three major types of spatial distributionsdistributions• ClumpedClumped• UniformUniform• RandomRandom


Clumped distribution – Clumped distribution – includes family and social includes family and social groups groups

Examples: elephant herds, wolf packs, prides of Examples: elephant herds, wolf packs, prides of lions, flocks of birds, and schools of fishlions, flocks of birds, and schools of fish

AdvantagesAdvantages• Provides many eyes that can search for localized Provides many eyes that can search for localized

food sourcesfood sources• Confuses predators with sheer numbersConfuses predators with sheer numbers• Cooperation for hunting more effectivelyCooperation for hunting more effectively

Ch. 26 42

Population Distributions: ClumpedPopulation Distributions: Clumped


UniformUniform distributiondistribution – constant distance – constant distance maintained between individuals; common maintained between individuals; common among territorial animals defending scarce among territorial animals defending scarce resources or defending breeding territoriesresources or defending breeding territories

Examples: iguanas, shorebirds, tawny owlsExamples: iguanas, shorebirds, tawny owlsAdvantage: a uniform distribution helps ensure Advantage: a uniform distribution helps ensure

adequate resources for each individualadequate resources for each individual

Ch. 26 44

Population Distributions: UniformPopulation Distributions: Uniform

Ch. 26 45 Spacial Distributions Spacial Distributions

Random distribution – Random distribution – rare, exhibited by rare, exhibited by individuals that do not form social groups; individuals that do not form social groups; occurs when resources are not scarce occurs when resources are not scarce enough to require territorial spacingenough to require territorial spacing

Examples: Trees and other plants in rain Examples: Trees and other plants in rain forestsforests

Ch. 26 46

Population Distributions: RandomPopulation Distributions: Random

Ch. 26 47 Survivorship in Populations Survivorship in Populations

Survivorship describes the pattern of Survivorship describes the pattern of survival in a populationsurvival in a population

Life tablesLife tables track groups of organisms born at track groups of organisms born at the same time throughout their life span, the same time throughout their life span, recording how many continue to survive in recording how many continue to survive in each succeeding yeareach succeeding year

Ch. 26 48


A A survivorship curvesurvivorship curve for a population can be for a population can be produced by graphing life table survivorship produced by graphing life table survivorship data data • Y-axis: the log of the number of individuals Y-axis: the log of the number of individuals

surviving to a particular age surviving to a particular age • X-axis: ageX-axis: age

Three types of survivorship curves can be Three types of survivorship curves can be distinguisheddistinguished• Late lossLate loss• Constant lossConstant loss• Early lossEarly loss

Ch. 26 50

Survivorship CurvesSurvivorship Curves


"Late loss" curves"Late loss" curves: seen in many animals : seen in many animals with few offspring that receive substantial with few offspring that receive substantial parental care; are convex in shape, with parental care; are convex in shape, with low mortality until individuals reach old low mortality until individuals reach old ageage• Examples: humans and many large Examples: humans and many large

mammalsmammals


"Constant loss" curves"Constant loss" curves: an approximate : an approximate straight line, indicates an equal chance of straight line, indicates an equal chance of dying at any age dying at any age • Example: some bird speciesExample: some bird species


"Early loss" curves"Early loss" curves: high early mortality as : high early mortality as most offspring fail to become established; most offspring fail to become established; are concave in shape are concave in shape • Typical of most plants and many animals that Typical of most plants and many animals that

do not receive parental caredo not receive parental care• Examples: most invertebrates and fishExamples: most invertebrates and fish

Ch. 26 54

Human PopulationHuman Population

Growing exponentiallyGrowing exponentially• Exponential growth will continue (age-structure)Exponential growth will continue (age-structure)• Developed countries with more stable populationsDeveloped countries with more stable populations• We continue to overcome environmental We continue to overcome environmental

resistanceresistance– Medical advances, agricultural revolutionMedical advances, agricultural revolution

U.S. populationU.S. population• Growing faster than world average (>1% annually)Growing faster than world average (>1% annually)• Immigration accounts for 30% of our growthImmigration accounts for 30% of our growth• Baby boomers are at reproductive ageBaby boomers are at reproductive age

Ch. 26 55 Rapid Human Population GrowthRapid Human Population Growth

In the last few centuries, the human In the last few centuries, the human population has grown at nearly an population has grown at nearly an exponential rateexponential rate• Follows a J-shaped growth curveFollows a J-shaped growth curve

Ch. 26 56 Technological AdvancesTechnological Advances

Most species must "make due" with the Most species must "make due" with the resources in an area resources in an area

Humans have manipulated the environment to Humans have manipulated the environment to increase the Earth’s carrying capacityincrease the Earth’s carrying capacity

Several technological “revolutions” have greatly Several technological “revolutions” have greatly influenced the human ability to make influenced the human ability to make resources availableresources available

• Technical and cultural revolutionTechnical and cultural revolution• Agricultural revolutionAgricultural revolution • Industrial-medical revolutionIndustrial-medical revolution

Ch. 26 57

Human Population GrowthHuman Population Growth

Ch. 26 58 Demographic TransitionDemographic Transition

In "developed" countries, the In "developed" countries, the industrial-industrial-medical revolutionmedical revolution resulted in an initial rise resulted in an initial rise in population, which then stabilizedin population, which then stabilized• Caused by a decrease in death rates, Caused by a decrease in death rates,

followed later by a decrease in birth ratesfollowed later by a decrease in birth rates

Ch. 26 59

Pop. Growth Rates:Pop. Growth Rates:Developed & Developing CountriesDeveloped & Developing Countries

DevelopingDevelopingCountriesCountries

Developed CountriesDeveloped Countries


Declining birth rates associated with Declining birth rates associated with demographic transition result from many demographic transition result from many factorsfactors• Better educationBetter education• Increased access to contraceptivesIncreased access to contraceptives• Shift of populations to cities (children Shift of populations to cities (children

provide fewer advantages than in agricultural provide fewer advantages than in agricultural areas)areas)

• More women working outside the homeMore women working outside the home


Demographic transition has occurred in Demographic transition has occurred in most developed countriesmost developed countries

When the adults of a population have just When the adults of a population have just enough children to replace themselves, the enough children to replace themselves, the situation is called situation is called replacement-level fertility replacement-level fertility (RLF)(RLF)

Because not all children survive to maturity, Because not all children survive to maturity, RLF is slightly higher than 2RLF is slightly higher than 2

Ch. 26 62 Uneven DistributionUneven Distribution

Many "developing" countries still have rapidly Many "developing" countries still have rapidly growing populations, as birth rates vastly growing populations, as birth rates vastly exceed death ratesexceed death rates• As in developed countries, death rates from As in developed countries, death rates from

infectious disease and starvation are lowinfectious disease and starvation are low

However, birth rates remain highHowever, birth rates remain high• Low incomes and the need for many children to Low incomes and the need for many children to

raise family income or produce foodraise family income or produce food• Knowledge of and access to contraception are Knowledge of and access to contraception are

limitedlimited

Ch. 26 63 Population Age StructurePopulation Age Structure

Age structureAge structure• Refers to the distribution of human Refers to the distribution of human

populations according to age groups populations according to age groups

Age structure can be shown graphically Age structure can be shown graphically • Age is shown on the vertical axis Age is shown on the vertical axis • The number of individuals in each age group The number of individuals in each age group

is shown on the horizontal axis, with males is shown on the horizontal axis, with males and females placed on opposite sidesand females placed on opposite sides


All age-structure diagrams peak at the All age-structure diagrams peak at the maximum life span, but the shape below maximum life span, but the shape below the peak reveals if the population is the peak reveals if the population is expanding, stable, or shrinkingexpanding, stable, or shrinking

Ch. 26 65

Generalized Age-Structure DiagramsGeneralized Age-Structure Diagrams

Post-ReproductivePost-Reproductive(46-100 y old)(46-100 y old)

ReproductiveReproductive(15-45 y old)(15-45 y old)Pre-ReproductivePre-Reproductive(0-14 y old)(0-14 y old)

ExpandingExpanding StableStable ContractingContracting

Ch. 26 66

Age Structures ComparedAge Structures Compared

Ch. 26 67

Age Structures ComparedAge Structures Compared


These diagrams reveal that even if These diagrams reveal that even if developing countries were to achieve RLF developing countries were to achieve RLF immediately, their population increases immediately, their population increases would continue for decadeswould continue for decades• A large population of children today create a A large population of children today create a

momentum for future growth as they enter momentum for future growth as they enter their reproductive yearstheir reproductive years

Ch. 26 69 Fertility in EuropeFertility in Europe

A comparison of growth rates for various A comparison of growth rates for various world regions shows Europe as the only world regions shows Europe as the only one with average rate of change that is one with average rate of change that is negativenegative

Ch. 26 70

00 0.50.5 11 1.51.5 22 2.52.5 33-0.5-0.5

World Average 1.4%

Developing Countries 1.7%

Africa 2.4%

Latin America/Caribbean 1.8%

Asia (excluding China) 1.7%

China 0.9%

NorthAmerica 0.6%

Developed Countries 0.1%

Europe -0.1%

Population GrowthPopulation Growthby World Regionsby World Regions

Natural Increase (annual %)Natural Increase (annual %)

Ch. 26 71 The U.S. PopulationThe U.S. Population

U.S. population is fastest growing of all U.S. population is fastest growing of all industrial nations industrial nations • U.S. fertility rate is only ~2.0, actually below U.S. fertility rate is only ~2.0, actually below

RLFRLF• However, immigration is adding people However, immigration is adding people

rapidlyrapidly

Ch. 26 72

U.S. Population GrowthU.S. Population Growth

U. S. over-consumes and U. S. over-consumes and over-pollutesover-pollutes

• U.S. citizen uses 6X U.S. citizen uses 6X energy of average world energy of average world citizencitizen

• 4.7% of world population 4.7% of world population

• 22% of carbon dioxide 22% of carbon dioxide and CFC emissionsand CFC emissions

CH. 26CH. 26

The EndThe End

population growth apbio

Technology

rate of population growth

population growth rn

population growth necessary

population growth death

population growth birth

population size change

exponential growth boom

maximum population size