Population DynamicsPopulation Dynamics

Characteristics of PopulationsCharacteristics of Populations Population ecologyPopulation ecology is the study of is the study of populationspopulations

in relation to the environment, including in relation to the environment, including environmental influences on population density environmental influences on population density and distribution, age structure, and population and distribution, age structure, and population size.size.

A A populationpopulation is a is a group of individuals of a group of individuals of a single speciessingle species that live in the that live in the same general same general area.area.

Members of a population rely on the same Members of a population rely on the same resourcesresources, are influenced by similar , are influenced by similar environmental factorsenvironmental factors, and have a high , and have a high likelihood of likelihood of interacting with and breeding interacting with and breeding withwith one another. one another.

Population DynamicsPopulation Dynamics: : change in change in size, density dispersion and age size, density dispersion and age distribution in response to distribution in response to environmental conditions.environmental conditions.

Population Size:Population Size: the the numbernumber of of individuals in a population at a individuals in a population at a given given time.time.

Ways of Examining PopulationWays of Examining Population

1) 1) densitydensity of a population is measured of a population is measured as as the number of individuals per unit the number of individuals per unit area or volumearea or volume..

Ex: Ex: # of # of trees per acretrees per acre

# of # of deer per square miledeer per square mile

# of # of Daphnia per liter of waterDaphnia per liter of water

Difficulties in Measuring DensityDifficulties in Measuring Density

- counting all of the individuals – - counting all of the individuals – nearly nearly impossibleimpossible

- population counts are estimates based on - population counts are estimates based on sampling techniquessampling techniques

Ex: Ex: For Plants and Sessile OrganismsFor Plants and Sessile Organisms::

In a forest In a forest count the # of oak trees in many count the # of oak trees in many different areasdifferent areas

Average the number and multiply by the Average the number and multiply by the area of the habitatarea of the habitat

- only accurate if the sample sizes are - only accurate if the sample sizes are the samethe same and the area being and the area being surveyed is surveyed is homogenoushomogenous which may which may not be the case due to not be the case due to possible possible variations in habitatvariations in habitat that may make that may make it inhospitable and different social it inhospitable and different social interactions interactions

For AnimalsFor Animals: : Mark and RecaptureMark and Recapture- a certain area is used and a - a certain area is used and a sample sample of the population is caught and of the population is caught and markedmarked – tags, electronic devices, – tags, electronic devices, paint – paint – and releasedand released- later the - later the same areasame area is used and is used and more organisms are caught and more organisms are caught and examined for markingsexamined for markings- based on the number of organisms - based on the number of organisms that have that have marks and don’t have marks and don’t have marks the actual population is marks the actual population is calculatedcalculated

- The mark-recapture method - The mark-recapture method assumes assumes that each marked individual has the that each marked individual has the same probability of being trapped as same probability of being trapped as each unmarked individual.each unmarked individual.

- This may not be a safe assumption, - This may not be a safe assumption, as trapped individuals may be more as trapped individuals may be more or less likely to be trapped a second or less likely to be trapped a second time.time.

- Also affected by the - Also affected by the changes in the changes in the populationpopulation - - birthbirth (including all forms of (including all forms of reproduction)reproduction)- - immigrationimmigration (the influx of new (the influx of new individuals from other areas).individuals from other areas).- - deathdeath (mortality) (mortality) - - emigrationemigration (the movement of (the movement of individuals out of a population).individuals out of a population).

- Immigration and emigration may - Immigration and emigration may represent biologically significant represent biologically significant exchanges between populationsexchanges between populations

2) 2) dispersiondispersion of a population is the of a population is the pattern pattern of spacing among individuals within the of spacing among individuals within the geographic boundariesgeographic boundaries..

Dispersion is Dispersion is clumpedclumped when individuals when individuals aggregate in patchesaggregate in patches..

Plants and fungi are often clumped where Plants and fungi are often clumped where soil conditions favor germination and soil conditions favor germination and growthgrowth..

Animals may clump in favorable Animals may clump in favorable microenvironmentsmicroenvironments (such as isopods under (such as isopods under a fallen log) or to facilitate a fallen log) or to facilitate matingmating interactions.interactions.

Group living may increase the Group living may increase the effectivenesseffectiveness of certain predators, such of certain predators, such as a wolf pack.as a wolf pack.

Dispersion is Dispersion is uniformuniform when individuals when individuals are are evenly spacedevenly spaced..

For example, some plants secrete For example, some plants secrete chemicals that chemicals that inhibit the inhibit the germination and growth of nearby germination and growth of nearby competitorscompetitors..

Animals often exhibit uniform Animals often exhibit uniform dispersion as a result of dispersion as a result of territorialityterritoriality,, the the defense of a bounded space defense of a bounded space against encroachment by othersagainst encroachment by others..

In In randomrandom dispersion, the position of dispersion, the position of each individual is each individual is independent of the independent of the others, and spacing is unpredictableothers, and spacing is unpredictable..

Random dispersion occurs in the Random dispersion occurs in the absence of absence of strong attractionstrong attraction or or repulsionrepulsion among individuals in a among individuals in a population, or when key physical or population, or when key physical or chemical factors are chemical factors are relatively relatively homogeneously distributedhomogeneously distributed..

For example, plants may grow where For example, plants may grow where windblownwindblown seeds land. seeds land.

Random patterns are Random patterns are not commonnot common in in nature nature

Dispersion PatternsDispersion Patterns

DemographyDemography is the study of the is the study of the vital vital statisticsstatistics of populations and of populations and how how they changethey change over time. over time.

Factors Influencing DemographyFactors Influencing Demography::

1. 1. Birth rates/Death RatesBirth rates/Death Rates

2. 2. Fecundity (fertility) of OrganismsFecundity (fertility) of Organisms

3. 3. Rate of Maturation to Reproductive Rate of Maturation to Reproductive CapacityCapacity

4. 4. Life SpanLife Span

5. 5. Reproductive StrategiesReproductive Strategies

Often represented by a Often represented by a life tablelife table - an - an age-specificage-specific summary of the summary of the survival survival patternpattern of a population usually by of a population usually by studying a group of individuals studying a group of individuals ((cohortcohort) of the same age as they ) of the same age as they proceed through lifeproceed through life noting when noting when they they diedie and calculating the and calculating the survival survival ratioratio of the group of the group

Tables can also be represented as a Tables can also be represented as a Survivorship CurveSurvivorship Curve – Shows the – Shows the overall pattern of surviving members overall pattern of surviving members of a population throughout the life of of a population throughout the life of the group of individuals. the group of individuals.

Types of Survivorship CurvesTypes of Survivorship Curves

Type I curveType I curve is relatively flat at the is relatively flat at the start, reflecting a start, reflecting a low deathlow death rate in rate in early and middle life, and drops early and middle life, and drops steeply as steeply as death rates increase death rates increase among older age groupsamong older age groups..

Ex: Ex: humans, elephantshumans, elephants Indicates a high level of Indicates a high level of parental investmentparental investment

in the raising of young. (in the raising of young. (IteroparityIteroparity))

Types of Survivorship CurvesTypes of Survivorship Curves

Type II curveType II curve is is intermediateintermediate, with , with constant mortalityconstant mortality over an over an organism’s life spanorganism’s life span..

Ex: Ex: rodents, insects, annual plantsrodents, insects, annual plants equal chance of dying at all times of lifeequal chance of dying at all times of life

Types of Survivorship CurvesTypes of Survivorship Curves

Type III curveType III curve drops quickly at the drops quickly at the startstart, reflecting , reflecting very high death rates very high death rates early in lifeearly in life, then flattens out as , then flattens out as death rates decline for the few death rates decline for the few individuals that survive to a individuals that survive to a critical critical ageage..

Reflect the production of Reflect the production of large numbers of large numbers of offspringoffspring with with little or no parental carelittle or no parental care. . ((SemelparitySemelparity))

Ex: Ex: long lived plants, sea turtles, marine long lived plants, sea turtles, marine invertebrates, most fishinvertebrates, most fish..

Types of Survivorship CurvesTypes of Survivorship Curves

Many species fall somewhere Many species fall somewhere between these basic types of between these basic types of survivorship curves or show more survivorship curves or show more complex curves.complex curves.

Some invertebrates, such as crabs, Some invertebrates, such as crabs, show a “show a “stair-steppedstair-stepped” curve, with ” curve, with increased increased mortality during moltsmortality during molts. .

POPULATION GROWTHPOPULATION GROWTH

Population GrowthPopulation Growth Change in population size = Births - DeathsChange in population size = Births - Deaths

- for a certain time period- for a certain time period

OrOr N/N/t = (B − D)t = (B − D) where where BB is the number of is the number of

birthsbirths and and DD is the number of is the number of deathsdeaths.. NN represents represents population sizepopulation size, and , and tt

represents represents timetime, then , then NN is the is the change in change in population sizepopulation size and and tt is the is the time intervaltime interval..

Ignores Ignores immigration and emigrationimmigration and emigration The The per capita birth rateper capita birth rate is the is the

number of offspring produced per number of offspring produced per unit time by an unit time by an average member of average member of the populationthe population..

If there are 34 births per year in a If there are 34 births per year in a population of 1,000 individuals, the population of 1,000 individuals, the annual per capita birth rate is annual per capita birth rate is 34/1000, or 0.03434/1000, or 0.034 or or 3.4%3.4%

ESTIMATING DOUBLING TIME: ESTIMATING DOUBLING TIME: Rule of Rule of 7070

- once a growth rate has been - once a growth rate has been determined the estimated time for determined the estimated time for the the population to doublepopulation to double can be can be determined by determined by dividing 70 by the dividing 70 by the rate as a percentagerate as a percentage..

Ex: Ex: 70/3.4% = 20.670/3.4% = 20.6

If we know the annual If we know the annual per capita birth rateper capita birth rate ((expressed as bexpressed as b),), we can use the formula we can use the formula B = bNB = bN to calculate the to calculate the expected number expected number of birthsof births per year in a population of any per year in a population of any size.size.

Similarly, the Similarly, the per capita death rateper capita death rate (symbolized by (symbolized by m for mortalitym for mortality) allows us ) allows us to calculate the expected number of to calculate the expected number of deaths per unit time for a population of deaths per unit time for a population of any size. any size. D = mND = mN

Population ecologists are most Population ecologists are most interested in the interested in the differencesdifferences between between the per capita birth rate and the per the per capita birth rate and the per capita death rate.capita death rate.

This This differencedifference is the is the per capita rate per capita rate of increaseof increase or or rr which equals which equals bb − − mm. . r = b - mr = b - m

The The value of rvalue of r indicates whether a indicates whether a population is growing (r > 0)population is growing (r > 0) or or declining (r < 0).declining (r < 0).

If If r = 0r = 0, then there is , then there is zero population zero population growth (ZPG).growth (ZPG).

Births and deaths still occur, but they Births and deaths still occur, but they balancebalance exactly. exactly.

Population Growth ModelsPopulation Growth Models 1) 1) ExponentialExponential: population growth in an : population growth in an

idealized, unlimited environment.idealized, unlimited environment. All populations have a All populations have a tremendous tremendous

capacity for growthcapacity for growth.. A hypothetical population living in an A hypothetical population living in an

ideal, unlimitedideal, unlimited environmentenvironment.. Under these conditions, we may assume Under these conditions, we may assume

the the maximum growth ratemaximum growth rate for the for the population (population (rmaxrmax), called the ), called the intrinsic rate intrinsic rate of increase.of increase.

The size of a population that is growing The size of a population that is growing exponentially increases at a exponentially increases at a constantconstant rate, rate, resulting in a resulting in a J-shaped growth curveJ-shaped growth curve when when the population size is plotted over time.the population size is plotted over time.

J-shaped curves are characteristic of J-shaped curves are characteristic of populations that are introduced into a populations that are introduced into a new new or unfilled environmentor unfilled environment or whose numbers or whose numbers have been have been drastically reduceddrastically reduced by a by a catastrophiccatastrophic event and are rebounding. event and are rebounding.

Exponential Growth Exponential Growth dN/dt = change in number over timedN/dt = change in number over time r = growth rate as a decimalr = growth rate as a decimal N = population sizeN = population size

EX: How many individuals will be added to a EX: How many individuals will be added to a population of 500 if the growth rate is 25% per population of 500 if the growth rate is 25% per year? year?

EX: If 100 individuals are added to a population EX: If 100 individuals are added to a population in a year, what is the rate if the original in a year, what is the rate if the original population was 400? population was 400?

EX: If a population has a growth rate of 15%, EX: If a population has a growth rate of 15%, what is the original size of the population if 150 what is the original size of the population if 150 new individuals are added to the population? new individuals are added to the population?

2) The 2) The logistic modellogistic model of population growth of population growth incorporates the concept of incorporates the concept of carrying carrying capacitycapacity..

Typically, resources are Typically, resources are limitedlimited.. As population density As population density increasesincreases, each , each

individual has access to an individual has access to an increasingly increasingly smaller sharesmaller share of available resources. of available resources.

Ultimately, there is a Ultimately, there is a limitlimit to the number to the number of individuals that can occupy a of individuals that can occupy a habitathabitat..

Ecologists define Ecologists define carrying capacity (K)carrying capacity (K) as as the the maximum stable population size that a maximum stable population size that a particular environment can supportparticular environment can support

FACTORS DETERMINING CARRYING CAPACITYFACTORS DETERMINING CARRYING CAPACITY

Environmental ResistanceEnvironmental Resistance – all the – all the factors acting jointly to factors acting jointly to limit the limit the growth of a populationgrowth of a population

1) 1) Energy LimitationEnergy Limitation

2) 2) Shelter and Breeding SitesShelter and Breeding Sites

3) 3) PredatorsPredators

4) 4) Soil NutrientsSoil Nutrients

5) 5) WaterWater

If individuals cannot obtain sufficient If individuals cannot obtain sufficient resources to reproduce, the per capita resources to reproduce, the per capita birth rate bbirth rate b will will declinedecline..

If they cannot find and consume enough If they cannot find and consume enough energyenergy to maintain themselves, the per to maintain themselves, the per capita capita death rate m may increasedeath rate m may increase..

the per capita rate of the per capita rate of increaseincrease declines as declines as carrying capacity is reachedcarrying capacity is reached

Population growth is Population growth is greatestgreatest when when the population is approximately the population is approximately half half of the carrying capacity.of the carrying capacity.

At this population size, there are At this population size, there are many reproducing individuals but still many reproducing individuals but still enough resourcesenough resources, and the per capita , and the per capita rate of increase remains relatively rate of increase remains relatively high.high.

The logistic model of population growth The logistic model of population growth produces a produces a sigmoid (S-shaped) growth sigmoid (S-shaped) growth curvecurve when when NN is plotted over time. is plotted over time.

Population growth rate slows dramatically Population growth rate slows dramatically as as N approaches KN approaches K..

In most natural populations, there is a In most natural populations, there is a lag lag timetime before the before the negativenegative effects of effects of increasing population are realized.increasing population are realized.

Populations may Populations may overshootovershoot their carrying their carrying capacity before settling down to a capacity before settling down to a relatively stable density.relatively stable density.

Logistic Growth Logistic Growth

dN/dt = rdN/dt = rmaxmaxN ((K-N)/K)N ((K-N)/K) dN/dt = growth of the population dN/dt = growth of the population

(number of new individuals added)(number of new individuals added) rrmaxmax = growth rate as a decimal = growth rate as a decimal K = carrying capacity populationK = carrying capacity population N = number of individuals in the N = number of individuals in the

populationpopulation If N < K then r > 0, if N = K then r = 0, If N < K then r > 0, if N = K then r = 0,

and if N > K then r < 0and if N > K then r < 0

Ex: If a population of 200 is at half Ex: If a population of 200 is at half the carrying capacity and growing at the carrying capacity and growing at a rate of 20 individuals per year, a rate of 20 individuals per year, what is the growth rate of the what is the growth rate of the population? population?

If a population of 500 is at its If a population of 500 is at its carrying capacity, what is the growth carrying capacity, what is the growth rate? rate?

A population exhibits logistic growth. If the carrying capacity is 200 for a given area. If the growth rate is 2% at the carrying capacity, what is the maximum growth rate?

(Hint: the population at the maximum growth rate is half the population at carrying capacity or N at rmax = K/2)

 

How Does Population Density How Does Population Density Affect Population Growth?Affect Population Growth?

Density-independent population Density-independent population controls affect a population size controls affect a population size regardless of its densityregardless of its density. .

Ex: Ex: floods, hurricanes, floods, hurricanes, earthquakes, landslides, earthquakes, landslides, drought, fire, habitat drought, fire, habitat destruction, pesticide sprayingdestruction, pesticide spraying

Density-dependent population Density-dependent population controlscontrols have a greater effect on a have a greater effect on a population as the population size population as the population size increasesincreases

Ex: Ex: competition for resources competition for resources (Intraspecific or Interspecific), (Intraspecific or Interspecific), territory. Health, predation, territory. Health, predation, parasitism, diseaseparasitism, disease

Dense populations have lower Dense populations have lower birthbirth rates, higher rates, higher deathdeath rates. rates.

Ex: In mice, Ex: In mice, overcrowdingovercrowding causes causes hormonalhormonal changes that inhibit sexual changes that inhibit sexual activity, lower sexual activity, reduced activity, lower sexual activity, reduced milk production. Stress from milk production. Stress from overcrowding reduces the number of overcrowding reduces the number of offspring produced (spontaneous offspring produced (spontaneous abortion)abortion). . May also lead to May also lead to cannibalismcannibalism and killing of the young.and killing of the young.

Population Change Curves Population Change Curves ((Population CyclesPopulation Cycles))

StableStable – population fluctuates – population fluctuates slightly slightly above and belowabove and below its its carrying capacitycarrying capacity

Irruptive Irruptive population occasionally population occasionally explodesexplodes

((irruptsirrupts) to a ) to a high peakhigh peak and then and then crashescrashes to a very low level. to a very low level.

caused by some factor that caused by some factor that temporarily temporarily increases the carrying capacityincreases the carrying capacity Ex: Ex: increase in rain = increased growth of increase in rain = increased growth of seeds seeds increase in mice increase in mice

CyclicCyclic – – boom-bust cyclesboom-bust cycles; poorly ; poorly understood and involve a number of understood and involve a number of factors.factors.

Factors Influencing Population GrowthFactors Influencing Population Growth

Life HistoriesLife Histories - the - the traits that traits that affect an organism’s schedule of affect an organism’s schedule of reproduction and survivalreproduction and survival

Factors Affecting Life HistoriesFactors Affecting Life Histories1) 1) when reproduction beginswhen reproduction begins2) 2) how often the organism how often the organism

reproducesreproduces3) 3) the number of offspring are the number of offspring are

produced during each produced during each reproductive episodereproductive episode..

Types of Life HistoriesTypes of Life Histories::

1) 1) SemelparitySemelparity: : big-bang big-bang reproductionreproduction - - an individual an individual produces a produces a large numberlarge number of of offspring and offspring and then diesthen dies..

2) 2) IteroparityIteroparity: : RepeatedRepeated reproductionreproduction - some organisms - some organisms produce only a produce only a few offspringfew offspring

Semeparity vs IteroparitySemeparity vs Iteroparity is is determined by the determined by the survival ratesurvival rate of the of the offspring.offspring.

When the survival of offspring is When the survival of offspring is lowlow, as in , as in highly variable or unpredictable highly variable or unpredictable environmentsenvironments, big-bang reproduction , big-bang reproduction ((semelparitysemelparity) is favored.) is favored.

Repeated reproduction (Repeated reproduction (iteroparityiteroparity) is ) is favored in favored in dependable environments dependable environments where competition for resources is where competition for resources is intenseintense..

However, all reproductive attempts are However, all reproductive attempts are limited by the limited by the resources availableresources available..

Ex: Plants and animals whose young are Ex: Plants and animals whose young are subject to subject to high mortality rates often high mortality rates often produce large numbers of relatively produce large numbers of relatively small offspringsmall offspring..

Plants that colonize Plants that colonize disturbed environmentsdisturbed environments usually produce many usually produce many small seedssmall seeds, only a few of , only a few of which reach suitable habitat.which reach suitable habitat.

Smaller seed size may increase the chance of Smaller seed size may increase the chance of seedling establishment by enabling seeds to be seedling establishment by enabling seeds to be carried longer distances to a broader range of carried longer distances to a broader range of habitats.habitats.

In other organisms, extra investment In other organisms, extra investment on the on the part of the parentpart of the parent greatly greatly increases the offspring’s increases the offspring’s chances of chances of survivalsurvival..

Oak, walnut, and coconut trees all have Oak, walnut, and coconut trees all have large seeds with a large store of energy and large seeds with a large store of energy and nutrients to help the seedlings become nutrients to help the seedlings become established.established.

However, a small population is not always However, a small population is not always beneficial for reproductive ratesbeneficial for reproductive rates

Some populations show an Some populations show an Allee effectAllee effect,, in which individuals may have a in which individuals may have a more more difficult time survivingdifficult time surviving or reproducing if or reproducing if the population is the population is too smalltoo small..

Animals may not be able to Animals may not be able to find matesfind mates in in the breeding season in small population the breeding season in small population sizes.sizes.

A plant may be protected in a clump of A plant may be protected in a clump of individuals but vulnerable to excessive individuals but vulnerable to excessive wind if it stands alone.wind if it stands alone.

Selection for life history traits that are Selection for life history traits that are sensitive to populationsensitive to population density is known density is known as as K-selectionK-selection,, or or density-dependent density-dependent selectionselection..

KK-selection tends to maximize population -selection tends to maximize population size and operates in populations living at a size and operates in populations living at a density near density near K (carrying capacity)K (carrying capacity)

Characteristics of KCharacteristics of K-selected Populations:-selected Populations:

- - fewer, larger offspringfewer, larger offspring- - high parental care of offspringhigh parental care of offspring- - later reproductive agelater reproductive age- - larger adultslarger adults- - adapted to climate and adapted to climate and environmental conditionsenvironmental conditions- - lower population growth rate (r)lower population growth rate (r)- - stable population size– near carrying stable population size– near carrying capacitycapacity- - high ability to compete for resourceshigh ability to compete for resources- - late successional specieslate successional species

Ex:Ex:

Selection for life history traits that Selection for life history traits that maximize reproductive success at low maximize reproductive success at low densitiesdensities is known as is known as r-selectionr-selection,, or or density-independent selectiondensity-independent selection..

rr-selection tends to maximize -selection tends to maximize rr, , the rate of the rate of increaseincrease, and occurs in environments in , and occurs in environments in which which population densities fluctuate population densities fluctuate well below Kwell below K, or when individuals face , or when individuals face little competitionlittle competition..

Characteristics of rCharacteristics of r-selected Populations-selected Populations

- - many small offspringmany small offspring- - little to no parental care of little to no parental care of

offspringoffspring- - early reproductive ageearly reproductive age- - most offspring die before most offspring die before

reaching reproductive agereaching reproductive age- - small adultssmall adults- - adapted to unstable climates adapted to unstable climates

and environmental conditionsand environmental conditions- - high population growth ratehigh population growth rate - - population size fluctuates wildly population size fluctuates wildly

and below carrying capacityand below carrying capacity- - low ability to competelow ability to compete- - early successional speciesearly successional speciesEx:Ex:

The Human PopulationThe Human Population

Human Population GrowthHuman Population Growth

1. The human population has been 1. The human population has been growing almost growing almost exponentiallyexponentially for for three centuriesthree centuries but cannot do so but cannot do so indefinitelyindefinitely..

How Have Humans Modified Natural How Have Humans Modified Natural Ecosystems?Ecosystems?

Global population = > Global population = > 7 billion people

Increasing by about Increasing by about 73 million each year73 million each year

Or Or 201,000 people each day201,000 people each day..

Population ecologists predict a population of Population ecologists predict a population of 7.3–8.4 billion7.3–8.4 billion people on Earth by the year people on Earth by the year 20252025..

Each color represents 1 billion Each color represents 1 billion people. people.

White areas is where 98% of White areas is where 98% of the Australian population livesthe Australian population lives

Although the global population is still Although the global population is still growing, the growing, the raterate of growth began to of growth began to slow approximately slow approximately 50 years ago50 years ago..

The rate of increase in the global The rate of increase in the global population peaked at population peaked at 2.19% in 19622.19% in 1962..

By By 20112011, it had declined to , it had declined to 1.17%1.17%.. Current Growth Rates

Current models project a Current models project a declinedecline in overall in overall growth rategrowth rate to just over to just over 0.4% by 20500.4% by 2050..

In the In the developed nationsdeveloped nations, populations are , populations are near near equilibriumequilibrium, with reproductive rates , with reproductive rates near near the replacement levelthe replacement level..

Most population growth is concentrated in Most population growth is concentrated in developing countriesdeveloping countries, where , where 80%80% of the of the world’s people live.world’s people live.

Examining Human Population Examining Human Population Growth:Growth:

Population growth seen in Population growth seen in Age Age structuresstructures - - the the proportion of proportion of individuals in each age group in a individuals in each age group in a populationpopulation..

prereproductiveprereproductive reproductivereproductive postreproductivepostreproductive

A population with a large percentage A population with a large percentage of its individuals in the of its individuals in the prereproductive and reproductive prereproductive and reproductive categories has a categories has a high potential for high potential for growthgrowth

Age structure is shown as a Age structure is shown as a pyramidpyramid showing the showing the percentage of the percentage of the population at each agepopulation at each age..

Significant Shapes of Age Significant Shapes of Age StructuresStructures

1. 1. PyramidPyramid: : developing countrydeveloping country – – high high birth rate, but low life expectancybirth rate, but low life expectancy

2. 2. Narrow at Top, but even below: Narrow at Top, but even below: developed country with a fairly developed country with a fairly stable populationstable population

3. 3. Narrow Top and Bottom: Usually a Narrow Top and Bottom: Usually a developed country – reproductive developed country – reproductive rates have fallen below replacement rates have fallen below replacement ratesrates

Changes Due to Economic Changes Due to Economic GrowthGrowth

Demographic Transition ModelDemographic Transition Model

Demographic Transition ModelDemographic Transition Model

Changes that occur as a population Changes that occur as a population moves from a LDC to a HDC.moves from a LDC to a HDC.

Stage 1: Stage 1: Pre-industrial – LDCPre-industrial – LDC

- high birth and death rate- high birth and death rate

- low life expectancy- low life expectancy

- lack of sanitation and medicine- lack of sanitation and medicine

- agricultural subsistence - agricultural subsistence

Demographic Transition ModelDemographic Transition Model

Stage 2: Stage 2: Transitional StageTransitional Stage• Movement toward industrializationMovement toward industrialization• Increase in sanitation and medicine and Increase in sanitation and medicine and

reliable food supplyreliable food supply• Birth rate still high but death rate is Birth rate still high but death rate is

lower = increase in population ratelower = increase in population rate• LDC-HDCLDC-HDC

Demographic Transition ModelDemographic Transition Model

Stage 3: Stage 3: Industrial StageIndustrial Stage• Sanitation and medicine readily Sanitation and medicine readily

availableavailable• Higher levels of educationHigher levels of education• Birthrate begins to lower = population Birthrate begins to lower = population

rate declines rate declines • MDC-HDCMDC-HDC

Demographic Transition ModelDemographic Transition Model

Stage 4: Stage 4: Post-industrial Post-industrial • High levels of :High levels of :

EducationEducation SanitationSanitation MedicineMedicine FoodFood

• Smaller familiesSmaller families Low birth rate and death rateLow birth rate and death rate Population Growth Rate is low, at ZPG or Population Growth Rate is low, at ZPG or

even decliningeven declining

Impact of A Large Human Population:Impact of A Large Human Population:

Fragmenting and degrading habitatFragmenting and degrading habitat Simplifying natural ecosystemsSimplifying natural ecosystems Strengthening some populations of Strengthening some populations of

pest species and disease-causing pest species and disease-causing bacteria by speeding up natural bacteria by speeding up natural selectionselection

Eliminating some predatorsEliminating some predators Deliberately or accidentally Deliberately or accidentally

introducing new speciesintroducing new species Overharvesting potentially renewable Overharvesting potentially renewable

resourcesresources Interfering with the normal chemical Interfering with the normal chemical

cycling and energy flows in cycling and energy flows in ecosystemsecosystems

SOLUTIONS: how can we rehabilitate SOLUTIONS: how can we rehabilitate and restore damaged ecosystems?and restore damaged ecosystems?

Prevention strategyPrevention strategy – reduce and – reduce and minimize the damage we do to natureminimize the damage we do to nature

Restoration ecologyRestoration ecology – renew, repair or – renew, repair or reconstruct damaged ecosystemsreconstruct damaged ecosystems• Natural restorationNatural restoration – an abandoned damaged – an abandoned damaged

ecosystem will eventually partially recover ecosystem will eventually partially recover through secondary ecological successionthrough secondary ecological succession

• RehabilitationRehabilitation – human intervention to make – human intervention to make degraded land productive again through degraded land productive again through stopping soil erosion, etc.stopping soil erosion, etc.

• Active RestorationActive Restoration – take a degraded site and – take a degraded site and reestablish a diverse, dynamic community of reestablish a diverse, dynamic community of organisms consistent with the climate and soil organisms consistent with the climate and soil of an area.of an area.