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S i  Section 4Professor Donald McFarlane

Lecture 18 Ecology: Population Growth

Population – group of interbreeding individuals occupying the same habitat at the same time

Water lilies in a particular lakeHumans in New York City

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Population ecology – study of what factors affect population size and how these factors change over space and timeUses the tools of demography – birth rates, death rates, age distributions, and sizes of populations

Understanding populations

Density – number of organisms in a given unit areaPopulation growth affects population

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Population growth affects population densityKnowledge can help us make decisions about the management of species

Quantifying population densitySimple visual countSampling methods to extrapolate captured organism number to size of populationMark-recapture method

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Dispersion patternsClumped

Most commonResources tend to be clustered in natureSocial behavior may promote this pattern

Uniform

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UniformCompetition may cause this patternMay also result from social interactions

RandomRarestResources are rarely randomly spacedMay occur where resources are common and abundant 6

Reproductive strategiesSemelparity – produce all offspring in single reproductive event, individuals reproduce

d di

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once and dieIteroparity – reproduce in successive years or breeding seasons

Seasonal iteroparity – distinct breeding seasonsContinuous iteroparity – reproduce repeatedly at any time of the year

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Age classesReproductive strategy has a strong effect on subsequent age classes of a population

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Semelparous organisms with same-aged young called cohortsIteroparous organisms have young of different agesExpect a population increasing in size to have many young and a decreasing population to have few young

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20

40

60

Perc

ent o

f tre

es

10 20 30 40 50 60 7010 20 30 40 50 60Age of trees (years)

(a) Undisturbed forest (b) Overgrazed forest

Life tablesData on the number of individuals alive in a particular age class

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p gMales are usually not includedNorth American beaver example

Trappers provided mandiblesTeeth extracted for age classification

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Survivorship curve – plots numbers of surviving individuals at each age

Use log scale to make it easier to examine

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Use log scale to make it easier to examine wide range of population sizesBeavers have a fairly uniform rate of death over the life span

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3 patterns of survivorship curvesType I – rate of loss of juveniles low and most individuals lost later in life

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Type II – fairly uniform death rateBeaver example

Type III – rate of loss for juveniles high and then loss low for survivors

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Age-specific fertility rate, mxProportion of female offspring born to females of reproductive age100 females produce 75 female offspring

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p p gmx=0.75

Age-specific survivorship rate, lxUse survivorship data to find proportion of individuals alive at the start of any given age class

lxmx = contribution of each age class to overall population growth

R0 = net reproductive rateOverall growth rate per generationNumber of offspring born to females of all ages

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Number of offspring born to females of all ages

To calculate future size of population, multiply number of individuals in the population by the net reproductive rate

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p p y pFor beaver example,

Nt+1 = NtR0= 1,000 x 1.1= 1,100

If R0>1, population growingIf R0<1, population decliningIf R0 = 1, population is at equilibrium

How populations grow

Life tables can provide accurate information about how populations grow from generation to generation

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Simpler models can give insight to shorter time periods

Exponential growth – resources not limiting, prodigious growthLogistic growth – resources limiting, limits to growth

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Per capita growth rateChange in population size over any time period

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pOften births and deaths expressed per individual100 births to 1000 deer = 0.1050 deaths in 1000 deer = 0.50

Exponential growthWhen r>0, population increase is rapidCharacteristic J-shaped curveIntrinsic rate of increase r = r at maximum

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Intrinsic rate of increase, rmax = r at maximumBecause population growth depends on the value of N as well as the value of r, the population increase is even greater as time passesReintroduction of a population to a habitat, growth of introduced exotic species, and global human population

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ulat

ion

size

600

500

300

400

400

200

mbe

r of a

nim

als

Predicted abundanceActual abundance

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19700

1980

Popu

1990 2000

200

100

(a) Tule elkYear

(b) Black-footed ferrets

100

0

Num

Survey year

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Logistic growthFor most species, resources become limiting as populations grow

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p p gCarrying capacity (K) or upper boundary for populationLogistic equation

Logistic growth – pattern where growth slows down as it approaches KModel fits some populations but not others

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Model fits some populations but not othersVariations in nature change resource levels that cause changes in carrying capacity

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Density-dependent factorsMortality factor whose influence varies with the density of the populationParasitism, predation, and competitionPredators kill few prey when the prey population is l th kill h th l ti i hi h

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low, they kill more prey when the population is higherDetected by plotting mortality against population density and finding positive slope

Density-independent factorMortality factor whose influence is not affected by changes in population size or densityGenerally physical factors – weather, drought, flood, fire 30

Life history strategiesContinuumr-selected species – high rate of per capita population growth, r, but poor competitive ability (weeds)

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ability (weeds)K-selected species – more or less stable populations adapted to exist at or near carrying capacity, K

Lower reproductive rate but better competitors (trees)

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Human population growth

In 2006, the world’s population was estimated to be increasing at the rate of 146 people every minute

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2 in developed nations and 144 in less developed nationsHuman growth fits an exponential pattern

Low until agriculture and animal domesticationBetween 1750 and 1998, population surged from 800 million to 6 billion

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atio

n (b

illio

ns)

3

4 1975

2000

5

6

7

8

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Popu

la

0

1

2

31950

19001800

Human populations can exist at equilibrium densities in one of two ways

1. High birth and high death ratesB f 1750 thi ft th ith hi h bi th

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Before 1750, this was often the case, with high birth rates offset by deaths from wars, famines, and epidemics

2. Low birth and low death ratesIn Europe, beginning in the 18th century, better health and living conditions reduced the death rateEventually, social changes such as increasing education for women and marriage at a later age reduced the birth rate

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Demographic transitionShift in birth and death rates with developmentFirst stage - birth and death rates are both high, and the population remains in equilibriumSecond stage death rate declines first while

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Second stage - death rate declines first, while the birth rate remains high - high rates of population growth resultThird stage - birth rates drop and death rates stabilize, so that population growth slowsFourth stage - both birth and death rates are low, and the population is again at equilibrium

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deat

h ra

tes

opul

atio

n si

ze

Stage 1 Stage 2 Stage 3 Stage 4

Birth ratedecreases

RelativepopulationsizeHigh birth rate

High death rate

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Relative population sizeBirth rateDeath rate

Birt

h an

d

Rel

ativ

e p o

Maximal DecreasingIncreasingLow

r

Lowr

Death ratedecreases

decreasesLow birth rateLow death rate

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Exact pace varies between countries depending on culture, economics, politics, and religion

Age structureRelative numbers of individuals in each defined age group

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g g pCommonly displayed as population pyramidHelps predict future population growth

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Earth’s carrying capacityMany and varied estimatesLifestyle has a huge influenceTotal fertility rate – average number of live births a woman has during her lifetime

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Global TFR declined from 4.47 in 1970s to 2.59 in 20072.3 needed for zero population growthDiffers considerably between geographic areasIn developed nations, population has stabilizedIn developing countries, population is still increasing dramatically

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2.0 2.05.0

5.0

A i

Europe6.7

4.72.3

1.52.2

2 4

NorthAmerica

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AfricaAsia

3.2

Oceania

2.3

1970–1975 2005

2.4

Latin America& Caribbean

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on (b

illio

ns)

9

10

11 TFR of 2.5 (high)TFR of 2.0 (medium)TFR of 1.5 (low)

10.8

9.2

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Popu

latio

20502000 2005 2010 2015 2020 2025 2030 2035 2040 2045

7

8

6

7.8

Year

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Ecological footprintAggregate total of productive land needed for survival in a sustainable world

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Average footprint size is about 3 hectares (1ha=10,000 m2)Wide variation is found around the globe

7.5 for Canadians, 10 for Americans

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