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Human population growthFrom here - end of course: Ecology - thestudy of the distribution and abundance oforganismsToday - population growth w/ humans as focusWednesday - Life histories - comparing

demographic parameters (e.g. birth rates, timing,mortality patterns) across species

Friday- Population regulation and competitionThen 3 more lectures on interactions between

species, mutualism, parasitism, herbivoryThen, community ecology, biogeography,

conservation ecology, invasive species,ecosystem ecology and the natural history of theSonoran desert

Human Population Growth

I. Concepts of population growthA. Unlimited, exponential growthB. Limited, logistic growth

II. Human population growthA. History of population growthB. Projecting into the future

Human Population Growth

I. Unlimited population growth

A. Exponential growth

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A. Exponential growth

1. Example of bacterial growthBacteria reproduce by binary

fission - 1 cell 2 cells

1. Example of bacterialgrowth

Generation No.

0 1

1. Example of bacterialgrowth

Generation No.

0 11 2

1. Example of bacterialgrowth

Generation No.0 11 22 4

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1. Example of bacterialgrowth

Generation No.0 11 22 43 8

1. Example of bacterialgrowth

Generation No.

1 12 23 44 85 1 6

1. Example of bacterialgrowth

Generation No.0 11 22 43 84 1 65 3 2

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1. Example of bacterialgrowth

Generation No.0 11 22 43 84 1 65 3 26 6 4

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1. Example of bacterialgrowth

Generation No.0 11 22 43 84 1 65 3 26 6 47 128

1. Example of bacterial growthGeneration No.

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1. Example of bacterial growthGeneration No.0 11 22 43 84 1 65 3 26 6 47 1288 2569 512 0

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1. Example of bacterial growthGeneration No.

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8 2569 5121 0 1024

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1. Example of bacterial growthGeneration No.

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3 84 1 65 3 26 6 47 128

8 2569 5121 0 10241 1 2048

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1. Example of bacterial growth

In this example,the generation time = thedoubling time (time untilpopulation doubles)

Features of simple exponentialgrowth• A constant doubling timeConstant growth rate?

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Features of simpleexponential growth

A constant doubling timeAn accelerating growth rate

A. Exponential growth

i. Example of bacterial growthii. The importance of

generation time

Imagine that 3 populations,bacteria, mice, and elephants, alldoubled their population size ateach generation. Each bacteriumproduced 2 offspring, each mouseand elephant couple produced 4.

Would their populations grow at the same rate?

When plotted against generations,they seem to be the same...

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BacteriaMiceElephants

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But of course, the generationtime of these organisms israther different!

Generation time in minutes:

Some species of bacteria 30

Mice (3 months) 132,480

Elephants (20 yrs) 10,512,0000

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Bacteria Mice Elephants

(190yrs!)

When plotted against minutes,the difference is apparent

Generation time: the meanlength of time between birthof parents and birth ofoffspring

We need an equation that will let uscompare the population growth oforganisms with different generation times

General differential equation for unlimitedpopulation growth:

dN/dt = rN

N is the population sizer is the instantaneous growth rate (the netpopulation growth at any one instant)

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The population growth rate, r, isdetermined by how many individualsare coming into the population atany one time (the instantaneous rateof births)

reduced by those leaving i.e. theinstantaneous rate of deaths

r=b-d

b - instantaneous birth rated - instantaneous death rate

Age distribution

Projections of population growthuse instantaneous rates of birth,rates of death, and the agedistribution

The age distribution is the way inwhich a population is distributedamong different age classes.

Examples of human agedistributions1) Cambodia, 1998

Examples of human agedistributions2) United States, 2000

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Which population is likelygrowing more rapidly?

Cambodia, 1998 United States, 2000

If the number of children per motherand mortality rates were heldconstant in these two countries,would population growth be thesame ?

Cambodia, 1998 United States, 2000

I. Unlimited population growth

A. Exponential growth

B. Limited (logistic) growth

In nature, we don’t generally seeexponential growth for very long

Thrips(flower feedinginsects)on rosebushes

1932 1938

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B. Limited (logistic) growth

What sorts of factors causepopulation growth to slow ordecline?

Limited food supplyLimited habitat availableDeterioration of habitatPredation, disease

B. Limited, logistic growthI. The carrying capacity

The carrying capacity (K) is themaximum stable population size theenvironment can support.

We observe that most populations innature have some limit, called thecarrying capacity

Logistic growth

K Logistic growth, the equation

d N/dt = rN[(K-N)/K] logistic function

Same equation as before, withlogistic function

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What’s the logisticfunction?

A handy, but somewhat arbitrarymathematical function that returnsthe population to a line, K

d N/dt = rN[(K-N)/K]

What happens to the sign of theright hand side of the equationwhen N=K? When N>K? WhenN< K?

What would happen if the habitatfor a particular population wasreduced?

What would happen to thepopulation if the habitat for aparticular population was reduced?

New K

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What would happen if the habitatfor a particular population wasreduced?

Pop. woulddeclineto new K

I. Unlimited population growth

II. Limited (logistic) growth

III. Human population growthAs of April 3, 2006 (17:19 GMT), the world population was

6,507,485,579 Every second: 4.1 births, 1.8 deathsA net increase of 2.3 individuals

II. Human population growthA. History of human population growth

What kind ofgrowth?

Year Global population1800 1 billion1930 2 billion1975 4 billion2000 6 billion

What’s the doubling time between1 and 2 billion?

}

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Year Global population1800 1 billion1930 2 billion1975 4 billion2000 6 billion

What’s the doubling time between2 and 4 billion?

}Year Global population1800 1 billion1930 2 billion1975 4 billion2000 6 billion

What can we say about thedoubling time between 1800 and1975?

If a constant doubling time is afeature of simple exponentialgrowth, and human populationsshow a decreasing doubling time…

then… ?

In recent years humanpopulations have grown fasterthan simple exponential growth

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II. Human population growthA. History of human population growth

Will it continue togrow like thisindefinitely?What populationparameter wouldwe like to know?

II. Human population growth

A. History of human populationgrowthB. Projecting into the futureWhat is the carryingcapacity of theplanet Earth?

B. Projecting into the futurei. Human carrying capacity, K

Been the subject of speculation sincevan Leeuwenhoek (1679) imagined aworld like 17th century Holland (13 bill)

Estimates range from 900 mill to “abillion billions”

Dependent on many assumptions, valuesystems

Unlike other animals, humans have thepotential to move towards a sustainablepopulation size

i. Human carrying capacity

What should be the standard ofliving?

Hardin (1986): 1 averageAmerican uses 300 times theamount of energy as an averageEthiopian. If everyone lived asEthiopians, the earth couldsupport 300 billion people…

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i. Human carrying capacity

How does one view sustainability?Many estimates are based on

maximum agricultural capacity andfossil fuels, no consideration fordegradation of farm land, pollution,global warming, or loss ofbiodiversity

Pimentel et al. (1994) assumed solarand other renewable energy atincreased rates, soil conservationin agriculture. K=3 billion, 1-2billion in relative prosperity

II. Human population growth

B. Projecting into the futurei. Human carrying capacityii. Population growth projections

ii. Population growth projections

Estimates of future populations donot take into consideration theenvironment or resources, or anyexplicit estimate of K - just projecttrends in birth rates and deathrates into the future 0.0E+00

2.0E+064.0E+066.0E+068.0E+061.0E+071.2E+071.4E+07

Constant High Medium Low

Const. Fert. 13b“High” 11b“Medium” 9b“Low” 7b

United Nations world populationpredictions to 2050 (2002)

We are here

}between 7and 11 bill.consideredmost likely

In “low”estimate, thepopulationdrops

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0.0E+002.0E+064.0E+066.0E+068.0E+061.0E+071.2E+071.4E+07

Constant High Medium Low

Const. Fert. 13b“High” 11b“Medium” 9b“Low” 7b

Note the “constant fertility” projection (i.e.fertility at current rates) is the highest andconsidered unrealistic. Why?

We are here

Note the “constant fertility” projection (i.e.fertility at current rates) is the highest andconsidered unrealistic. Why?

Fertility rates are droppingworldwide

Part of a robust pattern withoutone clear explanation - “thedemographic transition”

C. Projecting into the futurei. The demographic transitionCountries go from having small, slow

growing populations to having large, slowgrowing populations in 4 steps

1) High death rates, high birth rates fi no growth,small population

2) Death rate falls, birth rates stay same fi rapidgrowth, larger population

3) Birth rate falls, death rates stay the same fipopulation increases (because of agedistribution), large population

4) Birth rate and death rates stay the same fi nogrowth, large population

C. Projecting into the futurei. The demographic transitionIn the past 200 years, all “developed”

countries have undergone this processMost “developing” countries are

somewhere in the process - fertility ratesare dropping but populations are stillgrowing

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C. Projecting into the futurei. The demographic transitionFertility rates 1950 - 2050 (U.N. report

2002)

012345678

World

MoredevelopedLessdevelopedLeastdeveloped

Child

ren

per m

othe

r

We are here

If fertility rates are dropping why is theworld population still growing?

1) Fertility is still above replacement

012345678

World

MoredevelopedLessdevelopedLeastdeveloped

Child

ren

per m

othe

r We are here

2 per mother - replacement

If fertility rates are dropping why is theworld population still growing?

2) Age distribution - if most individuals ina population are young, population willstill grow after fertility drops belowreplacement

Most world population growth is comingfrom developing countries

C. Projecting into the futurei. The demographic transitionWhy the demographic transition?

What causes the dropping mortalityrates?

Better public healthIn developing countries the “public health

revolution” has been from 1945- present

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C. Projecting into the futurei. The demographic transitionLife expectancy from 1950 - 2050 (U.N.

2002)

0102030405060708090

World

MoredevelopedLessdevelopedLeastdeveloped

we are here

C. Projecting into the futurei. The demographic transitionWhy is life expectancy not currently rising

in the least developed countries?

0102030405060708090

World

MoredevelopedLessdevelopedLeastdeveloped

?

Why is life expectancy not currentlyrising in the least developed countries?HIV/AIDs : huge economic and social costs in

some of the least developed countries in AfricaSome tragic statistics:

-In Botswana, 1 in 3 adults is currently infected-Life expectancy has dropped from 63 years to44 years-Population is expected to be in decline by 2015

Outside of Africa, no. of countries where HIV/AIDSis a problem is growing, especially in Asia, LatinAmerica, Caribbean

Peak prevalence has passed in many countries,but prevalence expected to stay relatively highuntil 2050

C. Projecting into the futurei. The demographic transitionWhy the demographic transition?

What causes the dropping fertility rates?No one really knows.Some likely factors in recent decline in Africa:Drop in the “ideal family size” in many countries

coupled withEducation for girlsAccess to contraceptionIn some cases, costs of children borne increasingly

by familiesNo one factor appears universal to fertility decline

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C. Projecting into the futureWhere do we fall on this

curve?

1965-1970 max. world pop. growth

We may be somewhere in here

What is K?

Or possibly up here

C. Projecting into the futureiv. What else can we see ahead?

For some time, an ageing populationA much larger population, but one

that seems to be slowing itsgrowth, and may even start a slowdecline within 100 yrs.

With the prospect of a worldpopulation that is not growingexponentially in the future, we canbegin to think about whatpopulation size the earth cansustain