Chapter 5

Population Ecology

Counting individuals What constitutes an individual organism?

Variations on the individual Unitary organisms - one zygote per

embryo produces one organism

Variations on the individual Modular organisms - one zygote per embryo

produces one module, which eventually produces more modules like itself Branching or shoot development in plants, budding

in Hydra, sponges, fungi

How to count individuals in a population Some organisms - possible to easily count

all individuals Others must be subsampled and estimated

Plants and some animals - quadrat Soil, water dwellers - volume Animals - mark and recapture

Mark and recapture Random sample Release Try to recapture

Theory - marked individuals will remix within population; proportion marked in next sample represents proportion in entire population

Mark and recapture example

Population size (N)#marked on Day 1

= Total catch on Day 2# of recaptures

N = (# marked on Day 1) x (Total catch on Day 2)# of recaptures

Gilmore Creek Brown Trout200 m stream reach = 840 m2 area4.2 m average stream width

Day 1: 169 trout captured, marked, releasedDay 2: 178 trout captured

80 marked (recaptures), 98 unmarked

N = (# marked ) x (total catch Day 2)# of recaptures

N = 169 x 178 = 377 trout 377 trout/840 m2 = 80 0.45 trout m2

Life cycles Patterns of birth, death, growth are dictated

by an organism’s life cycle 5 main types of life cycles

Life cycle types Annual Overlapping iteroparity Overlapping semelparity Continuous semelparity Continuous iteroparity

Semelparous Individual organism has single

reproductive event during its life, then dies Invests large amount of energy in

reproduction

Iteroparous Individual may have many reproductive

events during season or life Invests lesser proportion of resources in

reproduction

Annuals 12 months or less to complete life cycle Discreet, non-overlapping generations May or may not overwinter as non-seed/egg May be either semelparous or iteroparous Annual may be a misnomer for some plants with

seeds that do not always germinate the year after being produced Seeds may lie dormant in seed bank for several years

before germinating

Overlapping iteroparity Overlapping generations (yearlings, 2-

year-olds, etc.), iteroparous Distinct breeding season Examples: temperate-zone trees, long-

lived, seasonally breeding vertebrates (deer, most fish, snakes, birds)

Overlapping semelparity Overlapping generations (several age classes

present [at least biennial]), semelparous New offspring in population every year (distinct

breeding season) Require 2 or more years to mature and reproduce,

then die Most common in plants, also in some species of

octopus, salmon

Continuous semelparity No distinct breeding season because of

favorable environmental conditions Many overlapping ages continually

growing, reproducing, dying Example: some animals in tropical oceans

Continuous iteroparity No distinct breeding season Many overlapping ages Example: humans

Life tables Used to follow changes in births, deaths,

growth of population through time Of differing complexity and usefulness

depending on life cycle of organism being examined Easiest for annuals, more difficult for other

types

Life table variables  x life stage or age classax total number of individuals observed at each stage or classlx proportion of original number of individuals surviving to the next stage or class; survivorshipdx proportion of original number of individuals dying during each stage or class; mortalityqx mortality rate for each stage or classkx "killing power;" Fx total fecundity, or reproductive output of entire population, for each stage or classmx individual fecundity, or mean reproductive output, for each stage or classlxmx number of offspring produced per original individual during each stage or class; product of survival and reproductionR0 basic reproductive rate

Cohort life table

Group of individuals “born” within same short time interval is followed from birth through death of last survivor

Grasshoppers - a cohort life table

Phlox - a cohort life table

Static life tables Life tables more difficult to construct for

longer-lived organisms, and those with many overlapping generations

Difficult to follow single cohort throughout entire life (many years)

Static life table is produced - snapshot in time

Static life tables Need information on total population size

and its age structure at some point in time Can get messy if older age classes have

more individuals than younger age classes Different mortality, recruitment

May need to smooth data to get things to work

Red deer - a static life table

Survivorship Curves Depict what

proportion of population remains alive at various points in life

3 basic patterns displayed by living things

Survivorship Curves Type I - little

mortality throughout early life

Mortality concentrated in older age groups

Example: humans

Survivorship Curves Type II - constant rate

of mortality throughout life

Constant proportion die age time/age interval

Example: corals, squirrels

Survivorship Curves Type III - high early

mortality Survivors have little

mortality until old age Example: plants,

fishes

Population Dynamics and GrowthPopulation Dynamics and Growth

Exponential Growth

Time (t)

Po

pu

lati

on

siz

e (N

)

-ideal habitat-maximum reproduction-unlimited resources

Increase oftenfollowed bycrash

2,000

1,500

Nu

mb

er o

f re

ind

eer

1910 1920 1930 1940 1950

Year

1,000

500

Reindeer on an Alaskan island

5,000

4,000

3,000

2,000

1,000

500

Nu

mb

er o

f m

oo

se

100

90

80

70

60

50

40

30

20

10

01900 1910 1930 1950 1970 1990 2000

1999Year

Nu

mb

er of w

olves

Moose population

Wolf population

Moose and wolves on Isle Royale

Logistic Growth

Time (t)

Po

pu

lati

on

siz

e (N

)

K-accelerating, deceleratingCarrying capacity

-growth slows aspopulation size approachescarrying capacity

-number that environmentcan support indefinitely

Carrying capacity set by limiting factor

2.0

1.5

1.0

.5

Nu

mb

er o

f sh

eep

(m

illio

ns)

1800 1825 1850 1875 1900 1925

Year

Sheep in Tasmania

Human population growth-exponential or logistic?

Human population growth-exponential or logistic?

-appears exponential-history may suggest logistic-periods of rapid growth followed by stability

Human population growth-exponential or logistic?

Cultural evolution

-tool-making revolution-agricultural revolution-industrial (technological) revolution

Carrying capacity for humans

Set by:

-famine-disease-warfare

Will these become more common aspopulation approaches carrying capacity?

Population DemographicsPopulation Demographics

What affects human population size and growth rate?

What affects human population size and growth rate?

1) Birth rate and death rate2) Migration rate3) Fertility rate4) Age structure5) Average marriage age

1) Birth rate and death rate2) Migration rate3) Fertility rate4) Age structure5) Average marriage age

Factors Affecting Human Population SizeFactors Affecting Human Population Size

Population change equationPopulation change equation

Zero population growth (ZPG)Zero population growth (ZPG)

Birth rate (number/1000 people/year)Birth rate (number/1000 people/year)

Death rate (number/1000 people/year)Death rate (number/1000 people/year)

PopulationChange

PopulationChange == (Births + Immigration) – (Deaths + Emigration)(Births + Immigration) – (Deaths + Emigration)

Birth and death rates

U.S. - 16 and 9 (7 or 0.7%) U.S. - 16 and 9 (7 or 0.7%) Rwanda - 52 and 18 (34 or 3.4%) Rwanda - 52 and 18 (34 or 3.4%)

32

30

28262422201816140

Bir

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po

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1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010

Year

Demographictransition

Depression

End of World War II

Baby boom Baby bust Echo baby boom

World - 26 and 9 (17 or 1.7%) World - 26 and 9 (17 or 1.7%)

Infant deathsper 1,000 live births

<10<10-35<36-70<71-100<100+Data notavailable

Factors Affecting Death Rate Life expectancy Life expectancy Infant mortality rate (IMR) Infant mortality rate (IMR)

Rate of Natural Increase

Developed Countries50

40

30

20

10

017751800 1850 1900 1950 2000 2050

Rate ofnatural increase

Crudebirth rate

Crudedeath rate

Rate of natural increase = crude birth rateRate of natural increase = crude birth rate––crude death rate crude death rate

Developing Countries50

40

30

20

10

017751800 1850 1900 1950 2000 2050

Rat

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,000

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birth rate

Rate ofnaturalincreaseCrude

death rate

Year

© 2004 B

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Natural Rate of Increase

<1%

1-1.9%

2-2.9%

3+%Data notavailable

Annual worldpopulation growth

1% - triple in 100 years2% - 7X in 100 years

Migration RatesMigration Rates

Affect regional populations Affect regional populations

e.g., United States e.g., United States

Net gain of 4/1000 people/year Net gain of 4/1000 people/year

Add to 7 from BR - DR = 11 (1.1%) Add to 7 from BR - DR = 11 (1.1%)

Fertility RatesFertility RatesAverage number of children born to a woman during her childbearing years (ages 15-44)

Average number of children born to a woman during her childbearing years (ages 15-44)

Replacement level fertility rates for ZPG

Replacement level fertility rates for ZPG

Total fertility ratesTotal fertility rates

Fertility RatesFertility Rates

Replacement level fertility rates for ZPG

- developed countries - 2.1/woman - developing countries - 2.5 - total world - 2.3-2.4

Replacement level fertility rates for ZPG

- developed countries - 2.1/woman - developing countries - 2.5 - total world - 2.3-2.4

Fertility RatesFertility Rates

Total fertility rates

- developed countries - 1.9 (U.S. 1.8) - developing countries - 3.8 (Rwanda-8.5, Kenya-8.0) - total world - 3.4

Total fertility rates

- developed countries - 1.9 (U.S. 1.8) - developing countries - 3.8 (Rwanda-8.5, Kenya-8.0) - total world - 3.4

Births per woman

< 2

2-2.9

3-3.9

4-4.9

5+

NoData

Fertility Rates

Fertility RatesFertility Rates

Time lag to ZPG

- about 3 generations (~70 years) required to achieve ZPG once replacement level fertility rates are reached

Time lag to ZPG

- about 3 generations (~70 years) required to achieve ZPG once replacement level fertility rates are reached

Ages 0-14Ages 0-14 Ages 15-44Ages 15-44 Ages 45-85+Ages 45-85+

Rapid GrowthGuatemala

NigeriaSaudi Arabia

Rapid GrowthGuatemala

NigeriaSaudi Arabia

Slow GrowthUnited States

AustraliaCanada

Slow GrowthUnited States

AustraliaCanada

MaleMale FemaleFemale

Zero GrowthSpainAustriaGreece

Zero GrowthSpainAustriaGreece

Negative GrowthGermanyBulgariaSweden

Negative GrowthGermanyBulgariaSweden

Population Age StructurePopulation Age Structure

Average Marriage AgeAverage Marriage Age

or age at birth of first childor age at birth of first child

Higher marriage age leads to reduced reproductive period, which leads to lower fertility rates

Higher marriage age leads to reduced reproductive period, which leads to lower fertility rates

Average Marriage AgeAverage Marriage Age

Current U.S. marriage age - 24 (F)Current U.S. marriage age - 24 (F)

Reduces 30-year reproductive period (15-44) to 21-year reproductive period (24-44)

- 30% reduction

Reduces 30-year reproductive period (15-44) to 21-year reproductive period (24-44)

- 30% reduction

Reduces 15-year prime reproductive period (15-29) to a 6-year prime reproductive period (24-29) - 60% reduction

Reduces 15-year prime reproductive period (15-29) to a 6-year prime reproductive period (24-29) - 60% reduction

Expectation: >25 needed to affect fertility rate Expectation: >25 needed to affect fertility rate

Current Needs for Large FamiliesCurrent Needs for Large Families

Increased incomeIncreased income

High infant mortalityHigh infant mortality

Support for elderlySupport for elderly

Few opportunities for women outside the home

Few opportunities for women outside the home

Family planning unavailableFamily planning unavailable

r strategistUnstable environment,

density independent

K strategistStable environment,

density dependent

Small size of organism Large size of organism

Low energy for reproduction High energy for reproduction

Many offspring produced Few offspring produced

Early maturity Late maturity (often after parental care)

Short life expectancy Long life expectancy

Reproduces once Reproduces more than once

Type III survivorship curve Type I or II survivorship curve

Life History Strategies