Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings – POPULATION ECOLOGY.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

– POPULATION ECOLOGY


Overview: Earth’s Fluctuating Populations

• To understand human population growth, we must consider general principles of population ecology


• Population ecology is the study of populations in relation to environment, including environmental influences on density and distribution, age structure, and population size

• The fur seal population of St. Paul Island, off the coast of Alaska, has experienced dramatic fluctuations in size



Concept 52.1: Dynamic biological processes influence population density, dispersion, and demography

• A population is a group of individuals of a single species living in the same general area


Density and Dispersion

• Density is the number of individuals per unit area or volume

• Dispersion is the pattern of spacing among individuals within the boundaries of the population


Density: A Dynamic Perspective

• Determining the density of natural populations is difficult

• In most cases, it is impractical or impossible to count all individuals in a population

• Density is the result of an interplay between processes that add individuals to a population and those that remove individuals

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

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LE 52-2

Populationsize

Emigration

Deaths

ImmigrationBirths


Patterns of Dispersion

• Environmental and social factors influence spacing of individuals in a population


• In a clumped dispersion, individuals aggregate in patches

• A clumped dispersion may be influenced by resource availability and behavior

Video: Flapping Geese (clumped)





LE 52-3a

Clumped. For many animals, such as these wolves, living in groups increases the effectiveness of hunting, spreads the work of protecting and caring for young, and helps exclude other individuals from their territory.


• A uniform dispersion is one in which individuals are evenly distributed

• It may be influenced by social interactions such as territoriality

Video: Albatross Courtship (uniform)





LE 52-3b

Uniform. Birds nesting on small islands, such as these king penguins on South Georgia Island in the South Atlantic Ocean, often exhibit uniform spacing, maintained by aggressive interactions between neighbors.


• In a random dispersion, the position of each individual is independent of other individuals

Video: Prokaryotic Flagella (Salmonella typhimurium) (random)





LE 52-3c

Random. Dandelions grow from windblown seeds that land at random and later germinate.


Demography

• Demography is the study of the vital statistics of a population and how they change over time

• Death rates and birth rates are of particular interest to demographers


Life Tables

• A life table is an age-specific summary of the survival pattern of a population

• It is best made by following the fate of a cohort

• The life table of Belding’s ground squirrels reveals many things about this population



Survivorship Curves

• A survivorship curve is a graphic way of representing the data in a life table

• The survivorship curve for Belding’s ground squirrels shows a relatively constant death rate





LE 52-4

Males

Females

10

Age (years)

Nu

mb

er o

f su

rviv

ors

(lo

g s

cale

)

4 6 80 2

1,000

100

10

1


• Survivorship curves can be classified into three general types: Type I, Type II, and Type III





LE 52-5

III

II

100

Percentage of maximum life span

Nu

mb

er o

f su

rviv

ors

(lo

g s

cale

)

0 50

1,000

100

10

1

I


Reproductive Rates

• A reproductive table, or fertility schedule, is an age-specific summary of the reproductive rates in a population

• It describes reproductive patterns of a population



Concept 52.2: Life history traits are products of natural selection

• Life history traits are evolutionary outcomes reflected in the development, physiology, and behavior of an organism


Life History Diversity

• Life histories are very diverse

• Species that exhibit semelparity, or “big-bang” reproduction, reproduce once and die

• Species that exhibit iteroparity, or repeated reproduction, produce offspring repeatedly



“Trade-offs” and Life Histories

• Organisms have finite resources, which may lead to trade-offs between survival and reproduction





LE 52-7

Female

Par

ents

su

rviv

ing

th

e fo

llo

win

g w

inte

r (%

)

Normalbrood size

100

80

60

0Reduced

brood sizeEnlarged

brood size

Male

40

20


• Some plants produce a large number of small seeds, ensuring that at least some of them will grow and eventually reproduce





LE 52-8a

Most weedy plants, such as this dandelion, grow quickly and produce a large number of seeds, ensuring that at least some will grow into plants and eventually produce seeds themselves.


• Other types of plants produce a moderate number of large seeds that provide a large store of energy that will help seedlings become established





LE 52-8b

Some plants, such as this coconut palm, produce a moderate number of very large seeds. The large endosperm provides nutrients for the embryo, an adaptation that helps ensure the success of a relatively large fraction of offspring.


• In animals, parental care of smaller broods may facilitate survival of offspring


Concept 52.3: The exponential model describes population growth in an idealized, unlimited environment

• It is useful to study population growth in an idealized situation

• Idealized situations help us understand the capacity of species to increase and the conditions that may facilitate this growth


Per Capita Rate of Increase

• If immigration and emigration are ignored, a population’s growth rate (per capita increase) equals birth rate minus death rate


• Zero population growth occurs when the birth rate equals the death rate

• Most ecologists use differential calculus to express population growth as growth rate at a particular instant in time:

dNdt rN


Exponential Growth

• Exponential population growth is population increase under idealized conditions

• Under these conditions, the rate of reproduction is at its maximum, called the intrinsic rate of increase


• Equation of exponential population growth:

dNdt rmaxN


• Exponential population growth results in a J-shaped curve





LE 52-9

Number of generations

Po

pu

lati

on

siz

e (N

)2,000

= 1.0N

1,000

1,500

500

0151050

dNdt

= 0.5NdNdt


• The J-shaped curve of exponential growth characterizes some rebounding populations





LE 52-10

Year

Ele

ph

ant

po

pu

lati

on

8,000

4,000

6,000

2,000

019801960194019201900


Concept 52.4: The logistic growth model includes the concept of carrying capacity

• Exponential growth cannot be sustained for long in any population

• A more realistic population model limits growth by incorporating carrying capacity


• Carrying capacity (K) is the maximum population size the environment can support


The Logistic Growth Model

• In the logistic population growth model, the per capita rate of increase declines as carrying capacity is reached

• We construct the logistic model by starting with the exponential model and adding an expression that reduces per capita rate of increase as N increases





LE 52-11

Population size (N)

Per

cap

ita

rate

of

incr

ease

(r)

Maximum

Positive

Negative

N = K0


• The logistic growth equation includes K, the carrying capacity

dNdt

(K N)

Krmax N



• The logistic model of population growth produces a sigmoid (S-shaped) curve





LE 52-12

Number of generations

Po

pu

lati

on

siz

e (N

)

K = 1,5001,500

2,000

1,000

500

1510500

Logistic growth

Exponentialgrowth

= 1.0NdNdt

= 1.0NdNdt

1,500 – N

1,500


The Logistic Model and Real Populations

• The growth of laboratory populations of paramecia fits an S-shaped curve





LE 52-13a

Time (days)

Nu

mb

er o

f P

aram

eciu

m/m

L

1,000

0

400

5

200

100

15

800

600

A Paramecium population in the lab


• Some populations overshoot K before settling down to a relatively stable density





LE 52-13b

Time (days)

Nu

mb

er o

f D

aph

nia

/50

mL

180

0

90

20

60

400

60

150

120

A Daphnia population in the lab

30

80 100 120 140 160


• Some populations fluctuate greatly around K





LE 52-13c

Time (years)

Nu

mb

er o

f fe

mal

es80

1975 1980

40

19850

1990

60

A song sparrow population in its natural habitat

20

1995 2000


• The logistic model fits few real populations but is useful for estimating possible growth


The Logistic Model and Life Histories

• Life history traits favored by natural selection may vary with population density and environmental conditions

• K-selection, or density-dependent selection, selects for life history traits that are sensitive to population density

• r-selection, or density-independent selection, selects for life history traits that maximize reproduction


• The concepts of K-selection and r-selection are somewhat controversial and have been criticized by ecologists as oversimplifications


Concept 52.5: Populations are regulated by a complex interaction of biotic and abiotic influences

• There are two general questions about regulation of population growth:

– What environmental factors stop a population from growing?

– Why do some populations show radical fluctuations in size over time, while others remain stable?


Population Change and Population Density

• In density-independent populations, birth rate and death rate do not change with population density

• In density-dependent populations, birth rates fall and death rates rise with population density





LE 52-14

Population density

Equilibriumdensity

Density-independentbirth rate

Density-dependentdeath rate

Population density

Equilibriumdensity

Density-independentdeath rate

Density-dependentbirth rate

Population density

Equilibriumdensity

Density-dependentdeath rate

Density-dependentbirth rate

pe

r c

ap

ita

Bir

th o

r d

ea

th r

ate


Density-Dependent Population Regulation

• Density-dependent birth and death rates are an example of negative feedback that regulates population growth

• They are affected by many factors, such as competition for resources, territoriality, health, predation, toxic wastes, and intrinsic factors


Competition for Resources

• In crowded populations, increasing population density intensifies intraspecific competition for resources





LE 52-15

10,000

Av

era

ge

nu

mb

er

of

se

ed

sp

er

rep

rod

uc

ing

in

div

idu

al

(lo

g s

ca

le)

1,000

100

100101Plants per m2 (log scale)

Plantain. The number of seeds produced by plantain (Plantago major) decreases as density increases.

Song sparrow. Clutch size in the song sparrow on Mandarte Island, British Columbia, decreases as density increases and food is in short supply.

Av

era

ge

clu

tch

siz

e

2.880

Females per unit area

3.0

3.8

4.0

3.4

3.6

3.2

60 705030 40200 10


Territoriality

• In many vertebrates and some invertebrates, territoriality may limit density


• Cheetahs are highly territorial, using chemical communication to warn other cheetahs of their boundaries



• Oceanic birds exhibit territoriality in nesting behavior



Health

• Population density can influence the health and survival of organisms

• In dense populations, pathogens can spread more rapidly


Predation

• As a prey population builds up, predators may feed preferentially on that species


Toxic Wastes

• Accumulation of toxic wastes can contribute to density-dependent regulation of population size


Intrinsic Factors

• For some populations, intrinsic (physiological) factors appear to regulate population size


Population Dynamics

• The study of population dynamics focuses on the complex interactions between biotic and abiotic factors that cause variation in population size


Stability and Fluctuation

• Long-term population studies have challenged the hypothesis that populations of large mammals are relatively stable over time





LE 52-18

1960

Year

Mo

ose

po

pu

lati

on

siz

e

2,500

Steady decline probably caused largely by wolf predation

2,000

1,500

1,000

500

01970 1980 1990 2000

Dramatic collapse caused by severe winter weather and food shortage, leading to starvation of more than 75% of the population


• Extreme fluctuations in population size are typically more common in invertebrates than in large mammals





LE 52-19

1960

Year

Co

mm

erci

al c

atch

(kg

) o

fm

ale

crab

s (l

og

sca

le)

730,000

100,000

10,000

1970 1980 19901950


Metapopulations and Immigration

• Metapopulations are groups of populations linked by immigration and emigration

• High levels of immigration combined with higher survival can result in greater stability in populations





LE 52-20

1988Year

Nu

mb

er o

f b

reed

ing

fem

ales

60

1989 1990 1991

Smallislands

MandarteIsland

50

40

30

20

10

0


• Many populations undergo boom-and-bust cycles

• Boom-and-bust cycles are influenced by complex interactions between biotic and abiotic factors





LE 52-21

Year

Ha

re p

op

ula

tio

n s

ize

(th

ou

sa

nd

s)

1850

Snowshoe hare

01875 1900 1925

40

80

120

160

Ly

nx

po

pu

lati

on

siz

e(t

ho

us

an

ds

)

Lynx

0

3

6

9


Concept 52.6: Human population growth has slowed after centuries of exponential increase

• No population can grow indefinitely, and humans are no exception


The Global Human Population

• The human population increased relatively slowly until about 1650 and then began to grow exponentially





LE 52-22

8000B.C.

Hu

man

po

pu

lati

on

(b

illi

on

s)

6

5

4

3

2

1

0

4000B.C.

3000B.C.

2000B.C.

1000B.C.

The Plague

0 1000A.D.

2000A.D.


• Though the global population is still growing, the rate of growth began to slow about 40 years ago





LE 52-23

An

nu

al p

erce

nt

incr

ease

2.2

2

1.8

1.6

1.4

1.2

1

2003

2050Year

2025200019751950

0.8

0.6

0.4

0.2

0


Regional Patterns of Population Change

• To maintain population stability, a regional human population can exist in one of two configurations:

– Zero population growth = High birth rate – High death rate

– Zero population growth =Low birth rate – Low death rate

• The demographic transition is the move from the first state toward the second state





LE 52-24

Bir

th o

r d

eath

rat

e p

er 1

,000

peo

ple

50

40

30

20

10 Sweden

2050

Year

20001900 195018500

18001750

Birth rate

Death rate

MexicoBirth rate

Death rate


• The demographic transition is associated with various factors in developed and developing countries


Age Structure

• One important demographic factor in present and future growth trends is a country’s age structure

• Age structure is the relative number of individuals at each age

• It is commonly represented in pyramids





LE 52-25

Rapid growthAfghanistan

AgeMale

Percent of population

Female

8 6 4 2 2 4 6 80

45–4940–4435–3930–3425–2920–2415–1910–14

5–90–4

85+80–8475–7970–7465–6960–6455–5950–54

Slow growthUnited States

AgeMale


Female

6 4 2 2 4 6 80

45–4940–4435–3930–3425–2920–2415–1910–14

5–90–4

85+80–8475–7970–7465–6960–6455–5950–54

8

Decrease Italy

Male


Female

6 4 2 2 4 6 808


• Age structure diagrams can predict a population’s growth trends

• They can illuminate social conditions and help us plan for the future


Infant Mortality and Life Expectancy

• Infant mortality and life expectancy at birth vary greatly among developed and developing countries but do not capture the wide range of the human condition





LE 52-26

Infa

nt

mo

rtal

ity

(dea

ths

per

1,0

00 b

irth

s)

50

40

30

20

10

0Developedcountries

60

Developingcountries

Lif

e ex

pec

tan

cy (

year

s)

80

40

20

0Developedcountries

60

Developingcountries


Global Carrying Capacity

• How many humans can the biosphere support?


Estimates of Carrying Capacity

• The carrying capacity of Earth for humans is uncertain


Ecological Footprint

• The ecological footprint concept summarizes the aggregate land and water area needed to sustain the people of a nation

• It is one measure of how close we are to the carrying capacity of Earth

• Countries vary greatly in footprint size and available ecological capacity





LE 52-27

Eco

log

ical

fo

otp

rin

t (h

a p

er p

erso

n)

14

12

10

8

6

4

16

0

2

02 4 6 8 10 12 14 16

Available ecological capacity(ha per person)

New Zealand

AustraliaCanada

Sweden

WorldChina

India

SpainUK

Japan

Germany

Norway

USA

Netherlands


• At more than 6 billion people, the world is already in ecological deficit

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings – POPULATION ECOLOGY.

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