chapter 53

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

PowerPoint® Lecture Presentations for

Biology Eighth Edition

Neil Campbell and Jane Reece

Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp

Chapter 53

Population Ecology


Concept 53.1: Dynamic biological processes influence population density, dispersion, and demographics.

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

• 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

Fig. 53-2

APPLICATION

Hector’s dolphins

Fig. 53-3

Births

Births and immigrationadd individuals toa population.

Immigration

Deaths and emigrationremove individualsfrom a population.

Deaths

Emigration


Patterns of Dispersion

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


Demographics

• 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

Table 53-1

Fig. 53-6

1,000

100

10

10 50 100

II

III

Percentage of maximum life span

Num

ber o

f sur

vivo

rs (l

og s

cale

)

I


Concept 53.2: Life history traits are products of natural selection.

• An organism’s life history comprises the traits that affect its schedule of reproduction and survival:

– The age at which reproduction begins

– How often the organism reproduces

– How many offspring are produced during each reproductive cycle

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


Evolution and 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

• Highly variable or unpredictable environments likely favor big-bang reproduction, while dependable environments may favor repeated reproduction


“Trade-offs” and Life Histories

• Natural selection cannot maximize all reproductive variables simultaneously.

• Organisms have finite resources, which may lead to trade-offs between survival and reproduction.– Plants and animals whose young are subject to

high mortality rates often produce large numbers of relatively small offspring.

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

Fig. 53-9

(a) Dandelion

(b) Coconut palm

Fig. 53-8

MaleFemale

100

RESULTS

80

60

40

20

0 Reducedbrood size

Normalbrood size

Enlargedbrood sizePa

rent

s su

rviv

ing

the

follo

win

g w

inte

r (%

)


Concept 53.3: 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

• 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:

Nt

rN

where N = population size, t = time, and r = per capita rate of increase = birth – death

Per Capita Rate of Increase


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

Fig. 53-10

Number of generations0 5 10 15

0

500

1,000

1,500

2,000

1.0N =dNdt

0.5N =dNdt

Popu

latio

n si

ze (N

)


Concept 53.4: The logistic model describes how a population grows more slowly as it nears its 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 approaches K

dNdt (K N)

Krmax N

Table 53-3

Fig. 53-12

2,000

1,500

1,000

500

00 5 10 15

Number of generations

Popu

latio

n si

ze (N

)

Exponentialgrowth

1.0N=dNdt

1.0N=dNdt

K = 1,500

Logistic growth1,500 – N

1,500

Fig. 53-13

1,000

800

600

400

200

00 5 10 15

Time (days)

Num

ber o

f Paramecium

/mL

Num

ber o

f Daphnia

/50

mL

0

30

60

90

180

150

120

0 20 40 60 80 100 120 140 160Time (days)

(b) A Daphnia population in the lab(a) A Paramecium population in the lab


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


Concept 53.5: Many factors that regulate population growth are density dependent.

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

– What environmental factors stop a population from growing indefinitely?

– 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

Fig. 53-15

(a) Both birth rate and death rate vary.

Population density

Density-dependentbirth rate

Equilibriumdensity

Density-dependentdeath rate

Birt

h or

dea

th ra

tepe

r cap

ita

(b) Birth rate varies; death rate is constant.

Population density

Density-dependentbirth rate

Equilibriumdensity

Density-independentdeath rate

(c) Death rate varies; birth rate is constant.

Population density

Density-dependentdeath rate

Equilibriumdensity

Density-independentbirth rate

Birt

h or

dea

th ra

tepe

r cap

ita


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, disease, predation, toxic wastes, and intrinsic factors

Fig. 53-16

Population size

100

80

60

40

20

0200 400 500 600300Perc

enta

ge o

f juv

enile

s pr

oduc

ing

lam

bs

Fig. 53-17

(a) Cheetah marking its territory

(b) Gannets


Other Density-Dependent Limiting Factors

• In dense populations, pathogens can spread more rapidly.

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

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

• Changes in predation pressure can drive population fluctuations

• Mathematical and computer models are used to illustrate and investigate population interactions within and environmental impacts on a community.

Fig. 53-19

Wolves Moose2,500

2,000

1,500

1,000

500

Num

ber o

f moo

se

0

Num

ber o

f wol

ves

50

40

30

20

10

01955 1965 1975 1985 1995 2005

Year


Calculating Lag Time

• Use the graph below to calculate lag time in months between the change in the densities of the prey and the predator populations.

• To find lag time, calculate the difference in the predator/prey size between peaks or valleys.


Population Cycles: Scientific Inquiry

• Some populations undergo regular boom-and-bust cycles

• Lynx populations follow the 10 year boom-and-bust cycle of hare populations

• Three hypotheses have been proposed to explain the hare’s 10-year interval

Fig. 53-20

Snowshoe hare

Lynx

Num

ber o

f lyn

x(th

ousa

nds)

Num

ber o

f har

es(th

ousa

nds)

160

120

80

40

01850 1875 1900 1925

Year

9

6

3

0


Variation and Population Dynamics

• The level of variation in a population affects population dynamics.

• A population’s ability to respond to changes in the environment is affected by genetic diversity.

• Species and populations with little genetic diversity are at risk for extinction.

– Illustrative Examples: California condors, prairie chickens


Variation and Population Dynamics

• Genetic diversity allows individuals in a population to respond differently to the same changes in environmental conditions:

– Not all animals in a population stampede

– Not all individuals in a population are equally affected in a disease outbreak


Concept 53.6: The human population is no longer growing exponentially but is still increasing rapidly.

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

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

• Though the global population is still growing, the rate of growth began to slow during the 1960s

Fig. 53-22

8000B.C.E.

4000B.C.E.

3000B.C.E.

2000B.C.E.

1000B.C.E.

0 1000C.E.

2000C.E.

0

1

2

3

4

5

6

The Plague

Hum

an p

opul

atio

n (b

illio

ns)

7

Fig. 53-23

2005

Projecteddata

Ann

ual p

erce

nt in

crea

se

Year1950 1975 2000 2025 2050

2.2

2.0

1.8

1.6

1.4

1.2

1.0

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

Fig. 53-24

1750 1800 1900 1950 2000 2050Year

1850

Sweden MexicoBirth rate Birth rate

Death rateDeath rate0

10

20

30

40

50B

irth

or d

eath

rate

per

1,0

00 p

eopl

e


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

Fig. 53-25

Rapid growthAfghanistan

Male Female Age AgeMale Female

Slow growthUnited States

Male Female

No growthItaly

85+80–8475–7970–74

60–6465–69

55–5950–5445–4940–4435–3930–3425–2920–2415–19

0–45–9

10–14

85+80–8475–7970–74

60–6465–69

55–5950–5445–4940–4435–3930–3425–2920–2415–19

0–45–9

10–14

10 10 8 866 4 422 0Percent of population Percent of population Percent of population

66 4 422 08 8 66 4 422 08 8

Fig. 53-26

Less indus-trialized

countries

Indus-trialized

countries

60

50

40

30

20

10

0 0

20

40

80

Life

exp

ecta

ncy

(yea

rs)

Infa

nt m

orta

lity

(dea

ths

per 1

,000

birt

hs)

Less indus-trialized

countries

Indus-trialized

countries

60


Global Carrying Capacity

• How many humans can the biosphere support?

• The carrying capacity of Earth for humans is uncertain

• The average estimate is 10–15 billion

• Our carrying capacity could potentially be limited by food, space, nonrenewable resources, or buildup of wastes

Fig. 53-27

Log (g carbon/year)13.4

9.85.8

Not analyzed

chapter 53

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