Chapter 52

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

PowerPoint Lectures for Biology, Seventh Edition

Neil Campbell & Jane Reece

Lectures by Chris Romero

Chapter 52Chapter 52

pop Ecology


Overview: Earth’s Fluctuating pop's

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


• pop ecology- study of pop’s in relation to environ, including density & distribution, age structure, & pop size

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



Concept 52.1: Dynamic biological processes influence pop density, dispersion, & demography

• pop-grp of indiv's of 1 spp living in the same general area


Density & Dispersion

• Density- # of indiv's per unit area or volume

• Dispersion- pattern of spacing among indiv's w/in the boundaries of the pop


Density: A Dynamic Perspective

• Determining the density of natural pop's is difficult

• In most cases, it is impractical or impossible to count all indiv's in a pop

• Density is the result of an interplay b/w processes that add indiv's to a pop & those that remove indiv's

LE 52-2

popsize

Emigration

Deaths

ImmigrationBirths


Patterns of Dispersion

• Environmental & social factors influence spacing of indiv's in a pop


• clumped dispersion- indiv's aggregate in patches

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

Video: Flapping Geese (clumped)

LE 52-3a

Clumped. For many animals, such as these wolves, living in groups ↑'s the effectiveness of hunting, spreads the work of protecting & caring for young, & helps exclude other indiv's from their territory.


• uniform dispersion- indiv's 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 b/w neighbors.


• random dispersion- position of each indiv is independent of other indiv's

Video: Prokaryotic Flagella (Salmonella typhimurium) (random)

LE 52-3c

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


Demography

• Demography- study of the vital statistics of a pop & how they change over time

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


Life Tables

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

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

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



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)

# o

f su

rviv

ors

(lo

g s

cale

)

4 6 80 2

1,000

100

10

1


• Survivorship curves can be classified into 3 general types: Type I, Type II, & Type III

LE 52-5

III

II

100

Percentage of maximum life span

# 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 pop

• It describes reproductive patterns of a pop



Concept 52.2: Life history traits are products of natural selection

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


Life History Diversity

• Life histories are very diverse

• Semelparity- “big-bang” reproduction, reproduce once & die

• Iteroparity- repeated reproduction, produce offspring repeatedly



“Trade-offs” & Life Histories

• Organisms have finite resources, which may lead to trade-offs b/w survival & 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 lg # of small seeds, ensuring that at least some of them will grow & eventually reproduce

LE 52-8a

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


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

LE 52-8b

Some plants, such as this coconut palm, produce a moderate # 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 pop growth in an idealized, unlimited environ

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

• Idealized situations help us understand the capacity of spp to ↑ & the cond’s that may facilitate this growth


Per Capita Rate of ↑

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


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

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

dNdt rN


Exponential Growth

• Exponential pop growth- pop ↑ under idealized cond’s; J-shaped curve

• Under these cond’s, the rate of reproduction is at its maximum, called the intrinsic rate of ↑


• Equation of exponential pop growth:

dNdt rmaxN

LE 52-9

# of generations

po

p s

ize

(N)

2,000

= 1.0N

1,000

1,500

500

0151050

dNdt

= 0.5NdNdt


• The J-shaped curve of exponential growth characterizes some rebounding pop's

LE 52-10

Year

Ele

ph

ant

po

p8,000

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 pop

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


• Carrying capacity (K)- max pop size the environ can support


The Logistic Growth Model

• logistic pop growth model- per capita rate of ↑ declines as carrying capacity is reached; sigmoid (S-shaped) curve

• We construct the logistic model by starting w/ the exponential model & adding an expression that reduces per capita rate of ↑ as N ↑'s

LE 52-11

pop size (N)

Per

cap

ita

rate

of

↑ (

r)

Maximum

Positive

Negative

N = K0


• The logistic growth equation includes K, the carrying capacity

dNdt

(K N)

Krmax N


LE 52-12

# of generations

po

p s

ize

(N) K = 1,500

1,500

2,000

1,000

500

1510500

Logistic growth

Exponentialgrowth

= 1.0NdNdt

= 1.0NdNdt

1,500 – N

1,500


The Logistic Model & Real pop's

• The growth of laboratory pop's of paramecia fits an S-shaped curve

LE 52-13a

Time (days)

# o

f P

aram

eciu

m/m

L 1,000

0

400

5

200

100

15

800

600

A Paramecium pop in the lab


• Some pop's overshoot K before settling down to a relatively stable density

LE 52-13b

Time (days)

# o

f D

aph

nia

/50

mL 180

0

90

20

60

400

60

150

120

A Daphnia pop in the lab

30

80 100 120 140 160


• Some pop's fluctuate greatly around K

LE 52-13c

Time (years)

# o

f fe

mal

es80

1975 1980

40

19850

1990

60

A song sparrow pop in its natural habitat

20

1995 2000


• The logistic model fits few real pop's but is useful for estimating possible growth


The Logistic Model & Life Histories

• Life history traits favored by natural selection may vary w/ pop density & environmental cond’s

• K-selection/density-dependent selection- selects for life history traits that are sensitive to pop density

• r-selection/density-independent selection- selects for life history traits that max reproduction


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


Concept 52.5: pop's are regulated by a complex interaction of biotic & abiotic influences

• There are 2 general questions about regulation of pop growth:

– What environmental factors stop a pop from growing?

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


pop Change & pop Density

• In density-independent pop's, birth rate & death rate do not change w/ pop density

• In density-dependent pop's, birth rates fall & death rates rise w/ pop density

LE 52-14

pop density

Equilibriumdensity

Density-independentbirth rate

Density-dependentdeath rate

pop density

Equilibriumdensity

Density-independentdeath rate

Density-dependentbirth rate

pop density

Equilibriumdensity

Density-dependentdeath rate

Density-dependentbirth rate

pe

r c

ap

ita

Bir

th o

r d

ea

th r

ate


Density-Dependent pop Regulation

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

• Density dependent limiting factors- affected by competition for resources, territoriality, health, predation, toxic wastes, & intrinsic factors


Competition for Resources

• In crowded pop's, increasing pop density intensifies intraspecific competition for resources

LE 52-15

10,000

Av

era

ge

# o

f s

ee

ds

pe

r re

pro

du

cin

g i

nd

ivid

ua

l(l

og

sc

ale

)

1,000

100

100101Plants per m2 (log scale)

Plantain. The # of seeds produced by plantain (Plantago major) decreases as density ↑'s.

Song sparrow. Clutch size in the song sparrow on Mandarte Island, British Columbia, decreases as density ↑'s & 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 & 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

• pop density can influence the health & survival of organisms

• In dense pop's, pathogens can spread more rapidly


Predation

• As a prey pop builds up, predators may feed preferentially on that spp


Toxic Wastes

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


Intrinsic Factors

• For some pop's, intrinsic (physiological) factors appear to regulate pop size

• **for example, white-footed mice stop reproduction when the pop density gets high, this is apparently due to aggressive interactions that increase w/pop density (even when there is abundant food & shelter)

• * scientists are not sure what is the mechanism by which aggression affects reproductive rate


• Density independent limiting factors- Natural disasters, weather, damming rivers, etc….


pop Dynamics

• The study of pop dynamics focuses on the complex interactions b/w biotic & abiotic factors that cause variation in pop size


Stability & Fluctuation

• Long-term pop studies have challenged the hypothesis that pop's of large mammals are relatively stable over time

LE 52-18

1960

Year

Mo

ose

po

p s

ize

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 & food shortage, leading to starvation of more than 75% of the pop


• Extreme fluctuations in pop 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 & Immigration

• Metapopulations are groups of pop's linked by immigration & emigration

• High levels of immigration combined w/ higher survival can result in greater stability in pop's

LE 52-20

1988Year

# o

f b

reed

ing

fem

ales

60

1989 1990 1991

Smallislands

MandarteIsland

50

40

30

20

10

0


• Many pop's undergo boom-&-bust cycles

• Boom-&-bust cycles are influenced by complex interactions b/w biotic & abiotic factors

LE 52-21

Year

Ha

re p

op

siz

e(t

ho

us

an

ds

)

1850

Snowshoe hare

01875 1900 1925

40

80

120

160

Ly

nx

po

p s

ize

(th

ou

sa

nd

s)

Lynx

0

3

6

9


Concept 52.6: Human pop growth has slowed after centuries of exponential ↑

• No pop can grow indefinitely, & humans are no exception


The Global Human pop

• The human pop increased relatively slowly until about 1650 & then began to grow exponentially

LE 52-22

8000B.C.

Hu

man

po

p (

bil

lio

ns)

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 pop is still growing, the rate of growth began to slow about 40 years ago

LE 52-23

An

nu

al p

erce

nt

↑

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 pop Change

• To maintain pop stability, a regional human pop can exist in one of 2 configurations:

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

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

• The demographic transition is the move from the 1st state toward the 2nd 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 w/ various factors in developed & developing countries


Age Structure

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

• Age structure- relative # of indiv's at each age

• It is commonly represented in pyramids

LE 52-25

Rapid growthAfghanistan

AgeMale

Percent of pop

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

Percent of pop

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

Percent of pop

Female

6 4 2 2 4 6 808


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

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


Infant Mortality & Life Expectancy

• Infant mortality & life expectancy at birth vary greatly among developed & 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 & 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 & 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

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