1

Chapter 53

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

Concept 53.1: Dynamic biological processes influence population

density, dispersion, and demographics• 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

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

• Sampling techniques can be used to estimate densities and total population sizes

• Population size can be estimated by either extrapolation from small samples, an index of population size, or the mark-

recapture method

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

• Immigration is the influx of new individuals from other areas

• Emigration is the movement of individuals out of a population

2

Fig. 53-3

Births

Births and immigrationadd individuals toa population.

Immigration

Deaths and emigrationremove individualsfrom a population.

Deaths

Emigration

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

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

Fig. 53-5

Age (years)

20 4 86

10

101

1,000

100

Nu

mb

er

of

su

rviv

ors

(lo

g s

cale

)

Males

Females

• Survivorship curves can be classified into three general types:

– Type I: low death rates during early and middle life, then an increase among older age groups

– Type II: the death rate is constant over the organism’s life span

– Type III: high death rates for the young, then a slower death rate for survivors

Fig. 53-6

1,000

100

10

10 50 100

II

III

Percentage of maximum life span

Nu

mb

er

of

su

rviv

ors

(lo

g s

ca

le)

I

3

Reproductive Rates

• For species with sexual reproduction, demographers often concentrate on females in a population

• 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

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

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

• 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:

∆N∆t

= rN

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

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

Fig. 53-10

Number of generations

0 5 10 150

500

1,000

1,500

2,000

1.0N=dN

dt

0.5N=dN

dt

Po

pu

lati

on

siz

e (

N)

4

Fig. 53-11

8,000

6,000

4,000

2,000

01920 1940 1960 1980

Year

Ele

ph

an

t p

op

ula

tio

n

1900

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

Fig. 53-12

2,000

1,500

1,000

500

00 5 10 15

Number of generations

Po

pu

lati

on

siz

e (

N)

Exponentialgrowth

1.0N=dN

dt

1.0N=dN

dt

K = 1,500

Logistic growth

1,500 – N

1,500

The Logistic Model and Real Populations

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

• These organisms are grown in a constant environment lacking predators and competitors

Fig. 53-13

1,000

800

600

400

200

0

0 5 10 15

Time (days)

Nu

mb

er

of

Para

me

ciu

m/m

L

Nu

mb

er

of

Da

ph

nia

/50 m

L

0

30

60

90

180

150

120

0 20 40 60 80 100 120 140 160

Time (days)

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

Population Dynamics

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

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Stability and Fluctuation

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

• Weather can affect population size over time

Fig. 53-18

2,100

1,900

1,700

1,500

1,300

1,100

900

700

500

01955 1965 1975 1985 1995 2005

Year

Nu

mb

er

of

sh

eep

• Changes in predation pressure can drive population fluctuations

Fig. 53-19

Wolves Moose

2,500

2,000

1,500

1,000

500

Nu

mb

er

of

mo

ose

0

Nu

mb

er

of

wo

lve

s50

40

30

20

10

01955 1965 1975 1985 1995 2005

Year

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

Nu

mb

er

of

lyn

x(t

ho

usa

nd

s)

Nu

mb

er

of

ha

res

(th

ou

sa

nd

s)

160

120

80

40

01850 1875 1900 1925

Year

9

6

3

0

6

The Global Human Population

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

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

Hu

ma

n p

op

ula

tio

n (

bil

lio

ns)

7

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

Fig. 53-23

2005

Projecteddata

An

nu

al p

erc

en

t in

cre

ase

Year

1950 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

Limits on Human Population Size

• 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

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Fig. 53-27

Log (g carbon/year)

13.4

9.85.8

Not analyzed

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

Food From the SeaFood From the Sea

•• What types of organisms are harvested?What types of organisms are harvested?

–– Finfish (about 90% of worldwide harvest)Finfish (about 90% of worldwide harvest)

–– ShellfishShellfish

–– Other species such as jellyfish, sea Other species such as jellyfish, sea

cucumbers, polychaetes and seaweedcucumbers, polychaetes and seaweed

–– While seafood represents only about 1% of While seafood represents only about 1% of

the food consumed each year, it represents the food consumed each year, it represents

about 30% of total animal protein consumedabout 30% of total animal protein consumed

Worldwide Worldwide

Marine Catch Marine Catch

and Maricultureand Mariculture

Atlantic bluefin tuna Thunnusthynnus

• Can grow >300 cm; 680 kg

• Extremely streamlined, one of the ocean’s fastest swimmers, endothermic

Bluefin as food

• 2001 440 pound tuna sold for $220,000 ($500/pound)

• Farm in oceanic pens

• Spotter planes and electric harpoons

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Optimal Yield and OverfishingOptimal Yield and Overfishing

•• SeaSea--life species are renewable resourceslife species are renewable resources

•• However, for a fishery to last longHowever, for a fishery to last long--term, it term, it

must be fished in a sustainable waymust be fished in a sustainable way

•• The sustainable yield is the amount that The sustainable yield is the amount that

can be caught and just maintain a can be caught and just maintain a constant population sizeconstant population size

Collapse of a FisheryCollapse of a Fishery

•• A fishery is regarded as collapsed if A fishery is regarded as collapsed if

numbers fall to 10% of historic highsnumbers fall to 10% of historic highs

•• It is estimated that oneIt is estimated that one--third of fisheries third of fisheries

are already collapsedare already collapsed

•• A 2006 study indicates that all major A 2006 study indicates that all major fisheries will collapse by 2050 if protective fisheries will collapse by 2050 if protective

measure are not taken to better manage measure are not taken to better manage and protect these resourcesand protect these resources

Managing the ResourcesManaging the Resources

•• Management can be difficult for many Management can be difficult for many

reasons:reasons:

–– Maximum sustainable yield is difficult to Maximum sustainable yield is difficult to

calculatecalculate

–– Harvested species may compete with other Harvested species may compete with other

species and fishing pressure may affect species and fishing pressure may affect

competitive balancecompetitive balance

–– Real fisheries are more complex than modelsReal fisheries are more complex than models

–– High seas are High seas are ““common propertycommon property””

• Bluefin tuna harpoon

• http://www.youtube.com/watch?v=tL-1te9SbLs&feature=related

• crab pot

• http://www.youtube.com/watch?v=Zd_OPFfpRdk

• tuna farming

• http://www.youtube.com/watch?v=XIbGTwLGZNU&feature=related