Chapter 52

Population Ecology

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

LE 52-2

Populationsize

Emigration

Deaths

ImmigrationBirths

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.

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.

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

LE 52-4

Males

Females

10

Age (years)

Num

ber o

f sur

vivo

rs (l

og s

cale

)

4 6 80 2

1,000

100

10

1

LE 52-5

III

II

100

Percentage of maximum life span

Num

ber o

f sur

vivo

rs (l

og s

cale

)

0 50

1,000

100

10

1

I

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 52-8a

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.

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

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

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

Territoriality

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

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

LE 52-22

8000B.C.

Hum

an p

opul

ation

(bill

ions

)

6

5

4

3

2

1

04000B.C.

3000B.C.

2000B.C.

1000B.C.

The Plague

0 1000A.D.

2000A.D.

Human Population Growth

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

Birt

h or

dea

th ra

te 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

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

Percent of population

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 population

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 m

orta

lity

(dea

ths

per 1

,000

birt

hs)

50

40

30

20

10

0Developedcountries

60

Developingcountries

Life

exp

ecta

ncy

(yea

rs)

80

40

20

0Developedcountries

60

Developingcountries

LE 52-27

Ecol

ogic

al fo

otpr

int (

ha 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

Competition

• Interspecific competition occurs when species compete for a resource in short supply

• Strong competition can lead to competitive exclusion, local elimination of a competing species

The Competitive Exclusion Principle

• The competitive exclusion principle states that two species competing for the same limiting resources cannot coexist in the same place

LE 53-2

Chthamalusfundamental niche

High tide

Low tideOcean

Chthamalusrealized niche

High tide

Low tideOcean

Balanusrealized niche

ChthamalusBalanus

Resource Partitioning

• Resource partitioning is differentiation of ecological niches, enabling similar species to coexist in a community

LE 53-3A. insolitususually percheson shady branches.

A. ricordii

A. insolitus

A. christophei

A. cybotesA. etheridgei

A. alinigerA. distichus

A. distichusperches onfence postsand othersunnysurfaces.

Predation

• Predation refers to interaction where one species, the predator, kills and eats the other, the prey

• Some feeding adaptations of predators are claws, teeth, fangs, stingers, and poison

• Prey display various defensive adaptations• Behavioral defenses include hiding, fleeing, self-

defense, and alarm calls• Animals also have morphological and

physiological defense adaptations

• Cryptic coloration, or camouflage, makes prey difficult to spot

Video: Seahorse Camouflage

• Animals with effective chemical defense often exhibit bright warning coloration, called aposematic coloration

• Predators are particularly cautious in dealing with prey that display such coloration

• In Batesian mimicry, a palatable or harmless species mimics an unpalatable or harmful model

LE 53-7

Hawkmoth larva

Green parrot snake

• In Müllerian mimicry, two or more unpalatable species resemble each other

LE 53-8

Cuckoo bee

Yellow jacket

Herbivory• Herbivory refers to an interaction in which an

herbivore eats parts of a plant or alga• It has led to evolution of plant mechanical and

chemical defenses and adaptations by herbivores

Parasitism

• In parasitism, one organism, the parasite, derives nourishment from another organism, its host, which is harmed in the process

• Parasitism exerts substantial influence on populations and the structure of communities

Disease

• Effects of disease on populations and communities are similar to those of parasites

• Pathogens, disease-causing agents, are typically bacteria, viruses, or protists

Mutualism

• Mutualistic symbiosis, or mutualism, is an interspecific interaction that benefits both species

Video: Clownfish and Anemone

Commensalism• In commensalism, one species benefits and the

other is apparently unaffected• Commensal interactions are hard to document

in nature because any close association of two species likely affects both

LE 53-12

Quaternaryconsumers

Tertiaryconsumers

Carnivore

Carnivore

Carnivore

Carnivore

Secondaryconsumers

CarnivoreCarnivore

Primaryconsumers

ZooplanktonHerbivore

Primaryproducers

PhytoplanktonPlant

A terrestrial food chain A marine food chain

LE 53-13

Euphausids(krill)

Carnivorousplankton

Phyto-plankton

Copepods

Squids

Elephantseals

FishesBirds

Crab-eaterseals

Leopardseals

Spermwhales

Smallertoothedwhales

Baleenwhales

Humans

LE 53-14

Zooplankton

Fish larvae

Fish eggs

Sea nettle Juvenile striped bass

Keystone Species

• In contrast to dominant species, keystone species are not necessarily abundant in a community

• They exert strong control on a community by their ecological roles, or niches

LE 53-16

Without Pisaster (experimental)

With Pisaster (control)

1963 ’64 ’65 ’66 ’67 ’68 ’69 ’70 ’71 ’72 ’73

20

15

10

5

0

Num

ber o

f spe

cies

pres

ent

LE 53-21

Before a controlled burn.A prairie that has not burned for several years has a high propor-tion of detritus (dead grass).

During the burn. The detritus serves as fuel for fires.

After the burn. Approximately one month after the controlled burn, virtually all of the biomass in this prairie is living.

LE 53-22

Soon after fire. As this photo taken soon after the fire shows, the burn left a patchy landscape. Note the unburned trees in the distance.

One year after fire. This photo of the same general area taken the following year indicates how rapidly the com-munity began to recover. A variety of herbaceous plants, different from those in the former forest, cover the ground.

Human Disturbance

• Humans are the most widespread agents of disturbance

• Human disturbance to communities usually reduces species diversity

• Humans also prevent some naturally occurring disturbances, which can be important to community structure

Ecological Succession

• Ecological succession is the sequence of community and ecosystem changes after a disturbance

• Primary succession occurs where no soil exists when succession begins

• Secondary succession begins in an area where soil remains after a disturbance

• Early-arriving species and later-arriving species may be linked in one of three processes:– Early arrivals may facilitate appearance of later

species by making the environment favorable– They may inhibit establishment of later species– They may tolerate later species but have no impact

on their establishment

LE 53-24Pioneer stage, with fireweed dominant

Dryas stage

Spruce stageNitrogen fixation by Dryas and alder

increases the soil nitrogen content.

Successional stageDryasPioneer Alder Spruce

Soil

nitr

ogen

(g/m

2 )

60

50

40

30

20

10

0

LE 54-2

Microorganismsand other

detritivores

Tertiaryconsumers

Secondaryconsumers

Detritus Primary consumers

Sun

Primary producers

Heat

Key

Chemical cycling

Energy flow

Decomposition

• Decomposition connects all trophic levels• Detritivores, mainly bacteria and fungi, recycle

essential chemical elements by decomposing organic material and returning elements to inorganic reservoirs

Pyramids of Production

• A pyramid of net production represents the loss of energy with each transfer in a food chain

LE 54-11

1,000,000 J of sunlight

10,000 J

1,000 J

100 J

10 JTertiaryconsumers

Secondaryconsumers

Primaryconsumers

Primaryproducers

Pyramids of Biomass

• In a biomass pyramid, each tier represents the dry weight of all organisms in one trophic level

• Most biomass pyramids show a sharp decrease at successively higher trophic levels

LE 54-12a

Trophic level Dry weight(g/m2)

Tertiary consumers

Secondary consumers

Primary consumers

Primary producers

1.5

11

37

809

Most biomass pyramids show a sharp decrease in biomass at successively higher trophic levels, as illustrated by data from a bog at Silver Springs, Florida.

• Certain aquatic ecosystems have inverted biomass pyramids: Primary consumers outweigh the producers

LE 54-12b

Trophic level Dry weight(g/m2)

Primary consumers (zooplankton)

Primary producers (phytoplankton)

21

4

In some aquatic ecosystems, such as the English Channel, a small standing crop of primary producers (phytoplankton) supports a larger standing crop of primary consumers (zooplankton).

Pyramids of Numbers

• A pyramid of numbers represents the number of individual organisms in each trophic level

LE 54-13

Trophic level Number ofindividual organisms

Tertiary consumers

Secondary consumers

Primary consumers

Primary producers

3

354,904

708,624

5,842,424

LE 54-17a

Transportover land

Precipitationover landEvaporation

from oceanPrecipitationover ocean

Net movement ofwater vapor by wind

Solar energy

Evapotranspirationfrom land

Runoff andgroundwater

Percolationthroughsoil

LE 54-17b

Cellularrespiration

Burning offossil fuelsand wood

Carbon compoundsin water

Photosynthesis

Primaryconsumers

Higher-levelconsumers

Detritus

Decomposition

CO2 in atmosphere

LE 54-17c

Assimilation

N2 in atmosphere

DecomposersNitrifyingbacteria

Nitrifyingbacteria

Nitrogen-fixingsoil bacteria

Denitrifyingbacteria

NitrificationAmmonification

Nitrogen-fixingbacteria in rootnodules of legumes

NO3–

NO2–NH4

+NH3

LE 54-17d

Sedimentation

Plants

Rain

Runoff

Weatheringof rocks

Geologicuplift

SoilLeaching

Decomposition

Plant uptakeof PO4

3–

Consumption

Agriculture and Nitrogen Cycling

• Agriculture removes nutrients from ecosystems that would ordinarily be cycled back into the soil

• Nitrogen is the main nutrient lost through agriculture; thus, agriculture greatly impacts the nitrogen cycle

• Industrially produced fertilizer is typically used to replace lost nitrogen, but effects on an ecosystem can be harmful

Toxins in the Environment

• Humans release many toxic chemicals, including synthetics previously unknown to nature

• In some cases, harmful substances persist for long periods in an ecosystem

• One reason toxins are harmful is that they become more concentrated in successive trophic levels

• In biological magnification, toxins concentrate at higher trophic levels, where biomass is lower

LE 54-23

Zooplankton0.123 ppm

Phytoplankton0.025 ppm

Lake trout4.83 ppm

Smelt1.04 ppm

Herringgull eggs124 ppm

Conc

entr

ation

of P

CBs

Depletion of Atmospheric Ozone

• Life on Earth is protected from damaging effects of UV radiation by a protective layer or ozone molecules in the atmosphere

• Satellite studies suggest that the ozone layer has been gradually thinning since 1975

LE 54-26

Ozo

ne la

yer t

hick

ness

(Dob

son

units

)

350

300

250

200

150

100

50

01960 1965 1970 1975 1980 1985 1990 1995 2000 2005

Year (Average for the month of October)

1955

• Destruction of atmospheric ozone probably results from chlorine-releasing pollutants produced by human activity

LE 54-27

Chlorine atoms

O3Chlorine

Cl2O2

CIO

O2

O2

CIO

Chlorine from CFCs interacts with ozone (O3), forming chlorine monoxide (CIO) and oxygen (O2).

Sunlight causes Cl2O2 to break down into O2 and free chlorine atoms. The chlorine atoms can begin the cycle again.

Two CIO molecules react, forming chlorine peroxide (Cl2O2).

Sunlight

LE 54-28

October 1979 October 2000