PowerPoint Lectures for Biology, Seventh Edition

Neil Campbell and Jane Reece

Chapter 53Chapter 53

Community Ecology

Overview: What Is a Community?

• A biological community is an assemblage of populations of various species living close enough for potential interaction

• Animals and plants surrounding a watering hole in southern Africa are members of a savanna community

Concept 53.1: A community’s interactions include competition, predation, herbivory, symbiosis, and disease

• Ecologists call relationships between species in a community interspecific interactions

• Interspecific interactions affect species survival and reproduction

• Examples are competition, predation, herbivory, symbiosis (parasitism, mutualism, and commensalism), and disease

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

Ecological Niches

• The total of a species’ use of biotic and abiotic resources is called the species’ ecological niche

• Ecologically similar species can coexist in a community if there are one or more significant differences in their niches

• As a result of competition, a species’ fundamental niche may differ from its realized niche

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.

Character Displacement

• Character displacement is a tendency for characteristics to be more divergent in sympatric populations of two species than in allopatric populations of the same two species

• An example is variation in beak size between populations of two species of Galapagos finches

LE 53-4

Beakdepth

Sympatricpopulations

G. fuliginosaG. fortis

Santa María, San Cristóbal40

20

0

Los Hermanos40

20

0

Daphne40

20

0

G. fuliginosa,allopatric

G. fortis,allopatric

Beak depth (mm)161412108

Perc

enta

ge o

f ind

ivid

uals

in e

ach

size

cla

ss

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

• 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 some cases, a prey species may gain significant protection by mimicking the appearance of another species

• 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

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

Interspecific Interactions and Adaptation

• Coevolution is reciprocal evolutionary adaptations of two interacting species

• The term is often used too loosely in describing adaptations within a community

• There is little evidence for true coevolution in most interspecific interactions

Concept 53.2: Dominant and keystone species exert strong controls on community structure

• In general, a few species in a community exert strong control on that community’s structure

• Two fundamental features of community structure are species diversity and feeding relationships

Species Diversity

• Species diversity of a community is the variety of organisms that make up the community

• It has two components: species richness and relative abundance

• Species richness is the total number of different species in the community

• Relative abundance is the proportion each species represents of the total individuals in the community

• Two communities can have the same species richness but a different relative abundance

• A community with an even species abundance is more diverse than one in which one or two species are abundant and the remainder are rare

LE 53-11

Community 1

AB

C

D

A: 25% B: 25% C: 25% D: 25%

Community 2A: 80% B: 5% C: 5% D: 10%

Trophic Structure

• Trophic structure is the feeding relationships between organisms in a community

• It is a key factor in community dynamics

• Food chains link trophic levels from producers to top carnivores

LE 53-12

Quaternaryconsumers

Tertiaryconsumers

Carnivore

Carnivore

Carnivore

Carnivore

Secondaryconsumers

CarnivoreCarnivore

Primaryconsumers

ZooplanktonHerbivore

Primaryproducers

PhytoplanktonPlant

A terrestrial food chain A marine food chain

Food Webs

• A food web is a branching food chain with complex trophic interactions

LE 53-13

Euphausids(krill)

Carnivorousplankton

Phyto-plankton

Copepods

Squids

Elephantseals

FishesBirds

Crab-eaterseals

Leopardseals

Spermwhales

Smallertoothedwhales

Baleenwhales

Humans

• Food webs can be simplified by isolating a portion of a community that interacts very little with the rest of the community

LE 53-14

Zooplankton

Fish larvae

Fish eggs

Sea nettle Juvenile striped bass

Limits on Food Chain Length

• Each food chain in a food web is usually only a few links long

• Two hypotheses attempt to explain food chain length: the energetic hypothesis and the dynamic stability hypothesis

• The energetic hypothesis suggests that length is limited by inefficient energy transfer

• The dynamic stability hypothesis proposes that long food chains are less stable than short ones

• Most data support the energetic hypothesis

LE 53-15

Productivity

Num

ber o

f tro

phic

links

Num

ber o

f spe

cies

No. of trophiclinks

No. of species

High(control)

6

5

4

3

2

1

0

6

5

4

3

2

1

0Medium Low

Species with a Large Impact

• Certain species have a very large impact on community structure

• Such species are highly abundant or play a pivotal role in community dynamics

Dominant Species

• Dominant species are those that are most abundant or have the highest biomass

• They exert powerful control over the occurrence and distribution of other species

• One hypothesis suggests that dominant species are most competitive in exploiting resources

• Another hypothesis is that they are most successful at avoiding predators

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

• Field studies of sea stars exhibit their role as a keystone species in intertidal communities

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

• Observation of sea otter populations and their predation shows how otters affect ocean communities

LE 53-17

100

806040

020

Sea otter abundance

Otte

r num

ber

(% m

ax. c

ount

)

400300200

0100

Sea urchin biomass

Gra

ms

per

0.25

m2

10864

02

Total kelp density

Num

ber p

er0.

25 m

2

19971993198919851972Year

Food chain beforekiller whaleinvolvement inchain

Food chain afterkiller whales startedpreying on otters

Ecosystem “Engineers” (Foundation Species)

• Some organisms exert influence by causing physical changes in the environment that affect community structure

• For example, beaver dams can transform landscapes on a very large scale

• Some foundation species act as facilitators that have positive effects on survival and reproduction of some other species in the community

LE 53-19

Num

ber o

f pla

nt s

peci

es

8

6

4

0

2

Conditions

WithJuncus

WithoutJuncusSalt marsh with Juncus

(foreground)

Bottom-Up and Top-Down Controls

• The bottom-up model of community organization proposes a unidirectional influence from lower to higher trophic levels

• In this case, presence or absence of mineral nutrients determines community structure, including abundance of primary producers

• The top-down model proposes that control comes from the trophic level above

• In this case, predators control herbivores, which in turn control primary producers

• Long-term experimental studies have shown that communities can shift periodically from bottom-up to top-down controls

LE 53-20

Perc

enta

ge o

fhe

rbac

eous

pla

nt c

over

100

75

50

0

25

Rainfall (mm)200 300 4000 100

• Pollution can affect community dynamics

• Biomanipulation can help restore polluted communities

LE 53-UN1171

Polluted State Restored State

Fish Abundant Rare

Zooplankton AbundantRare

Algae Abundant Rare

Concept 53.3: Disturbance influences species diversity and composition

• Decades ago, most ecologists favored the view that communities are in a state of equilibrium

• Recent evidence of change has led to a nonequilibrium model, which describes communities as constantly changing after being buffeted by disturbances

What Is Disturbance?

• A disturbance is an event that changes a community, removes organisms from it, and alters resource availability

• Fire is a significant disturbance in most terrestrial ecosystems

• It is often a necessity in some communities

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.

• The intermediate disturbance hypothesis suggests that moderate levels of disturbance can foster higher diversity than low levels of disturbance

• The large-scale fire in Yellowstone National Park in 1988 demonstrated that communities can often respond very rapidly to a massive disturbance

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

• Retreating glaciers provide a valuable field- research opportunity for observing succession

LE 53-23

McBride glacier retreating

CanadaAlaska

1940

1941

1931 1911

19481912

1907

1879 1879

1879

1899

1948

GrandPacific Gl.

1900

1935 194919131892

Riggs Gl.

Muir Gl.Plateau Gl.

McB

ride

Gl.

Case

men

t Gl.

1860Johns Hopkins

Gl.

Pleasant Is.

Reid Gl.1879

GlacierBay

1760

18301780

Miles

15Kilometers

10

105

5

0

0

• Succession on the moraines in Glacier Bay, Alaska, follows a predictable pattern of change in vegetation and soil characteristics

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

Concept 53.4: Biogeographic factors affect community diversity

• Two key factors correlated with a community’s species diversity are geographic location and size

Equatorial-Polar Gradients

• Two key factors in equatorial-polar gradients of species richness are probably evolutionary history and climate

• Species richness generally declines along an equatorial-polar gradient and is especially great in the tropics

• The greater age of tropical environments may account for the greater species richness

• Climate is likely the primary cause of the latitudinal gradient in biodiversity

• Two main climatic factors correlated with biodiversity are solar energy and water availability

• They can be considered together by measuring a community’s rate of evapotranspiration

• Evapotranspiration is evaporation of water from soil plus transpiration of water from plants

LE 53-25

Trees

Tree

spe

cies

rich

ness

180

160

140

120

100

80

0

Vert

ebra

te s

peci

es ri

chne

ss(lo

g sc

ale)

200

100

50

10

60

40

20

1,100900700500300100Actual evapotranspiration (mm/yr)

Vertebrates

2,000Potential evapotranspiration (mm/yr)

1,5001,000500

Area Effects

• The species-area curve quantifies the idea that, all other factors being equal, a larger geographic area has more species

• A species-area curve of North American breeding birds supports this idea

LE 53-26

Num

ber o

f spe

cies

(log

sca

le)

1,000

Area (acres)1010109106 107 108103 104 1051 10 100

100

10

1

Island Equilibrium Model

• Species richness on islands depends on island size, distance from the mainland, immigration, and extinction

• The equilibrium model of island biogeography maintains that species richness on an ecological island levels off at a dynamic equilibrium point

LE 53-27

Immigration and extinction ratesNumber of species on island

Equilibriumnumber

Imm

igration

Rat

e of

imm

igra

tion

or e

xtin

ctio

n

Extin

ction

Effect of island sizeNumber of species on island

Smallisland

Immigration

Rat

e of

imm

igra

tion

or e

xtin

ctio

n

Extin

ctio

n

Largeisland

(large island)

Immigration

(small island)

Extin

ction

(large

islan

d)(sm

all is

land)

Effect of distance from mainlandNumber of species on island

Farisland

Imm

igration

Rat

e of

imm

igra

tion

or e

xtin

ctio

n

Extin

ctio

n

Nearisland

(near island)

Immigration

(far island) Extin

ction

(near

island

)(fa

r isla

nd)

• Studies of species richness on the Galápagos Islands support the prediction that species richness increases with island size

LE 53-28

Area of island (mi2)(log scale)

400N

umbe

r of p

lant

spe

cies

(log

sca

le)

1,0001001010.1

200

100

50

25

10

5

Concept 53.5: Contrasting views of community structure are the subject of continuing debate

• In the 1920s and 1930s, two views on community structure emerged: the integrated hypothesis and the individualistic hypothesis

Integrated and Individualistic Hypotheses

• The integrated hypothesis describes a community as an assemblage of closely linked species, locked into association by mandatory biotic interactions

• The individualistic hypothesis proposes that communities are loosely organized associations of independently distributed species with the same abiotic requirements

• The integrated hypothesis predicts that presence or absence of particular species depends on presence or absence of other species

LE 53-29a

Environmental gradient(such as temperature or moisture)

Popu

latio

nde

nsiti

es o

fin

divi

dual

spec

ies

Integrated hypothesis

• The individualistic hypothesis predicts that each species is distributed according to its tolerance ranges for abiotic factors

LE 53-29b

Environmental gradient(such as temperature or moisture)

Popu

latio

nde

nsiti

es o

fin

divi

dual

spec

ies

Individualistic hypothesis

• In most actual cases, composition of communities seems to change continuously, with each species more or less independently distributed

LE 53-29c

Trees in the Santa Catalina Mountains

Num

ber o

fpl

ants

per h

ecta

re

Moisture gradientWet

600

400

200

0Dry

Rivet and Redundancy Models

• The rivet model suggests that all species in a community are linked in a tight web of interactions

• It also states that loss of even a single species has strong repercussions for the community

• The redundancy model proposes that if a species is lost, other species will fill the gap

• Community hypotheses and models represent extremes; most communities probably lie somewhere in the middle

Chapter 53Overview: What Is a Community?�Slide Number 3Concept 53.1: A community’s interactions include competition, predation, herbivory, symbiosis, and diseaseSlide Number 5CompetitionThe Competitive Exclusion PrincipleEcological NichesLE 53-2Resource PartitioningLE 53-3Character DisplacementLE 53-4PredationSlide Number 15Slide Number 16Slide Number 17Slide Number 18Slide Number 19Slide Number 20Slide Number 21LE 53-7Slide Number 23LE 53-8HerbivoryParasitismDiseaseMutualismSlide Number 29CommensalismSlide Number 31Interspecific Interactions and AdaptationConcept 53.2: Dominant and keystone species exert strong controls on community structureSpecies DiversitySlide Number 35LE 53-11Trophic StructureLE 53-12Food WebsLE 53-13Slide Number 41LE 53-14Limits on Food Chain LengthSlide Number 44LE 53-15Species with a Large ImpactDominant SpeciesSlide Number 48Keystone SpeciesSlide Number 50LE 53-16Slide Number 52LE 53-17Ecosystem “Engineers” (Foundation Species)Slide Number 55Slide Number 56LE 53-19Bottom-Up and Top-Down ControlsSlide Number 59Slide Number 60LE 53-20Slide Number 62LE 53-UN1171Concept 53.3: Disturbance influences species diversity and compositionWhat Is Disturbance?LE 53-21Slide Number 67LE 53-22Human DisturbanceEcological SuccessionSlide Number 71Slide Number 72LE 53-23Slide Number 74LE 53-24Concept 53.4: Biogeographic factors affect community diversityEquatorial-Polar GradientsSlide Number 78LE 53-25Area EffectsLE 53-26Island Equilibrium ModelLE 53-27Slide Number 84LE 53-28Concept 53.5: Contrasting views of community structure are the subject of continuing debateIntegrated and Individualistic HypothesesSlide Number 88LE 53-29aSlide Number 90LE 53-29bSlide Number 92LE 53-29cRivet and Redundancy Models