Chapter 54CommunityEcology

Overview: A Sense of Community

A biological community is an assemblage ofpopulations of various species living close enoughfor potential interaction

Concept 54.1Community interactions areclassified by whether they help,harm, or have no effect on thespecies involved

Ecologists call relationships between species in acommunity interspecific interactionsExamples are competition, predation, herbivory, andsymbiosis (parasitism, mutualism, and commensalism)Interspecific interactions can affect the survival andreproduction of each species, and the effects can besummarized as positive (+), negative ( ), or no effect (0)

CompetitionInterspecific competition ( / interaction) occurswhen species compete for a resource in shortsupply

Competition: Competitive Exclusion

Strong competition can lead to competitiveexclusion, local elimination of a competing speciesThe competitive exclusion principle states that twospecies competing for the same limiting resourcescannot coexist in the same place

Competition: Ecological Niches

ecological nicheAn ecological niche can also be thought of as an

Ecologically similar species can coexist in acommunity if there are one or more significantdifferences in their nichesResource partitioning is differentiation of ecologicalniches, enabling similar species to coexist in acommunity

A. ricordii

A. insolitus usually percheson shady branches.

A. distichus perches on fenceposts and other sunny surfaces.

A. alinigerA. distichus

A. insolitus

A. christophei

A. cybotesA. etheridgei

niche may differ from its realized niche

Ocean

ChthamalusBalanus

EXPERIMENT

RESULTS

High tide

Low tide

Chthamalusrealized niche

Balanusrealized niche

High tide

Chthamalusfundamental niche

Low tideOcean

Competition: CharacterDisplacement

Character displacement isa tendency forcharacteristics to be moredivergent in sympatricpopulations of two speciesthan in allopatricpopulations of the sametwo speciesAn example is variation inbeak size betweenpopulations of two speciesof Galápagos finches

Los Hermanos

G. fuliginosaG. fortis

Beakdepth

Daphne

G. fuliginosa,allopatric

G. fortis,allopatric

Sympatricpopulations

Santa María, San Cristóbal

Beak depth (mm)

6040200

6040200

6040200

8 10 12 14 16

PredationPredation (+/ interaction) refers to interactionwhere one species, the predator, kills and eats theother, the preySome feeding adaptations of predators are claws,teeth, fangs, stingers, and poisonPrey display various defensive adaptationsBehavioral defenses include hiding, fleeing, formingherds or schools, self-defense, and alarm callsAnimals also have morphological and physiologicaldefense adaptations

PredationCryptic coloration, orcamouflage, makes preydifficult to spotAnimals with effectivechemical defense oftenexhibit bright warningcoloration, calledaposematic coloration

Predators are particularlycautious in dealing withprey that display suchcoloration

Canyon tree frog(a)Cryptic

coloration

Poison dart frog(b)Aposematic

coloration

PredationIn some cases, a prey species may gain significantprotection by mimicking the appearance ofanother speciesIn Batesian mimicry, a palatable or harmless speciesmimics an unpalatable or harmful modelIn Müllerian mimicry, two or more unpalatablespecies resemble each other

(c) Batesian mimicry: A harmless species mimics a harmful one.Hawkmothlarva

Green parrot snakeYellow jacketCuckoo bee

Müllerian mimicry: Two unpalatable speciesmimic each other.

(d)

HerbivoryHerbivory (+/ interaction) refers to an interaction inwhich an herbivore eats parts of a plant or algaIt has led to evolution of plant mechanical andchemical defenses and adaptations by herbivores

SymbiosisSymbiosis is a relationship where two or morespecies live in direct and intimate contact with oneanother

Symbiosis: ParasitismIn parasitism (+/ interaction), one organism, theparasite, derives nourishment from anotherorganism, its host, which is harmed in the processParasites that live within the body of their host arecalled endoparasites; parasites that live on theexternal surface of a host are ectoparasitesMany parasites have a complex life cycle involvinga number of hostsSome parasites change the behavior of the host toincrease their own fitness

Symbiosis:Mutualism

Mutualistic symbiosis, ormutualism (+/+interaction), is aninterspecific interactionthat benefits bothspeciesA mutualism can be

Obligate, where onespecies cannot survivewithout the otherFacultative, where bothspecies can survivealone

(a) Acacia tree and ants (genus Pseudomyrmex)

(b) Area cleared by ants at the base of an acacia tree

Symbiosis: CommensalismIn commensalism (+/0 interaction), one speciesbenefits and the other is apparently unaffectedCommensal interactions are hard to document innature because any close association likely affectsboth species

Concept 54.2Dominant and keystone speciesexert strong controls on communitystructure

In general, a few species in a community exertstructure

Two fundamental features of communitystructure are species diversity and feedingrelationships

Species DiversitySpecies diversity of a community is the variety oforganisms that make up the communityIt has two components: species richness andrelative abundanceSpecies richness is the total number of differentspecies in the communityRelative abundance is the proportion each speciesrepresents of the total individuals in the community

Community 1A: 25% B: 25% C: 25% D: 25%

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

A B C D

Species DiversityTwo communities can have the same speciesrichness but a different relative abundanceDiversity can be compared using a diversity index

Shannon diversity index (H):H = [(pA ln pA) + (pB ln pB) + (pC ln pC) +

Determining the number and abundance ofspecies in a community is difficult, especially forsmall organismsMolecular tools can be used to help determinemicrobial diversity

Soil pH

3.6

RESULTS

3.4

3.2

3.0

2.8

2.6

2.4

2.23 4 5 6 7 8 9

Trophic StructureTrophic structure is thefeeding relationshipsbetween organisms in acommunityIt is a key factor incommunity dynamicsFood chains link trophiclevels from producers totop carnivores

Carnivore

Carnivore

Carnivore

Herbivore

Plant

A terrestrial food chain

Quaternaryconsumers

Tertiaryconsumers

Secondaryconsumers

Primaryconsumers

Primaryproducers

A marine food chainPhytoplankton

Zooplankton

Carnivore

Carnivore

Carnivore

Trophic Structure: Food WebsA food web is a branching food chain with complextrophic interactions

Humans

Smallertoothedwhales

Baleenwhales

Spermwhales

Elephantseals

Leopardseals

Crab-eaterseals

Birds Fishes Squids

Carnivorousplankton

CopepodsEuphausids(krill)

Phyto-plankton

Trophic Structure: Food WebsSpecies may play a role at more than one trophiclevelFood webs can be simplified by isolating a portionof a community that interacts very little with the restof the community

Sea nettle

Fish larvae

Juvenile striped bass

Fish eggs Zooplankton

Trophic Structure: Limits on FoodChain Length

Each food chain in a food web is usually only a fewlinks longTwo hypotheses attempt to explain food chainlength: the energetic hypothesis and the dynamicstability hypothesisThe energetic hypothesissuggests that length is limitedby inefficient energy transferThe dynamic stabilityhypothesis proposes that longfood chains are less stablethan short onesMost data support the energetic hypothesis

Productivity

012345

High (control):natural rate of

litter fall

Medium: 1/10natural rate

Low: 1/100natural rate

Species with a Large ImpactCertain species have a very large impact oncommunity structureSuch species are highly abundant or play a pivotalrole in community dynamics

Species with a Large Impact:Dominant Species

Dominant species are those that are most abundant orhave the highest biomassBiomass is the total mass of all individuals in a populationDominant species exert powerful control over theoccurrence and distribution of other speciesOne hypothesis suggests that dominant species are mostcompetitive in exploiting resourcesAnother hypothesis is that they are most successful atavoiding predatorsInvasive species, typically introduced to a newenvironment by humans, often lack predators or disease

Species with a Large Impact:Keystone Species

Keystone species exert strong control on acommunity by their ecological roles, or nichesIn contrast to dominant species, they are notnecessarily abundant in a communityField studies of sea stars exhibit their role as akeystone species in intertidal communities

EXPERIMENT

With Pisaster (control)

Without Pisaster (experimental)

Year

20

15

10

5

01963

RESULTS

Species with aLarge Impact:KeystoneSpecies

Observation of seaotter populationsand their predationshows how ottersaffect oceancommunities

(a) Sea otter abundance

100

80

60

40

200

400300200100

0(b) Sea urchin biomass

1086420

1972 1985 1997Year

(c) Total kelp densityFood chain

1989 1993

Species with a Large Impact:Foundation Species (Ecosystem

physical changes in the environment that affectcommunity structureFor example, beaver dams can transformlandscapes on a very large scale

Species with a Large Impact:Foundation Species (Ecosystem

Some foundation species act as facilitators thathave positive effects on survival and reproductionof some other species in the community

With Juncus Without Juncus0

2

4

6

8

Salt marsh with Juncus(foreground)

(a) (b)

Bottom-Up and Top-Down Controls

The bottom-up model of community organizationproposes a unidirectional influence from lower tohigher trophic levelsIn this case, presence or absence of mineralnutrients determines community structure, includingabundance of primary producersThe top-down model, also called the trophiccascade model, proposes that control comes fromthe trophic level aboveIn this case, predators control herbivores, which inturn control primary producers

Bottom-Up and Top-Down ControlsLong-term experimental studies have shown thatcommunities vary in their relative degree of bottom-up to top-down control

Control plotsWarmed plots

E. antarcticus S. lindsayae0

100

200

300

RESULTS

Bottom-Up and Top-Down ControlsPollution can affect community dynamicsBiomanipulation can help restore pollutedcommunities

Polluted State Restored State

Rare

Rare Abundant

AbundantFish

Zooplankton

Algae Abundant Rare

Concept 54.3Disturbance influences speciesdiversity and composition

Decades ago, most ecologists favored the view thatcommunities are in a state of equilibriumThis view was supported by F. E. Clements who suggestedthat species in a climax community function as asuperorganismOther ecologists, including A. G. Tansley and H. A.Gleason, challenged whether communities were atequilibriumRecent evidence of change has led to a nonequilibriummodel, which describes communities as constantlychanging after being buffeted by disturbances

Characterizing DisturbanceA disturbance is an event that changes acommunity, removes organisms from it, and altersresource availabilityFire is a significant disturbance in most terrestrialecosystemsIt is often a necessity in some communities

The intermediate disturbance hypothesis suggeststhat moderate levels of disturbance can fostergreater diversity than either high or low levels ofdisturbanceHigh levels of disturbance exclude many slow-growing speciesLow levels of disturbance allow dominant species toexclude less competitive species

Log intensity of disturbance

30

20

15

101.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0

35

25

1.00.9

Characterizing DisturbanceThe large-scale fire in Yellowstone National Park in1988 demonstrated that communities can oftenrespond very rapidly to a massive disturbance

(a) Soon after fire (b) One year after fire

Ecological SuccessionEcological succession is the sequence ofcommunity and ecosystem changes after adisturbancePrimary succession occurs where no soil exists whensuccession beginsSecondary succession begins in an area where soilremains after a disturbanceEarly-arriving species and later-arriving species maybe linked in one of three processes:

Early arrivals may facilitate appearance of laterspecies by making the environment favorableThey may inhibit establishment of later speciesThey may tolerate later species but have no impacton their establishment

Retreating glaciers provide a valuable field-research opportunity for observing successionSuccession on the moraines in Glacier Bay, Alaska,follows a predictable pattern of change invegetation and soil characteristics

Pioneer stage, withfireweed dominant

1

19411907

1860

1760

Alaska

GlacierBay

Kilometers5 10 150

Dryas stage2

Alder stage3Spruce stage4

Succession is the result of changes induced by thevegetation itselfOn the glacial moraines, vegetation lowers the soilpH and increases soil nitrogen content

Successional stagePioneer Dryas Alder Spruce

0

10

20

30

40

50

60

Human DisturbanceHumans have the greatest impact on biologicalcommunities worldwideHuman disturbance to communities usually reducesspecies diversityHumans also prevent some naturally occurringdisturbances, which can be important tocommunity structure

Concept 54.4Biogeographic factors affectcommunity biodiversity

Latitude and area are two key factors that affect

Latitudinal GradientsSpecies richness generally declines along anequatorial-polar gradient and is especially great inthe tropicsTwo key factors in equatorial-polar gradients ofspecies richness are probably evolutionary historyand climateThe greater age of tropical environments mayaccount for the greater species richness

Latitudinal GradientsClimate is likely the primary causeof the latitudinal gradient inbiodiversityTwo main climatic factorscorrelated with biodiversity aresolar energy and water availabilityThey can be considered together

of evapotranspirationEvapotranspiration is evaporationof water from soil plus transpirationof water from plants

(a) TreesActual evapotranspiration (mm/yr)

180

160

140

120

100

80

60

20

0

40

100 300 500 700 900 1,100

(b) Vertebrates

200

100

50

100 500 1,000 1,500 2,000

Potential evapotranspiration (mm/yr)

Area EffectsThe species-area curve quantifies the idea that, allother factors being equal, a larger geographic areahas more speciesA species-area curve of North American breedingbirds supports this idea

Area (hectares)

1,000

100

10

10.1 1 10 100 103 104 105 106 107 108 109 1010

Island Equilibrium ModelSpecies richness on islands depends on island size,distance from the mainland, immigration, andextinctionThe equilibrium model of island biogeographymaintains that species richness on an ecologicalisland levels off at a dynamic equilibrium point

Number of species on islandEquilibrium number

(a) Immigration and extinction rates

Number of species on island

(b) Effect of island size

Small island Large island

(c) Effect of distance from mainland

Number of species on islandFar island Near island

Island Equilibrium ModelStudies of species richness on the Galápagos Islandssupport the prediction that species richnessincreases with island size

Area of island (hectares)(log scale)

10 100 103 104 105 106

10

2550

100200

400

5

Concept 54.5Community ecology is useful forunderstanding pathogen life cyclesand controlling human disease

Ecological communities are universally affectedby pathogens, which include disease-causingmicroorganisms, viruses, viroids, and prionsPathogens can alter community structure quicklyand extensively

Pathogen and Community Structure

Pathogens can have dramatic effects oncommunitiesFor example, coral reef communities are beingdecimated by white-band diseaseHuman activities aretransporting pathogensaround the world atunprecedented ratesCommunity ecology isneeded to help studyand combat them

Community Ecology and ZoonoticDiseases

Zoonotic pathogens have been transferred fromother animals to humansThe transfer of pathogens can be direct or throughan intermediate species called a vector

zoonoticAvian flu is a highly contagious virusof birdsEcologists are studying the potentialspread of the virus from Asia toNorth America through migratingbirds