Sinh học - Chapter 54: Community ecology

Chapter 54Community EcologyOverview: A Sense of CommunityA biological community is an assemblage of populations of various species living close enough for potential interactionFig. 54-1Concept 54.1: Community interactions are classified by whether they help, harm, or have no effect on the species involvedEcologists call relationships between species in a community interspecific interactionsExamples are competition, predation, herbivory, and symbiosis (parasitism, mutualism, and commensalism)Interspecific interactions can affect the survival and reproduction of each species, and the effects can be summarized as positive (+), negative (–), or no effect (0)CompetitionInterspecific competition (–/– interaction) occurs when species compete for a resource in short supplyCompetitive ExclusionStrong competition can lead to competitive exclusion, local elimination of a competing speciesThe competitive exclusion principle states that two species competing for the same limiting resources cannot coexist in the same placeEcological NichesThe total of a species’ use of biotic and abiotic resources is called the species’ ecological niche An ecological niche can also be thought of as an organism’s ecological roleEcologically similar species can coexist in a community if there are one or more significant differences in their nichesResource partitioning is differentiation of ecological niches, enabling similar species to coexist in a communityFig. 54-2A. ricordiiA. insolitus usually percheson shady branches.A. distichus perches on fenceposts and other sunny surfaces.A. alinigerA. distichusA. insolitusA. christopheiA. cybotesA. etheridgeiAs a result of competition, a species’ fundamental niche may differ from its realized nicheFig. 54-3OceanChthamalusBalanusEXPERIMENTRESULTSHigh tideLow tideChthamalusrealized nicheBalanusrealized nicheHigh tideChthamalusfundamental nicheLow tideOceanFig. 54-3aOceanChthamalusBalanusEXPERIMENTHigh tideLow tideChthamalusrealized nicheBalanusrealized nicheFig. 54-3bRESULTSHigh tideChthamalusfundamental nicheLow tideOceanCharacter DisplacementCharacter displacement is a tendency for characteristics to be more divergent in sympatric populations of two species than in allopatric populations of the same two speciesAn example is variation in beak size between populations of two species of Galápagos finches Fig. 54-4Los HermanosG. fuliginosaG. fortisBeakdepthDaphneG. fuliginosa,allopatricG. fortis,allopatricSympatricpopulationsSanta María, San CristóbalBeak depth (mm)Percentages of individuals in each size class604020060402006040200810121416PredationPredation (+/– interaction) refers to interaction where one species, the predator, kills and eats the other, the preySome feeding adaptations of predators are claws, teeth, fangs, stingers, and poisonPrey display various defensive adaptationsBehavioral defenses include hiding, fleeing, forming herds or schools, self-defense, and alarm callsAnimals also have morphological and physiological defense adaptationsCryptic coloration, or camouflage, makes prey difficult to spotVideo: Seahorse CamouflageFig. 54-5Canyon tree frog(a)Crypticcoloration(b)AposematiccolorationPoison dart frog(c) Batesian mimicry: A harmless species mimics a harmful one.HawkmothlarvaGreen parrot snakeYellow jacketCuckoo beeMüllerian mimicry: Two unpalatable speciesmimic each other.(d)Fig. 54-5aCanyon tree frog(a)CrypticcolorationAnimals with effective chemical defense often exhibit bright warning coloration, called aposematic colorationPredators are particularly cautious in dealing with prey that display such colorationFig. 54-5bPoison dart frog(b)AposematiccolorationIn some cases, a prey species may gain significant protection by mimicking the appearance of another speciesIn Batesian mimicry, a palatable or harmless species mimics an unpalatable or harmful modelFig. 54-5cHawkmothlarva(c) Batesian mimicry: A harmless species mimics a harmful one.Green parrot snakeIn Müllerian mimicry, two or more unpalatable species resemble each otherFig. 54-5dCuckoo beeMüllerian mimicry: Two unpalatable speciesmimic each other.Yellow jacket(d)HerbivoryHerbivory (+/– interaction) refers to an interaction in which an herbivore eats parts of a plant or algaIt has led to evolution of plant mechanical and chemical defenses and adaptations by herbivoresFig. 54-6SymbiosisSymbiosis is a relationship where two or more species live in direct and intimate contact with one anotherParasitismIn parasitism (+/– interaction), one organism, the parasite, derives nourishment from another organism, its host, which is harmed in the processParasites that live within the body of their host are called endoparasites; parasites that live on the external surface of a host are ectoparasitesMany parasites have a complex life cycle involving a number of hostsSome parasites change the behavior of the host to increase their own fitnessMutualismMutualistic symbiosis, or mutualism (+/+ interaction), is an interspecific interaction that benefits both speciesA mutualism can beObligate, where one species cannot survive without the otherFacultative, where both species can survive aloneVideo: Clownfish and AnemoneFig. 54-7(a) Acacia tree and ants (genus Pseudomyrmex)(b) Area cleared by ants at the base of an acacia treeFig. 54-7a(a) Acacia tree and ants (genus Pseudomyrmex)Fig. 54-7b(b) Area cleared by ants at the base of an acacia treeCommensalismIn commensalism (+/0 interaction), one species benefits and the other is apparently unaffectedCommensal interactions are hard to document in nature because any close association likely affects both speciesFig. 54-8Concept 54.2: Dominant and keystone species exert strong controls on community structureIn general, a few species in a community exert strong control on that community’s structureTwo fundamental features of community structure are species diversity and feeding relationshipsSpecies DiversitySpecies diversity of a community is the variety of organisms that make up the communityIt has two components: species richness and relative abundanceSpecies richness is the total number of different species in the communityRelative abundance is the proportion each species represents of the total individuals in the communityFig. 54-9Community 1A: 25% B: 25% C: 25% D: 25%Community 2A: 80% B: 5% C: 5% D: 10%ABCDTwo communities can have the same species richness but a different relative abundanceDiversity can be compared using a diversity indexShannon diversity index (H):H = –[(pA ln pA) + (pB ln pB) + (pC ln pC) + ]Determining the number and abundance of species in a community is difficult, especially for small organismsMolecular tools can be used to help determine microbial diversityFig. 54-10Soil pHShannon diversity (H)3.6RESULTS3.43.23.02.82.62.42.23456789Trophic StructureTrophic structure is the feeding relationships between organisms in a communityIt is a key factor in community dynamicsFood chains link trophic levels from producers to top carnivoresVideo: Shark Eating a SealFig. 54-11CarnivoreCarnivoreCarnivoreHerbivorePlantA terrestrial food chainQuaternaryconsumersTertiaryconsumersSecondaryconsumersPrimaryconsumersPrimaryproducersA marine food chainPhytoplanktonZooplanktonCarnivoreCarnivoreCarnivoreFood WebsA food web is a branching food chain with complex trophic interactionsFig. 54-12HumansSmallertoothedwhalesBaleenwhalesSpermwhalesElephantsealsLeopardsealsCrab-eatersealsBirdsFishesSquidsCarnivorousplanktonCopepodsEuphausids(krill)Phyto-planktonSpecies may play a role at more than one trophic level Food webs can be simplified by isolating a portion of a community that interacts very little with the rest of the communityFig. 54-13Sea nettleFish larvaeJuvenile striped bassFish eggsZooplanktonLimits on Food Chain LengthEach food chain in a food web is usually only a few links longTwo hypotheses attempt to explain food chain length: the energetic hypothesis and the dynamic stability hypothesisThe energetic hypothesis suggests that length is limited by inefficient energy transferThe dynamic stability hypothesis proposes that long food chains are less stable than short onesMost data support the energetic hypothesisFig. 54-14ProductivityNumber of trophic links012345High (control):natural rate oflitter fallMedium: 1/10natural rateLow: 1/100natural rateSpecies with a Large ImpactCertain species have a very large impact on community structureSuch species are highly abundant or play a pivotal role in community dynamicsDominant SpeciesDominant species are those that are most abundant or have the highest biomassBiomass is the total mass of all individuals in a populationDominant species exert powerful control over the occurrence and distribution of other speciesOne hypothesis suggests that dominant species are most competitive in exploiting resourcesAnother hypothesis is that they are most successful at avoiding predatorsInvasive species, typically introduced to a new environment by humans, often lack predators or diseaseKeystone SpeciesKeystone species exert strong control on a community by their ecological roles, or nichesIn contrast to dominant species, they are not necessarily abundant in a communityField studies of sea stars exhibit their role as a keystone species in intertidal communitiesFig. 54-15With Pisaster (control)Without Pisaster (experimental)Number of speciespresentYear201510501963’64’65’66’67’68’69’70’71’72’73RESULTSEXPERIMENTFig. 54-15aEXPERIMENTFig. 54-15bWith Pisaster (control)Without Pisaster (experimental)Number of speciespresentYear201510501963’64’65’66’67’68’69’70’71’72’73RESULTSObservation of sea otter populations and their predation shows how otters affect ocean communitiesFig. 54-16(a) Sea otter abundanceOtter number(% max. count)1008060402004003002001000(b) Sea urchin biomassGrams per0.25 m210864201972Number per0.25 m219851997Year(c) Total kelp densityFood chain19891993Foundation Species (Ecosystem “Engineers”)Foundation species (ecosystem “engineers”) cause physical changes in the environment that affect community structureFor example, beaver dams can transform landscapes on a very large scaleFig. 54-17Some foundation species act as facilitators that have positive effects on survival and reproduction of some other species in the communityFig. 54-18With JuncusWithout Juncus02468Number of plant speciesSalt marsh with Juncus(foreground)(a)(b)Fig. 54-18aSalt marsh with Juncus(foreground)(a)Fig. 54-18bWith JuncusWithout Juncus02468Number of plant species(b)Bottom-Up and Top-Down ControlsThe bottom-up model of community organization proposes a unidirectional influence from lower to higher trophic levelsIn this case, presence or absence of mineral nutrients determines community structure, including abundance of primary producersThe top-down model, also called the trophic cascade model, proposes that control comes from the trophic level aboveIn this case, predators control herbivores, which in turn control primary producersLong-term experimental studies have shown that communities vary in their relative degree of bottom-up to top-down controlFig. 54-19Control plotsWarmed plotsE. antarcticusS. lindsayae0100200300Nematode density(number of individualsper kg soil)RESULTSPollution can affect community dynamicsBiomanipulation can help restore polluted communitiesFig. 54-UN1Polluted StateRestored StateRareRareAbundantAbundantFishZooplanktonAlgaeAbundantRareConcept 54.3: Disturbance influences species diversity and compositionDecades ago, most ecologists favored the view that communities are in a state of equilibriumThis view was supported by F. E. Clements who suggested that species in a climax community function as a superorganismOther ecologists, including A. G. Tansley and H. A. Gleason, challenged whether communities were at equilibriumRecent evidence of change has led to a nonequilibrium model, which describes communities as constantly changing after being buffeted by disturbancesCharacterizing DisturbanceA disturbance is an event that changes a community, removes organisms from it, and alters resource availabilityFire is a significant disturbance in most terrestrial ecosystemsIt is often a necessity in some communitiesThe intermediate disturbance hypothesis suggests that moderate levels of disturbance can foster greater diversity than either high or low levels of disturbanceHigh levels of disturbance exclude many slow-growing speciesLow levels of disturbance allow dominant species to exclude less competitive speciesFig. 54-20Log intensity of disturbanceNumber of taxa302015101.11.21.31.41.51.61.71.81.92.035251.00.9The large-scale fire in Yellowstone National Park in 1988 demonstrated that communities can often respond very rapidly to a massive disturbanceFig. 54-21(a) Soon after fire(b) One year after fireFig. 54-21a(a) Soon after fireFig. 54-21b(b) One year after fireEcological SuccessionEcological succession is the sequence of community and ecosystem changes after a disturbancePrimary succession occurs where no soil exists when succession beginsSecondary succession begins in an area where soil remains after a disturbanceEarly-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 favorableThey may inhibit establishment of later speciesThey may tolerate later species but have no impact on their establishmentRetreating glaciers provide a valuable field-research opportunity for observing successionSuccession on the moraines in Glacier Bay, Alaska, follows a predictable pattern of change in vegetation and soil characteristicsFig. 54-22-1Pioneer stage, withfireweed dominant11941190718601760AlaskaGlacierBayKilometers510150Fig. 54-22-2Pioneer stage, withfireweed dominant11941190718601760AlaskaGlacierBayKilometers510150Dryas stage2Fig. 54-22-3Pioneer stage, withfireweed dominant11941190718601760AlaskaKilometers510150Dryas stage2Alder stage3GlacierBayFig. 54-22-4Pioneer stage, withfireweed dominant11941190718601760AlaskaGlacierBayKilometers510150Dryas stage2Alder stage3Spruce stage4Fig. 54-22aPioneer stage, with fireweed dominant1Fig. 54-22bDryas stage2Fig. 54-22cAlder stage3Fig. 54-22dSpruce stage4Succession is the result of changes induced by the vegetation itselfOn the glacial moraines, vegetation lowers the soil pH and increases soil nitrogen contentFig. 54-23Successional stagePioneerDryasAlderSpruceSoil nitrogen (g/m2)0102030405060Human DisturbanceHumans have the greatest impact on biological communities worldwideHuman disturbance to communities usually reduces species diversityHumans also prevent some naturally occurring disturbances, which can be important to community structureFig. 54-24Fig. 54-24aFig. 54-24bConcept 54.4: Biogeographic factors affect community biodiversityLatitude and area are two key factors that affect a community’s species diversityLatitudinal GradientsSpecies richness generally declines along an equatorial-polar gradient and is especially great in the tropicsTwo key factors in equatorial-polar gradients of species richness are probably evolutionary history and climateThe greater age of tropical environments may account for the greater species richnessClimate is likely the primary cause of the latitudinal gradient in biodiversityTwo main climatic factors correlated with biodiversity are solar energy and water availabilityThey can be considered together by measuring a community’s rate of evapotranspirationEvapotranspiration is evaporation of water from soil plus transpiration of water from plantsFig. 54-25(a) TreesActual evapotranspiration (mm/yr)Tree species richness1801601401201008060200401003005007009001,100(b) VertebratesVertebrate species richness(log scale)200100501005001,0001,5002,000Potential evapotranspiration (mm/yr)Fig. 54-25a(a) TreesActual evapotranspiration (mm/yr)Tree species richness1601201001401808060402001003005009001,100700Fig. 54-25b(b) VertebratesVertebrate species richness(log scale)200100501005001,0001,5002,000Potential evapotranspiration (mm/yr)Area EffectsThe species-area curve quantifies the idea that, all other factors being equal, a larger geographic area has more speciesA species-area curve of North American breeding birds supports this ideaFig. 54-26Area (hectares)Number of species1,0001001010.11101001031041051061071081091010Island Equilibrium ModelSpecies richness on islands depends on island size, distance from the mainland, immigration, and extinctionThe equilibrium model of island biogeography maintains that species richness on an ecological island levels off at a dynamic equilibrium pointFig. 54-27Number of species on islandEquilibrium number(a) Immigration and extinction ratesRate of immigration or extinctionExtinctionImmigrationRate of immigration or extinctionNumber of species on island(b) Effect of island sizeSmall islandLarge islandImmigrationImmigration(small island)(large island)ExtinctionExtinction(large island)(small island)(c) Effect of distance from mainlandNumber of species on islandRate of immigration or extinctionImmigrationImmigration(far island)(near island)Extinction(far island)Extinction(near island)Far islandNear islandFig. 54-27aNumber of species on islandEquilibrium number(a) Immigration and extinction ratesRate of immigration or extinctionExtinctionImmigrationFig. 54-27bRate of immigration or extinctionNumber of species on island(b) Effect of island sizeSmall islandLarge island(large island)ImmigrationImmigration(small island)ExtinctionExtinction(large island)(small island)Fig. 54-27c(c) Effect of distance from mainlandNumber of species on islandRate of immigration or extinctionFar islandNear islandImmigrationImmigration(far island)(near island)Extinction(far island)(near island)ExtinctionStudies of species richness on the Galápagos Islands support the prediction that species richness increases with island sizeFig. 54-28Area of island (hectares)(log scale)Number of plant species (log scale)101001031041051061025501002004005Concept 54.5: Community ecology is useful for understanding pathogen life cycles and controlling human diseaseEcological communities are universally affected by pathogens, which include disease-causing microorganisms, viruses, viroids, and prionsPathogens can alter community structure quickly and extensivelyPathogens and Community StructurePathogens can have dramatic effects on communitiesFor example, coral reef communities are being decimated by white-band diseaseFig. 54-29Human activities are transporting pathogens around the world at unprecedented ratesCommunity ecology is needed to help study and combat themCommunity Ecology and Zoonotic DiseasesZoonotic pathogens have been transferred from other animals to humansThe transfer of pathogens can be direct or through an intermediate species called a vectorMany of today’s emerging human diseases are zoonoticAvian flu is a highly contagious virus of birdsEcologists are studying the potential spread of the virus from Asia to North America through migrating birdsFig. 54-30Fig. 54-UN2Fig. 54-UN3You should now be able to:Distinguish between the following sets of terms: competition, predation, herbivory, symbiosis; fundamental and realized niche; cryptic and aposematic coloration; Batesian mimicry and Müllerian mimicry; parasitism, mutualism, and commensalism; endoparasites and ectoparasites; species richness and relative abundance; food chain and food web; primary and secondary succession Define an ecological niche and explain the competitive exclusion principle in terms of the niche conceptExplain how dominant and keystone species exert strong control on community structureDistinguish between bottom-up and top-down community organizationDescribe and explain the intermediate disturbance hypothesisExplain why species richness declines along an equatorial-polar gradientDefine zoonotic pathogens and explain, with an example, how they may be controlled