Bài giảng Biology - Chapter 53: Community Ecology

Chapter 53Community EcologyOverview: What Is a Community?A biological communityIs an assemblage of populations of various species living close enough for potential interactionThe various animals and plants surrounding this watering holeAre all members of a savanna community in southern AfricaFigure 53.1Concept 53.1: A community’s interactions include competition, predation, herbivory, symbiosis, and diseasePopulations are linked by interspecific interactionsThat affect the survival and reproduction of the species engaged in the interactionInterspecific interactionsCan have differing effects on the populations involvedTable 53.1CompetitionInterspecific competitionOccurs when species compete for a particular resource that is in short supplyStrong competition can lead to competitive exclusionThe local elimination of one of the two competing speciesThe Competitive Exclusion PrincipleThe competitive exclusion principleStates that two species competing for the same limiting resources cannot coexist in the same placeEcological NichesThe ecological nicheIs the total of an organism’s use of the biotic and abiotic resources in its environmentThe niche concept allows restatement of the competitive exclusion principleTwo species cannot coexist in a community if their niches are identicalHowever, ecologically similar species can coexist in a communityIf there are one or more significant difference in their nichesWhen Connell removed Balanus from the lower strata, the Chthamalus population spread into that area.The spread of Chthamalus when Balanus was removed indicates that competitive exclusion makes the realizedniche of Chthamalus much smaller than its fundamental niche.RESULTSCONCLUSIONOceanEcologist Joseph Connell studied two barnacle speciesBalanus balanoides and Chthamalus stellatus that have a stratified distribution on rocks along the coast of Scotland.EXPERIMENTIn nature, Balanus fails to survive high on the rocks because it isunable to resist desiccation (drying out) during low tides. Its realized niche is therefore similar to its fundamental niche. In contrast, Chthamalus is usually concentrated on the upper strata of rocks. To determine the fundamental of niche of Chthamalus, Connell removed Balanus from the lower strata. Low tideHigh tideChthamalusfundamental nicheChthamalusrealized nicheLow tideHigh tideChthamalusBalanusrealized nicheBalanusOceanFigure 53.2As a result of competitionA species’ fundamental niche may be different from its realized nicheA. insolitususually percheson shady branches.A. distichus perches on fence posts and other sunny surfaces.A. distichusA. ricordiiA. insolitusA. christopheiA. cybotesA. etheridgeiA. alinigarFigure 53.3Resource PartitioningResource partitioning is the differentiation of nichesThat enables similar species to coexist in a communityG. fortisBeak depth (mm)G. fuliginosaBeak depthLos HermanosDaphneSanta María, San CristóbalSympatric populationsG. fuliginosa, allopatricG. fortis, allopatricPercentages of individuals in each size class402004020040200810121416Figure 53.4Character DisplacementIn character displacementThere is a tendency for characteristics to be more divergent in sympatric populations of two species than in allopatric populations of the same two speciesPredationPredation refers to an interactionWhere one species, the predator, kills and eats the other, the preyFeeding adaptations of predators includeClaws, teeth, fangs, stingers, and poisonAnimals also displayA great variety of defensive adaptationsCryptic coloration, or camouflageMakes prey difficult to spotFigure 53.5Aposematic colorationWarns predators to stay away from preyFigure 53.6In some cases, one prey speciesMay gain significant protection by mimicking the appearance of anotherIn Batesian mimicryA palatable or harmless species mimics an unpalatable or harmful model(a) Hawkmoth larva(b) Green parrot snakeFigure 53.7a, bIn Müllerian mimicryTwo or more unpalatable species resemble each other(a) Cuckoo bee(b) Yellow jacketFigure 53.8a, bHerbivoryHerbivory, the process in which an herbivore eats parts of a plantHas led to the evolution of plant mechanical and chemical defenses and consequent adaptations by herbivoresParasitismIn parasitism, one organism, the parasiteDerives its nourishment from another organism, its host, which is harmed in the processParasitism exerts substantial influence on populationsAnd the structure of communitiesDiseaseThe effects of disease on populations and communitiesIs similar to that of parasitesPathogens, disease-causing agentsAre typically bacteria, viruses, or protistsMutualismMutualistic symbiosis, or mutualismIs an interspecific interaction that benefits both speciesFigure 53.9CommensalismIn commensalismOne species benefits and the other is not affectedFigure 53.10Commensal interactions have been difficult to document in natureBecause any close association between species likely affects both speciesInterspecific Interactions and AdaptationEvidence for coevolutionWhich involves reciprocal genetic change by interacting populations, is scarceHowever, generalized adaptation of organisms to other organisms in their environmentIs a fundamental feature of lifeConcept 53.2: Dominant and keystone species exert strong controls on community structureIn general, a small number of species in a communityExert strong control on that community’s structureSpecies DiversityThe species diversity of a communityIs the variety of different kinds of organisms that make up the communityHas two componentsSpecies richnessIs the total number of different species in the communityRelative abundanceIs the proportion each species represents of the total individuals in the communityTwo different communitiesCan have the same species richness, but a different relative abundanceCommunity 1A: 25% B: 25% C: 25% D: 25%Community 2A: 80% B: 5% C: 5% D: 10%DCBAFigure 53.11A community with an even species abundanceIs more diverse than one in which one or two species are abundant and the remainder rareTrophic StructureTrophic structureIs the feeding relationships between organisms in a communityIs a key factor in community dynamicsFood chainsQuaternary consumersTertiary consumersSecondary consumersPrimary consumersPrimary producersCarnivoreCarnivoreCarnivoreHerbivorePlantCarnivoreCarnivoreCarnivoreZooplanktonPhytoplanktonA terrestrial food chainA marine food chainFigure 53.12Link the trophic levels from producers to top carnivoresFood WebsA food webHumansBaleen whalesCrab-eater sealsBirdsFishesSquidsLeopardsealsElephant sealsSmaller toothed whalesSperm whalesCarnivorous planktonEuphausids (krill)CopepodsPhyto-planktonFigure 53.13Is a branching food chain with complex trophic interactionsFood webs can be simplifiedBy isolating a portion of a community that interacts very little with the rest of the communitySea nettleFish larvaeZooplanktonFish eggsJuvenile striped bassFigure 53.14Limits on Food Chain LengthEach food chain in a food webIs usually only a few links longThere are two hypothesesThat attempt to explain food chain lengthThe energetic hypothesis suggests that the length of a food chainIs limited by the inefficiency of energy transfer along the chainThe dynamic stability hypothesisProposes that long food chains are less stable than short onesMost of the available dataSupport the energetic hypothesisHigh (control)MediumLowProductivityNo. of speciesNo. of trophic linksNumber of speciesNumber of trophic links 01234560123456Figure 53.15Species with a Large ImpactCertain species have an especially large impact on the structure of entire communitiesEither because they are highly abundant or because they play a pivotal role in community dynamicsDominant SpeciesDominant speciesAre those species in a community that are most abundant or have the highest biomassExert powerful control over the occurrence and distribution of other speciesOne hypothesis suggests that dominant speciesAre most competitive in exploiting limited resourcesAnother hypothesis for dominant species successIs that they are most successful at avoiding predatorsKeystone SpeciesKeystone speciesAre not necessarily abundant in a communityExert strong control on a community by their ecological roles, or nichesField studies of sea starsExhibit their role as a keystone species in intertidal communitiesFigure 53.16a,b(a) The sea star Pisaster ochraceous feeds preferentially on mussels but will consume other invertebrates.With Pisaster (control)Without Pisaster (experimental)Number of species present051015201963´64´65´66´67´68´69´70´71´72´73(b) When Pisaster was removed from an intertidal zone, mussels eventually took over the rock face and eliminated most other invertebrates and algae. In a control area from which Pisaster was not removed, there was little change in species diversity.Observation of sea otter populations and their predationFigure 53.17Food chain beforekiller whale involve-ment in chain(a) Sea otter abundance(b) Sea urchin biomass(c) Total kelp densityNumber per 0.25 m21972198519891993199702468100100200300400Grams per 0.25 m2Otter number (% max. count)040206080100YearFood chain after killerwhales started preyingon ottersShows the effect the otters haveon ocean communitiesEcosystem “Engineers” (Foundation Species)Some organisms exert their influenceBy causing physical changes in the environment that affect community structureBeaver damsCan transform landscapes on a very large scaleFigure 53.18Some foundation species act as facilitatorsThat have positive effects on the survival and reproduction of some of the other species in the communityFigure 53.19Salt marsh with Juncus (foreground)With JuncusWithout JuncusNumber of plant species02468ConditionsBottom-Up and Top-Down ControlsThe bottom-up model of community organizationProposes a unidirectional influence from lower to higher trophic levelsIn this case, the presence or absence of abiotic nutrientsDetermines community structure, including the abundance of primary producersThe top-down model of community organizationProposes that control comes from the trophic level aboveIn this case, predators control herbivoresWhich in turn control primary producersLong-term experiment studies have shownThat communities can shift periodically from bottom-up to top-downFigure 53.200100200300400Rainfall (mm)0255075100Percentage of herbaceous plant coverPollutionCan affect community dynamicsBut through biomanipulationPolluted communities can be restoredFishZooplanktonAlgaeAbundantRareRareAbundantAbundantRarePolluted StateRestored StateConcept 53.3: Disturbance influences species diversity and compositionDecades ago, most ecologists favored the traditional viewThat communities are in a state of equilibriumHowever, a recent emphasis on change has led to a nonequilibrium modelWhich describes communities as constantly changing after being buffeted by disturbancesWhat Is Disturbance?A disturbanceIs an event that changes a communityRemoves organisms from a communityAlters resource availabilityFireIs a significant disturbance in most terrestrial ecosystemsIs often a necessity in some communities(a) Before a controlled burn.A prairie that has not burned forseveral years has a high propor-tion of detritus (dead grass).(b) During the burn. The detritus serves as fuel for fires.(c) After the burn. Approximately one month after the controlled burn, virtually all of the biomass in this prairie is living.Figure 53.21a–cThe intermediate disturbance hypothesisSuggests that moderate levels of disturbance can foster higher species diversity than low levels of disturbanceThe large-scale fire in Yellowstone National Park in 1988Demonstrated that communities can often respond very rapidly to a massive disturbanceFigure 53.22a, b(a) 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.(b) One year after fire. This photo of the same general area taken the following year indicates how rapidly the community began to recover. A variety of herbaceous plants, different from those in the former forest, cover the ground.Human DisturbanceHumansAre the most widespread agents of disturbanceHuman disturbance to communitiesUsually reduces species diversityHumans also prevent some naturally occurring disturbancesWhich can be important to community structureEcological SuccessionEcological successionIs the sequence of community and ecosystem changes after a disturbancePrimary successionOccurs where no soil exists when succession beginsSecondary successionBegins in an area where soil remains after a disturbanceEarly-arriving speciesMay facilitate the appearance of later species by making the environment more favorableMay inhibit establishment of later speciesMay tolerate later species but have no impact on their establishmentMcBride glacier retreating0510MilesGlacierBayPleasant Is.Johns HopkinsGl.Reid Gl.GrandPacific Gl.CanadaAlaska1940191218991879187919491879193517601780183018601913191118921900187919071948193119411948Casement Gl.McBride Gl.Plateau Gl.Muir Gl.Riggs Gl.Retreating glaciersProvide a valuable field-research opportunity on successionFigure 53.23Succession on the moraines in Glacier Bay, AlaskaFollows a predictable pattern of change in vegetation and soil characteristicsFigure 53.24a–d(b) Dryas stage(c) Spruce stage(d) Nitrogen fixation by Dryas and alder increases the soil nitrogen content. Soil nitrogen (g/m2)Successional stagePioneerDryasAlderSpruce0102030405060(a) Pioneer stage, with fireweed dominantConcept 53.4: Biogeographic factors affect community diversityTwo key factors correlated with a community’s species diversityAre its geographic location and its sizeEquatorial-Polar GradientsThe two key factors in equatorial-polar gradients of species richnessAre probably evolutionary history and climateSpecies richness generally declines along an equatorial-polar gradientAnd is especially great in the tropicsThe greater age of tropical environmentsMay account for the greater species richnessClimateIs likely the primary cause of the latitudinal gradient in biodiversityThe two main climatic factors correlated with biodiversityAre solar energy input and water availability(b) Vertebrates5001,0001,5002,000Potential evapotranspiration (mm/yr)1050100200Vertebrate species richness(log scale)11003005007009001,100180160140120100806040200Tree species richness(a) TreesActual evapotranspiration (mm/yr)Figure 53.25a, bArea EffectsThe species-area curve quantifies the idea thatAll other factors being equal, the larger the geographic area of a community, the greater the number of speciesA species-area curve of North American breeding birdsSupports this ideaArea (acres)1101001031041051061071081091010Number of species (log scale)1101001,000Figure 53.26Island Equilibrium ModelSpecies richness on islandsDepends on island size, distance from the mainland, immigration, and extinctionFigure 53.27a–cThe equilibrium model of island biogeography maintains thatSpecies richness on an ecological island levels off at some dynamic equilibrium pointNumber of species on island(a) Immigration and extinction rates. The equilibrium number of species on anisland represents a balance between the immigration of new species and theextinction of species already there. (b) Effect of island size. Large islands may ultimately have a larger equilibrium num-ber of species than small islands because immigration rates tend to be higher and extinction rates lower on large islands.Number of species on islandNumber of species on island(c) Effect of distance from mainland. Near islands tend to have largerequilibrium numbers of species thanfar islands because immigration ratesto near islands are higher and extinctionrates lower.Equilibrium numberSmall islandLarge islandFar islandNear islandImmigrationExtinctionExtinctionImmigrationExtinctionImmigration(small island)(large island)(large island)(small island)ImmigrationExtinctionImmigration(far island)(near island)(near island)(far island)ExtinctionRate of immigration or extinctionRate of immigration or extinctionRate of immigration or extinctionStudies of species richness on the Galápagos IslandsSupport the prediction that species richness increases with island sizeThe results of the study showed that plant species richness increased with island size, supporting the species-area theory.FIELD STUDYRESULTSEcologists Robert MacArthur and E. O. Wilson studied the number of plant species on the Galápagos Islands, which vary greatly in size, in relation to the area of each island.CONCLUSION2001005025100Area of island (mi2) (log scale) Number of plant species (log scale)0.11101001,0005400Figure 53.28Concept 53.5: Contrasting views of community structure are the subject of continuing debateTwo different views on community structureEmerged among ecologists in the 1920s and 1930sIntegrated and Individualistic HypothesesThe integrated hypothesis of community structureDescribes a community as an assemblage of closely linked species, locked into association by mandatory biotic interactionsThe individualistic hypothesis of community structureProposes that communities are loosely organized associations of independently distributed species with the same abiotic requirementsThe integrated hypothesisPredicts that the presence or absence of particular species depends on the presence or absence of other speciesPopulation densities of individual speciesEnvironmental gradient(such as temperature or moisture) (a) Integrated hypothesis. Communities are discrete groupings of particular species that are closely interdependent and nearly always occur together.Figure 53.29aThe individualistic hypothesisPredicts that each species is distributed according to its tolerance ranges for abiotic factorsPopulation densities of individual species Environmental gradient(such as temperature or moisture) (b) Individualistic hypothesis. Species are independently distributed along gradients and a community is simply the assemblage of species that occupy the same area because of similar abiotic needs. Figure 53.29bIn most actual cases the composition of communitiesSeems to change continuously, with each species more or less independently distributedNumber of plantsper hectareWetMoisture gradient Dry(c) Trees in the Santa Catalina Mountains. The distribution of tree species at one elevation in the Santa Catalina Mountains of Arizona supports the individualistic hypothesis. Each tree species has an independent distribution along the gradient, apparently conforming to its tolerance for moisture, and the species that live together at any point along the gradient have similar physical requirements. Because the vegetation changes continuously along the gradient, it is impossible to delimit sharp boundaries for the communities.0200400600Figure 53.29cRivet and Redundancy ModelsThe rivet model of communitiesSuggests that all species in a community are linked together in a tight web of interactionsAlso states that the loss of even a single species has strong repercussions for the communityThe redundancy model of communitiesProposes that if a species is lost from a community, other species will fill the gapIt is important to keep in mind that community hypotheses and modelsRepresent extremes, and that most communities probably lie somewhere in the middle