Community StructureSlide 2Pine community in ArizonaScaleTemporal PerspectivesOpen versus Closed CommunitiesEcotones – clear community boundaries California hills; live oaks between ridgesQuantifying ecotonesNew Brunswick – ecotone of bare rock from salt waterSoil conditions cause ecotoneEdge of serpentine barrenTrees of Great Smoky Mountains overlap spaciallySpecies Richness/DiversitySpecies ListsSlide 15Relative AbundanceSlide 17Niche PreemptionBroken StickSlide 20Slide 21Quantifying DiversityRarefaction curve – avoiding sampling biasSlide 24Slide 25Slide 26Slide 27Species-Area RelationshipSpecies – Area RelationshipsSlide 30Slide 31Slide 32Slide 33Slide 34Slide 35Food web structureFood WebsFood web may be bottom-up or top-down controlledKeystone speciesKeystone predator (seastars)Slide 41Food web complexity varies with species richnessCommunity productivity depends on trophic levelsSummaryCommunity Structure1. Scale & Perspective2. Richness3. Relative Abundance4. Diversity5. Species-Area6. Food WebsCommunity StructurePatterns of species abundanceInteractions among populationsHow many species are in a given habitat?How common is each species?Community stability & resiliencePine community in ArizonaScaleLocal•Ecologically uniform•Daily activities of individuals•Structure is defined by •richness•relative abundance •feeding relationships•resource partitioningTemporal PerspectivesEvolutionary•Origin & extinction of speciesEcological•Routes of dispersal•catastrophic history•stochastic changeOpen versus Closed CommunitiesEcotones – clear community boundariesCalifornia hills; live oaks between ridgesQuantifying ecotonesNew Brunswick – ecotone of bare rock from salt waterSoil conditions cause ecotoneEdge of serpentine barrenTrees of Great Smoky Mountains overlap spaciallySpecies Richness/DiversitySpecies ListsBasic structure: a list of species presentSpecies Richness = # of species presentTropical forests most specioseRelative AbundanceDominant species – the most abundant ones in a communityRelative AbundanceModelling the abundance distributionAllows different communities to be comparedInfer causative processesNiche pre-emptionBroken-stickNiche PreemptionBased on a geometric seriesAs each species arrives, it takes over a fixed proportion of the available habitat.Broken StickBased on random nichesThe habitat is broken into random “pieces.”A species’ abundance is proportional to it’s pieceBroken StickBased on random nichesThe habitat is broken into random “pieces.”A species’ abundance is proportional to it’s pieceGroup of spp simultaneously partition a habitatSequential invasion, with random portion occupiedBroken StickBased on random nichesAbundance is expected to be a linear function of log(rank)Quantifying DiversityDiversity: species richness & relative abundanceProblem 1: samplingRate of discoveryRarefaction curvesRarefaction curve – avoiding sampling biasQuantifying DiversityDiversity: species richness & relative abundanceProblem 2: not all spp should be given equal weightWhy not?Quantifying DiversitySimpson’s Index:D = 1/p2iDiversityproportion of individuals that are species iQuantifying DiversitySimpson’s Index:D = 1/p2i9 White Pine2 Tulip tree2 Chapman oak8 Dogwood4 Striped mapleQuantifying DiversityIndices quantify taxonomic diversitywhat about ecological diversity?addresses the role each species plays in the environmentsometimes they match, sometimes they don’t.Species-Area RelationshipSpecies – Area RelationshipsSpecies richness, S, is a power function of areaWhat causes this?Species – Area RelationshipsEquilibrium HypothesisS-A relationships arise from a balance of colonization and extinctionSteep slopes mean high isolationSpecies – Area RelationshipsEquilibrium HypothesisHabitat Diversity HypothesisLarge areas have more habitat varietySpecies – Area RelationshipsEquilibrium HypothesisHabitat Diversity HypothesisPassive Sampling HypothesisLarge areas are a bigger target for immigrationFood web structureFood WebsFood Chains: passage of energy through consumers at higher trophic levelsWho eats whomProducersPrimaryConsumersSecondaryConsumersSecondaryConsumersSecondaryConsumersSecondaryConsumersTop PredatorFood web may be bottom-up or top-down controlled•Keystone predator has large impact on community (top-down)•Robert Paine (1960s-70s) studied intertidal communities–Space is a limited resource–Experiment to remove predator•Expected species richness to decline•Result was species richness declined from 15 to 8•Therefore, Pisaster (seastar) is a keystone speciesKeystone speciesKeystone predator (seastars)Sea stars (Pisaster)Sea star preys on musselsWithout sea stars - only mussels & barnacles remainFood web complexity varies with species richnessCommunity productivity depends on trophic levels•Ecotones – clear community boundary•Species diversity measure•Species-area relationship – species richness correlated with habitat area•Keystone predator – important in maintenance of