17.0147.014Lectures 33,34,35Lectures 33,34,35May 4,6,9May 4,6,9Species Interactions Species Interactions ––20052005Consumptive competition occurs when organisms compete for the same resources. These trees are competing for nitrogen and other nutrients.Figure 49.14 upper left2Preemptive competition occurs when individuals occupy space and prevent access to resources by other individuals. The space preempted by these barnacles is unavailable to competitors.Figure 49.14 upper rightOvergrowth competition occurs when an organism grows over another, blocking access to resources. This large fern has overgrown other individuals and is shading them.Figure 49.14 middle left3Chemical competition occurs when one species produces toxins that negatively affect another. Note how few plants are growing under these Salvia shrubs.Figure 49.14 middle rightTerritorial competition occurs when mobile organisms protect a feeding or breeding territory. These red-winged blackbirds are displaying to each otherat a territorial boundary.Figure 49.14 lower left4Encounter competition occurs when organisms interfere directly with each other’s access to specific resources. Here, spotted hyenas and vultures fight over a kill.Figure 49.14 lower leftThe Fundamental Ecological Niche: “An n-dimensional hyper-volume every point on which a species can survive and reproduce indefinitely in the absence of other species” (Hutchinson)Food sizetemperaturehumidity5Food sizetemperaturehumidityniche overlap leads to competitionThe Realized Ecological Niche:the niche actually occupied in the presence of other speciesFood sizetemperaturehumidity6One species eats seeds of one size rangeNumber consumedSeed size (one niche dimensionFigure 49.13aPartial niche overlap can lead to Niche Partitioning and Competitive CoexistencePartial niche overlap: competition for seeds ofintermediate sizeSpecies 1Species 2Number consumedSeed sizeFigure 49.13b7Complete niche overlapSpecies 1: Strong competitorSpecies 2: Weak competitor,driven to extinctionNumber consumedSeed sizeFigure 49.13cStrong niche overlap can lead to Competitive ExclusionThe Spread of Introduced Zebra Mussels in the United States (1998 – 2001)Extensive niche overlap can lead to competitive exclusion8Connell’s BarnaclesA classic ecological experiment demonstrating niche partitioningPelagic larvae are sessile as adultsObserved Distributions: Is this due to competition? Or differential tolerance of desiccation?Do an Experiment!Chthamalusin upperintertidal zoneMean tidal levelBalanusin lowerintertidal zoneFigure 49.15a9COMPETITION EXPERIMENTUpperintertidalLower intertidal1. Transplant rockscontaining youngChthamalus tolower intertidal.2. Let Balanuscolonize the rocks.3. Remove Balanusfrom one-half ofeach rock.Monitor survivalof Chthamaluson both sides.On which side of the rocks doChthamalus survive better?ChthamalusBalanusFigure 49.15b100806040200Young Chthamalus Older ChthamalusCompetitorabsentCompetitor presentCompetitorabsentCompetitor presentFigure 49.15cChthamalus survives better without competition.Percent mortality10Connell’s BarnaclesPelagic larvae are sessile as adultsBalanus outcompetes Chthamalusbut is more vulnerable to desiccationChthamalus is resistant to desiccation but is outcompeted by BalanusSpring high tideNeap high tideMean tidal levelNeap low tideCould not become adults due to competitionLarval Settlement ZoneAdult DistributionRealized NicheBarnacles subject to desiccationSpring low tideLarval Settlement ZoneAdult DistributionDistribution of Two Barnacle Species – Balanus and ChthamalusBalanusChthamalusFundamentalNiche11Stabilizing SelectionNumber of Individuals with phenotype01020304050Medium-sized individuals favoredReduces Variation01020304050Mean stays the sameSelectionPeak gets higher and narrowerDirectional SelectionNumber of Individuals with phenotype01020304050Mean ShiftsSelectionPeak shifts in one direction01020304050Larger individuals favored12Number of Individuals with phenotype30 40 50Two populations formingSelectionTwo PeaksDirectional Selection01020304050Large and small individuals favoredAfrican Seedcracker(Pyrenestes ostrinus)Birds with smaller bills can consume soft seeds more efficientlyBirds with large bills can crack hard seedsBirds with intermediate beak size survive poorlyWidth of lower bill (mm)Number of Birds10 2015100050Competition can lead to character displacement13Galapagos FinchesANCESTOR FINCHBlue-back grassquitWarbler finchVegetarian finchSmall tree finchLarge tree finchMedium tree finchWoodpecker finchMangrove finchLarge ground finchMedium ground finchSharp-billed ground finchLarge cactus finchCactus finchInsect Eaters Bud EatersSeed EatersNiche Partitioning can lead to Adaptive RadiationHawaiian HoneycreepersAdaptive Radiation to Exploit a Variety of Food Sources Results in Speciation and a variety of Beak Shapes14PredationPredationChapter 4915What drives the oscillation?Snowshoe Hare and Lynx Population Population Cycles Lepus americausLynx canadensis1840 193011 years1870 1900Number of Pelts (thousands)15050100Hudson Bay Company Trapping Records (1840 –1940)HareLynxField Experiments - Lloyd Keith (1960’s to 1980’s)What accounts for population cycles? Food Supply or Predation?Control Phase of Hare PopulationRatio of Hare Density to ControlsFood Supply IncreasedPredators ExcludedFood Supply Increased & Predators Excluded1351351359111315CONCLUSION: Food Supply & Predation both affect population cycles.16Source: Ricklefs, R. The Economy of Nature 3rdEd., p.372Predatory mitepreyPrey miteKeystone predator presentFigure 49.9 left17Keystone predator absentFigure 49.9 rightKeystone predator presentFigure 49.10 left18Keystone predator absentFigure 49.10 rightEvidenceEvidenceforforpredationpredationasasananevolutionaryevolutionaryagentagent:-Cryptic Coloration- Physical and Chemical Defenses- Mimicry1920Cottonwood tree felled by beaversFigure 49.12a20151050ControltreesBrowsed andresprouted treesSalicortin concentration(mg/g dry mass)Figure 49.12bResprouted trees have more defense compounds.214003002001000Larvae fromcontrol treesLarvae frombrowsed andresprouted treesLarval survival time (sec)Figure 49.12cHerbivorous larvae have higher survivorship in the face of their own predators (ants) when they have been feeding on resprouted trees.Thus --- Leaf beetle larvae sequester anti-beaver compounds and use them as a defense against antsFigure 49.8aInducible Defenses – Only induced in presence of
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