17.0147.014Lecture 35Lecture 35May 14May 14Species Interactions Species Interactions –– MutualismMutualismMutualismMutualism• Mutualism increases the relative fitness ofindividuals of both species• It does not involve altruism.• The costs and benefits of mutualism varywidely between partners, over time, and fromone area to the next.• It is a powerful evolutionary force23Treehopper excreting honeydew, which is harvested by antsAnts protect the treehoppers from spider predatorsAre ants beneficial to treehoppers?1000 m2study plotPlants withantsPlants withants removedFigure 49.17b100806040200PlantswithantsPlantswithoutants2025 30510 20AugustJulyAverage number of youngtreehoppers per plantFigure 49.17cBut in years where spidersare less abundant, theants provide no advantageTreehoppersurvivorshipincreased bypresence ofantsMutualism between ants and fungusFigure 49.16aFILM CLIPFILM CLIP4Mutualism between fishFigure 49.16bcleaner wrassehard at work5Community Structure and SpeciesCommunity Structure and SpeciesDiversityDiversityFigure 50.1Climate Dictates Community StructureHigh and ConstantTemperatureHigh and Variable RainfallFigure 50.2bRainforest6Figure 50.4bDesertFigure 50.5bMid latitude SteppeFigure 50.6bMid Latitude - Temperate ForestFigure 50.7bBoreal Forest – Yukon Terratory7Figure 50.8bTundra – Point Barrow AlaskaCommunity 1 Community 2 Community 3 6 6 5 0.59 0.78 0.69Species richness:Species diversity H:ABCDEFSpeciesQuantifying Species DiversityQuantifying Species DiversityH = -Σ pi log (pi)Shannon Weaver Index - H pi = proportion oftotal individualsthat belong tospecies iGiant sequoias after a fireFigure 50.10aRegularDisturbanceis critical tomaintainingcommunitystructureEssay 50.1, Figure 1, right8Fire scars in the growth ringsFigure 50.10bReconstructing history from fire scars504030201000 400 800 1200 1600 2000Years A.D.Number of firesper centuryFigure 50.10c1 species per plot 24 species per plot65554535251 42 6 8 12 24Number of plant species per plotTotal plant cover (%)Tilman Experiments: Relationship between structure and function of ecosystems0.0– 0.5–1.0–1.50.350.00–0.35–0.70CompletelyresistantCompletelyresilient(a) Resistance to disturbance(b) Resilience after disturbance0 5 10 15 20 25 300 5 10 15 20Number of plant species before droughtNumber of plant species 2 years after droughtChange in biomass:Before drought to fouryears afterChange in biomass:One year before drought topeak of droughtFigure 50.16
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