Diversity and StabilityDoes Increased Diversity Lead to Increased Stability?What is stability? - MacArthurComplexity & Stability - MacArthurImplications of MacArthur’s Stability DefinitionEcological Implications, if this is true…Along Comes Robert May…Lotka-Volterra Models - MaySlide 9Mathematical Models - MayTilman’s StudyTilman - ResultsSlide 13Slide 14Reconciling MacArthur and MayHow Can This Be??How can this be??What About Omnivores?Fagan - ExperimentFagan - ResultsReferencesDiversity and Stability By Ariana Strandburg-PeshkinQuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.Does Increased Diversity Lead to Increased Stability?What is stability? - MacArthur•Abundances of species staying relatively constant–Perturb one species --> Others relatively unaffected•The amount of choice which the energy has in following paths up through the food webComplexity & Stability - MacArthurVS.QuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.It’s Intuitive!Implications of MacArthur’s Stability Definition•Stability can be increased by…–Increasing the number of links–Increasing the number of species•Complexity --> Stability is implied in the definition•Evolution optimizes for maximum efficiency subject to a certain necessary stabilityQuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.QuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.Ecological Implications, if this is true…•Small number of species --> hard to fulfill stability requirement --> must diversify diet•Large number of species --> stability requirement easily fulfilled --> can specializeAlong Comes Robert May…QuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.I know math. You’re all wrong!•In mathematical models, complexity tends to lead to instabilityLotka-Volterra Models - MayLotka-Volterra Models - MayMathematical Models - May•Randomly-Constructed Community Matrices–Too rich web connectance --> instability–Too large average interaction strength --> instabilityTilman’s Study• Effects of Nitrogen addition to soil•207 plots of land, 9 different nutrient treatments•Sampled biomass every year•Major droughts in 1987 and 1988Tilman - Results•Species-rich plots more drought resistantTilman - Results•Total biomass variability (c.v.) negatively correlated with SR (both drought and non-drought)Tilman - Results•Biomass of individual species varied more in SR plots•For COMMUNITIES - diversity --> stability•For SPECIES - diversity --> instability•But wait…QuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.Stop arguing! You’re both right!Reconciling MacArthur and MayHow Can This Be??How can this be??•Competitive Release / Compensatory Growth•Monocultures - Avg. response to perturbation = Avg. of individuals•Mixtures - Avg. response to perturbation includes compensatory growth•Mixtures do better --> Species-rich plots do better b/c higher probability of perturbation resistant plantsWhat About Omnivores?•Omnivore - species that feeds on more than one trophic level•Classical Mathematical Results: Omnivory --> Instability•Fagan: Omnivory --> StabilityFagan - Experiment•Studied arthropod communities•Manipulated level of omnivory•Perturbed populationsFagan - Results•Omnivory decreases coefficient of variation after a perturbation•Probably due to rapid response of omnivores to changing pressuresReferences•MacArthur, R.H. Fluctuations of animal populations and a measure of community stability. Ecology 36, 533-536 (1955)•May, R.M. Stability and Complexity in Model Ecosystems (Princeton Univ. Press, 1973).•Tilman, D. Biodiversity: population versus ecosystem stability. Ecology 77, 350-363 (1996).•Fagan, W.F. Omnivory as a stabilizing feaure of natural communities. Am. Nat. 150, 554-567
View Full Document
Unlocking...