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Diversity and Stability

Diversity and Stability. By Ariana Strandburg-Peshkin. Does Increased Diversity Lead to Increased Stability?. What is stability? - MacArthur. Abundances of species staying relatively constant Perturb one species --> Others relatively unaffected

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Diversity and Stability

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  1. Diversity and Stability By Ariana Strandburg-Peshkin

  2. Does Increased Diversity Lead to Increased Stability?

  3. 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 web

  4. Complexity & Stability - MacArthur It’s Intuitive! VS.

  5. 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 stability

  6. 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 specialize

  7. Along Comes Robert May… I know math. You’re all wrong! • In mathematical models, complexity tends to lead to instability

  8. Lotka-Volterra Models - May

  9. Lotka-Volterra Models - May

  10. Mathematical Models - May • Randomly-Constructed Community Matrices • Too rich web connectance --> instability • Too large average interaction strength --> instability

  11. Tilman’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 1988

  12. Tilman - Results • Species-rich plots more drought resistant

  13. Tilman - Results • Total biomass variability (c.v.) negatively correlated with SR (both drought and non-drought)

  14. Tilman - Results • Biomass of individual species varied more in SR plots

  15. Reconciling MacArthur and May Stop arguing! You’re both right! • For COMMUNITIES - diversity --> stability • For SPECIES - diversity --> instability • But wait…

  16. How Can This Be??

  17. 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 plants

  18. What About Omnivores? • Omnivore - species that feeds on more than one trophic level • Classical Mathematical Results: Omnivory --> Instability • Fagan: Omnivory --> Stability

  19. Fagan - Experiment • Studied arthropod communities • Manipulated level of omnivory • Perturbed populations

  20. Fagan - Results • Omnivory decreases coefficient of variation after a perturbation • Probably due to rapid response of omnivores to changing pressures

  21. References • 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 (1997).

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