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 VS. It’s Intuitive!

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

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

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).