biolgy

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

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. • 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).