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EEBB Seminar
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Kevin McCann
Department of Integrative Biology
University of Guelph
Zen and the Art of Food Web Maintenance: From Modules to Whole Ecosystems
Thursday, October 4, 3:30 p.m.
115 Eppley Center
---------------------------------How does diversity influence stability of food web??
Research history of food web (…., May 1973, Robert M. Pirsig)
How does the number of parts influence the operation of motorcycle?
Yodzis P. 1981, The stability of real ecosystem, Nature,
-----------------------------------Ecological Networks & Building Blocks
C-R theory
Relative Interaction Strength (ISrel)
--------------------------------------Strong (Excitable) interaction and Weak (Non-excitable) interaction
Increasing Relative Interaction strength → Top heavy interaction
i.e. high C:R biomass ratio
--------------------------------------Increasing relative interaction strength → Decrease the stability of interaction
Conjunction: Networks, therefore, with strong vertical energy flow should tend to be top
heavy and less stability.
---------------------------------------Corollary to the principle of interaction strength
Given that increased relative Interaction Strength → unstable
How Ecological Mechanism??
Weak Interaction Theory (Extend to Modules)
Works for many modules and stabilizes → both equilibrium and non-equilibrium
---------------------------------------Weak interaction Module Experiment (Rip et al. 2010, Proc. Roy. Soc. London. B)
Rotifer and algae (maybe….)
---------------------------------------Weak interaction mutes and stabilizes strong interaction.
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Towards Whole Food Webs
General approach: Employ random constructed matrices that have base building block
(C-R) aligned with ecological models.
Specific: Unfold Implications of “symmetry property” often assumed in matrix theory
(e.g. Sommers et al. 1988)
(Gellner & McCann, in prep??)
Weak to intermediate stabilization followed by destabilization of network except for the
degenerate symmetry case (f = 1)
----------------------------------------Randomly assembled high diversity webs
Perfect Symmetry Interaction strength (f = 1) (Allesina & Tang 2012)
Genetic Interaction Strength Experiments (f < 1)
-----------------------------------------Whole food webs
Statistical mechanics of food webs suggest that random webs boil down to behave like a
“mean C-R interaction module”
So, strong interactions are destabilizing
------------------------------------------Longstanding argument:
Aquatic vs. Terrestrial ecosystem
Predictions: Aquatic pyramids should have higher predator: prey biomass and should be
more variable (less stable).
i) higher herbivore, resource biomass
ii) should be less stable (more variable)
(Rip & McCann 2011, Ecol. Lett.)
-------------------------------------------Summary
i) Increasing the flux thru an interaction relative to its loss rate tends to destabilize it
(Note: perfect symmetry case different)
ii) Any biological mechanism that inhibits flux of strong excitatory interactions tends to
stabilize it.