1. Extinction resistance in prehistoric food webs A bottom-up model

2. What effect does the structure of food webs have on their stability?  Ancient communities with a large population of carnivores  Modern communities with mainly herbivores  Do models of such communities show a difference in the amount of extinction when perturbed?

3. The model

4. C 1 guild C 2 guild

5. Why a bottom-up approach?  Biodiversity crises involved declines in primary production: end-Permian and Cretaceous-Tertiary extinctions  Modern disruptions to biodiversity from the top of the food chain (e.g., habitat loss) have not yet led to top- down extinction cascades  Bottom-up disturbances can lead to top-down effects

6. CEG model  CEG=Cascading Extinction on Graphs  Test resistance to disturbances –Remove primary producer, watch how secondary extinctions propagate through food web  Extinction when population size drops below a minimum viable population size

7. Cistecephalus zone Dicynodon zone Lystrosaurus zone Cynognathus zone Wuchiapingian Eodicynodon zone Tapinocephalus zone Pristerognathus zone Tropidostoma zone Wordian P-Tr boundary Wuchiapingian Results: community reaction to disruption

8. Lystrosaurus zone is different  Why such variability?  Presence of amphibians? –Feed at multiple trophic levels –Link between land and water allows cascades to propagate –Larger feedback network

9. Effect of connectance  Greater number of connections in Lystrosaurus communities may have amplified secondary effects

10. Paradox of connectance?  Species with more connectance to others (more generalized in what they can eat) are more resistant to secondary extinction BUT  The large number of connections in the Lystrosaurus community may have contributed to instability

11. Selection for stable food web structure  Selection can act on food web as a whole  End-Permian mass extinction  amphibian-dominated food web  instability  Concentration of biomass in herbivores may lead to more stable communities