Extinction resistance in Extinction resistance in prehistoric food websprehistoric food webs

A bottom-up modelA bottom-up model

What effect does the structure of What effect does the structure of food webs have on their stability?food webs have on their stability?

Ancient communities with a large Ancient communities with a large population of carnivorespopulation of carnivores

Modern communities with mainly Modern communities with mainly herbivoresherbivores

Do models of such communities Do models of such communities show a difference in the amount of show a difference in the amount of extinction when perturbed?extinction when perturbed?

The modelThe model

C1

guild

C2

guild

Why a bottom-up approach?Why a bottom-up approach?

Biodiversity crises involved declines Biodiversity crises involved declines in primary production: end-Permian in primary production: end-Permian and Cretaceous-Tertiary extinctionsand Cretaceous-Tertiary extinctions

Modern disruptions to biodiversity Modern disruptions to biodiversity from the top of the food chain (e.g., from the top of the food chain (e.g., habitat loss) have not yet led to top-habitat loss) have not yet led to top-down extinction cascadesdown extinction cascades

Bottom-up disturbances can lead to Bottom-up disturbances can lead to top-down effectstop-down effects

CEG modelCEG model

CEG=Cascading Extinction on GraphsCEG=Cascading Extinction on Graphs Test resistance to disturbancesTest resistance to disturbances

– Remove primary producer, watch how Remove primary producer, watch how secondary extinctions propagate secondary extinctions propagate through food webthrough food web

Extinction when population size Extinction when population size drops below a minimum viable drops below a minimum viable population sizepopulation size

Cistecephaluszone

Dicynodonzone

Lystrosauruszone

Cynognathuszone

Wuchiapingian

Eodicynodonzone

Tapinocephaluszone

Pristerognathuszone

Tropidostomazone

Wordian

P-Tr boundaryWuchiapingian

Results: community reaction to Results: community reaction to disruptiondisruption

LystrosaurusLystrosaurus zone is different zone is different

Why such variability?Why such variability? Presence of amphibians?Presence of amphibians?

– Feed at multiple trophic levelsFeed at multiple trophic levels– Link between land and water allows Link between land and water allows

cascades to propagatecascades to propagate– Larger feedback networkLarger feedback network

Effect of connectance Effect of connectance

Greater Greater number of number of connections connections in in LystrosaurusLystrosaurus communitiescommunitiesmay have may have amplified amplified secondary secondary effectseffects

Paradox of connectance?Paradox of connectance?

Species with more connectance to Species with more connectance to others (more generalized in what others (more generalized in what they can eat) are more resistant to they can eat) are more resistant to secondary extinction BUTsecondary extinction BUT

The large number of connections in The large number of connections in the the LystrosaurusLystrosaurus community may community may have contributed to instabilityhave contributed to instability

Selection for stable food web Selection for stable food web structurestructure

Selection can act on food web as a Selection can act on food web as a wholewhole

End-Permian mass extinction End-Permian mass extinction amphibian-dominated food webamphibian-dominated food web instabilityinstability

Concentration of biomass in Concentration of biomass in herbivores may lead to more stable herbivores may lead to more stable communities communities