FOSSIL FOOD WEBS: QUANTIFYING CHANGES IN MARINE ECOSYSTEMS DURING TIMES OF ESCALATING PREDATOR-PREY INTERACTIONS

Predator-prey interactions affect modern ecosystem dynamics with important consequences, and predation is perhaps the most intensely studied biotic interaction in the fossil record. The importance of biotic interactions such as predation on macroevolutionary timescales, however, remains controversial. The examination of ancient food webs presents an opportunity to understand the mechanisms controlling community structure, and determine whether trophic organization has changed through time.

To test whether observed escalatory trends in increasing predation intensity and changes in biodiversity during the Mesozoic correspond with changes in community structure and stability, community dynamics in marine communities from the Jurassic and Cretaceous are examined using Cascading Extinctions on Graphs (CEG), a mathematical model designed to explore the role of ancient food web structure and species interactions. Changes in community structure are measured as community stability after minor perturbation, and robustness, or resistance to the propagation of secondary extinctions in stochastically generated species level food web networks. Comparison of perturbation thresholds between communities are used to examine community robustness, with higher perturbation thresholds representing more robust communities. If higher diversity and increasing predation pressure result in structural changes in marine communities, then lower perturbation thresholds should be observed in Late Cretaceous communities relative to Early Jurassic communities. Differences in community dynamics during the Mesozoic are therefore reflected by differences in stability and robustness (model response to simulated perturbations).

If lower perturbation thresholds are observed in CEG models generated from Late Cretaceous marine communities, relative to those from the Early Jurassic, this would suggest that trophic restructuring and increasing taxonomic diversity may also be partially responsible for the severity of the end-Cretaceous extinction in the marine realm. Increasing predation intensity may affect community structure, stability, and resilience, which would imply that biotic interactions scale up to larger temporal and geographic scales, and are an important cause of evolutionary change.