A NEW LOOK AT PALEO-FOOD WEBS AND THE EVOLUTION OF ECOSYSTEM STRUCTURE

The reconstruction of paleo-food webs was a significant focus of paleoecological research during the 1970's-80's. However, interest in this approach faded because of a lack of precision that precluded meaningful principles from emerging about the possible evolution of food-web structure. Since the early 1990's, ecologists have addressed contemporary food webs with increasing rigor by using more sophisticated data collection, analysis, and modeling techniques. It is now possible to quantify complex food-web network structure, the effects of resolution and sampling effort on structure, and the relationship of structure to ecosystem function, dynamics, and stability. These advances, in turn, provide a new conceptual basis for constructing and analyzing food webs based on fossil data. We have compiled detailed food webs for the marine Middle Cambrian Burgess Shale in British Columbia (505 Ma), the marine Late Carboniferous Ames Limestone and Shale in Ohio (320 Ma), and the earliest Middle Eocene Messel Shale in Germany (49 Ma), which encompasses both terrestrial and freshwater species. These food webs comprise 154, 203, and 367 taxa, respectively. Taxa that share the same set of predators and prey are aggregated into "trophic species," resulting in "trophic webs" with 53, 41, and 346 functionally distinct taxa. For each trophic web, the "connectance," which specifies the proportion of possible links that are actually realized (links per species2=0.04, 0.10, and 0.01), as well as other parameters, are within or near ranges observed for modern food webs. Average trophic levels of the webs are 2.0, 2.7, and 2.1 and maximum trophic levels observed for a taxon in each web are 3.0, 5.6, and 5.0. Despite the very different temporal, spatial, and ecological contexts of these three paleo-food webs, there appear to be no intractable problems in reconstructing complex webs for carefully chosen and well-documented fossil assemblages. Analyses using these and other data sets will enable us to test the generality of current food-web theory and assess whether and how food-web structure and function shifts. Thus, our investigation points to new opportunities for robust, quantitative analysis of the evolution of ecosystem structure through the Phanerozoic.