MODELING AND ASSESSING THE BEHAVIOR OF FOSSIL FOOD WEBS

Food webs are graphical representations of feeding relationships among members of a community. Understanding food web function is crucial to conservation biology; studies of web composition and stability can help predict the outcomes of extinction events and direct conservation efforts more efficiently. In such studies, food webs constructed from fossil assemblages are frequently used to provide grounds for comparison with modern webs and to evaluate changes in web dynamics over time. These webs, however, may be skewed due to the incomplete nature of the fossil record. We have been exploring potential differences between the structure and function of fossilized and modern food webs. Concurrently we have been examining how differently constructed webs behave in top-down and bottom-up perturbations, and defining the concept of robustness as it pertains to food webs.

We used a numerical model to evaluate a modern, Indo-Pacific coral reef food web (~ 110 species), and two hypothetical, fossilized food webs generated from the modern web and taxonomic records of preservation. We also examined six randomly generated, incomplete (fossilized) webs. The model tests robustness by removing particular species and evaluating the subsequent impact on the remaining members of the community, leading to several observations. First, high connectance does not necessarily correspond to increased robustness. Fossilized food webs tend to be more unstable than full food webs despite possessing higher connectances and shorter average path lengths. Second, "fossilization" increases both the variance of responses to perturbation as well as the magnitude of the responses. This is in contrast to the random webs, where magnitude of the responses is greatly elevated. Finally, these results are compared to similar manipulations of the modern soft-bottom marine community of San Francisco Bay, a food web of much greater diversity (>1,400 species).