EFFECT OF RICHMONDIAN INVASION ON LATE ORDOVICIAN COMMUNITY FOOD WEB STRUCTURE

Biotic invasions have become increasingly common in modern ecosystems as the physical barriers that define environments (e.g., temperature) shift geographically due to the accumulating effects of the climate crisis. For example, tropical species are increasingly found at higher latitudes, and temperate species in the North and Southern hemispheres have begun to invade polar latitudes in the Arctic and Antarctic, respectively. It is well understood in ecology that invasion events change species interactions within an ecosystem, but how these changes specifically alter food web structure, and if and how community responses have varied over geologic time, remains poorly understood. The Cincinnati Series represents a well documented incursion of species into shallow marine ecosystems of the Katian of the Late Ordovician, termed the Richmondian Invasion. Here we use fossil occurrence data from the Paleobiology Database and museum collections to reconstruct food web networks of marine communities that occurred before and after the Richmondian Invasion event. We generated model food web networks that were consistent with functional structure and taxon richness in communities. Networks were reconstructed from two third-order stratigraphic sequences which occurred prior to and after the invasion. Fossil taxa were organized into functional groups (e.g., Mobile Grazers) based on ecological traits, such as life mode, habitat, diet, and predators, which were further subdivided by body size into guilds. We compare distributions of guild network trophic position (ntp) and guild maximum chain length (mcl), and used Bayesian linear regression to assess and compare the relationship between guild ntp and mcl, in the two Ordovician communities. We find clear differences in the overall ntp distribution between pre- and post-invasion communities, though mcl remains very similar in both networks. We also find that the relationship between guild ntp and mcl also varies, indicating that the invasion event significantly altered the balance between long and short food chains, and thus the energy flow within the network. These results demonstrate community structural responses to invasion, which has important implications for predicting how modern communities will respond to similar events as the climate crises progresses.