LATE ORDOVICIAN FOOD WEBS ACROSS THE RICHMONDIAN INVASION

Biotic invasions have become increasingly common in modern ecosystems as humans have altered or circumvented the physical barriers that define environments. As a result, biogeographic patterns are reshaped either directly via species introductions, or indirectly through the cumulative effects of anthropogenic climate change (e.g., temperature change). It is well documented in ecology that invasions fundamentally reorganize species interactions, though how this transformation affects food web network structure and functional dynamics remains uncharacterized. The Cincinnati Series records a well documented incursion of species into shallow marine ecosystems during the Late Ordovician (i.e., the Richmondian Invasion). We used fossil occurrence data from collections and the Paleobiology Database to reconstruct food web network models of marine paleocommunities from six third-order stratigraphic sequences that occurred prior to, during, and after the invasion. To assess changes in functional structure among communities, we compared quantitative network statistics, such as connectance, as well as distributions of guild parameters, such as mean network trophic position (ntp) and mean maximum food chain length (mcl) using Bayesian statistics implemented in the brms package in R. Results indicate that there are no credible differences in the distributions of mean guild ntp or mcl among communities. However, we do find that the linear relationship between ntp and mcl has a credibly lower slope in the C7 sequence, which occurred after the invasion, than all other sequences: C2 and C3 (pre-invasion), C4 (isolated invasion), and C5 and C6 (main invasion pulses). This indicates that while the invasion did not alter the size of food web networks in terms of overall chain length, invading taxa did indeed substantially increase the proportion of consumers that participated in trophic omnivory. Connectance was highest for C5 and lowest in C6, indicating that while both communities were subjected to major invasion pulses, the different ecological functions of invaders in each case likely had distinct effects on food web structure. Our analysis contributes new insights into how community food web networks respond structurally to biological invasion over ecological timescales relevant to modern conservation.