CHANGES IN ORDOVICIAN FOOD WEB STRUCTURE AND FUNCTIONING ACROSS THE RICHMONDIAN INVASION (CINCINNATI SERIES, USA)

The Cincinnati Series preserves an ideal sequence to examine the effects of biotic immigrations, and can provide unique insights into immigrations and invasions anticipated in the coming century. Invasion alters fundamental ecosystem properties such as nutrient cycling, productivity, and the intensity or number of interactions among species and between trophic levels. Therefore, here, changes in paleocommunity structure and functioning were examined using networks of trophic interactions (food webs).

Shallow marine food webs from the Late Ordovician (Katian) were constructed before and after the Richmondian Invasion (RI), a well-documented influx of non-native species. Network structure and functioning were compared using descriptive metrics and Cascading Extinction on Graphs (CEG) models, to account for the uncertainty in paleocommunity data. Richness decreased from 358 to 239 species, with radical turnover (only 67 species persisted post-RI). Despite topological similarities, species loss corresponded to loss of trophic guilds. Interestingly, invaders occupied all of the pre-RI functional guilds, with the exception of 4 incumbent guilds that were lost and the addition of 1 new guild. This suggests that the preservation of functional richness may promote robustness post-invasion, and corroborates the notion that invaders displace incumbents and fill pre-existing niche space. CEG models exhibited a uniform response among species level networks with strong resistance to secondary extinction. However, the post-RI community was less resistant and stable relative to the pre-RI community. Response to perturbation was also more variable post-RI, although also well constrained. Despite similarities in overall structure, the post-invasion community was less stable and resistant suggesting that functional richness may play a more critical role in ecosystem stability than biodiversity.

These results could have important consequences for conservation, which currently focuses on preservation of biodiversity. If functional richness plays a larger role in stability than richness, conservation efforts may need to focus on preserving functional diversity (types of behaviors such as grazing or predation), or preventing biotic invasions if more diverse ecosystems are not more stable.