MARINE FOOD WEB STRUCTURE AND THE CONSEQUENCES OF NON-NATIVE SPECIES DURING THE LATE ORDOVICIAN

Invasions are thought to trigger significant ecosystem restructuring and changes in energy transfer pathways, such as patterns of interactions and the distribution of taxa among trophic levels. Here we examine the evolution of Late Ordovician (Katian) shallow marine food webs in response to the Richmondian Invasion, a well-documented influx of non-native species preserved in the Cincinnati Series.

Food webs were constructed for six stratigraphic sequences, using ~1,300 species and >70,000 individuals identified largely from museum collections. Consistent with previous studies assessing species composition, ordinations of species follow a directional trajectory, becoming progressively less similar to the initial C2 community. However, compositional change did not necessarily equate to consistent changes in ecosystem structure or functioning. Despite an increase in the number of guilds, species, interactions, and link density, measures of complexity and stability, such as connectance, network diameter, and the number of modules, fluctuated. Ordinations of metanetwork food web properties and module composition show that communities in the sequences prior to the ‘invasion’ were not more structurally similar to one another relative to later sequences with more invaders, and that the guild composition of modules remained similar across sequences. 8 new functional groups were present in the latest community after ‘invaders’ became fully integrated; these groups had the greatest number of guilds overall (70 relative to 62 in the earliest food web that predates the arrival of most invaders). Guilds at higher trophic positions were linked with more resources in communities after the onset of the main phases of invasion (C5-C7), suggesting a change in prey vulnerability, and guilds at intermediate trophic positions were more commonly missing in the C5 and C6 sequences.

Although additional assessments of species level networks are needed to determine the reliability of these results, these preliminary comparisons suggest that changes in composition, i.e., species diversity and dominance, do not directly translate to structural changes, which are complex and nuanced. This underscores the importance of including food web and functional approaches in the conservation and management of marine ecosystems, as opposed to species composition alone.