Canary in An Ordovician Coalmine: Using Rank-Abundance Curves to Quantify the Ecological Impact of the End-Ordovician Mass Extinction on Brachiopod Communities, Anticosti Island, Quebec

The ecological impact of extinction events, compared to taxonomic impact, is difficult to quantify. This study applies rank-abundance curves (RACs), an ecological tool used in measuring ecosystem health, to quantify changing community structure of brachiopods during the two pulses of the Ordovician extinction. Curve-shape is measured using kurtosis; a high kurtosis value reflects a convex-down RAC, or a stressed community. By applying RACs, communities before and after the extinction can be compared to detect early warning signs, as well as to recognize the point of ecological recovery, which may differ from recovery of taxonomic diversity.

Brachiopod communities were sampled from Paul Copper's Anticosti Island collection. Anticosti Island is the most complete, shallow water, fossiliferous record of the Ordovician-Silurian boundary in North America. Upper Ordovician through lowermost Silurian samples were collected from limestone surfaces that showed no evidence of taphonomic overprinting. Individuals were identified to the species level; abundance and biovolume data were collected. RACs were generated for every sample with species richness greater than two. The frequency of communities dominated by one taxon was compared to the kurtosis of the RACs within each unit.

The units below each extinction pulse show increasing stress; while units following the extinction pulses have significantly lower stress (t-test, p<0.05). The frequency of communities dominated by one taxon shows the same pattern as average kurtosis. While the taxonomic recovery has not been identified until the Middle Silurian, there is an ecological recovery relatively quickly after each extinction pulse. These findings support previous qualitative work that suggests that the ecological impact of the Ordovician extinction is not as severe as the taxonomic impact. RACs have been used successfully in modern systems; our results suggest this technique can also be applied to the fossil record to understand the progression of and recovery from extinction events better.