ESTIMATING ECOLOGICAL HEALTH INDICES (AMBI AND M-AMBI) FROM THE MARINE SUBFOSSIL RECORD: A CASE EXAMPLE FROM COASTAL NORTH CAROLINA, USA

Differences in faunal composition between living benthic invertebrate communities and sympatric dead remains have been used to assess ecological changes induced by human activities. However, we lack empirical studies that compare live and dead assemblages in terms of ecological indices like AMBI and M-AMBI, which are effective at identifying various human-induced impacts. To calculate AMBI and M-AMBI and assess the concordance of those indices between modern communities and their sympatric subfossil death assemblages, dredge samples were collected at 52 sites in Onslow Bay, North Carolina, USA. The samples included live and dead specimens from six phyla: Annelida, Arthropoda, Brachiopoda, Cnidaria, Echinodermata, and Mollusca. AMBI and M-AMBI were calculated in R using the function “mambisimpl”, “vegan”, and “psych” packages. The strength of the association was estimated using a Spearman correlation coefficient. A preliminary analysis resulted in the following estimates: M-AMBI dead vs M-AMBI live (rho = 0.64, p << 0.0001), AMBI dead vs AMBI live (0.04, p = 0.75), M-AMBI dead vs AMBI dead (0.20, p = 0.14), and M-AMBI live vs AMBI live (0.53, p << 0.001). These results suggest a relatively high, positive correlation between live M-AMBI and dead M-AMBI estimates. There is no notable relationship between AMBI live and AMBI dead, mostly reflecting the elevated presence of dead specimens in the AMBI category EG-IV. Because all species belonging to EG-IV are mollusks, this live-dead discordance may be taphonomic in origin. There is a weak correlation between the M-AMBI and AMBI indicating that these two indices are not congruent for this study system. The live-dead M-AMBI congruence suggests that the subfossil record may provide meaningful estimates of the M-AMBI index of local communities. The contradictory AMBI results call for caution in interpreting these preliminary results. If the AMBI results more accurately reflect the actual relation between dead and live assemblages, they suggest that either taphonomic biases undermined the validity of subfossil estimates or anthropogenic changes caused recent shifts in local communities. These pilot results are insufficient to assess the validity of those two alternative explanations, and additional analyses may provide further insights.