The current biodiversity crisis is beginning to affect humanity negatively, and the need to understand the processes underlying extinction grows as the rate of species’ loss rises. This study uses rank-abundance curves (RACs, also known as species-abundance distributions) to examine the health of communities during the faunal turnover and extinctions caused by global warming during the Paleocene-Eocene Thermal Maximum (PETM), which is an appropriate analogy for the rapid modern climate change. RACs change predictably when communities are stressed, and this sensitivity can also be scaled-up to fossil communities and extinction events. When a community is exposed to continuing disturbances, then the succession never proceeds, and the community structure reflects the high stress level. An extinction event is a period of high stress levels, and as such the community response will be quantified by RACs. In order to test the viability of RACs to measure community health in the fossil record, curves were generated from benthic foraminifera and ostracode species level data from a series of samples before, during and after the PETM from ODP site 690. RACs were quantified using kurtosis and curve-area, both of which accurately reflect the overall shape of the curves.
Results/Conclusions
The communities were relatively stable before the onset of the event, but each responded differently. Kurtosis quantifies the shape of RACs and increased values reflect higher levels of stress in the community. The ostracode communities were relatively stressed before the onset of the PETM, and then became less stressed during the event before returning to the pre-event levels of stress. The benthic foraminifera community was stable before the event, but with the onset of the event the community became increasingly stressed leading up to an extinction, before stabilizing and finally recovering after the event. The pre- and post-event and extinction RACs are both significantly different (t-test, p<0.001). The event is marked by a negative carbonate δ13C excursion, which mirrors the increased temperature and CO2 levels. Kurtosis values are negatively correlated to the carbonate δ13C values (Pearson’s, r = -0.52, p<0.01), showing that RACs successfully indicate the increased stress level before the recognized critical extinction level. These patterns show the utility of RACs for measuring the response and health of different communities during environmental stress, and the approach used in this study has great potential for examining other extinction events, including the current crisis.
