Global warming causes the poleward shift of the trailing edges of marine ectotherm species distributions. In the semi-enclosed Mediterranean Sea, continental masses and oceanographic barriers do not allow natural connectivity with thermophilic species pools: as trailing edges retreat, a net diversity loss occurs.

This research quantified this diversity loss on the Israeli shelf, among the warmest areas in the Mediterranean, by comparing current native molluscan richness with the historical one obtained from surficial death assemblages.

Only 12% and 5% of historically present native species on shallow subtidal soft and hard substrates, respectively, were recorded. The authors state this is the largest climate-driven regional-scale diversity loss in the oceans documented to date.

Assemblages in the intertidal, more tolerant to climate extremes, and in the cooler mesophotic zone showed approximately 50% of the historical native richness. The authors also highlight that 60% of the recorded shallow subtidal native species do not reach reproductive size, making the shallow shelf a demographic sink.

The authors predict that, as climate warms, this native biodiversity collapse will intensify and expand geographically, counteracted only by Indo-Pacific species entering from the Suez Canal. It is concluded that, these assemblages, shaped by climate warming and biological invasions, give rise to a “novel ecosystem” whose restoration to historical baselines is not achievable.

https://royalsocietypublishing.org/doi/10.1098/rspb.2020.2469

Albano et al. (2021) Native biodiversity collapse in the eastern Mediterranean. Proceedings of the Royal Society B 288: 20202469.