Mollusc species response to drivers in space and time: Rise and demise of the unique Pontocaspian fauna

The evolution of the Pontocaspian lakes and seas (Caspian Sea, Black Sea, Aral Sea) is characterised by major changes in water levels, which developed into a pulsating system of connected and isolated basins. During the late Quaternary (Late Pleistocene and Holocene), these basins have hosted a unique endemic biodiversity that experienced species turnover events. There are indications that the Pontocaspian species diversity is now in demise because of anthropogenic modifications of the ecosystem, such as habitat alteration, poaching, pollution and invasive species. Little data is available on faunal changes and resilience of Pontocaspian species to help discerning the effect of natural drivers and anthropogenic drivers on the endemic fauna, which is of direct relevance for conservation strategies. This thesis aims to characterize species richness and abundance of fossil faunas to establish a baseline for comparison with the changing biodiversity of today. Mollusc species are used as the study group as they easily fossilize and are abundant in the geological record. They inform us about the composition of communities and how they change, and enable us to reconstruct environments (‘habitats’). This is necessary in order to compare biodiversity through different time intervals and establish whether comparable habitats existed as those of today. A baseline of Pontocaspian mollusc faunas was achieved by analysing snapshots from the Caspian Sea basin and the Black Sea basin. Caspian mollusc assemblages are analysed from late Pleistocene (corresponding to the late Khazarian, Hyrcanian, early Khvalynian and late Khvalynian regional stages) and pre-20th century Holocene (Novocaspian) assemblages. Black Sea assemblages from the late Neoeuxinian and late Holocene are described. They are both compared with modern day assemblages. This study shows clear indications of a Pontocaspian mollusc diversity crisis. Overall, in the 20th–21st century, Pontocaspian molluscs have experienced a severe decline in species richness and abundance: a strong turnover towards invasive species in the Caspian basin, a strong habitat decline in parts of the Black Sea basin and a total obliteration in the Aral basin. The late Quaternary mollusc fauna snapshots demonstrate that endemic species dominated the Caspian communities with minor contributions of native species until the late Holocene. The Pontocaspian mollusc species richness in the Caspian Sea basin is higher than in the Pontocaspian habitats of the Black Sea basin, or the Aral Sea basin. Throughout the Late Quaternary, the strong natural fluctuations of Caspian Sea level affected shallow water bivalve species of the genus Didacna, but barely changed the overall composition of the mollusc faunas. Late Quaternary mollusc fauna snapshots of the Black Sea basin show very little change in the Pontocaspian species community since humans impacted the habitats. However, the Black Sea Pontocaspian mollusc communities are currently under severe threat of habitat deterioration and destruction. This research indicates that the Caspian Sea could act as a source for Pontocaspian biota, hence conservation efforts are imperative there. The deeper parts (>50 m water depth) possibly present a (partial) refuge, and urgent research is required to assess these habitats and their faunas. Future research should focus on improving the taxonomic framework to elucidate the identity of some Pontocaspian cryptic and sister species. We need more distribution and ecological data of current Pontocaspian species and more detailed biodiversity time series coupled with detailed environmental proxies to assess the nature of the Pontocaspian biodiversity crisis more comprehensively and plan conservation actions accordingly.