Geohistorical Data in Conservation Biology: Historical Changes and Natural Range of Variability to Long-term Climate Changes in the Adriatic

Using ordinations and network analyses, we quantified responses of nearshore mollusk assemblages to climate and sea level changes. We used ~300 samples (~130,000 specimens) from shoreface settings of the Adriatic Basin to compare present-day mollusk assemblages to fossil ones derived from Quaternary sediment cores. The fossiliferous sediments provided a historical archive for measuring long-term term dynamics of mollusk associations in response to climate-driven environmental changes during times when globalization and industrialization were absent or very limited (i.e., prior to the Modern Period). Fossil data indicate that nearshore mollusks assemblages of the last interglacial shifted in their faunal composition during the subsequent ice age, and then reassembled again, with the return of interglacial conditions (Holocene pre-Modern Era). In contrast, the comparison of Holocene vs. modern nearshore assemblages indicates a geologically recent shift in mollusk associations. Contemporary coastal Adriatic mollusk assemblages differ from older fossil assemblages by increased relative abundance of sessile species and presence of non-native taxa. Differences between past and present-day assemblages are particularly notable in lower shoreface settings, with present-day samples showing lower heterogeneity, reduced standardized species richness, and a more simplified trophic network. These recent changes have been likely driven primarily by increasing anthropogenic stressors, including aquaculture, invasive species, and fishing. The resilient response of the mollusk assemblages to long-term environmental changes over the past 125 thousand years highlights the historically unprecedented nature of the ongoing reorganization of Adriatic coastal ecosystems, which are shifting into novel states outside the range of natural