Non-indigenous Assemblages Cannot Buffer Lost Ecological Functions of Declining Native Biota in the Eastern Mediterranean Sea

The south-eastern Mediterranean Sea has been successfully colonized by hundreds of non-indigenous species introduced through the Suez Canal, a process termed the ‘Lessepsian Invasion’. Although Lessepsian species often dominate contemporary shallow water communities in the region, the effects of the invasion on ecosystem functioning are poorly understood. We used biological traits analysis of molluscan living (LAs) and death assemblages (DAs, the shelly remains preserved in the sediments) from the Israeli Mediterranean shelf to assess whether co-occurring native and Lessepsian community components differ in their functional properties. Time-averaged DAs – natural archives of past community states – were analyzed to (i) gather information on the trait composition of pre-invasion baseline communities, and (ii) to evaluate if any functional segregation between native and Lessepsian LAs could be the result of past, unobserved competitive interactions. Such interactions would be reflected in a functional overlap of Lessepsian LAs with both, native and their own DAs. Our results show that across a variety of subtidal habitats (soft and hard substrates), depths (10-40 m) and in different seasons (spring and autumn), native and Lessepsian sub-communities were clearly separated by their functional traits composition. Native DAs were either functionally similar to their source LAs (particularly in soft-substrate habitats) or, if distinct, also differed from Lessepsian DAs and LAs. The lack of functional overlap suggests that non-indigenous community components differ in ecological functioning from co-occurring native assemblages. These findings imply that interspecific competition is likely not the main driver of the recently observed massive population declines in formerly common species, and that functional extinctions of native species cannot be compensated for by Lessepsian species, with unknown but potentially severe consequences for ecosystem functioning.