Non-indigenous molluscs in the Eastern Mediterranean have distinct traits and cannot replace historic ecosystem functioning

Aim A large body of ecological theory predicts that non-indigenous species (NIS) are successful invaders if their niches overlap little with native taxa. Native-non-indigenous trait dissimilarity, however, may also be observed if NIS have outcompeted ecologically similar native species. Discriminating these scenarios is essential for assessing invasion impacts but requires baseline assemblage data that are frequently unavailable. We overcome this impediment by analysing death assemblages - identifiable organism remains in the seafloor - which are natural community archives. Focusing on molluscs from the heavily invaded Eastern Mediterranean, we gain insights into the contentious role of competitive displacement by NIS as the primary driver of the massive regional declines of native populations, and their potential to alter ecosystem functioning. Location Israel/Eastern Mediterranean. Time period Pre-Lessepsian invasion (pre-1869) to contemporary. Major taxa studied Mollusca. Methods We sampled molluscan living and death assemblages from various substrates on the Israeli shelf and compiled trait information on all constituent species. We then compared the abundance-weighted trait composition and functional diversity of native and non-indigenous assemblage components. Death assemblage time-coverage was quantified radiometrically. Results Native and non-indigenous assemblage components consistently differed in trait composition, both in present-day (i.e., living) and historical (i.e., death) assemblages, irrespective of habitat conditions. Furthermore, present-day non-indigenous assemblage components had a different trait composition than historical native assemblages. These findings suggest that the increasing NIS dominance has considerably altered the functional properties of shallow-water molluscan assemblages. Main conclusions By utilizing death assemblages, we show that native and non-indigenous assemblage components have differed in trait composition since the onset of the invasion, suggesting that competition was unlikely the primary driver of the regional-scale native biodiversity loss. Our findings, however, also imply that NIS cannot functionally compensate for native species disappearance. Instead, the transition towards increasingly NIS-dominated assemblages has profoundly altered ecosystem functioning, with unknown consequences.