Invasive shallow-water foraminifera impacts local biodiversity mostly at densities above 20 %: the case of Corfu Island

Corfu Island (Greece) is located in the northern Ionian Sea and exhibits unique and diverse marine coastal habitats suitable for high-diversity assemblages such as shallow-water foraminifera. The island also lies near the current range expansion front of the invasive species Amphistegina lobifera. We analyzed the foraminiferal assemblages of 51 samples from 25 sites around the island, calculated diversity indices, and analyzed the community structures of foraminiferal assemblages in comparison to local environmental variables. In addition to that, using the spatial structure or relative abundances, we evaluated the effect of A. lobifera on the species richness of all benthic foraminifera and habitat-specific groups. With 200 benthic foraminiferal species found, the high species richness and other diversity indices indicate Corfu as an area of high diversity. The main ecological drivers for the assemblage compositions were water depth, sediment texture, and habitat (especially vegetation), resulting in three main assemblage clusters around the island: (1) sandy or rocky, shallow-water areas from the south and west; (2) deeper areas from the west; and (3) rocky, vegetated areas of variable depths from the northwest and northeastern parts of the island. Our analyses suggest that the invasive species A. lobifera affects local diversity of the foraminiferal assemblage and that these effects become apparent when the invasive species accounts for more than 10%-20% of the total abundance. We also observed significant negative correlations with sessile epiphytes and smaller miliolids. Both groups share similar microhabitats with A. lobifera and might be outcompeted, which is probably further facilitated by ongoing ocean warming. However, other warm-affiliated taxa (e.g., other symbiont-bearing species) initially show a positive correlation with the increasing presence of A. lobifera until the latter exceeds 20%. We expect that A. lobifera and other warm-adapted species will play an increasing role in shaping future biodiversity and assemblage composition in this area, a feature that supports the prognosed tropicalization of the Mediterranean Sea.