Spatial distribution shifts in two temperate fish species associated to a newly-introduced tropical seaweed invasion

Invasion by an ecosystem engineer shifts the abundance and distribution of fish but does not decrease diversity

Negative impacts of invasive species are widely accepted, but there is increasing evidence that neutral or positive effects are more prevalent than initially recognized, particularly for species which are of different functional / trophic group than the invader. We used a BACI design to examine how fish communities responded to the invasion of Halimeda incrassata, an ecosystem engineer which colonizes sandy habitats in the Western Mediterranean. While invasion did not alter overall species richness or diversity, we detected positive, negative and neutral responses by different fish species, which has important ecological and socio-economic implications. Contrasting responses likely result from different alterations that this alga conferred, putatively increasing prey availability via habitat creation, or limiting burial and camouflage abilities of fish. Our results highlight that effects of ecosystem engineers can be multiple and complex, and that predictions of invasive species are not straight forward.