The age dependent invasibility of assemblages concurs with the functional distinctness within a marine urban species pool

Jean-Charles Leclerc^1,2, Naily Nashira Figueroa², Christian Díaz², Frédérique Viard³, Antonio Brante²

¹Sorbonne Université, CNRS, UMR 7144 AD2M, Station Biologique de Roscoff, France - ²Universidad Católica de la Santísima Concepción, Chile - ³ISEM, Université de Montpellier, CNRS, EPHE, IRD, France

The biotic resistance paradigm considers that non-indigenous species (NIS) establishment and spread fail in diverse communities because strong biotic interactions with native species. This paradigm is often evoked in urban management strategies worldwide such as those increasing the structural complexity of artificial coastal structures in order to facilitate native assemblages. Nonetheless, the influence of native diversity under realistic assembly rules remains understudied in marine environments. Diversity-invasibility relations can vary according to the distribution of competitive traits in the species pool and may be skewed in several scenarios, depending e.g. on the distribution of facilitative traits. We explored these relations in fouling communities using a controlled translocation approach from a “pristine” to an “invaded” site (experiencing higher propagule pressure) at two seasons. Translocated substrates were previously left being colonized for either 0, 1, 3, 7 or 13 mo. Following a first sampling at the time of translocation (procedural control), panels were retrieved after 1 and 4 mo. NIS recruitment and colonization was generally greater on early stages-assemblages (0, 1, 3 mo), regardless of their species richness. A trait-based analysis, using 12 functional traits, revealed that non-indigenous, cryptogenic and native taxon pools were functionally distinct, though overlapping in the trait space. More specifically, the NIS the most affected by biotic resistance were generally the more opportunistic and less defended against predators – as further confirmed by exclusion experiments. Yet several NIS were either indifferent to the successional stage of the recipient assemblage or positively influenced by habitat-forming species. While niche-based processes are important in these urban habitats, their management must account for the increasing taxonomic and functional diversity of NIS, their propagule pressure and the disturbance regimes affecting them.