Invasive moss alters patterns in life-history traits and functional diversity of spiders and carabids

Invasive plants can modify terrestrial habitats and affect the natural faunal composition. In acidic coastal dunes the invasive moss Campylopus introflexus can form dense carpets that largely replace native vegetation. As shown in a previous study, moss invasion affects habitat structure and ground-dwelling arthropod diversity. We suggested that including the functional diversity concept in the analysis of moss invasion impacts may offer further insights. We used pitfall trap data to compare trait composition and functional diversity of spiders and carabids in (a) invaded, moss-rich (C. introflexus) and (b) native, lichen-rich (Cladonia spp.) acidic coastal dunes. Moss invasion induced shifts in the trait values body size and feeding preference (carabids) and hunting mode (spiders): Species were smaller in native sites, and the percentages of web-building spiders and phytophagous beetles were reduced in invaded sites. Furthermore, moss invasion led to a more heterogeneous trait composition for spiders, and changed functional diversity of both arthropod groups, although with the opposite effects: While spiders were functionally more diverse in invaded sites, moss invasion reduced carabid beetles' functional diversity. We also observed changes in the relationship between species richness and functional diversity that indicate a high functional similarity for spiders but a lower one for carabid beetles in native grey dunes. C. introflexus invasion not only alters the arthropod diversity and assemblage structure of endangered acidic coastal dunes but also interferes at a functional level. These results provide further insight into the way plant invasions might alter the structure and function of ecosystems.