Food or furniture: Separating trophic and non‐trophic effects of Spanish moss to explain its high invertebrate diversity

Foundation species are typically suggested to enhance community diversity non‐trophically by increasing habitat structure and mitigating physical stress, while their trophic role is considered of minor importance. Yet, there is little experimental evidence on the relative importance of trophic and non‐trophic effects and the interaction with patch size. Here, we transplanted different festoon sizes of living Tillandsia usneoides (Spanish moss) and structural mimics assessing the trophic and non‐trophic roles of this habitat‐forming epiphyte in mediating the invertebrate community. Compared to bare branches, mimics enhanced species and feeding guild richness and abundances, but living festoons even more so, demonstrating that trophic and non‐trophic effects jointly stimulated the community. Specifically, our results show that, independent of patch size, 40% of the total species richness and 46% of total guild richness increase could be contributed to habitat structure alone, while Spanish moss trophically stimulated these metrics by another 60% and 54%. As detritivores were particularly enhanced in living festoons, our findings suggest that trophic stimulation occurred primarily through the provisioning of Spanish moss detritus. Our results highlight that foundation species can facilitate their associated communities through both trophic and non‐trophic pathways, calling for studies addressing their indirect trophic role via the brown food web. Foundation species are spatially dominant, habitat‐forming organisms that enhance the richness and abundance of ecological communities (Bertness and Callaway 1994, Bruno et al. 2003). Trees, freshwater macrophytes, seagrasses, reef‐forming bivalves, and corals are all examples of such foundation species which create habitat for other species with their own body tissue (Jeppesen et al. 1992, Ellison et al. 2005, Coker et al. 2014, Christianen et al. 2016, van der Zee et al. 2016, Ali and Yan 2017). A major factor thought to underlie foundation species’ enhancements of associated communities is their positive effect through their ability to modify their habitat (Govenar 2010). Habitat structure is suggested to enhance species richness through a number of potentially codependent non‐trophic mechanisms (Kovalenko et al. 2012). First, it can enhance niche availability by creating new microhabitats (Cunha et al. 2012), modify predator–prey interactions (Klecka and Boukal 2014), and mitigate physical stress in harsh environments (Kovalenko et al. 2012, St Pierre and Kovalenko 2014). Secondly, habitat structure can also