In habitat-forming facilitation cascades, primary foundation species facilitate additional (secondary) foundation species colonization and, combined, they enhance biodiversity. Whether some terrestrial assemblages exhibit such hierarchical organization, and if secondary foundation species simply increase species diversity or additionally modify trophic, food web structure remains unclear. We investigated potential mechanisms generating a tree (oak)-epiphyte (Spanish moss)-invertebrate facilitation cascade and quantified effects of Spanish moss on multiple metrics of community structure.
Field experiments confirmed Spanish moss obligately depends on oak limbs to ameliorate light and temperature stress. Within oaks, Spanish moss locally elevates humidity, forming refugia for invertebrates vulnerable to desiccation. In both experimental removals and a 1000-km latitudinal survey, invertebrate species richness, trophic diversity, adult, and immature abundance were all dramatically higher on limbs colonized by Spanish moss, relative to bare-limb controls. These findings reveal that facilitation cascades can regulate invertebrate diversity in epiphyte-dominated canopies and suggest secondary foundation species may commonly augment both species abundance and associated food web complexity.