Facilitation cascades occur where foundation species (FS) facilitate secondary, dependent foundation species (SFS), which, in turn, facilitate associated communities that are far larger and more diverse than those facilitated by the FS alone. Recent meta-analyses indicate these hierarchical positive interaction chains are as widespread and powerful drivers of biodiversity as their more intensively studied conceptual analogs, trophic cascades. Despite increasing awareness of their importance, little is know about how the environmental context in which they arise or FS characteristics moderate facilitation cascade strength, information critical to defining where they most (and least) effectively enhance biodiversity. Here, we use transplant experiments and surveys to examine the relative effects of habitat type, tree host identity, and crown volume on the growth and abundance of the prevalent SFS epiphyte Tillandsia usneoides (Spanish moss, hereafter Tillandsia), and on the strength of Tillandsia facilitation of arthropods in the southeastern US coastal plain.
We discovered that Tillandsia grows faster in savanna and marsh upland than in coastal dune or hardwood forest habitats, and reaches maximal densities in large-crowned trees, regardless of their identity. Tillandsia-laden branches consistently supported larger and more functionally diverse arthropod communities than epiphyte-free branches; however these effects were far stronger in savannas, marsh uplands, and dunes than hardwood forests, and weaker when the FS host was a pine tree or cedar, compared to the seven other tree species examined. This research reveals that Tillandsia-based facilitation cascades are: 1) at least tripling arthropod density and diversity at the branch scale, regardless of environmental context, tree species, or crown volume, 2) enhancing biodiversity most powerfully at the tree scale in open, sunny habitats where the SFS can achieve rapid, sustained growth and the FS can form sprawling crowns amenable to hosting high SFS densities, and 3) supporting arboreal communities similar in functional composition across habitat types and tree hosts, hinting Tillandsia forms a network of protective habitat across the coastal plain that may be critical for the dispersal of a diverse suite of arthropod functional groups.