Spillover of Secondary Foundation Species Transforms Community Structure and Accelerates Decomposition in Oak Savannas

Abstract

Secondary foundation species facilitate biodiversity by providing complex habitat within the protective structures of many primary foundation species; whether they retain this role when they are dispersed in novel environments is unclear. Here, we explore how secondary foundation species’ density within hosts controls the rate at which individuals accrue and test if dislodged secondary foundation species then bolster the abundance and diversity of associated species and decomposition in a widespread system: southern live oaks, Quercus virginiana, hosting festoons of the bromeliad, Tillandsia usneoides, in southeastern US savannas. One year of monitoring showed that as Tillandsia density within oaks increases, Tillandsia deposition beneath oaks increases linearly in kind. Once on the ground, two manipulative experiments revealed that: on small scales, individual festoons retain moisture, reduce temperature, and facilitate invertebrates and fungi which combine to accelerate litter decomposition and, on larger scales, continual deposition of festoons dramatically reduces grass cover and increases litter layer depth, invertebrate and fungi richness and density, and decomposition. Surveys further suggest that Tillandsia populations within oaks, in moderating festoon accumulation, are shaping understory communities throughout this geographic region. Thus, disturbance-induced spillover enables secondary foundation species to define biodiversity and ecosystem functioning beyond the boundaries of their hosts.