Secondary foundation species enhance biodiversity.

It has long been recognized that primary foundation species (FS), such as trees and seagrasses, enhance biodiversity. Among the species facilitated are secondary FS, including mistletoes and epiphytes. Case studies have demonstrated that secondary FS can further modify habitat-associated organisms ('inhabitants'), but their net effects remain unknown. Here we assess how inhabitants, globally, are affected by secondary FS. We extracted and calculated 2,187 abundance and 397 richness Hedges' g effect sizes from 91 and 50 publications, respectively. A weighted meta-analysis revealed that secondary FS significantly enhanced the abundance and richness of inhabitants compared to the primary FS alone. This indirect facilitation arising through sequential habitat formation was consistent across environmental and experimental conditions. Complementary unweighted analyses on log response ratios revealed that the magnitude of these effects was similar to the global average strength of direct facilitation from primary foundation species and greater than the average strength of trophic cascades, a widely recognized type of indirect facilitation arising through sequential consumption. The finding that secondary FS enhance the abundance and richness of inhabitants has important implications for understanding the mechanisms that regulate biodiversity. Integrating secondary FS into conservation practice will improve our ability to protect biodiversity and ecosystem function.