Significant phorophyte (substrate) bias is not explained by fitness benefits in three epiphytic orchid species

Abstract

Epiphytes rely on their phorophyte (host substrate) for support; epiphytic orchids also rely on mycorrhizal fungi for germination. Previous studies have proposed a degree of specificity in both interactions. Epiphytic orchids therefore provide an interesting system in which to examine multispecies interactions and the evolution of specialization. We examined the potential and actual distributions of three co-occurring, related epiphytic orchid species: Sarcochilus hillii, Plectorrhiza tridentata, and Sarcochilus parviflorus on phorophytes in Australia's temperate dry rainforests. These three small epiphytic orchid species were all biased toward certain woody plant species, in particular, the tree Backhousia myrtifolia, though the extent of specificity varied. Biases toward the most common phorophyte species were not explained by increases in adult orchid fitness, nor did probability of flowering increase on B. myrtifolia. Indeed, individuals on this woody phorophyte tended to have fewer inflorescences than those on other woody phorophytes. Only S. hillii benefited from establishment on B. myrtifolia; it had more leaves on this phorophyte than on others. In many cases what appear to be simple interactions between two species may be mediated by more complex symbioses. For this system, we propose that the cause for bias in orchid distribution occurs much earlier in an orchid's life and may be due to a bias of their mycorrhizal fungi for the dominant orchid phorophytes.