What constrains the distribution of orchid populations?

The distribution and abundance of orchid populations depend on a suite of biological and ecological factors, including seed production and dispersal, availability of mycorrhizal fungi and appropriate environmental conditions, with the weighting of these factors depending on the spatial scale considered. Disentangling the factors determining successful orchid establishment represents a major challenge, involving seed germination experiments, molecular techniques and assessment of environmental conditions. Identification of fungi from large-scale surveys of mycorrhizal associations in a range of orchid species has shown that mycorrhizal fungi may be widespread and occur in varied habitats. Further, a meta-analysis of seed introduction experiments revealed similar seed germination in occupied and unoccupied habitat patches. Orchid rarity was also unrelated to mycorrhizal specificity. Nonetheless, seed germination within sites appears to depend on both biotic and abiotic conditions. In the few cases that have been examined, coexisting orchids have distinct mycorrhizal communities and show strong spatial segregation, suggesting that mycorrhizal fungi are important drivers of niche partitioning and contribute to orchid coexistence. A broader investigation of orchid mycorrhizal fungus distribution in the soil, coupled with fungus and recruitment mapping, is needed to translate fungal abundance to orchid population dynamics and may lead to better orchid conservation.