Symbiotic fungi underlie the regeneration potential of island rainforests.

Symbioses can be vital on islands, where low species diversity leaves few alternative partners and the failure of associations can cascade into broader community collapse. Key to the functioning of many island ecosystems is the rainforest tree, Pisonia grandis (pisonia). Pisonia attracts nesting seabirds whose guano delivers intense nutrient pulses that fuel coral reef ecosystems. Symbiotic mycorrhizal fungi have been hypothesized to be crucial for capturing and distributing these nutrients to pisonia trees. However, little is known about the factors that influence the distribution of mycorrhizal fungi on islands. Here, we map the diversity and distribution of mycorrhizal fungi in relation to pisonia and other tree species across Palmyra Atoll, the most remote island on Earth that is a US territory in the Northern Line Islands. We found that pisonia is obligately associated with specific Tomentella fungi that are able to survive in the extreme nutrient environments created by seabird feces (guano). Tomentella was widespread in soils across different habitats, and its abundance was predicted by distance to pisonia. In addition, burrowing by crabs, the dominant group of land animals on Palmyra Atoll, was associated with increased fungal diversity, including new or globally rare fungal species. These findings support the hypothesized critical role of mycorrhizal fungi for key atoll tree species, indicating that fungal distributions may affect the success of restoration projects. More broadly, this work highlights the importance of specific interactions between species in isolated island ecosystems. VIDEO ABSTRACT.