Islands are known for high rates of species extinction, especially for large birds, entailing losses of ecological functions such as seed dispersal. Improving our knowledge of interactions between the flora and fauna is critical to better understand the functioning of island ecosystems and protect their unique biodiversity. Here, we assessed the role of the endemic New Caledonian imperial pigeon, Ducula goliath Gray, 1859, as a key disperser of native rainforest trees. D. goliath is one of the largest species of fruit pigeons worldwide and is classified as “nearly threatened” on the IUCN Red List of threatened species due to overhunting and habitat shrinkage. We analysed the seed dispersal syndrome of 460 tree species and found that 82% of these species are likely dispersed by internal animal transportation. Diaspore consumption observations support that D. goliath would be the only bird disperser for about 20% of these species. We suggest that besides birds the only native alternate dispersers for these species could be the three flying fox species (genus Pteropus). Conservation actions and hunting regulations should continue to be improved to mitigate further adverse effects on D. goliath and the key dispersal function it provides.