The vast majority of flowering plants are pollinated and about half of are dispersed by animals, therefore the impacts of changes in the animal community are likely to cascade to affect plants. Recent insect declines have highlighted the importance of insect pollination, yet fewer studies have explored the impact of vertebrate pollinator or seed disperser declines on plants. We draw from a case study on the island of Guam, where the invasive brown treesnake has caused the functional extirpation of all native vertebrate pollinators and frugivores. Nearby islands retain native birds and bats, and thus serve as comparisons to Guam. To identify the importance of animal pollinators, we use a combination of flower observations, pollinator sampling, and pollinator exclusion experiments across Guam and nearby islands. Most plant species exhibit an insect-pollinator floral syndrome, but we found birds commonly visited these species. Yet, our pollinator exclusion experiment showed that insects were the primary pollinators, but that seed set was still lower on Guam in some species suggesting changes in insect community possibly as a result of changes in the food web. To determine how frugivores affect forest composition and structure, we used seed traps, gut passage experiments, seed and seedling additions near and far from conspecific plants, and seedling community surveys. We demonstrated that vertebrate seed dispersal affects the spatial pattern of seed rain, generally increases germination, but that few plants required escape from conspecific adults. At the community-level, seed dispersal increases species richness and influences the physical structure of forests, with treefall gap regeneration slowing in its absence. Small-seeded, fleshy-fruited, quick-growing tree species are the most demographically dependent upon vertebrate dispersal, and thus the most likely to experience reductions in a complete disperser loss scenario. However, these also tend to be the species with the most frugivore partners, which limits their risk of coextinction to the most extreme defaunation situations. The conservation concern for small-seeded plants is in contrast to most of the world, where large-bodied frugivores have declined orphaning large-seeded plants from their frugivores. Given the widespread effects of defaunation on plant communities, mutualistic rewilding must be a conservation priority. Guam provides an unprecedented demonstration of the importance of vertebrate seed dispersal for biodiversity, a cautionary tale for systems experiencing defaunation around the world, and a potential model for mutualistic rewilding. We also discuss the need to consider plant reproduction in a more holistic manner, pairing plant-pollinator, fruit-frugivore, and food web networks to identify plant species most susceptible to the effects of defaunation.