<p>Most plant species rely on animals for the dispersal of their seeds, and seed dispersal is the most widespread mutualistic ecosystem function provided by terrestrial vertebrates. Because seed dispersal affects plant gene flow, fitness and distributions, seed-dispersing animals influence ecosystem biodiversity, functioning and resilience to environmental changes. In this Review, we evaluate the decline of seed dispersers and the consequences for biodiversity and ecosystem functioning, including their maintenance, recovery and future trajectories under land-use change, wildfire, biotic invasions and climate change. Human-driven changes (habitat loss, landscape fragmentation, invasive species and direct exploitation of animals) have reduced the species diversity, trait diversity, abundance and movement of seed dispersers. These changes lead to declines in seed dispersal function along gradients of anthropogenic impact. Field studies and process-based models that evaluate resulting impacts on plant communities indicate pervasive negative consequences ranging from plant biodiversity decline to reduced carbon storage, with ongoing research seeking to quantify the magnitude of these effects and their impacts on human wellbeing. Addressing seed disperser decline represents an important pathway by which protection and restoration of animal biodiversity can support biodiversity, connectivity and functioning of vegetation communities, thus confronting the joint challenges of biodiversity loss and climate change.</p>