Interactions between plants and animals are ubiquitous in ecosystems, and include seed dispersal, pollination, and cross-ecosystem nutrient fluxes. There is growing evidence that these critical functions may disappear before the species involved in them do. We have unravelled the ecological consequences of changes in animal and plant populations in the context of seed dispersal and pollination, and quantified the loss of ocean-land nutrient flux due to seabird population declines in Aotearoa-New Zealand. To explore these issues we use a combination of field-based data collection and computational ecology.