Most recent vertebrate extinctions have happened on islands, often associated with the introduction of non-native species. This has led to drastic changes in island community compositions and their ecological functions, with unknown consequences for island ecosystems. Species interactions have been particularly strongly impacted, such as interactions between plants and the animals that eat fruits and thereby disperse seeds. Loss of seed dispersal may limit plant movement and recruitment success, increasing the risk of associated secondary extinctions of fleshy fruited plants. However, plants differ in the degree of their dependency on interactions with animals and there is very little direct empirical evidence that co-extinction has happened. Here, we provide an overview of what we know, and do not know, about the consequences of losing frugivore seed dispersers for island plants and discuss the main challenges for quantifying the problem.