While cassowaries have been known to eat fungi, flowers, snails, insects, frogs, birds, rats, mice, and even carrion, their diet consists primarily of fruit. They will eat the fruit of several hundred species of tree and bush, and one tree, the cassowary plum (which is toxic to other species but eaten readily by the cassowary), has even been named for the birds. Cassowaries can become extremely aggressive about their food; when they find a tree that is dropping fruit, they will stay there and eat, chasing away any other cassowaries who try to approach and feed, until the fruit is gone.
Cassowaries will swallow fruits whole, even large ones like apples and plums. Because of this, seeds and pits will go through the cassowary’s digestive system and be passed in their droppings. These birds have been known to distribute seeds over distances of over a kilometre, making them hugely important in the dispersal and germination of fruit trees through the rainforests. Some seeds, such as those of the Ryparosa trees, are shown to have much greater germination rates when they have been through the gut of a cassowary. These makes these birds a keystone species for the rainforests they inhabit.