Did you know that English ivy (Hedera helix) that is never allowed to climb upwards won’t reproduce by making flowers and seeds? Once it grows vertically for some time, however, an internal switch seems to flip. Thereafter, even if the tree it was climbing falls, English ivy is capable of producing fleshy blue berries.
Birds help native AND non-native plants spread by eating their beautiful plump berries and flying away. When nature calls, so to speak, a bird deposits seeds that are ready to grow. By that time, the bird could be in the middle of a pristine forest.
Moral of the story for gardeners: Don't let your ivy climb! If you garden with other ornamental non-native plants that have berries, harvest the berries for indoor decorations.