All organisms need energy and certain basic nutrients to survive. Many organisms find these resources by eating other organisms. Shrubs serve as nutrition for many herbivores--which can sometimes be a problem for gardeners.
Different types of shrubs come under attack from different herbivores. Deer graze on many shrubs like golden currant and chokecherry, for example, but are much less likely to graze on a yucca. Many shrubs have evolved adaptations that help to defend them from herbivores; Japanese privet, for example, is poisonous to horses.
Deer, elk, hares, rabbits, sheep and goats are all common herbivores that can graze on shrubs, depending on the species of shrub. In some ecosystems, shrubs may come under attack from other herbivores like antelope or rhinoceros. Birds also eat the seeds and berries of some shrubs as well.
Shrubs can also fall victim to insect herbivores like aphids and caterpillars. A wide variety of different insects eat shrubs, and many of these insects are garden or agricultural pests.