Why are the bacteria present in the caecum of ruminants?

Ruminants have green plants as their food. These plants contain a type of complex carbohydrate, called cellulose. In the cecum, a kind of symbiotic bacteria helps digest cellulose. In ruminants, a major part of all carbohydrates, including the complex carbohydrates such as cellulose and hemi-cellulose, is digested by bacterial action.