Beetles are the group of insects with the largest number of known species. They are classified in the order Coleoptera, which contains more described species than in any other order in the animal kingdom, constituting about 25% of all known life-forms. About 40% of all described insect species are beetles and new species are frequently discovered. Beetles can be found in almost all habitats, but are not known to occur in the sea or in the polar regions. They often feed on plants and fungi, break down animal and plant debris, and eat other invertebrates. Some species are prey of various animals including birds and mammals. Certain species are agricultural pests, while other species of beetles are important controls of agricultural pests.