Herbivores have adapted to a diet of cellulose. They have specialized stomachs, teeth, and intestines which allow them to consume plant material and convert the fibre into energy. Herbivores are necessary in an ecosystem because they are the only animals that break down the plant fibre into animal tissue. Without herbivores, carnivores would not exist in the ecosystem. A heterotrophic organism cannot produce its own food.
All animals are heterotrophic and obtain their food from plants.
A horizon is a layer of soil running more or less parallel to the soil surface. The different soil horizons are the result of the actions of living organisms on the original parent materials. Organic matter is added to the soil gradually by the plants and animals living in the soil. Over a long period of time, this organic material or humus builds up. The largest amounts of organic material are near the surface in what is known as the topsoil.