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. Below the topsoil is the subsoil which contains many of the nutrients washed out of the topsoil by rain. The final layer contains the parent material but no humus. Humus is decaying organic matter such as dead plants, leaf litter and the remains of dead animals in an advanced state of decomposition.