Animals have very diverse feeding habits, but all are heterotrophs and thus obtain their nutrition by eating pre-existing organic matter. All other organisms are heterotrophs because they must rely on the primary producers to produce food. Heterotrophs are therefore consumers. Grazers are herbivores that consume plants directly. Predators are carnivores which typically search for and consume smaller prey.
Deposit feeders ingest sediment and extract food particles.
When food is abundant animals generally increase in number. When food is scarce animals decrease in abundance. A lag usually exists between the peak in food abundance and the peak in consumer abundance. Bacterial decomposition of dead organic matter returns nutrients to the system so that they can be used again to generate new organic matter. Without decomposition the generation of new organic matter would cease.