The survival of some organisms depends on a close relationship with another species. Two important examples are parasitism and mutualism.
In parasitism a parasite lives on or in another organism, called the host, and feeds on it. The parasite benefits but the host is harmed. A flea lives on a dog or cat and feeds on its blood. A tapeworm lives in the gut of its host and takes some of the digested food. Parasites may weaken the host but usually do not kill it quickly, because the parasite needs the host to survive.
In mutualism both species benefit. Bees visit flowers and feed on nectar, and in doing so they pollinate the flowers. Bacteria live in the root nodules of legume plants such as peas and clover. The bacteria get sugars from the plant, and they make nitrates that the plant uses to build proteins.
In both cases the species show interdependence, because the survival of one depends on the other.