Some organisms can only survive because of a close relationship with another species. Two important types are parasitism and mutualism.
In parasitism one organism, the parasite, lives on or in another organism, the host. The parasite gets food from the host and benefits, but the host is harmed. A tapeworm lives in the gut of a person and absorbs digested food, so the person loses nutrients. A flea feeds on the blood of a dog or cat. The parasite depends on its host for survival, but it usually does not kill the host quickly.
In mutualism both species benefit from the relationship, and each helps the other to survive. Nitrogen-fixing bacteria live in nodules on the roots of pea and bean plants. The bacteria receive sugars from the plant, and the plant receives nitrates that it can use to make proteins. Another example is a clownfish living among the stinging tentacles of a sea anemone. The fish is protected from predators and the anemone receives nutrients from the fish's waste.