An association between living organisms in which one organism lives on and feeds at the expense of the other organism is known as?

Step 1: Analyze the question The question describes a relationship where one organism:

• lives on (or in) another organism, and
• feeds at the expense of that organism, meaning the host is harmed.

Step 2: Explain the concepts

| Term | Meaning | Effect on each organism | |---|---|---| | Mutualism | Both organisms benefit | + / + | | Commensalism | One benefits, the other is neither helped nor harmed | + / 0 | | Parasitism | One (the parasite) lives on or in the other (the host) and feeds on it, harming it | + / − | | Predation | One organism (the predator) kills and eats another (the prey) | + / − (prey is killed) |

Step 3: Work through the options

• A. Mutualism: Both benefit, but the question says one organism suffers. ✗
• B. Commensalism: The host is not harmed, but the question says "at the expense of the other." ✗
• C. Parasitism: The parasite lives on the host and feeds on it, which harms the host. Examples are a tapeworm in the human gut, a tick on a dog, and Plasmodium in humans. ✔
• D. Predation: The predator does not live on the prey. It hunts, kills and eats it. ✗

Step 4: Answer The key phrase is "lives on and feeds at the expense of." This describes a parasite and its host, so the answer is C. parasitism.

Exam tip: Look for the words "lives on or in" in the question.

• Lives on or in the host and harms it means parasitism.
• Kills and eats means predation.
• Both benefit means mutualism.
• One benefits and the other is unaffected means commensalism.

A quick memory trick: a parasite is like a bad house guest who stays, eats your food and gives nothing back.