The association between two organisms in which one of the organisms gains and the other loses is referred to as

Step 1: Analyze the question The question describes a relationship between two organisms where one gains (benefits) and the other loses (is harmed).

Step 2: Explain the concepts Let's look at each option:

• Saprophytism: An organism feeds on dead or decaying organic matter (e.g., mushrooms, bread mold). No living partner is harmed, so this is not a two-organism association of gain and loss.
• Commensalism: One organism benefits and the other is neither helped nor harmed (e.g., an epiphytic orchid growing on a tree).
• Mutualism: Both organisms benefit (e.g., nitrogen-fixing Rhizobium bacteria in legume root nodules).
• Parasitism: One organism (the parasite) benefits by taking nutrients from another (the host), which is harmed (e.g., tapeworm in humans, Plasmodium in humans, Striga on maize or sorghum).

Step 3: Match the description

• One gains and the other loses, so this is parasitism.

Step 4: Answer The correct option is D. parasitism.

Quick tip: Use the "+/−" memory trick for associations:

• Mutualism = +/+
• Commensalism = +/0
• Parasitism = +/−

If you see "one gains, the other loses," think of a "+/−" relationship and choose parasitism. Keep it up, you're doing great!