Which term describes a relationship where one species benefits and the other is harmed?

• A. Parasitism
• B. Biodiversity
• C. Dominant allele
• D. Pioneer species

Answer: (A)

When one species benefits at the expense of another, the relationship is parasitism. The parasite gains nutrients, energy, or a habitat by feeding on or exploiting the host, while the host suffers harm such as reduced growth, illness, or even death in extreme cases. This is exactly the pattern described—one partner gains at the other’s cost.

Biodiversity, by contrast, refers to the variety of life in an area and isn’t about a specific interspecies interaction. A dominant allele is a genetic term describing how certain alleles are expressed, not how species interact. Pioneer species are the first organisms to colonize a disrupted or barren environment, which is about succession rather than a host–parasite relationship.