Final answer:
The relationship where one species is harmed while the other benefits and the affected species has no influence in return is known as parasitism.
Step-by-step explanation:
When one species reduces or adversely affects the population of another, but the affected species has no influence in return, the relationship is referred to as parasitism. In a parasitic relationship, one species, known as the parasite, benefits by deriving nutrients at the host's expense. The host species is harmed by this interaction, often suffering health detriments and sometimes even death. Parasitism is distinct from other interactions such as commensalism, where one species benefits while the other is unaffected, and predation, which also involves one species (the predator) consuming another (the prey), but usually results in the immediate death of the prey.