In ecology, __________ is the type of interaction between two species in which one species benefits while the other species is neither harmed nor benefited in any significant way.

Introduction / Context:
Species in an ecosystem interact with each other in many different ways, ranging from cooperation to competition and exploitation. Ecologists classify these interactions based on how each species is affected in terms of benefit, harm or neutrality. This question asks you to identify the specific type of interaction in which one species gains a benefit while the other species is essentially unaffected, experiencing neither significant harm nor benefit.

Given Data / Assumptions:

• One species clearly benefits from the relationship.
• The other species is neither harmed nor helped; its fitness remains largely unchanged.
• The interaction is stable enough to be recognised as an ecological relationship.
• We are choosing between common interaction types: predation, commensalism, competition and parasitism.

Concept / Approach:
Commensalism is defined as an interaction between two species where one species benefits and the other is neither harmed nor helped. Examples include birds building nests in trees, where the bird gains shelter and the tree is generally unaffected, and epiphytic plants growing on branches of larger trees without drawing nutrients from them. In predation, one species benefits while the other is harmed. In competition, both species are negatively affected by the interaction as they compete for limited resources. In parasitism, one species benefits at the expense of the other, which is harmed. Therefore, the only term that matches the pattern one benefits, other unaffected is commensalism.

Step-by-Step Solution:
Step 1: Translate the question description into ecological notation: one species benefits, the other is neutral, which can be written as plus and zero. Step 2: Recall that commensalism is often represented as plus and zero because one species gains and the other is unaffected. Step 3: Consider predation, which is plus and minus; the predator benefits while the prey is harmed or killed. Step 4: Consider competition, which is minus and minus; both species experience reduced growth or reproduction due to shared limited resources. Step 5: Consider parasitism, which is plus and minus; the parasite benefits while the host is harmed, and choose commensalism as the only interaction matching plus and zero.

Verification / Alternative check:
Ecology textbooks list major interspecific interactions and often summarise them in a small table showing effects on both species. In that table, commensalism is consistently described as a relationship in which one partner benefits and the other experiences no significant effect. Examples such as remora fish hitching rides on sharks or cattle egrets feeding on insects disturbed by grazing cattle illustrate this pattern. Predation and parasitism are shown as interactions where the host or prey is harmed, and competition is shown as harmful to both. This consistent presentation confirms that commensalism is the correct term for the interaction described in the question.

Why Other Options Are Wrong:
Option a, predation, involves one species eating another, clearly benefiting the predator while harming the prey, which is not neutral.

Option c, competition, reduces the success of both species because they vie for the same limited resources, so neither is neutral.

Option d, parasitism, benefits the parasite but harms the host by stealing nutrients or causing disease, which again does not match the description of the second species being unaffected.

Common Pitfalls:
Students may confuse commensalism with mutualism or parasitism because all involve close living arrangements. A useful trick is to remember that in mutualism both species benefit, in parasitism one benefits and one is harmed, and in commensalism one benefits while the other experiences no significant effect. Another pitfall is to assume that any minor change must count as harm or benefit, but ecologists focus on significant, consistent effects on survival and reproduction when classifying interactions.

Final Answer:
The interaction where one species benefits and the other is neither harmed nor benefited is called Commensalism.