Ecological relationships – commensalism: The phenomenon of commensalism refers to a relationship between organisms in which…

Introduction / Context:
Ecological interactions are categorized by net effects on participants. Accurately distinguishing commensalism, mutualism, parasitism, and amensalism is foundational to environmental and medical microbiology.

Given Data / Assumptions:

• We compare sign conventions of interactions (+/0, +/+, +/−, −/−).
• Question asks specifically about commensalism.
• No mathematical modeling is required, only definitions.

Concept / Approach:
Commensalism is a +/0 interaction: one species benefits (e.g., uses metabolic by-products, shelter, or transport), while the other experiences no significant positive or negative effect. This is distinct from mutualism (+/+), parasitism (+/−), and amensalism (0/− or −/0 depending on framework).

Step-by-Step Solution:
Recall definition mapping: commensalism = +/0. Identify examples: skin commensals feeding on sebum without harming the host. Contrast with mutualism (+/+) and parasitism (+/−). Select the option stating benefit to one with no effect on the other.

Verification / Alternative check:
Clinical microbiology labels species like Staphylococcus epidermidis as commensals under normal conditions, lacking overt harm or benefit to the host.

Why Other Options Are Wrong:
Mutualism – both benefit; not commensalism.

Parasitism – one benefits at the other’s expense.

Both harmed – not a standard commensal pattern.

None – incorrect because a precise definition exists.

Common Pitfalls:
Assuming neutrality always; context can shift commensals to opportunists, but the definition remains +/0 under baseline conditions.

Final Answer:
One species of a pair benefits while the other is neither helped nor harmed.