Biology · Organisms and Populations · NEET
The sign shows what happens to that one species in the interaction. '+' means the species is benefitted (it gains). '-' means it is harmed or loses. '0' means it is neither benefitted nor harmed (neutral). NEET assigns one sign to Species A and one to Species B, and you name the interaction from that pair.
In both, exactly one species benefits and the other is harmed. In predation the predator gains (+) and the prey is harmed (-). In parasitism the parasite gains (+) and the host is harmed (-). So both are (+, -). NEET uses this on purpose to confuse you. Trick: if the options include both, read carefully; if only one is listed, that is the answer (as in NEET 2016 where only Parasitism was given).
Both have one species at '0' (unaffected). The difference is the other species. Commensalism = (+, 0): one species is HELPED, the other feels nothing (e.g. cattle egret and cattle). Amensalism = (-, 0): one species is HARMED, the other feels nothing (e.g. Penicillium releasing an antibiotic that kills bacteria). Memory hook: coMMensalism has an extra letter for the extra benefit (+); aMensalism is the sad one (-).
Only mutualism and competition. Mutualism = (+, +) because both species win. Competition = (-, -) because both species lose. All the other interactions (predation, parasitism, commensalism, amensalism) have two DIFFERENT signs. This is the fastest way to eliminate options.
NCERT Table 11.1 does not list a (0, 0) interaction, so for NEET no interaction is fully neutral on both sides. If an option shows (0, 0), it is a distractor. Neutralism as a term is not part of the class 12 sign table you must match.
Match List I with List II: List I (Interaction): A. Mutualism B. Commensalism C. Amensalism D. Parasitism List II (Effect on Species A and B): I. + (A), 0 (B) II. - (A), 0 (B) III. + (A), - (B) IV. + (A), + (B) Choose the correct answer:
(+) is beneficial, (-) is detrimental, (0) is neutral. Which interaction can be assigned (+) for one species and (-) for another species?
If '+' is beneficial, '-' is detrimental and '0' is neutral, then the population interaction represented by '+' '-' refers to:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Memorise only the pattern, not lines: Same-sign pairs = Mutualism (+,+) and Competition (-,-). The (+, -) 'one wins one loses' pair = Predation AND Parasitism. The '0' pair = Commensalism (+,0) and Amensalism (-,0). With these three groups you can eliminate options in seconds.
The order does not change the name of the interaction. (+, -) and (-, +) are both predation/parasitism; NEET just labels one column Species A and the other Species B. Focus on the pair of signs, not which side is first.
Amensalism, which is (-, 0). One organism (like the mould Penicillium) harms another (bacteria) by releasing a chemical, while the mould itself is unaffected. NEET 2018 asked exactly this.
No. In the NCERT sign table only mutualism is (+, +). If a match-the-column option shows (+, +), it must be mutualism. This makes mutualism the easiest one to lock in first.
