Third attempt and I still mix up amensalism, commensalism and competition signs. Cleanest way to fix the (+,-,0) table?
Rahul DubeyDropper Sample
15 Oct 2026 · 2 min read
Third attempt. I will say it plainly: I know this chapter, and I still lose marks on it because of the sign table. I look at a question like "one species benefits and the other is neither harmed nor benefited" and for a second I go blank.
Here's my current table, please tell me if any row is wrong:
| Interaction | Species A | Species B |
|---|---|---|
| Mutualism | + | + |
| Competition | - | - |
| Predation | + | - |
| Parasitism | + | - |
| Commensalism | + | 0 |
| Amensalism | - | 0 |
I know the examples for the more common ones, such as orchid growing on a mango branch, barnacles on a whale, cattle egret near grazing cattle for commensalism, and lichens and mycorrhiza for mutualism. But I am unsure about:
- Cuckoo laying eggs in the crow's nest. Is that predation or parasitism?
- Competition: NCERT says it's not only for the same food, but it can also occur when resources are not limiting. Did I get that right? I thought it was a typo.
- Penicillium and bacteria — which row?
Any sensible mnemonic would help. I have used nothing so far. Just brute force, and it clearly isn't working.
7 answers & replies
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[+][–]Siddharth RaoDropperSample· 4d ago
Your table is right, all six rows. Don't change it. Your problem is with examples:
- Cuckoo-crow: parasitism (brood parasitism). The cuckoo lays its eggs in the host's nest and the host incubates them and raises the chick. The host loses, the cuckoo gains.
- Penicillium: the antibiotic it produces harms or kills bacteria, while Penicillium itself isn't affected. That is the classic textbook example for amensalism.
[+][–]Rahul DubeyDropperSample· 4d agoAuthor
Thank you. The table was fine, it was the examples. I'm going to write each example on the back of the table. Clean.
[+][–]Kavya NaikDropperSample· 4d ago
Mnemonic from my study group: look at the zeros. Only two interactions have a zero, and they both start with vowels: Amensalism (−/0) and... hmm, commensalism starts with C. No wait. OK the real one is: "Com" = together at the table (commensal = eating at the same table), one gets food (+), the other doesn't mind (0). "A-" = not, amensalism = "not at the table" → one is harmed, the other isn't affected. Hope that works for you. It worked for me, anyway, amma was impressed.
[+][–]Ishita GoyalDropperSample· 4d ago
On competition: yes, you read it correctly, and it's a subtle point in NCERT. Competition is not limited to resource-limited cases. Superficially, it's when closely related species compete for the same resources that are limiting, but NCERT notes that competition can also occur when resources are abundant, such as through interference where one species harms another's feeding efficiency. NCERT's example is Connell's barnacle study, where Balanus dominated the Chthamalus on the rocky coast.
[+][–]Farhan ShaikhDropperSample· 4d ago
Is Ophrys (the orchid that imitates the female bee) mutualism or parasitism? It's in the mutualism section of my book but the bee gets nothing.
[+][–]Lakshmi TripathiDropperSample· 4d ago
It's discussed along with the mutualism/pollination examples as co-evolution, via "sexual deceit" so the male bee tries to mate with the flower and pollinates it. The flower benefits; the bee does not get a reward. If someone asks for an example of co-evolution or of sexual deceit, write Ophrys. If it's a strict sign question, then it's not a real +/+ mutualism. Just know it's flagged in the textbook under the chapter on interactions.
[+][–]Mehak AroraDropperSample· 4d ago
Cattle egret and cattle: cattle graze and disturb insects from the grass, the egret feeds on those insects, so the egret benefits (+) and the cattle are neither benefited nor harmed (0). That is commensalism. Write that and you're safe. The similar-looking trap is "cattle and cattle egret = mutualism", which is wrong because the cattle don't gain anything noticeable.
