Question
Classify the major types of species interactions using the +, -, 0 notation system. Give one example of each type from the NCERT syllabus. Why is interspecific competition considered a (-/-) interaction?
(NEET 2023 pattern — classification + reasoning)
Solution — Step by Step
Each interaction is described by how it affects two species (A and B). The symbols:
- + means the species benefits
- - means the species is harmed
- 0 means no significant effect
This gives us a clean classification framework that NEET loves to test.
| Interaction | Species A | Species B | Example |
|---|---|---|---|
| Mutualism | + | + | Lichen (algae + fungus) |
| Predation | + | - | Tiger eating deer |
| Parasitism | + | - | Cuscuta (dodder) on host plant |
| Competition | - | - | Flamingos and resident fish competing for zooplankton |
| Commensalism | + | 0 | Cattle egret with grazing cattle |
| Amensalism | - | 0 | Penicillium inhibiting bacteria |
In interspecific competition, both species are trying to use the same limited resource — food, space, nesting sites. Neither species benefits; both lose fitness. The competitive exclusion principle (Gause’s principle) states that two species competing for the exact same niche cannot coexist indefinitely — one will be eliminated.
This is why competition is the only interaction where BOTH species suffer. Even the “winner” expends energy and resources fighting for dominance.
Both are (+/-), but the mechanism differs:
- Predator kills and consumes the prey (immediate death)
- Parasite lives on/in the host and weakens it over time (usually does not kill immediately)
NEET often asks you to distinguish between these. The key: predators are typically larger than prey; parasites are typically smaller than their host.
graph TD
A[Species Interactions] --> B["+/+ Mutualism"]
A --> C["+/- Predation"]
A --> D["+/- Parasitism"]
A --> E["-/- Competition"]
A --> F["+/0 Commensalism"]
A --> G["-/0 Amensalism"]
B --> B1["Lichen, Mycorrhiza"]
C --> C1["Tiger-Deer"]
D --> D1["Cuscuta on host"]
E --> E1["Gause exclusion"]
F --> F1["Egret-Cattle"]
G --> G1["Penicillium-Bacteria"]
style A fill:#fbbf24,stroke:#000,stroke-width:2px
style B fill:#86efac,stroke:#000
style E fill:#fca5a5,stroke:#000Why This Works
The +/- system works because every ecological interaction boils down to energy and resource flow. Mutualism is cooperative resource sharing. Predation and parasitism are one-way exploitation. Competition is mutual resource depletion. Commensalism is a one-sided benefit with no cost to the other.
Understanding this framework helps you answer any NEET question about species interaction — even for organisms you have never heard of. Just ask: who benefits, who is harmed, and who is unaffected?
Common Mistake
Students frequently confuse commensalism with mutualism. In commensalism, only ONE species benefits and the other is unaffected (+/0). In mutualism, BOTH benefit (+/+). The classic test case: is the cattle-egret relationship mutualism or commensalism? NCERT says commensalism — the egret benefits by catching insects disturbed by cattle, but the cattle are neither helped nor harmed significantly.
NEET loves asking about Cuscuta (parasitism), lichen (mutualism), and cattle egret (commensalism). Memorise these three NCERT examples with their interaction type — they appear almost every other year.