Question
Describe the major types of population interactions. For each type, indicate whether the interaction is beneficial (+), harmful (-), or neutral (0) for each species involved. Give two examples of each.
(NEET 2023, similar pattern)
Solution — Step by Step
| Interaction | Species A | Species B | Example |
|---|---|---|---|
| Mutualism | + | + | Mycorrhiza (fungus + plant roots) |
| Competition | - | - | Flamingoes and resident fish competing for zooplankton |
| Predation | + | - | Tiger (predator +) and deer (prey -) |
| Parasitism | + | - | Cuscuta (parasite +) on host plant (-) |
| Commensalism | + | 0 | Orchid growing on mango tree; cattle egret with grazing cattle |
| Amensalism | - | 0 | Penicillium mould inhibits bacteria; large tree shading out small plants |
Both species gain from the relationship.
- Lichens: Fungus provides shelter and moisture; alga provides food through photosynthesis
- Mycorrhiza: Fungal hyphae increase mineral absorption for the plant; plant provides sugars to the fungus
- Fig tree and wasp: Wasp pollinates the fig; fig provides nursery for wasp larvae
Many mutualistic relationships are obligate — neither partner can survive alone (e.g., lichens).
The predator benefits; the prey is harmed. Predation is essential for ecosystem balance — it controls prey population and selects for fitter individuals.
Prey defences: camouflage (insects resembling leaves), warning colouration (monarch butterfly), mimicry (viceroy butterfly mimics toxic monarch), thorns, chemical defence (calotropis produces toxic latex).
Predator strategies: speed, stealth, venom, pack hunting.
The parasite benefits; the host is harmed but usually not killed immediately.
- Ectoparasites: Live on the host surface — ticks, leeches, lice, Cuscuta (dodder plant)
- Endoparasites: Live inside the host — tapeworm, Plasmodium (malaria), liver fluke
- Brood parasitism: Cuckoo lays eggs in crow’s nest — crow raises cuckoo’s chick
Parasites show adaptations like loss of unnecessary organs (tapeworm has no digestive system), high reproductive output, and adhesive organs (suckers, hooks).
Why This Works
Population interactions drive natural selection and shape community structure. Predation keeps prey populations in check (prevents overgrazing). Competition promotes niche differentiation (Gause’s competitive exclusion principle — two species can’t occupy the same niche indefinitely). Mutualism allows species to access resources they couldn’t obtain alone.
These interactions create the complex web of relationships that maintains ecosystem stability. Remove one interaction (say, eliminate predators) and the entire ecosystem can cascade into imbalance.
Alternative Method — The +/- Notation System
For NEET, the +/0/- notation is the fastest way to identify interactions:
- Both benefit: Mutualism (+/+)
- Both harmed: Competition (-/-)
- One benefits, other harmed: Predation or Parasitism (+/-)
- One benefits, other unaffected: Commensalism (+/0)
- One harmed, other unaffected: Amensalism (-/0)
NEET distinguishes predation from parasitism by asking whether the harmed organism is killed immediately (predation) or exploited over time (parasitism).
Common Mistake
Students confuse commensalism with mutualism. In commensalism, only ONE partner benefits and the other is neither helped nor harmed. The orchid on a mango tree gets sunlight and support, but the mango tree is unaffected. If both benefited, it would be mutualism. Also, don’t say “parasites always kill the host” — most parasites keep the host alive because a dead host means a dead parasite. It’s predators that typically kill prey.