Types of ecological interactions — mutualism parasitism competition predation

Ecologists classify species interactions by their effect on each species involved:

• + = beneficial (population grows or individual fitness increases)
• − = harmful (population decreases or fitness reduces)
• 0 = neutral (no significant effect)

Every two-species interaction can be described as a combination of these signs.

Both species benefit. Neither can survive as well (or at all) without the other in obligate mutualism.

Examples relevant to NEET:

• Lichens: fungus + alga. Fungus provides water/minerals; alga provides photosynthate.
• Mycorrhizae: fungi + plant roots. Fungus extends root absorption area; plant provides carbon.
• Fig and fig wasp (Ficus + Agaonid wasps): fig provides shelter/breeding site; wasp pollinates the fig. Neither can reproduce without the other.
• Rhizobium + legumes: bacteria fix N₂; plant provides habitat in root nodules.

The fig–wasp mutualism is a NEET favourite — it’s an example of co-evolution and obligate mutualism in one.

Predator benefits; prey is harmed (killed and consumed). Predation shaped much of animal evolution — anti-predator adaptations (camouflage, warning coloration, toxins) arose because of predation pressure.

Ecological importance:

• Keeps prey populations in check (prevents competitive exclusion)
• Drives co-evolution (arms race between predator and prey)
• Can increase prey population diversity (keystone predation effect)

Examples:

• Tiger eating deer (Sundarbans)
• Sundew (carnivorous plant) catching insects — yes, plants can be predators
• Brood parasitism in cuckoos is sometimes classified separately but shares the +/− logic

Cryptic coloration (prey hiding) and aposematism (warning colours like monarch butterfly’s orange) are direct evolutionary products of predation.

Parasite benefits; host is harmed (but usually not killed immediately). The parasite lives on or inside the host.

Two types:

• Ectoparasites: live on the host’s surface (lice, ticks, fleas)
• Endoparasites: live inside the host (Plasmodium, tapeworm, Ascaris)

NEET examples:

• Plasmodium (malaria parasite) in human RBCs: +/−
• Cuscuta (dodder) on host plant: haustoria penetrate host stem to extract nutrients — parasite has no chlorophyll
• Liver fluke (Fasciola) in sheep liver

Parasites typically evolve towards reduced virulence over time — killing the host too quickly is bad for the parasite’s own reproduction.

Both species are harmed by the interaction. They compete for the same limited resource (food, space, light, mates).

Two types:

• Interspecific competition: between different species
• Intraspecific competition: within the same species (actually drives logistic growth)

Gause’s Competitive Exclusion Principle: Two species competing for the identical ecological niche cannot coexist indefinitely — one will be driven to extinction locally (or the two species will diverge to occupy different niches = character displacement).

Indian example: Abingdon tortoise in Galapagos was driven to extinction by goats (introduced competitors for the same vegetation). In India, encroachment by invasive water hyacinth outcompetes native aquatic plants.

Resource partitioning is how species coexist despite overlap — they use slightly different resources or microhabitats, reducing competition intensity.