🌿 Chapter 01 · Topic 02 · Food Chain & Food Web

Food Chain, Food Web & Trophic Levels

Grazing and detritus food chains, trophic levels, food web stability, producers, consumers, decomposers, and key feeding terminology.

Food Chain

A food chain is a linear sequence showing the transfer of energy and nutrients from one organism to another through feeding relationships. Each organism occupies a specific trophic level.

Types of Food Chains:

1. Grazing Food Chain (GFC) — starts with living green plants (producers)

🌿 Plant → 🦗 Grasshopper → 🐸 Frog → 🐍 Snake → 🦅 Eagle
🌿 Phytoplankton → 🦐 Zooplankton → 🐟 Small fish → 🐬 Large fish

2. Detritus Food Chain (DFC) — starts with dead organic matter (detritus)

🍂 Dead organic matter → 🦠 Bacteria/Fungi (decomposers) → 🪱 Detritivores → Predators
More common in forest and aquatic ecosystems; dominant in most terrestrial ecosystems

Key Difference: GFC starts with living plants; DFC starts with dead matter. In forests, DFC dominates. In open water (pelagic zone), GFC dominates.

Trophic Levels

Each step in a food chain is called a trophic level (from Greek trophe = nourishment). Energy flows from lower to higher trophic levels.

Trophic Level	Organisms	Examples
T1 — Producers	Green plants, algae, phytoplankton	Grass, wheat, algae
T2 — Primary Consumers	Herbivores (plant-eaters)	Deer, rabbit, grasshopper, cow
T3 — Secondary Consumers	Carnivores eating herbivores	Frog, fox, small fish
T4 — Tertiary Consumers	Carnivores eating secondary consumers	Snake, eagle, shark
T5 — Quaternary Consumers	Top predators (apex predators)	Tiger, orca, crocodile

Why are food chains limited to 4–5 levels? Because only ~10% of energy is transferred at each trophic level (Lindeman’s 10% Law). By T5, very little energy remains to support organisms.

Food Web

A food web is a network of interconnected food chains in an ecosystem. It represents the actual feeding relationships more realistically than a single food chain.

Food webs are more stable than food chains — if one species is removed, energy can flow through alternate pathways
Greater the number of food chains in a web, greater the stability of the ecosystem
Food webs show that most organisms feed at multiple trophic levels (e.g., omnivores)
Removal of a keystone species can collapse an entire food web

Food Chain vs Food Web: Food chain = single linear path. Food web = multiple interconnected paths. Food web is more realistic and more stable.

Producers, Consumers & Decomposers

Category	Type	Description	Examples
Producers	Autotrophs	Make own food via photosynthesis or chemosynthesis	Plants, algae, cyanobacteria
Consumers	Herbivores	Feed only on plants (T2)	Cow, deer, rabbit
	Carnivores	Feed only on animals (T3/T4)	Lion, eagle, snake
	Omnivores	Feed on both plants and animals	Human, bear, crow
	Scavengers	Feed on dead animals	Vulture, hyena, jackal
Decomposers	Saprotrophs	Break down dead organic matter; release nutrients	Bacteria, fungi

Key Terms

Term	Definition
Omnivore	Organism that eats both plants and animals (e.g., humans, bears)
Carnivore	Organism that eats only animals (e.g., lion, eagle)
Herbivore	Organism that eats only plants (e.g., cow, deer)
Scavenger	Feeds on dead animals (e.g., vulture, hyena)
Detritivore	Feeds on dead organic matter/detritus (e.g., earthworm, millipede)
Parasite	Lives on/in host, benefits at host’s expense (e.g., tapeworm, lice)
Saprophyte	Absorbs nutrients from dead organic matter (e.g., mushrooms, moulds)

Detritivore vs Decomposer: Detritivores physically break down dead matter (earthworms, millipedes). Decomposers chemically break it down (bacteria, fungi). Both are part of the detritus food chain.

Revision Checklist

✅ Food chain = linear sequence of energy transfer
✅ Grazing food chain starts with living plants
✅ Detritus food chain starts with dead organic matter
✅ T1 = Producers, T2 = Primary consumers, T3 = Secondary, T4 = Tertiary
✅ Food web = interconnected food chains; more stable than food chain
✅ Greater food web complexity = greater ecosystem stability
✅ Producers = autotrophs (photosynthesis/chemosynthesis)
✅ Consumers = heterotrophs (herbivore, carnivore, omnivore, scavenger)
✅ Decomposers = bacteria + fungi; break down dead matter
✅ Detritivore = physically breaks detritus (earthworm)
✅ Decomposer = chemically breaks detritus (bacteria, fungi)
✅ Food chains limited to 4–5 levels due to 10% energy transfer rule