Noesis
5.1.5Ecology & Ecosystems

Describe trophic levels and energy flow

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🇮🇳 Yeh note Hinglish mein padho →

WHAT is a trophic level?

Level Name Example Role
1 Producers (autotrophs) grass, algae make food via photosynthesis
2 Primary consumers (herbivores) grasshopper eat producers
3 Secondary consumers (carnivores) frog eat primary consumers
4 Tertiary consumers snake eat secondary consumers
— Decomposers bacteria, fungi break down dead matter, recycle nutrients

WHY does energy flow (and why only one way)?

This is just the laws of thermodynamics applied to biology:

  • 1st law: energy is conserved — none vanishes, it changes form.
  • 2nd law: every transfer wastes some usable energy as heat — so a level always has less usable energy than the one below it.

HOW MUCH is lost? — Deriving the 10% rule from first principles

We don't just memorise "10%". Let's build it.

Let a trophic level receive an amount of energy EnE_n per unit area per time (units: kJ m−2yr−1\text{kJ m}^{-2}\text{yr}^{-1}). What happens to it?

En=R⏟respiration (heat)+Eeg⏟egested/not eaten+P⏟stored as new biomassE_n = \underbrace{R}_{\text{respiration (heat)}} + \underbrace{E_{\text{eg}}}_{\text{egested/not eaten}} + \underbrace{P}_{\text{stored as new biomass}}

Only the stored biomass PP is available to be eaten by the next level. So:

En+1=P=En−R−EegE_{n+1} = P = E_n - R - E_{\text{eg}}

Define the ecological (transfer) efficiency:

Because it multiplies at each step, energy after nn transfers starting from producers E1E_1 is:

En=E1×(0.10) n−1E_{n} = E_1 \times (0.10)^{\,n-1}

The consequence: energy pyramids and short chains

Figure — Describe trophic levels and energy flow

Common mistakes (steel-manned)

Flashcards

What is a trophic level?
An organism's position in a food chain — the number of energy-transfer steps it is from the Sun.
Which trophic level are producers?
Level 1 (autotrophs, e.g. plants/algae, make food by photosynthesis).
Why does energy flow one-way while nutrients cycle?
Energy is lost as heat at each step (2nd law) and can't be re-eaten; atoms of matter are never destroyed so they recycle.
State the equation for ecological efficiency.
Efficiency = energy passed on / energy received = E(n+1)/E(n), roughly 0.10.
Roughly what fraction of energy passes to the next trophic level?
About 10%.
Where does the ~90% lost energy go?
Respiration (heat), plus egested/uneaten parts (faeces, bones, roots) and death from non-predation.
Why are food chains usually only 4–5 links long?
Energy shrinks ~10× per level, so after a few steps too little remains to support another population.
Why is a pyramid of ENERGY always upright?
Each level holds ~10% of the one below, so bars must shrink upward.
Give the formula for energy at the nth level from producers E1.
E(n) = E1 × (0.10)^(n-1).
Why is eating plants more energy-efficient than eating meat?
Fewer 10% transfer steps, so less energy is wasted per kJ of food obtained.
What role do decomposers play in energy flow?
They break down dead matter/faeces from all levels, releasing energy as heat and recycling nutrients.
Recall Feynman: explain to a 12-year-old

Imagine the Sun hands the grass a big bag of 100 coins. The grass keeps most and only passes 10 coins to the rabbit that eats it. The rabbit passes just 1 coin to the fox. The fox has almost nothing left to give — that's why there aren't lots of foxes on top, and why food chains are short. The coins (energy) get "spent" as body heat and never come back, so the Sun must keep sending new coins every day.

Connections

  • Food chains and food webs
  • Photosynthesis — how producers capture light energy
  • Cellular respiration — where the heat loss comes from
  • Ecological pyramids (number, biomass, energy)
  • Nutrient cycles (carbon, nitrogen) — the matter that cycles
  • Laws of thermodynamics
  • Biomagnification — why chains being short matters for toxins

Concept Map

captured by photosynthesis

eaten by

eaten by

eaten by

dead matter

dead matter

recycle nutrients

reused by

classifies

2nd law wastes energy

only 10% stored biomass passes on

En = E1 x 0.10^n-1

Sunlight

Producers L1

Primary consumers L2

Secondary consumers L3

Tertiary consumers L4

Decomposers

Nutrient cycle

Trophic level = feeding position

Laws of thermodynamics

~90% lost as heat

10% rule

Short chains, rare top predators

Hinglish (regional understanding)

Intuition Hinglish mein samjho

Dekho, ecosystem me energy Sun se aati hai. Plants (producers) ise photosynthesis se pakadte hain — ye Level 1 hai. Fir herbivore plant khata hai (Level 2), carnivore herbivore ko khata hai (Level 3), aur aage. Har step ko trophic level kehte hain — matlab food chain me kaunsi position par ho.

Sabse important baat: energy ek hi direction me flow karti hai aur har level par sirf 10% hi upar jaati hai, baaki ~90% heat ki tarah waste ho jaati hai (respiration, movement, aur jo parts khaaye nahi jaate jaise haddi/faeces). Isko 10% rule bolte hain. Formula simple: En=E1×(0.10)n−1E_n = E_1 \times (0.10)^{n-1}. Yaad rakho — energy recycle NAHI hoti (heat wapas nahi kha sakte), lekin nutrients (carbon, nitrogen) cycle karte hain.

Iska bada consequence: kyunki har level par energy 10 guna kam ho jaati hai, food chain sirf 4-5 links lambi ho sakti hai — top predators (jaise sher, baaz) bahut kam number me hote hain. Energy pyramid hamesha upright (seedha) hota hai. Aur isiliye plants khaana meat khaane se zyada energy-efficient hai — kam 10% steps, kam loss. Exam me efficiency ka formula En+1/EnE_{n+1}/E_n zaroor yaad rakhna aur data se percentage nikaalna aa jaana chahiye.

Test yourself — Ecology & Ecosystems

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Connections

Explain food chains and food websBiology · 5.1.4Explain the laws of thermodynamics in biologyBiology · 2.5.2Describe predation and its effectsBiology · 5.2.7Describe biodiversity and its importanceBiology · 5.2.11