The arrow always points in the direction energy flows — i.e. from the eaten to the eater. Grass → grasshopper means "grasshopper eats grass, energy moves into grasshopper."
Deriving why energy shrinks (from first principles):
An organism eats food carrying energy Ein.
It cannot absorb all of it — some passes out as faeces (egestion). Call absorbed energy A.
Of the absorbed energy, a big chunk is burned in respiration to move, grow, keep warm — released as heat (R).
Only what's left builds body tissue (new biomass) P:
P=A−R=(Ein−faeces)−R
The next level can only eat the biomass P, not the heat (you can't eat heat — it's gone into the environment and, by the second law of thermodynamics, cannot be recaptured as food).
So each step loses energy mostly to heat and waste, leaving roughly a tenth. Repeating this:
En≈E1×(0.10)n−1
A linear sequence showing energy/nutrient transfer from a producer through successive consumers.
What is a food web?
An interconnected network of many food chains showing multiple feeding relationships.
Which direction does the arrow in a food chain point?
From the organism eaten to the organism that eats it (direction of energy flow) — "is eaten by."
What is a trophic level?
A feeding position/step in a food chain (e.g. producer, primary consumer).
What sits at trophic level 1, always?
Producers (autotrophs) — they capture sunlight via photosynthesis.
State the 10% law.
Only ~10% of energy at one trophic level is transferred to the next; ~90% is lost.
Where does the ~90% lost energy go?
Mostly heat from respiration, plus undigested waste (faeces/egestion).
Why are food chains usually only 4–5 links long?
Because 90% of energy is lost per step, so too little remains to support higher levels.
Difference between energy and nutrient movement in an ecosystem?
Energy flows one-way (Sun → heat, not reused); nutrients are recycled by decomposers.
Why do food webs make ecosystems more stable than single chains?
Multiple food sources provide backups, so losing one species is less catastrophic.
Role of decomposers?
Break down dead matter and waste, returning nutrients to soil for producers.
Formula for energy at level n given level 1?
En≈E1×(0.10)n−1.
Recall Feynman: explain it to a 12-year-old
The Sun is like a giant free food-truck, but only plants know the secret recipe to catch its energy. A rabbit eats the plant and grabs some of that energy. A fox eats the rabbit and grabs some of what's left. But every animal is leaky — it "spends" most of its energy just running around and staying warm, and that spent energy escapes as heat you can't eat. So by the time you reach the top animal, there's barely any energy left — that's why there are lots of grass but very few hawks. A food chain is one straight story; a food web is all the stories tangled together, because everyone snacks on more than one thing.
Dekho, sabse pehle yeh samajh lo: saari energy Sun se aati hai, lekin sirf producers (jaise ghaas, plants) hi Sun ki energy ko photosynthesis se pakad sakte hain. Fir jab grasshopper ghaas khata hai, energy uske andar chali jaati hai — yahi ek chain ban jaati hai: Grass → Grasshopper → Frog → Snake → Hawk. Yaad rakho, arrow hamesha "khaya jaata hai by" ki taraf point karta hai, matlab energy jis direction mein ja rahi hai.
Ab asli baat: har step par sirf 10% energy aage jaati hai, baaki 90% heat aur waste mein loss ho jaati hai (kyunki animal saans lene, chalne, warm rehne mein energy kharch karta hai). Isi wajah se food chain zyada lambi nahi hoti — top par itni kam energy bachti hai ki 5th ya 6th level ka animal survive hi nahi kar paata. Isiliye ghaas bahut hoti hai lekin hawk bahut kam.
Food web bas food chains ka jaal hai. Real life mein frog sirf grasshopper nahi khata, aur hawk sirf snake nahi khata — sab log multiple cheezein khaate hain. Isse ecosystem ko stability milti hai: agar ek prey khatam ho jaaye, toh predator doosra food source use kar leta hai. Ek important point: energy ek hi direction mein flow karti hai (recycle nahi hoti), lekin nutrients recycle hote hain — yeh kaam decomposers (fungi, bacteria) karte hain, jo dead bodies ko todkar nutrients wapas mitti mein daalte hain. Yahi cheez ecology ka core funda hai.
