Relevance of Elasticity Concepts in Explaining the Impact on the Durian Market and Related Goods

Answer

The rise in demand for durians affects the durian market and its related goods through equilibrium price, quantity and total revenue/expenditure, $TR = P \times Q$. The question asks which elasticity concepts are relevant in explaining that impact — and relevance, not magnitude, is the discriminating idea: an elasticity is relevant only if the change it governs actually occurs given the events in the preamble. I argue that CED is the most relevant concept, PES is relevant for the durian market itself, PED is relevant only for the related goods, and YED is not relevant at all.

PES in the durian market. The preamble gives a demand increase for durians, raising equilibrium price and quantity. The price elasticity of supply governs the extent of those changes: durians have $PES < 1$ because trees take long to plant and bear fruit, so the demand increase raises price more than quantity. But for a pure demand increase, total revenue rises regardless of $PES$ — so $PES$ is relevant for the split between price and quantity, but not for the direction of $TR$.

PED in the durian market — relevant only conditionally. PED governs the impact of a supply change on price, quantity and $TR$. Were durian supply to fall — say from bad weather — an inelastic demand ($|PED| < 1$, since die-hard durian lovers find no close substitute) would mean $TR$ rises, whereas an elastic demand would mean $TR$ falls. But the preamble mentions no change in the supply of durians, only a demand increase. So for the durian market itself, $PED$ is not relevant in this context.

PED in related goods — derived demand and competitive supply. $PED$ becomes relevant for goods whose supply falls as a consequence of the events. Durian-flavoured food such as durian cake derives its demand from durians and uses durian as a factor input, so the higher durian price raises its cost of production and reduces its supply; the impact on the cake market’s price, quantity and $TR$ then turns on the $PED$ of durian cake (inelastic ⇒ $TR$ rises; elastic ⇒ $TR$ falls). The same logic applies to palm trees, whose supply falls through competitive supply. So $PED$ is relevant — but for the related markets, not the durian market.

CED in related goods — the most relevant. Cross elasticity of demand measures the responsiveness of one good’s demand to a change in the durian price, and so is directly relevant to substitutes and complements. Mangoes, a substitute, have positive but low CED ($0 < CED < 1$) with durians — die-hard fans see no good substitute — so the rise in durian price raises mango demand and revenue only slightly. A complement such as green tea has negative CED, so dearer durians cut its demand and revenue, again by a small extent given weak complementarity. CED captures both the direction and the extent of the cross-market impact, making it the most broadly relevant concept here.

YED — not relevant. Income elasticity would matter only if income changed. Durian is plausibly a normal, even luxury good ($YED > 1$, eaten for pleasure once necessities are met), but the preamble reports no change in income — so $YED$ is not relevant to explaining the impact of these particular events.

Evaluation and judgement. Relevance is decided by which changes the events actually trigger, not by the size of any elasticity coefficient. On that test: $YED$ is irrelevant (no income change); $PED$ is irrelevant for the durian market (no durian supply change) but relevant for the related markets where supply does fall (durian cake via derived demand and rising costs, palm trees via competitive supply); $PES$ is relevant for the extent of the price-versus-quantity split in the durian market, though not for the direction of its revenue. The most relevant concept is $CED$, because it alone speaks directly to the impact on durian’s substitutes and complements — the “related goods” the question foregrounds — explaining both the direction and the (small) extent of those effects, given durian’s standing as the unique “king of fruits” with few close substitutes. The reasoned stand is therefore that CED is the most relevant elasticity, with PED and PES relevant in their proper places and YED not relevant at all — a verdict that rests on matching each concept to a change the events genuinely cause.