Why would the tertiary consumers have the smallest population?

Because of something known as the 10% rule This means that only 10% of the energy can go to the next consumer level. That may not have been worded well, so here's an unrealistic but explanatory example; 1000g of algae (producer) is consumed by a fish (primary consumer), but only 10% of the energy (100g worth) can go to the fish. Then a bird eats that fish, which has 100g worth of energy from the algae.

That bird get's 10% of the energy in the fish, so 10g worth. Now a wolf eats the bird, which only had 10g worth of energy from the fish. That wolf get's 10% of the energy from the bird, so the wolf (tertiary consumer), only gets 1g worth of the original 1000g.

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