I remember watching a documentary once and learned that whales need to eat tonnes (literally) of krill per day to survive. I don’t understand why this is better than eating small amounts of large fishes instead.
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Because of a thing called trophic levels. This is the term given to each step in a food chain. Level one is your primary producers: anything that has chlorophyll and converts sunlight to energy. Next level up are your herbivores, followed by your first level predators and then your apex predators.
The food chain is basically a long line of sales people trying to make money off one bit of labour. Plants/phytoplankton (lvl1) are your farmers, taking raw ingredients and turning them into food. They use some of this food to live: moving, reproducing etc. This is the profit. They store what remains in their bodies either as energy reserves or in growing bigger.
Herbivores(lvl2) are your wholesalers. They take what the farmers have stored and cut themselves a profit to live off. The rest they store in their bodies as either energy reserves or growth.
Predators (lvl3-lvl4) are the companies that buy from wholesalers to divvy up to retailers. They take the energy the wholesalers have, cut themselves a profit to live off, and then store the leftovers as energy reserves or growth.
Apex Predetors (lvl4-lvl5) are the last in the chain, and are the consumer-level retailers. Like everyone else, they take what they need and store the rest.
So you have a line of parties clipping the ticket everytime the energy (money) passes from one level to the next. It’s worth keeping in mind here that moving, reproducing, keeping warm etc counts for 90% of the energy lost at each level. If the phytoplankton produces 1,000 units of energy, the herbivore is only getting 100 units per phytoplankton. The first level of predators is only getting 10 units. The apex predators gets 1. So for an apex predator to get the energy it needs, that’s a lot of individuals that need to be eaten in the course of a food-chain. For this reason, food chains rarely extend beyond 4-5 trophic levels. There’s just no way to extract more value from the same amount of farmers in a given area.
(Edit: I was wrong, it’s actually a limit of 5-6 levels. The concept still holds though)
Whales are huge, and need a lot of energy to maintain that mass. There’s also only so many phytoplankton in the sea. If whales waited for the wholesalers to take their cut, there wouldn’t be enough for the whales to live on. So instead, the whales have evolved to literally cut out the middle men and go straight to the source.
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