BrightKidz Library
Subjects
Four rows of counters, each row far shorter than the one below it A counting picture with four rows stacked up the plate. The bottom row is packed with counters and runs off both edges of the picture, so you cannot see where it ends. The row above it has nine counters. The row above that has three. The top row has a single counter on its own. Every counter is drawn separately with a clear gap around it, so the rows can be counted rather than guessed at. Each row stands for one step up a food chain, and the shrinking is the point: almost all of the energy at one step is spent before it can reach the step above, so each row can only support a small fraction of the row below.

Why There Are Always More Small Animals Than Big Ones

About 11 minutes

Go and count the animals in a garden and something odd shows up straight away.

Greenfly: hundreds, easily, on one rose stem. Ladybirds: a handful, if you look properly. Blackbirds: two, maybe three. Sparrowhawks: none most days, one occasionally, and never two at once.

It is the same in a wood, the same in a river, the same on a grass plain in Africa. Blades of grass, then zebra, then lions — and always in that order, always fewer at each step. Nobody arranged it. Nothing is stopping the lions from having a big family.

So why does it come out the same way every single time?

Food is fuel, and fuel gets burned

Here is the thing that is easy to get wrong. When a rabbit eats a kilogram of grass, that grass does not become a kilogram of rabbit. Not even close.

Most of it goes somewhere else, and it goes there in three different ways.

Some of it is never eaten at all. Grass has roots underground and tough stems the rabbit leaves behind. A tree has bark and wood that almost nothing chews.

Some of it is eaten and not digested. It goes in one end and out the other without ever becoming rabbit.

And most of it — by a long way — is spent. Every hop costs energy. Every heartbeat costs energy. Sitting perfectly still in a cold field and staying warm costs energy, all night, whether the rabbit does anything or not. That energy does not vanish; it leaves the rabbit as heat and drifts off into the air, and no animal can ever get it back.

What is left over, actually built into the rabbit and available to whatever eats the rabbit, is a small slice.

Where a hundred mouthfuls of grass actually go Three bars drawn one under another, all starting from the same left edge, showing what becomes of a hundred units of food an animal eats. The top bar is much the longest and is the share spent simply staying alive: moving about, growing, and keeping warm. The middle bar is half that length and is the share never eaten at all, or eaten and passed straight through undigested. The bottom bar is by far the shortest and is drawn solid so it stands out, and it is the share left over and passed up to whatever eats this animal. The numbers beside the bars read sixty, thirty and ten, and the bars get visibly shorter down the picture. Where a hundred mouthfuls of grass go spent staying alive: moving, growing, keeping warm60never eaten, or eaten and not digested30left over, and passed up to whatever eats this animal10 Only the bottom bar reaches the next animal along.

Follow one big pile of grass all the way up. Watch what is left at each step.

  1. Start with 1000 mouthfuls of grass. Rabbits get through the lot.
  2. After all the hopping, warming, breathing and wasting, about 100 mouthfuls of it has turned into rabbit. The other 900 are gone.
  3. A fox eats the rabbits and does exactly the same thing to them. Roughly 10 mouthfuls now exist as fox.
  4. Something that hunted foxes would find about 1 mouthful waiting for it. One, from a thousand. There is not enough there to live on.

About a tenth. Sometimes a little more, often less, but a tenth is the number to carry around in your head.

The same sum, over and over

That tenth is the whole answer, because it happens again at every step.

A tenth of a tenth is a hundredth. A tenth of that is a thousandth. Divide by ten four times and you have taken a thousand down to one.

This is why food chains are short. Grass, rabbit, fox — and then it stops. You almost never find a chain with six or seven links in it, and it is not because animals ran out of ideas. There is simply nothing left at the top of a long chain to make a living from.

Nobody hunts lions

Here is the part that surprises people.

There is no animal whose living is made by hunting lions. Not one. And it is not because lions are frightening — plenty of animals hunt things that could hurt them.

It is because the sums do not work. To feed itself, a lion-hunter would need a good supply of lions. That many lions would need an enormous supply of zebra. That many zebra would need more grass than the plain has got. The animal is impossible before it is dangerous.

The same is true in the sea, where orcas hunt almost everything and nothing at all hunts orcas.

Being the biggest hunter around is not a prize for being fierce. It is a description of where the food ran out.

Warm blood costs more

One more thing, because it explains a real difference between places.

You and I, and every bird and every mammal, burn a large share of our food simply keeping our bodies warm. A lizard does not. A fish does not. They are the temperature of whatever is around them, and they spend nothing on it.

So a chain of cold-blooded animals wastes less at each step, and can run a little longer before it runs out. A pond can support a slightly taller stack of creatures than a field of the same size — not because the pond has more food in it, but because less of the food is quietly leaving as heat.

Go and check it

You do not have to take anyone's word for this. It is a thing you can count.

Pick one small patch — a single bush, a square metre of long grass, a jar of pond water. Count the smallest creatures you can see, then the middle-sized ones, then the big ones. Do not try to be exact; you only need the shape.

Every time, the shape is the same. Wide at the bottom, thin at the top.

Why is there no animal that makes its living by hunting lions?

The next time somebody says a hunter sits at the top of the food chain, you will know something better than that. It is not sitting at the top. It is standing on a very large pile of grass.