As you pass along a food chain, the number of individual animals decreases at each trophic level. If you arrange these numbers on top of each other, they form a pyramid shape. Here’s an example from a woodland food chain:
All food chains form this shape. It is called a pyramid of numbers. It has a broad base of plants (producers), supporting a smaller number of plant-eaters (primary consumers), and so on up the pyramid, with the numbers getting smaller as you get higher. The smallest number is always the one that shows the predators at the top.
The reason for the pyramid shape is simple. There must always be enough plants producing food at the bottom, or else the whole food chain would collapse. Higher up, no predator (consumer) can be as common as its prey, otherwise both animals would soon die out. This is why sparrowhawks can never be more common than blue tits in an ecosystem that they share.
Birds lower down the food chain have more young, more quickly, than those higher up. Blue tits lay ten or more eggs, and their babies leave the nest after five or six weeks. Sparrowhawks lay only four to six eggs, and their babies leave the nest after nine or ten weeks. By the time the young sparrowhawks hatch, there are plenty of young blue tits for the parents to feed them on.
Plants reproduce in huge numbers, making sure that the pyramid keeps its shape. They send out millions of seeds – think of a dandelion clock – that sprout all over the place – offering a constant feast for consumers. Animals low down the chain, such as insects or frogs, tend to lay lots of eggs. This way plenty make it to adulthood, even though thousands get eaten along the way.
View this page online at http://www.rspb.org.uk/youth/learn/foodchains/pyramidsofnumbers.asp