Food Chains, Pyramids & Webs

Showing energy flow in an ecosystem is complicated, which diagram you should use depends on what you want to show.

A Food Chain is the simplest and easy to understand.
An Energy Pyramid is a Food Chain that shows how energy is lost through an ecosystem.
A Food Web is complicated and tries to show every possible energy pathway in an ecosystem. These are usually simplified, because showing, let alone knowing every energy pathway is very difficult.

Here's the Amoeba Sisters give their insights into Energy Flow & Biodiversity.