At the top of every food pyramid sit the carnivores, the last link in an energy transfer chain that begins with the plants - the initial food synthesizers. This food in the form of leaves and fruit is passed on to the herbivores, lying higher up the pyramid, and ultimately through them to the carnivores. Similar pyramids exist throughout the world in all environments from the tropics to the poles.
Sometimes a predator, represented here by the polar bear, may lie at the top of a pyramid that embraces both land- and water-based organisms. The complex feeding relationships which exist between plants and animals living together in the same environment amount to a self-sufficient organization known as an ecosystem. Ecosystems in tropical areas may consist of thousands of species.