A forest ecosystem is defined as an area dominated by trees and other woody plants. Forests aren't only trees, however. Healthy forests have a lot going on in them, and many different species of both animals and plants that call them home. There are many different types of forests in the world, ranging from tropical rain forests to the dense sub-polar taiga. To truly understand a forest ecosystem, it is easiest to break it down into the five layers that most healthy forests have. Animals that live in a forest move between the layers to feed and hunt.

The Canopy

The canopy section of a forest is the very top, and consists of the tallest, oldest trees, which can reach heights of 150 to 200 feet. This layer is the harshest of the five layers because it is exposed to everything that nature throws its way. It gets whipped by the wind, exposed to the sun without shade, receives the brunt of downpours, and is the most likely to be struck by lightning. Animal that live in this layer are those adapted to living tough, and include birds, tree frogs, snakes, lizards and hard-bodied insects.

The Understory

The understory is the layer just below the canopy, and consists of those trees that are still growing but haven't reached full height. This environment is protected from the elements somewhat by the canopy layer, and is therefore less harsh. Trees in the understory are growing slower because they have less light, and tend to be a bit thinner in foliage. There is a greater variety of animals that live in this layer, including birds, butterflies and caterpillars, frogs and tree mammals like squirrels and raccoons, in the north, and monkeys, in the tropics.

The Shrub Layer

The shrub layer is the next level down, and is dominated by woody plants that never grow very tall. Some of these are very young trees or trees that remain shorter, but most are shrubs, which are woody plants that have more than one stem. Shrubs can get as tall as 15 to 20 feet, but most top out at around 10, and many are shorter than that. Lichens can grow on tree bark between the shrub layer and the understory, and animal life also thrives. The shrub layer is home to many different kinds of insects and spiders, birds, snakes and lizards.

The Herbaceous Layer and Forest Floor

The herbaceous layer is the layer just above the forest floor, and consists of tree seedlings and non-woody plants. These include mosses and a variety of flowers. The forest floor consists of the leaf litter-- a thick bed of leaves dropped from the trees-- and the soil. These layers are the backbone of the forest. Without good soil, trees have nothing to root into, and in the north, the leaf litter acts as insulation for tree roots and soil-based animals. Hornets, butterflies, birds, worms, slugs, snails, centipedes, millipedes, spiders, snakes live at this level, as well as billions of microbes, all of which contribute to soil health.