However, this begs the question of how these species became so specialized and diverse, and here we turn to an evolutionary perspective. First, let's examine two examples of the high degree of specialization found in tropical forests. The forests themselves are multi-layered. A temperate forest usually is two or three layered. In an undisturbed forest the trees are fairly similar in height, there usually is some ground vegetation, and in between there may or may not be a shade-adapted middle layer.
Tropical forests are vertically more complex.
As many as perhaps five layers exist, although the distinctness of layers is arguable. Human examples include cobblers, brewers, and professors. In the animal kingdom we use guild as a finer sub-division than trophic level. As Figure 10 illustrates for birds, there are more guilds in the tropics, and more species per guild. Let us turn to the deeper question, of how this greater specialization came about.