The classic example is the San Francisco Mountains in northern Arizona. On the north side of the mountains the vegetation at the base is a scrub desert changing to a desert dominated by the dry adapted trees pinyon pine and juniper. As the elevation increases the forest changes to Ponderosa Pine, then to a spruce-douglas fir habitat very similar to the taiga, and finally changes to an alpine zone similar to tundra in its appearance on the top of the mountain peaks. Ecologists have proposed a series of biomes for the oceans of the world as well, although most of their attention has focused on terrestrial biomes. Other more complicated classifications of ocean habitats have been proposed.
Biomes are a crude way of classifying the ecosystems of the world.
Their very size is also their most severe weakness. They lack precision and many parts of the world do not fit comfortably into one biome type or another. The world is almost infinitely complex. Biomes are an extremely useful concept but are not very good at dealing with the particular problems or characteristics of specific ecosystems. In addition no two ecologists are able to agree on a standard list of biomes.