How are biomes formed?
Scientists divide up the places where life exists on Earth into several distinct zones, which they call biomes. Biologists distinguish biomes primarily by the amount of precipitation that they receive and the average temperature. For example, a really dry area may be called a desert. A really wet areas may be called a tropical rain forest. An area with seasonal changes in temperature and lots of grasses may be called a grassland. These biome names are a convenient way to talk about parts of the Earth that are similar in climate and in species diversity but may be far apart.
The boundaries between biomes are always shifting as the climate changes*, but few scientists would say that biomes are "forming" right now. Life exists almost everywhere on Earth, so almost every area is part of some biome.
However, in some cases a community can be completely eliminated. For example, a volcanic eruption may eliminate all existing life in an area. A process called primary succession follows. Life soon colonizes the bare rock, starting with species such as lichens. The lichens begin the process of breaking down the rock into soil. Other hardy species follow, like mosses, herbs, weeds, and then grasses. Centuries may pass before a mature forest forms.
Footnote: The consequences of shifting biomes are important areas of research. As biomes shift the species that inhabit them often must move as well. Right now, biome shifts are occurring faster than the pace of evolution in some species. If species do not adapt fast enough to the changing environment, then extinctions occur.