Biomes

The biome is a concept that allows spatial organization of vegetation and animals at the planetary scale. More specifically, it accounts for the integration of climateplant-animal relationships over large areas. Biomes are the world’s major life communities classified by the predominant vegetation, often the most prevalent feature on the natural landscape. The biome concept implies that plants and animals have adapted to each biome in ways specific to the biome. Each biome has species that the geographer views as emblematic of that biome. For instance, the solitary baobab tree in the midst of the savanna grasses and the grove of scrubby trees near the Mediterranean Sea evoke regional relationships of vegetation, animals, and people.

The zone of transition between two biomes is an ecotone. Although biomes usually grade gradually from one to another, small-scale biome maps such as that shown below give the misimpression that there are sharp boundaries. Horizontally, biomes usually grade into each other. For instance, the Sahel—the semiarid zone south of the Sahara—contains the transition between the desert and the tropical savanna biomes. In the southern Sahel there is greater biomass with grasses and shrubs found more plentifully than in the northern Sahel, where there is sparse biomass emblematic of the Sahara. Yet, when pressed to find an exact boundary, it becomes apparent that the edges of biomes blend together as do the climates that cause them. Contrastingly, in mountainous areas there are often several sharply defined vertical biomes visible.

The biomes that can be observed today have not existed throughout Earth history, even relatively recent history (see accompanying map). Over very long periods evolution and continental drift have impacted the extents of biomes. Climate changes have shifted ecotones over scales of hundreds and thousands of years. In North America the Pleistocene ice age saw biomes shifted hundreds of kilometers south of their current positions because of the presence of the continental ice sheets. In places, the boreal forest reached all the way to the Gulf of Mexico.