In a short article about the cycles it is impossible to convey a full sense of how
interconnected they are. A change in one cycle can markedly affect the function-
ing of another. There are a number of other biogeochemical cycles that have not
been mentioned here (for example, calcium, zinc, sulfur, and phosphorus).
Although vital at various times and places, they do not have the overall volumes
and energy contents of the major cycles illustrated above; life can be threatened
if these other cycles are somehow interrupted.
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 climate-
plant-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 ani-
mals 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 anecotone. Although biomes usu-
ally 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 bio-
mass 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 throughoutEarthhis-
tory, even relatively recent history (see accompanying map). Over very long peri-
ods 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.
Biomes 31