Biomes, Zonation and Climate Determinants

Two numbers make a biome. A biome is a group of ecosystems sharing a climate and therefore similar vegetation and productivity: tundra, boreal forest (taiga), temperate forest, temperate grassland, desert, savanna, tropical rainforest. Their global distribution is predicted almost entirely by mean annual temperature and annual precipitation, because those two decide how long plants can grow and how much water they have while growing. Insolation drives temperature, and the tricellular circulation of the atmosphere decides where air rises (rain at the equator and around 60° latitude) and where it sinks (deserts around 30°). Hot and wet gives rainforest; hot and dry gives desert; cold gives tundra whatever the rain; and at a given temperature, rainfall decides forest or grassland.

What follows from climate. Tropical rainforest has the highest net primary productivity and species diversity on land because it grows all year with abundant water — yet its soils are poor, because nutrients are held in the living biomass and cycled so fast that little accumulates; clear the forest and the fertility goes with it. Deserts have the lowest productivity because water is the limiting factor; tundra is limited by temperature, with a growing season of weeks and permafrost beneath. Temperate biomes lie between, with a marked seasonal cycle. Knowing the limiting factor of a biome lets you predict what climate change will do to it.

Zonation. Where an abiotic factor changes steadily with distance, communities change in bands: up a mountain (temperature falls with altitude, so the sequence of biomes from base to summit repeats the sequence from equator to pole), down a rocky shore (time exposed to air rises up the shore, so species are sorted by tolerance of drying), across a sand dune or into a forest from its edge. Zonation is a pattern in space, measured with a transect; succession is change in time at one place. Confusing the two is a standard error — a transect across a shore shows zonation, not succession, however tempting the sequence looks.

Climate change and biome shift. Warming moves every boundary on the temperature–rainfall diagram: boreal forest advances into tundra; thawing permafrost releases methane, a positive feedback; the dry edge of temperate forest turns to grassland; deserts expand. Species can track a shifting climate only if they can move fast enough and have somewhere to go — mountain species run out of mountain, island species run out of island, and species in fragmented habitats cannot cross the farmland between fragments. A good answer names the biome, its limiting factor, the direction of shift and the species that cannot follow.