Biome boundaries are drawn arbitrarily, because nature refuses sharp edges
Maps show deserts, grasslands and forests as neat coloured patches, but real landscapes blend gradually from one to the next, with small-scale variation everywhere. Ecologists admit that biome borders are drawn somewhat arbitrarily, based on average conditions, and the sheer number of rival classification schemes shows how imperfectly life fits them.
A biome is a large region defined by its climate, plants and animals, a community of life shaped by its physical surroundings. Arthur Tansley enriched the idea in 1935 by folding in climate and soil and naming the result an ecosystem, and the International Biological Program, running from 1964 to 1974, made biome a household term among scientists. Usage still differs: German writers in the Walter tradition treat it more like a specific local habitat, and some Brazilian authors use it for a province defined by which species live there.
The American botanist Leslie Holdridge offered an influential scheme in 1947. Assuming temperature and rainfall matter most to vegetation, he built a diagram dividing the world into 30 humidity provinces. He knew soil and sunlight counted too but largely set them aside. Robert Whittaker later simplified the approach to two measures, average yearly rainfall plotted against average yearly temperature, trading some precision for clarity.
Whittaker distinguished a formation, a plant community on one continent, from a biome, which includes animals as well. Similar biomes on different continents form a biome-type. Using what he called gradient analysis, he traced how communities change along gradients of moisture, temperature and altitude. As conditions grow harsher, productivity falls, vegetation loses its layered structure and species diversity declines. Each growth form, such as grass or shrub, peaks at its own point along these gradients, and the same forms dominate similar climates in distant parts of the world.
Field data back the logic. A 1978 study of North American grasslands found that water use tracked above-ground plant growth, while sunlight and temperature were tied to root growth, and warmth and moisture together decided whether plants grew in cool or warm seasons.
Source: Biome
