Fragile environment

The Savory brittleness scale is used to describe the position on a simple 1 to 10 scale of any environment based upon the distribution of humidity throughout each year, to assist with management decisions. It was developed by Allan Savory, a Zimbabwean biologist, because scientists were only recognizing desertification taking place in low rainfall or “fragile” environments. Savory recognized that desertification was the extreme or terminal form of man-made land degradation, but that this began with simple biodiversity loss that was occurring at all levels of rainfall from high to low. Fragile environments did not cover the situation because Savory thought all environments were fragile. The scale is used in land management because the annual distribution of humidity affects the way entire biological communities function on the land, and in particular to conservation or resting the environment to maintain or restore biodiversity. He also noted that the distribution of humidity and thus position on the brittleness scale influenced the effects of such things as fire and grazing by large herbivores.

The scale ranges from 1 to 10 with 1 being non-brittle and 10 being very brittle. The scale is subjective; there is no formula for its calculation. A tropical rainforest is considered a 1 on the scale, and an arid desert such as the Namib or Gobi is considered a 10.

Position on the scale is determined not from rainfall records but from field observation because humidity is influenced by several factors such as altitude or prevailing winds. Savory used the term brittleness because an easy practical observation is whether or not a dead grass stem or small dead twig is soft and easily bent by hand or so brittle it snaps. For these reasons it differs from an aridity index. Thus, some high rainfall environments, e.g., Zambia, with 2,000 mm annual rainfall and distinct wet and dry seasons, is higher on the brittleness scale because of the long portions of the year without rainfall. An environment with lower total rainfall distributed fairly evenly throughout the year, such as parts of England with 600 mm annually, are lower on the brittleness scale.

Grass leaves and stems that die off above ground every year following the growing season break down slowly and mainly through oxidation in sunlight and physical weathering in the more brittle environments. And trees dying in such environments tend to remain standing for many years again gradually breaking down through oxidation and weathering from the top down. At the opposite end of the scale where humidity distribution is more even dead plant material tends to break down rapidly through high micro-organism activity or biological decay. Commonly a dead tree will decay at the base, fall and decay on the ground. Other observations Savory made were that at the lower end of the scale most herbivores naturally were insects with few large herbivores and solitary predators, while moving toward the higher end large herding herbivores and pack-hunting predators increased.

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