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 of 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.
The effect of resting the environment to allow recovery and maintenance of biodiversity is different across the brittleness scale. At the low end resting the environment as in establishing wilderness or conservation is the most powerful action possible to restore biodiversity. Moving across the scale the same practice can become increasingly damaging to the health of entire communities. This is why Savory argued that abandoned cities of past ages low on the scale are today found under recovered tropical forest, while abandoned cities at the other end in former savannas are today found under desert sands. Total protection by removing all livestock leads to recovery of biodiversity in the perennially humid environments in the US, but the same practice in places such as New Mexico has led to severe desertification.
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Deserts and xeric shrublands are a habitat type defined by the World Wide Fund for Nature. Deserts and xeric shrublands form the largest terrestrial biome, covering 19% of Earth's land surface area. Ecoregions in this habitat type vary greatly in the amount of annual rainfall they receive, usually less than 250 millimetres (10 in) annually. Generally evaporation exceeds rainfall in these ecoregions. Temperature variability is also diverse in these lands. Many deserts, such as the Sahara, are hot year-round but others, such as Asia's Gobi, become quite cold in winter.
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Clifford Allan Redin Savory is a Zimbabwean ecologist, livestock farmer, and president and co-founder of the Savory Institute. He originated Holistic management (agriculture), a systems thinking approach to managing resources.
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The environmental impact of agriculture is the effect that different farming practices have on the ecosystems around them, and how those effects can be traced back to those practices. The environmental impact of agriculture varies based on the wide variety of agricultural practices employed around the world. Ultimately, the environmental impact depends on the production practices of the system used by farmers. The connection between emissions into the environment and the farming system is indirect, as it also depends on other climate variables such as rainfall and temperature.
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