The Savory brittleness scale is used to describe the annual distribution of humidity in a particular environment. It was developed by Allan Savory, a Zimbabwean biologist. The scale is used in land management because the distribution of humidity affects the way that land, particularly when degraded, responds to being rested. Most commonly the scale is used to guide the use of cattle or other herding animals in the maintenance and restoration of land which is brittle or subject experiencing desertification.
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The scale ranges from 1 to 10 with 1 being non-brittle and 10 being very brittle. The scale is not precise and is a matter of judgment rather than applying a formula. A tropical rainforest would measure 1 on the scale, and an arid desert such as the Sahara would rate a 10.
The scale reflects the distribution of humidity throughout the year, not the amount of rainfall. For this reason, 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, can be high 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, can be low on the brittleness scale.
While total rainfall is important for understanding the ability of an area to be productive during the growing season, brittleness gives an indication of the ability of insects and bacteria to degrade dead plant material throughout the year. A brittle environment may be characterized by grasses which grew during the rainy season and which died during periods of extended dryness and are still standing at the time the rains return. This lack of decay means the dead grasses shade new plant growth, reducing the productivity of the area during the rainy season.
Non-brittle lands, such as rain forests, will not remain empty of plant life for long periods of time if they are cleared and fully rested. Continuous availability of moisture promotes a continuous cycle of the growth of new plants, and upon their death, their decomposition so space is made for following generations.
At the other end of the scale are brittle and very brittle lands. If left rested after being cleared these lands often do not recover.
Brittle landscapes have erratic distribution of moisture throughout the year. The main significance of this scale is that the "tool" of resting land in either form - total rest or partial rest - leads to grasslands weakening and being replaced by either woody vegetation or bare soil covered with algae and lichens depending on the amount of rainfall. Partial rest, a new concept named by Savory, describes situations in which there are large herbivores on the land who are scattered and exhibit calm unbunching behavior. Research plots across the brittleness scale indicate that the effects of partial and total rest are almost identical in grasslands and savannas. Partial rest in high end brittle environments, particularly where combined with overgrazing of plants, is the single greatest influence leading to desertification.