What does the 'brittleness' of an ecosystem mean?
The 'brittleness' of an ecosystem, in the management sense, is a statement regarding it's capacity to support biological decay of dead plant material. The term 'brittleness' comes from the simple snap test where a stem of a plant is bent. If it snaps, it's dry and if it bends, it's moist. If humidity is high enough, dead vegetation will retain enough moisture to support biological decay. If humidity is too low, the dead vegetation will not decay, it will oxidize, a chemical process.
A 'brittle' environment is one in which humidity is very low and/or seasonally sporadic. The more 'brittle' the environment, the greater the number of days per year in which dead vegetation is dry. It is not a matter of total precipitation per year, but the distribution of humidity throughout the year.
The brittleness scale is not an objective scale, but a subjective judgement based on observation. In a 'wet' year, east-central Alberta would probably rate in the 4-5 range, but in a drought year, it would probably rate a 7-8. In general, it could be referred to as a 'semi-brittle' region but managing grasslands in this region will require adjustment based on year-on-year variation in rainfall.
Most of the world's grasslands ecosystems fit in the 'semi-brittle' to 'brittle' end of the scale.
Why does 'brittleness' matter?
Brittleness matters for a simple reason, it determines the mechanism(s) required to produce biological decay and prevent oxidative/chemical decay of dead vegetation. In order to support healthy soil, dead plant matter must break down biologically and the nutrients liberated from the old material be added to the soil. In addition, dead plant material that is left standing will prevent sunlight penetrating to the new leaves of young plants. With insufficient sunlight, this new growth is hampered in much the same way that a wood chip mulch will help prevent weed growth in a flowerbed. If left to decay chemically, grasslands lose biodiversity and biomass over time, causing the soil to develop a hard surface (poor water infiltration) and slowly open patches of soil open up where the plant life has died off. Exposed to sunlight, it overheats, killing remaining biology. Exposed to rain/wind, it begins to erode.
Therefore, in a semi-brittle or brittle environment, another mechanism is needed to force biological decay of plant material. In nature, this is provided by animal impact. Vast herds of bison, wildebeest, zebra, etc, eat and trample old vegetation. This trampled vegetation and dung is deposited in a shallow mat on the soil surface, protecting it from wind, rain and sun. This layer of soil is called the detritusphere. Because it is protected from wind and sun, humidity from the soil is trapped below and inside this mat. This creates a safe and hospitable living space for a multitude of insects, worms and other organisms that get to work breaking down the dead plant material and recycling it into the soil. In addition, this mat of decaying material, combined with the action of soil biology hard at work, create a spongy porous topsoil able to rapidly infiltrate precipitation, where it becomes available for use by plants and soil biology.
Above the detritusphere, the new plants now have only to put up shoots a short distance to find full sun. With unimpeded access to sunlight above the mat of dead vegetation, new plants can rapidly get on with photosynthesis and restarting the whole cycle.
It was the discovery of the vital importance of animal impact to semi-brittle and brittle grasslands ecosystems that lead to regenerative rangeland management systems.