I'll use rangelands in the western US as the example, but many of the principles are generalizable to rangelands elsewhere in the world.
(1) Many rangelands historically have supported native grasslands, originally dominated by perennial grasses. These grasses are useful for soil stabilization (they maintain cover of the ground year around and have extensive root systems),are quite productive, and also are very palatable to cattle.
(2) As the size of livestock herds (or the duration of grazing) increases, the perennial grasses are consumed, trampled, and decline. Their roots suffer from loss of above-ground parts and soil compaction. In addition, when they are heavily grazed (particularly at flowering and seed set times) they produce fewer seeds, decreasing recruitment of new individuals into the population.
In addition, biotic soil crusts (lichens and algae), which cover much of soil in native systems, get trampled, churned up, and decline. The biotic crusts are important nitrogen fixers and soil stabilizers, and their presence also encourage retention and infiltration of water. Hence, with their decline, there is increased erosion and runoff of water, as well as decreased nitrogen input, which worsens conditions for the native perennial grasses further, and also make it more difficult for seedlings of these species to establish.
Perennial bunchgrasses in these systems are called "decreasers" because they decrease with exposure to heavy grazing.
(3) The decline of the perennial bunchgrasses is followed by increased abundance of less palatable annual (or short-lived perennial) grasses and herbs. Space has been freed by the decline of the native perennials and the annuals seed in well. These annuals (and short-lived perennials) often produce abundant seed and have effective dispersal mechanisms, having life history traits characteristics of such "weedy annuals."
(I am defining a "Weed" here simply as a plant out of place.)
The species that invade and increase following the decline of the bunchgrasses are often aggressive nonnative species ("exotics"), such as cheat grass (Bromus tectorum) and other non-palatable species such as golden star thistle, spotted knap weed, medusa head, and others. (We'll talk more about some of these nonnative invaders when we discuss threats to biological diversity later in the term.)
These kinds of species are termed "increasers" because they increase in abundance following grazing.
These species (in part because they are often annuals) don't maintain cover of the ground year around as the perennials grasses did.
(4) So, rates of soil erosion increase. (Less plant cover means less of the rain drops' energy is absorbed by vegetation, increasing its ability to dislodge soil particles. In addition, lower cover of the ground by vegetation means decreased water uptake by vegetation, so there is more runoff of water and soil. In addition, wind erosion losses increase with lowered vegetation cover.)
Compaction of soil by wind and water also increases in the presence of lowered vegetation cover.
Thus, soil blows and washes away, rain and wind form hard crusts on the surface, infiltration of water into the soil decreases because of the increased compaction, and so more water runs off. The consequence of all of these changes is lowered water availability in the soil.
Gradually, it becomes harder for any plants to establish in the compacted, eroded and drier soil.
(5) As overgrazing and trampling continue, plant cover of all types -- even the annuals -- decreases further. The soil gets more and more compacted by trampling and wind and water, and increasingly eroded, making it increasingly difficult for plants to establish at all.
Also, in absence of severely reduced plant cover, humus formation slows. (There is less input of organic material from vegetation (although manure inputs are maintained!).) The consequences of diminished humus in the soils include decreased nutrient status, water-holding capacity, and porosity of soils, (as we described earlier when discussing reliance on inorganic fertilizers in agriculture) making it hard for plants to reestablish.
The wind-blown sands and dirt are also very abrasive, adding to the difficulties in plant establishment.
(6) Overall, we end up with vicious cycle -- grazing decreases vegetation cover, soil loss and compaction increase, water infiltration decreases and runoff increases, cover decreases still further, and so forth. In the severest cases, it is difficult for vegetation to reestablish even when grazing pressure is reduced. The initial plant destruction leads to effects that make it more difficult for plants to reestablish, which lead to greater losses of vegetatin and so forth. (In fact, I realized while giving this lecture in 1993, that what people mean when they say "vicious cycle" is really a positive feedback loop!)
You may wish to diagram this system as a positive feedback loop. Your diagram could include two boxes, indicating land degradation and plant cover, respectively, with the influences of each on the other indicated by positive or negative signs. See if you can make it work out as a positive feedback system. (I did this in lecture, but...).
Finally, the increasers also tend to be more fire prone (they dry earlier in season, for example) and so fire frequencies increase. Native vegetation can normally recover from fire, but under conditions of overgrazing, it can be so stressed that recovery from fire is more difficult, allowing the increasers to increase still further.
In the most extreme cases, overgrazing essentially results in a desert -- area basically devoid of plants. In some cases, effects of overgrazing are aggravated by drought -- like those experienced in Africa, China, India, and N. America in the late 1980's. Drought is an added stress on the plants, decreasing their ability to grow and accomplish valuable soil stabilization, protection, and building. In addition, when stressed by lack of water, the vegetation is less capable of coping with the animal-related stresses.
(Incidentally, sagebrush doesn't necessarily indicate overgrazing -- it can increase with grazing, but many areas historically had lots of sagebrush.)
To review notes on public rangelands in the western US as an example of management of "common" grazing lands, click >> at the bottom of this page; for general reminders on how to navigate within and among these pages, click "Navigate ."