Overgrazing occurs when a grazing animal eats too much of the grass or other plant life in an ecosystem. This can cause severe damage to the ecosystem, including the disruption of local food webs, increased soil erosion, and the spread of noxious weeds.
The most obvious effect on local food webs is that the grazing animals will have to move on to new grazing grounds (which will in turn be at increased risk for overgrazing), meaning the loss of a food source for predators (which may include humans) and birds that feed on bugs attracted to the ruminant droppings. Grasslands also serve as a home to numerous small insects and mammals, and their predators.
Soil erosion is a major effect of severe overgrazing. In some cases the local plants are grazed to the point where the plants die, and without leaves to protect the soil from drying to dust and without the root systems in place to hold the soil, enormous amounts of soil can be blown away in the wind or washed away into rivers. This can, and does, turn grassland into desert, but also has negative effects on property and crops downwind, and can severely damage river ecosystems and municipal water supplies downstream. In some cases overgrazing can also remove plants that protect coastlines, causing loss of grass- and farmlands into the sea.
Once all of the food plants are eaten, all that are left are the poisonous plants; these now have an obvious advantage, and may take over meadow land. In addition, many pioneer species and invasive weeds are noxious. While this can be a serious problem for both farmers and local ecologies, it is generally a step up from the more serious problem of desertification, and farmland overtaken by noxious weeds is likely to recover more quickly than farmland blown away in the wind.
There are a number of solutions to overgrazing, the most obvious being to limit the amount of time grazers spend on any given patch of land. When dealing with wild populations, this may mean reopening pathways for migration; in domesticated species, this may mean fencing off fields and rotating the animals through them. In some cases, it may be necessary to thin herds, particularly if overgrazing is due to the elimination of natural predators such as wolves.
One of the primary early indicators of overgrazing is a dearth of the grazer's favorite food plants. The preferred species will vary from one area to another. In a traditional farm meadow for grazing cows, an absence of clover and other sweet forbs is a warning sign of overgrazing; for other species and other ecosystems, other plant species will be useful indicators. If you are able to see patches of dirt between plants, this is also generally a warning sign.
However, overgrazing may be occurring even in the absence of obvious warning signs. When grazed, grasses let their root systems die back. Once the grazers move on, first the leaves recover, and then the roots spread out again. If an area is grazed again every time the leaves come back then the root system is never given a chance to fully recover, and after a sufficient number of grazings apparently healthy plants will suddenly begin to fail as their nutrient stores give out.
If soil erosion is so severe that the grassland loses all or most of the soil's O horizon, it may take centuries for the ecosystem to recover even once full ground cover returns. Building up a healthy layer of nutrient-rich soil aerated by roots and earthworms and thick enough to absorb a healthy buffer of moisture takes decades at best.
Overgrazing, along with overfarming and deforestation, are causing serious large-scale damage to large parts of Asia, Africa, and South America. Currently Antarctica is the only continent that is not suffering from some form of large-scale ecological degradation from agriculture, including overgrazing.