and disturbed ground, particularly under wet conditions and thereby increase
birds’ access to soil invertebrates.
The effects of grazing vary depending on the vegetation already present, the
densities, and type of stock and the timing and frequency of its access. Grazing at
medium stocking levels will selectively remove only a moderate proportion of the
sward, and tends to create variation in the vegetation structure. Very high or very
low levels of grazing will in time remove, respectively, virtually all, or almost
none, of the vegetation, and thereby produce little variation in sward structure.
High densities of small mammals can survive under light cattle grazing regimes,
which preserve tussocks and maintain a dense litter layer. Grazing creates coarse
scale variation in sward structure and composition by accentuating existing vari-
ation in plant composition resulting from differences in topography and previ-
ous management. If such variation is not present, then grazing is unlikely to
create it. The effects of prescribed grazing levels on the sward vary from year to
year, primarily due to weather-related differences in vegetation growth. Thus
stocking levels often have to be adjusted by eye to achieve the desired conditions.
Nevertheless, it is still useful to measure the height and variation in structure of
the sward at key times of year, for example, the beginning of the breeding season,
340 |Habitat management
Fig. 14.3Grazing is important in certain habitats to prevent vegetation succession
and maintain suitable vegetation structure for birds. A simple but effective way to
monitor its effects is to erect grazing exclosures and compare the vegetation inside
and outside of them. Yellowstone National Park. (William J. Sutherland)