Biology Reference
In-Depth Information
wk clipping sequences. Interestingly, although the leaves of plants from
the regularly grazed area were held more toward horizontal under the 8-
and 4-wk clipping regimes, those of plants from the ungrazed area were
held toward horizontal under the 2-wk clipping regime.Thus, plants from
the ungrazed area showed greater plasticity of growth in response to dif-
ferent intensities of grazing.
Nevertheless, both studies (Carman and Briske 1985; Smith 1998)
document adaptations and responses that show that tolerance is a major
mechanism of adaptation of perennial grasses to grazing. In these
responses, however, native North American species are probably not
greatly different from Eurasian species. Both of the above studies suggest
that perennial grasses tend to possess a range of genotypes that permit
many species to adjust to major ecological pressures that characterize the
grassland environment, such as grazing, fire, and drought.
Adaptability of perennial grasses to impacts of ungulate grazers is espe-
cially evident in the North American Great Plains, where bison (
Bison
bison
) and other ungulates were abundant in pre-European time. Evolu-
tionary adaptability in growth form and silicon content of foliage is also
shown by various perennial grasses of the Great Plains in response to graz-
ing by prairie dogs (
Cynomys
spp.) (see, e.g., Brizuela et al. 1986; Detling
and Painter 1983; Painter et al. 1989; Polley and Detling 1990). In the
intermontane region farther west, as well as in the central valley of Cali-
fornia, dominant perennials before European settlement were cespitose
grasses that were vulnerable not only to grazing but to damage by tram-
pling (Mack and Thompson 1982). Grasslands dominated by cespitose
species also occurred in several other world regions, such as New
Zealand. In these areas, the introduction of domestic ungulates in large
numbers has resulted not in selection for growth forms tolerant of ungu-
late impacts but instead the extensive replacement of perennial bunch-
grasses by alien annual species.
A number of broad-leafed herbaceous plants also show evolutionary
adaptation to grazing by livestock. One of the most adaptable species is
the ribwort plantain (
Plantago lanceolata
), a species native to Eurasia but
now cosmopolitan in distribution. This species exhibits a basal rosette of
leaves and a number of naked flowering stalks. In the Netherlands, popu-
lations of ribwort plantain in pasture habitats tend to possess shorter
leaves, smaller and more numerous flowering stalks, and more vegetative
daughter rosettes than plants in ungrazed habitats (Van der Toorn and Van
Tienderen 1992). A number of other weedy broad-leafed herbs, such as
shepherd's purse (
Capsella bursa-pastoris
), also show similar ecotypic
