Yellowstone's Northern Range - Greater Yellowstone Science ...
Yellowstone's Northern Range - Greater Yellowstone Science ...
Yellowstone's Northern Range - Greater Yellowstone Science ...
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THE NORTHERN RANGE<br />
34<br />
indicates the number of grass, forb, and shrub<br />
species on the northern range was the same in<br />
grazed and ungrazed sampling plots. Shannon's<br />
index to plant species diversity was very similar;<br />
2.38 in ungrazed grasslands compared to 2.30 in<br />
grazed grasslands. Community or Beta diversity<br />
on the northern range, however, may have experienced<br />
a slow decline in the past century due to the<br />
decline in aspen and willow stands, but this is<br />
somewhat unclear due to the greater Alpha<br />
diversity outside the exclosures versus inside the<br />
excIosures (Singer 1996a; Coughenour and Singer,<br />
Colo. State Univ., and U.S. Geol. Surv., unpubl.<br />
data.)<br />
Wallace (1991) monitored community<br />
structure of five northern range sites, on summer,<br />
winter, and transitional ranges, collecting data both<br />
inside and outside of exclosures. She found "no<br />
significant correlations between changes in<br />
diversity and grazing intensities," and concluded<br />
that "climate has a stronger control on system<br />
structure than does grazing."<br />
However, one study of mid-elevation willow<br />
communities and nearby grasslands reported<br />
significantly more plant species outside the<br />
exclosure than inside (Chadde and Kay 1988).<br />
According to this study, there were a total of l7<br />
species inside the exc1osure: 1 tree, 5 shrubs, 7<br />
forbs, and 4 graminoids. There were 35 species<br />
outside the excIosure: 2 trees, 6 shrubs, 16 forbs,<br />
and 11 graminoids. If only native species are<br />
considered, there were 15 native species in the<br />
ungrazed inside and 31 in the grazed outside. This<br />
does not suggest that ungulate grazing decreases<br />
species diversity, and seems to suggest that grazing<br />
allows for more diversity.<br />
In mid-elevation grasslands, climatic<br />
variation from year to year caused greater variation<br />
in plant species composition than did grazing<br />
(Wallace and Macko 1993). In one study, bison<br />
and elk grazing were found to be complementary,<br />
the two grazers selecting different plants and plant<br />
parts (Wallace 1996).<br />
Three investigations focused on determining<br />
plant species changes after 30 years of protection<br />
from grazing in exc10sures (Coughenour et al.<br />
1994, 1996; Reardon 1996; Singer 1996a). Minor<br />
changes in species abundance (based on biomass<br />
measures) due to grazing were found in the first<br />
study (Singer 1996a). Two grasses resistant to<br />
grazing, Junegrass and thick-spiked wheatgrass,<br />
were more abundant on grazed sites. There was a<br />
slight tendency for Sandberg's bluegrass and<br />
Hood's phlox to be less abundant on grazed sites,<br />
even though Hood's phlox is not known to be a<br />
grazed plant. No effects on 127 other plant species<br />
were found. A second investigation that monitored<br />
plant basal areas found no effect upon any species<br />
that could be attributed to grazing (plant basal area<br />
is the area occupied by the root crown of the bunch<br />
grasses; it is regarded as a more useful long-term<br />
indicator of the plant's success than is the area<br />
covered by the canopy formed by the plant's<br />
leaves, because the canopy is more dramatically<br />
affected by yearly variations in moisture) (Reardon<br />
1996).<br />
A third study based on plant frequencies<br />
showed that total plant frequency, Idaho fescue<br />
frequency, and Junegrass frequency increased on<br />
grazed areas over the 1958-1981 period, even<br />
while elk numbers increased fourfold (Coughenour<br />
et al. 1994). As mentioned above, in the 1960s, elk<br />
numbers were artificially reduced to less than<br />
4,000 on the northern range. Bison numbers were<br />
reduced to 400 parkwide at the same time. Since<br />
then, the northern range herds of elk and bison<br />
have increased substantially, to as many as an<br />
estimated 22,000 elk and 900 bison, their numbers<br />
varying depending upon a variety of environmental<br />
conditions. Coughenour et aJ. (1994) found that<br />
"dominant perennial grasses either maintained their<br />
relative abundance or increased. Forbs (broadleaf,<br />
herbaceous plants that are neither grasses nor<br />
shrubs) decreased in relative abundance, and<br />
increased after 1986 both in and out of exclosures,<br />
in response to drought. Total plant cover decreased<br />
after 1981 due to climatic conditions, as shown by<br />
parallel declines both inside and outside<br />
excIosures." The authors concluded, "On the basis<br />
of these trends, we conclude that elk grazing has<br />
not degraded the <strong>Yellowstone</strong> northern winter<br />
range."