THE NORTHERN RANGE 36 Data were gathered for two standard rangeland measures. One was net aboveground production. This is the total of all green annual production for the growing season, and includes an estimate of the production that is removed by grazing ungulates during the growing period. A second measure taken was peak standing crop biomass. This single measure includes the total vegetation biomass produced during the current season still standing at the peak of the growing season. Peak: standing crop biomass on summer range was not correlated with elk numbers. Biomass fluctuations apparently were related to climate and snowpack changes (Merrill and Boyce 1991, 1996). Peak standing crop biomass on lower elevation winter ranges was less on grazed sites in one study in 1986, but no difference was detected in 1987, 1989, or 1990 (Singer 1996a, Singer and Harter 1996). Grassland production was stimulated by elk and bison grazing on a wide variety of sites and elevations (Frank 1990, Singer 1996a, Wallace 1996), except that production was not stimulated on some sites during the severe summer drought of 1988 (Frank and McNaughton 1992, Wallace 1996). Frank and McNaughton studied the interactive ecology of plants, large mammals, and drought on the northern range (Frank 1990; Frank and McNaughton 1991, 1992, 1993, 1996a, 1996b). They concluded that the park's ungulates, because of their high mobility and ability to make all their own decisions regarding forage choice, track young vegetation as it grows across the <strong>Yellowstone</strong> landscape. Elk and bison consumption rates of yearly grass production was found to be high, about 45 percent (Frank 1990). Peak consumption was linked to peak periods of plant growth. Nutrient cycling, that is, the return of minerals necessary for plant growth to the soil, occurred at a high rate on heavily grazed sites. Ungulates excrete 90 percent of the phosphorus they ingest, and 65-95 percent of their ingested nitrogen. By consuming and excreting plant matter at this scale, grazers stimulated aboveground production of their preferred food plants. This is a milestone scientific finding in <strong>Yellowstone</strong>, but it has been substantiated in other large wildland ranges, such as the African Serengeti. The discovery that grazers • stimulated aboveground production of their preferred plant foods dramatically reverses traditional views by demonstrating that not only do wild ungulates not harm the plants, they facilitate and enhance plant growth: In <strong>Yellowstone</strong>, herbivores stimulated production at sites that were explicitly selected at the beginning of the study for their high herbivore use. Moreover, stimulationoccurred in 1988 when elk and bison populations were at their highest levels in recent history. The only exception was the summer range site, mentioned above, where the drought was the severest and, notably, grazers had no effect on production. Some .. . have argued that the increase of northern range elk since <strong>Yellowstone</strong> Park's implementation of the "natural regulation" policy in 1969 ... has led to grassland deterioration in the northern range. These data clearly refute this argument by demonstrating no evidence for ecosystem process degradation, and show that quite the contrary, even during a year of unusually high elk and bison numbers, grazers stimulated grassland production in the northern range (Frank 1990). It should be pointed out that some livestock managers are now discovering similar responses to grazing. When these managers move dense herds of their stock in an attempt more nearly to mimic the concentrations and seasonal movements of wild ungUlates, they find that some plant species respond with vigorous growth (Dagget 1995). Timing of ungulate use of the northern range was another critical element in understanding grazing there. Frank (1990) determined that use of plants on the northern range is timed to allow the plants to sustain heavy use year after year. The elk follow the "growth pulse" of greenup as it moves from winter ranges to higher elevation summer ranges. Unlike fenced, penned, or even herded livestock, elk do not remain on any given range for long, but move as environmental conditions dictate. This means that they seldom graze winter
~ ,., ~ ,~ 100,000 10,000 GRASSLANDS 37 loge ;::;; 5 j'jj'j 2.04 (log NFP) - 4.80 5 ",' 1,000 * *
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THE NORTHERN RANGE Figure 7.7. Envi
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THE NORTHERN RANGE 88 not aware of
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THE NORTHERN RANGE 90 tion size, an
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THE NORTHERN RANGE 92 activity (95
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THE NORTHERN RANGE 94 pursue the su
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THE NORTHERN RANGE 96 factors were
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THE NORTHERN RANGE Figure 7.13. Num
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, .. , , " , ! I ." ~, ~ " ,.\ ,•
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THE NORTHERN RANGE 102 complex set
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THE NORTHERN RANGE 104 practically
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THE NORTHERN RANGE 106 appear suppr
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AppendixA. Conferences, Meetings, a
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lJO Year Estimate Comment Source 19
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lJ2 , Estimated # of Elk Year Date
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lJ4 Winter Total Count 1967-68 397
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116 Year Date of Count Parkwide Par
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118 Counts and estimates of mule de
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120 Northern Yellowstone winter ran
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122 Population estimates and reduct
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124 Actual count Inside Outside Est
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Pronghorn counted during spring sur
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128 Year 1939 1940 1941 1942 1943 1
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Ground and aerial counts of bighorn
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REFERENCE LIST NOTES ON THIS REFERE
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Pages 199-210 in Carbyn, L.N., S.H.
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Yellowstone National Park. Technica
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mammal mortality in the 1988 Yellow
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National Park. International Journa
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Mehus, C.A. 1995. Influences ofbraw
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__ , and __ . 1996a. Preburn root b
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Pages 127-138 in F.J. Singer, ed. E
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some northern Yellowstone elk. M.S.
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I I I I I