Yellowstone's Northern Range - Greater Yellowstone Science ...

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THE NORTHERN RANGE 74 Montana State Univ., pers. commun.) on population data from 1969 through 1995 shows strong natural regulation dimensions, but minimize the notion that weather is a significant factor. IS NATURAL REGULATION A THREAT TO BIODIVERSITY? National Park Service policy contains a potential internal tension because it directs park managers both to protect native species and to protect ecological processes. But even without human influences, native species come and go from a landscape. Virtually all of the Yellowstone Plateau was covered by ice only 20,000 years ago (Good and Pierce 1996); all of that land, once relieved of its ice cover, had to be recolonized by the many species of plants and animals that now reside and are considered native there. The tension in park policy, then, involves what to do if ecological processes pose a threat to a native species; which part of the policy mandate is given preference? In some cases, were such a situation to develop, other laws would override policy. If. ecological processes threaten grizzly bears, for example, the Endangered Species Act would presumably require the National Park Service to find ways to protect and sustain the grizzly bear population. But in most cases, which would involve species that are probably abundant elsewhere and need no special local protection, it is more likely that the species would not receive special protection. No such case has arisen in Yellowstone, or to our knowledge in any other national park; native species are routinely threatened by the introduction or invasion of non-native species, or by other causes, but not by the ongoing ecological functioning of the ecosystem. Biological diversity, commonly referred to as biodiversity, has become a central concern of conservation biologists because human activities around the globe have drastically accelerated the rate of extinctions of species (Grumbine 1992, Soule 1996), and national parks have become recognized as reservoirs of native species diversity. Biodiversity has several levels of meaning in a landscape. At a basic level, it may involve the genetic diversity within an individual species (for example a rare predator whose genetic diversity is facing a "bottleneck" as its numbers decline). At another level, it may involve the diversity of species that inhabit a landscape, and is often referred to Alpha diversity. At a more complex level, it may involve the diversity of communities of species (Beta diversity), communities that occupy different parts of a landscape. As explained earlier, in the history of the northern range issue, the first concerns over species diversity involved the herbivores that some people believed were being pushed out by the elk. Recent studies have demonstrated that the history and real status of two of these species-beaver and whitetailed deer-were badly misunderstood (see Chapter Seven, under "White-tailed deer" and "Beaver" for more details). White-tailed deer were quite rare in the park except for a slight increase in numbers around the turn of the last century, and elk seem to have had a secondary effect on beaver numbers following the collapse of the beaver population because of a beaver-caused food shortage (Jonas 1955, Houston 1982). More important, elk are not threatening the more common northern range ungulates. No significant relationship has been found between elk and pronghorn numbers, and bison and mule deer numbers have increased along with elk numbers during the past three decades. Moose and bighorn sheep offer more complicated scenarios, but even if significant head-to-head competition for resources should be conclusively demonstrated in the future, neither species is believed to be in danger of disappearing from the northern range. Numerous investigators, cited earlier, have demonstrated that grazing and browsing of northern range plant species by ungulates does not decrease native plant species diversity. Singer (I996a) found that the number of grass, forb, and shrub species in grassland-steppe communities was the same in grazed and ungrazed plots. On the other hand, community diversity has experienced some reduction, as aspen and willow stands have declined. No communities have disappeared, but they are relatively less abundant than they were on
ELK POPULATION ISSUES 75 the northern range. Yet on the other hand, as mentioned earlier, species diversity in the understories of browsed willow stands has increased, probably because browsing opened the willow canopy and increased the amount of light reaching the ground layer. It is important to point out, however, that National Park Service policy mandates do not place a value judgment on this process. In other words, the policy does not judge management as somehow more successful because it increases species diversity. The primary goal is not the most possible species stockpiled in each park; the goal is the protection of the ecological processes that determine species diversity. In some areas, then, there is ample reason to consider the natural regulation policy as a benign force for or against species diversity. On the other hand, the intensive research of the past decade has posed important questions about other kinds of diversity. However, as northern range willows have declined, and as the aspen groves that started in the late 1800s grow old and fall, certain habitats are reduced. In the case of aspen, and eventually some other deciduous species such as cottonwoods, it is entirely possible that they will largely disappear from the northern winter range, affecting a variety of vertebrate and invertebrate species that use them for habitat. In recent years, it is these changes in the northern range that have caused the most alarms to be raised about biodiversity and natural regulation, so they merit more consideration here. Conversely, all of these water-loving woody plants could return to the nOlthern range with a change to a cooler, wetter climate. During the past century, willows declined on the n0l1hern range during periods of drought rather than during periods of exceptionally high elk numbers. Elk are, quite obviously, the immediate reason that winter range willows do not grow taller, because they browse them. But under a different environmental regime a century ago those same willows did grow tall in the presence of large numbers of wintering elk. It appears that the investigators who believe that much more is going on here than a mere "overpopulation" of elk are right, and it is to be hoped that continued research will clarify this situation, In the meantime, and as mentioned earlier, llOlthern range willows constitute only about half of one percent of park's willow communities, Their decline does not presage the disappearance of willow communities and their habitats from the park, because they are robust elsewhere in the park and in the greater Yellowstone ecosystem. If, as appears likely, the decline in northern range willow is the result of climatic changes, then it may be necessary to accept that decline as an aesthetically regrettable but ecologically inevitable part of the system's adaptation to a changing environment. But it is also possible that the return of a cooler, wetter climate to the northern range, if it persists, might result in the return of tall willows. Aspen, on the other hand, owe their present abundance on the llOlthelTI range to one demonstrably brief period of success in escaping browsing in the late 1800s. It may be necessary for all of us who enjoy aspen for their great beauty to admit that aspen are only an occasional occupant of the northern range, and that the park's first century was one period when we were fortunate enough to enjoy them here. It is to be hoped that further study will improve our understanding of aspen on the northern range, but in the meantime, they will not disappear entirely, either in or outside the park. National parks are enjoyed by people for a wide variety of subjective reasons. Just as there are people for whom the sight of large herds of elk are an impOltant part of the Yellowstone experience, there are other people who like aspen more than they like elk, and they will argue on that subjective basis that aspen should be protected and preserved because they are such a lovely, even spectacular part of the histOlic Yellowstone landscape. Eventually, National Park Service management may wish to address that question, perhaps by considering the establishment of discreetly protected "refugia". in which aspen can thrive without ungulate browsing. That esthetic issue should, however, be kept separate from the reality that under the current set of environmental
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YELLOW"STONE'S NORTHERN RANGE COMPL
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Suggested citation: Yellowstone Nat
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c=J Northern Winter Range _ Boundar
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Aspen, willows, and biodiversity ..
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vii
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THE NORTHERN RANGE xii range criter
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THE NORTHERN RANGE 2 Figure 1.1. Ph
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THE NORTHERN RANGE tion. There was
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THE NORTHERN RANGE 6 Figure 1.6. Gr
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THE NORTHERN RANGE 8 Figure 1.8. El
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THE NORTHERN RANGE 10 unpopular or
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THE NORTHERN RANGE 12 earlier Yello
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THE NORTHERN RANGE 16 Northern rang
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Figure 2.3. Sum ot rotal annual riv
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THE NORTHERN RANGE 20 Figure 2.4 Tr
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THE NORTHERN RANGE 22 dendrochronol
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Page 42 and 43: THE NORTHERN RANGE 26 et al. (1991)
Page 44 and 45: THE NORTHERN RANGE 28 earlier time,
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Page 48 and 49: THE NORTHERN RANGE 32 systems-range
Page 50 and 51: THE NORTHERN RANGE 34 indicates the
Page 52 and 53: THE NORTHERN RANGE 36 Data were gat
Page 54 and 55: THE NORTHERN RANGE 38 grazed and un
Page 56 and 57: THE NORTHERN RANGE showed that graz
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Page 60 and 61: Figure 4.1. Northern range, showing
Page 62 and 63: Figure 4.2. Willow distribution in
Page 64 and 65: THE NORTHERN RANGE 48 Figure 4.4. I
Page 66 and 67: THE NORTHERN RANGE 50 public lands,
Page 68 and 69: THE NORTIlERN RANGE 52 Figure 4.5.
Page 70 and 71: THE NORTHERN RANGE 54 able uncertai
Page 72 and 73: THE NORTHERN RANGE 56 to confirm an
Page 74: THE NORTHERN RANGE 58 RESEARCH RECO
Page 77 and 78: SOIL, EROSION, SEDIMENTS, AND WATER
Page 79 and 80: SOIL, EROSION, SEDIMENTS, AND WATER
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Page 83 and 84: ELK POPULATION ISSUES IS YELLOWSTON
Page 85 and 86: ange grazing issue, Yellowstone man
Page 87 and 88: ELK POPULATION ISSUES 71 ... Other
Page 89: ELK POPULATION ISSUES 73 1996d), so
Page 93 and 94: ELK POPULATION ISSUES 77 It has evo
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Page 98 and 99: THE NORTHERN RANGE 82 Competition t
Page 100 and 101: Figure 7.3. Bison winter range. Sin
Page 102 and 103: THE NORTHERN RANGE Figure 7.7. Envi
Page 104 and 105: THE NORTHERN RANGE 88 not aware of
Page 106 and 107: THE NORTHERN RANGE 90 tion size, an
Page 108 and 109: THE NORTHERN RANGE 92 activity (95
Page 110 and 111: THE NORTHERN RANGE 94 pursue the su
Page 112 and 113: THE NORTHERN RANGE 96 factors were
Page 114 and 115: THE NORTHERN RANGE Figure 7.13. Num
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Page 118 and 119: THE NORTHERN RANGE 102 complex set
Page 120 and 121: THE NORTHERN RANGE 104 practically
Page 122 and 123: THE NORTHERN RANGE 106 appear suppr
Page 124 and 125: AppendixA. Conferences, Meetings, a
Page 126 and 127: lJO Year Estimate Comment Source 19
Page 128 and 129: lJ2 , Estimated # of Elk Year Date
Page 130 and 131: lJ4 Winter Total Count 1967-68 397
Page 132 and 133: 116 Year Date of Count Parkwide Par
Page 134 and 135: 118 Counts and estimates of mule de
Page 136 and 137: 120 Northern Yellowstone winter ran
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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
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