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

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