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

ELK POPULATION ISSUES
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1996d), so, perhaps not surprisingly, "population
recluitment appeared to be strongly influenced by
precipitation" (Coughenour and Singer 1996d).
Evidence of food limitation was observed (Men-iII
and Boyce 1991). "Population growth rates
decline as elk numbers increase towards a fluctuating
upper limit set by total available forage."
Undernutrition at higher elk densities, densitydependent
population responses, increased overwinter
mortality of elk at higher densities, and
population modeling (Barmore 1980; Houston
1982; DelGiudice et al. 1991a, 1991b, 1994, 1996;
Dennis and Taper 1994; Coughenour and Singer
1996d; DelGiudice and Singer 1996; M. Taper,
Mont. State Univ., pel's. commun.) all indicate
natural regulation processes are working on the
northem Yellowstone elk herd. Fames (1996b)
showed that winter severity was the major factor
limiting elk survival.
There are other population-regulating
mechanisms acting on northern range elk. Elkcalf
winter survival was inversely related to elk
population size (Singer et al. 1997). Both
yearling and adult cow elk pregnancy rates
declined at higher densities (Houston 1982).
Yearling elk recruitment (that is, the number of
yearling elk added to the population each year)
declined at higher population densities (Houston
1982). Population growth rates were also strongly
density dependent (Men-ill and Boyce 1991;
Coughenour and Singer 1996a, d; M. Taper,
Montana State Univ., pers. commun.).
Following the fIres of 1988, concern was
expressed that forage growth enhancement due to
burning might cause a large increase in elk
numbers, but Pearson et al. (1995) concluded that,
though ungulates did preferentially feed in burned
areas, fIre effects on ungulates would be "relatively
Short-lived," and ultimately would be less important
to ungulate population dynamics than winter
conditions. Conversely, Turner et al. (1994)
theorized that the number of postflfe seedlings and
propagules varied considerably due to fire intensity
and other factors, and that a longer view of the
effects of fires on ungulates might be wan-anted.
Because the drought of 1988 and the harsh the
winter of 1988-1989 saw such a dramatic mortality
of elk, Turner et al. (1994) developed a simulation
model that Wu et al. (1996) then used "to explore
how elk mortality that winter might have been
different under alternative weather conditions,
spatial patterning of the burn, and initial elk
numbers." ConfIrming Farnes (1996b), as well as
what Men-ill and Boyce (1991,1996) projected in
their extensive modelling work in the 1980s, Wu et
al. found that winter weather was by far the most
impOltant factor in determining elk overwinter
survival:
Ironically, the single most important
determinant of elk winter survival
appears to be the weather, over which
managers have no control. Those
factors that can be controlled to some
degree-fire pattem and initial elk
numbers-are important, but their
effects interact with the effects of winter
weather conditions and may be completely
overshadowed by the influence
of snow depth and snow water equivalent
(Wu et al. 1996).
Others have disagreed with Wu et al. (1996)
of the importance of weather. Dennis and Taper
(1994) applied a statistical test that they call a
parametric bootstrap ratio (PBLR) test to the
nOl1hem elk population data from the end of the elk
reduction era (1969) through 1979, and concluded
that "the population appears to have subsequently
attained a stochastic equilibrium." Later and
additional work by Taper and Gogan (M. Taper,
Figure 6.4. Mountain
lions were eradicated
from Yellowstone
National Park by
about 1930, but have
since reestablished a
viable population
around the northern
range. NPS photo.