Predation Effects
Relationships between deer populations and predator populations are complex yet poorly
understood. Each time mule deer populations have declined in Colorado and other western states,
predation has been implicated as a potential cause for the decline. Yet, given the current state of
knowledge about mule deer populations and predation, the only certainty is that predators kill and
eat mule deer (Fig. 11). The evidence concerning the effects of these predatory activities on the
performance of mule deer populations is much less clear, partly because there are few well
designed experiments to examine responses of mule deer populations to predator control.
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The relationship of a mule deer population to its food supply is a critical factor governing the
impacts of predation. When mule deer numbers are at or near the food production capacity of
their habitats, deer numbers are unlikely to increase when predators are removed. In contrast,
when severe winters or other natural calamities temporarily drop mule deer numbers well below
the food producing potential of their habitats, it is theoretically possible for predators to keep deer
numbers depressed for long periods of time. Under these circumstances, reduction in predator
numbers can result in a substantial increase in the size of the deer population. Evidence from past
studies, nonetheless, fail to demonstrate that deer herds increase when predators are removed. At
least part of the problem with earlier studies was that they often lacked control areas or were
conducted on small study areas where applicability of results to entire deer herds was questionable.
Predator Control Study in the Piceance Basin of Northwestern Colorado: 1981-1988:
Here in Colorado, we studied the mortality rates of mule deer fawns and adult does in the
Piceance Basin area west of Meeker, Colorado beginning in 1981. We found fewer than 40% of
fawns born each year survived the entire year while annual survival rates of does ($1 year of age)
continually exceeded 85%. Fawns and does differed markedly in their abilities to survive each
year, particularly during winter. The ability of fawns to survive from year to year had much
greater impacts upon the growth rate of the deer population than the survival rates of does.
Although both circumstantial and experimental evidence implicated food shortages as the major
cause of low fawn survival, coyotes were responsible for the deaths of many fawns which perished
during the winter months.
Subsequently, we studied the effects of coyote control on fawn survival during winter.
Mortality rates of fawns were documented for 4 years before coyote control was started and
compared to mortality rates for the following 3 years during which 218 coyotes were killed (1.3
coyotes/mi2/yr). If coyote predation was a major factor limiting deer, fawn survival was expected
to increase during the periods when coyotes were killed. If food shortage was a major factor, fawn
mortality rates were not expected to change during periods when coyotes were controlled. It was
anticipated that the mortality rates would not change but deaths from coyote predation would
decline and be replaced with deaths from other causes. Prior to coyote control (1981-82 through
1984-85), an average of 83% of the fawns died during winter. Coyote predation on deer fawns
varied from year to year, accounting for 49-77% of the total winter fawn mortality. Winter fawn
mortality during years when coyotes were controlled averaged 76% and was not significantly
different from mortality rates during years when coyotes were not controlled (Fig. 12). Decreases
in fawn deaths due to coyote predation were largely offset by increases in starvation rates.
Recent Studies of the Effects of Coyote Control on Mule Deer Populations:
Montana - Montana Fish, Wildlife, and Parks initiated studies in 1997 to assess the benefits of
coyote control to pronghorn and mule deer populations in central Montana. Coyote control was
conducted on one herd unit, and not on 2 adjacent units. Mule deer and pronghorn population
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Figure 12. Fawn mortality was studied in the Piceance Basin of northwestern Colorado before and after
coyote control. Mortality rates before coyote control were slightly higher than after coyotes were controlled,
but the difference was not statistically significant.
responses from the unit where coyotes were controlled were compared to those where coyotes
were not controlled. Coyotes were killed via aerial gunning from helicopters and fixed-wing
aircraft and snaring. During the first 2 years of study (1997 and 1998), nearly 200 coyotes were
killed, 68% from helicopter gunning, 21% from fixed-wing aircraft gunning, and 11% from
snaring. Aerial gunning required slightly more than 1 hour of aircraft time per coyote killed at a
cost of approximately $225 per hour. Although pronghorn populations increased on the unit
subjected to coyote control, they also increased in one area where coyotes were not controlled and
decreased in the other. Ratios of fawns per 100 mule deer does on areas where coyotes were
controlled did not differ from areas where coyotes were not controlled and deer populations
declined across all 3 areas. Researchers summarized the early results of the project by
commenting, “It would appear, that to this point, the killing of coyotes in HD 530 has had little
positive affect (sic) on mule deer fawn:doe ratios or populations.”
Utah - Beginning in 1997, the Utah Division of Wildlife Resources began implementing coyote
management plans to increase deer numbers in herd units where: 1) mule deer herds were 50% or
more below herd objectives, 2) where winter ratios of fawns per 100 does were 50:100 or lower,
and 3) where populations were not increasing. Fawn production and survival, as indexed by winter
ratios of fawns:100 does, seemed to increase in these units during 1997 and 1998 when coyotes
were being controlled compared to 1995 and 1996 when coyotes were not controlled. However,
when the Utah data were subjected to statistical analyses, ratios of fawns:100 does increased over
time regardless of whether or not coyotes were controlled (Fig. 13). An alternative explanation for
the observed increase in ratios of fawns per 100 does is favorable weather. The past 4 winters
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Figure 13. In Utah, 15 mule deer herds were selected for coyote control to increase fawn survival. Coyotes
were controlled in 1997 and 1998, but not in 1995 and 1996. Although the ratio of fawns per 100 does
increased over time, this increase could not be attributed to coyote control.
have been unusually mild in both Utah and Colorado and one would expect deer populations to
increase following mild winters.
Any inferences to the effectiveness of coyote control based on this study will be controversial
because the experimental design was incomplete. Ordinarily, areas where coyotes were controlled
would be paired with similar units without coyote control to adequately separate the effects of
controlling coyotes from possible effects of weather. In this study, if we were to assume the data
are perfect predictors of mule deer responses to coyote control, increasing ratios of fawns per 100
does from 43 (1995 levels) to 69 (1998 levels) would have required the removal of over 500
coyotes per year.
Idaho - Idaho Department of Fish and Game initiated a study in 1997 to assess the effects of
coyote control on mule deer populations. In their experiment, predators (primarily coyotes and
mountain lions) were controlled in several deer herd units. Deer responses in coyote control units
were compared to herd responses in similar units where no predator control occurred. Adult does
and fawns on all herd units were equipped with radio transmitters and used to compare mortality
rates of deer on coyote control units with those units where coyotes were not controlled.
After 2 years of study, mortality rates of adult does and fawns in areas where predators were
controlled were not different from areas where predators were not controlled. Coyote numbers in
spring were reduced significantly by coyote control, but by fall coyote numbers had rebounded to
levels comparable to herd units where coyotes were not controlled. Although coyotes were
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responsible for fewer fawn deaths in herd units where they were controlled, overall, fawn mortality
did not decrease because more fawns died from starvation even in mild winters. The studies are
ongoing, but preliminary results indicate that deer populations have not benefitted from predator
control.
New Mexico - Deer biologists from the Jicarilla Apache Reservation in northern New Mexico
reported that mule deer numbers increased from 1991 through 1999 (Fig. 14a) and attributed much
of that increase to aggressive predator control programs. However, most of the increase in deer
counted each year can be accounted for by the number of hours flown while counting them. For
example, in winter 1990-91, 2024 mule deer were counted during 50 hours of aerial counting
effort. In contrast, 6719 mule deer were counted during winter 1996-97 with 117 hours of aerial
counting effort. When number of deer counted is related to the number of hours flown per count,
nearly 70 additional deer are counted for every 1 hour that count time is increased, and counting
time is the only variable that is consistently correlated with temporal increases in deer numbers
(Fig. 14b). When the number of coyotes killed each year is related to the ratio of fawns per 100
does the following year, killing 800 coyotes per year is predicted to increase the fawn:doe ratio by
only a single fawn per 100 does (Fig. 14c). The results from the Jicarilla Apache Reservation are
further confounded because, in addition to coyote control, biologists improved deer habitats in
several areas and greatly increased law enforcement effort to reduce mule deer poaching losses.
Coyote control, habitat improvement, counting effort, increased law enforcement, climate effects,
etc. collectively influenced mule deer numbers and ratios of fawns per 100 does. There is no way
to unravel their combined effects to reliably assess the effects of any one of these factors acting
alone. Despite all of these efforts to improve deer populations, fawn:doe ratios on the Jicarilla
Apache Reservation (Fig. 14d) are s
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