effect of fire, predation and vegetation quality
on mule deer habitat use and fawn survival
Western states biologists have documented 2 declines in mule deer numbers
since the 1960s. The reasons for decline are varied and often difficult
to pinpoint. Because mule deer are so important to sportsman and the
general public, the Arizona Game and Fish Department began to research
desert mule deer decline by constructing the Walnut Canyon Enclosure
at the Three Bar Wildlife Area in 1970. This predator proof enclosure
allowed us to closely examine the effects of predation on fawn survival.
In the initial study, we determined that fawn survival was approximately
30% greater in the absence of predators. In 1988 the gates to the enclosure
were opened to allow animals to move in and out of the area until 1997.
In early May 1996, the Lone Fire burned approximately half of the enclosure,
so we once again used the enclosure and examined mule deer habitat
use in predator-free, burned and unburned environments. In 2000, we
introduced coyotes for 6 months to see if their presence changed deer
habitat use. Even during our current drought, the predator free deer
population continued to increase, prompting us to continue investigating
the effects of different mule deer densities, vegetation quantity and
quality, predation, drought, and deer nutritional condition in and
outside of the enclosure have on mule deer fawn survival.
The 602-acre Walnut Canyon Enclosure (slightly
smaller than a square mile) on the Three Bar
Wildlife Area in the Tonto National Forest
south of Roosevelt Lake in central Arizona.
data are collected each spring, summer, autumn,
and winter, and deer density counts are taken
each fall in a deer drive. We measure vegetation
inside and outside the enclosure to look for
differences among seasons and years at different
deer densities (high inside, low outside).
The deer drive is conducted using 60-100 employees
and volunteers who form a line across the entire
enclosure and walk from one end to the other.
Each animal that passes through the line along
the way is counted once, providing accurate
information on the number deer and peccaries
in the enclosure each year. Two graduate students
from the University of Arizona completed the
fire ecology portion of the study in December
2002, and 2 new graduate students (ASU East
and Texas Tech Universities) will continue
to work with research branch biologists over
the next three years to look at habitat quality
and deer nutritional condition.
The use of a large enclosure is similar to having an outdoor laboratory,
and allows to control the predator component of population regulation.
The original study that found fawn survival was greater inside the enclosure
was during a six year wet period. However, since we closed the gates in
1997, Gila County, as well as much of the rest of the state, is in the
worst drought that has occurred in the last 700 to 1,000 years. We did
not expect the mule deer fawns within the enclosure to survive at as high
of rates as they did in the wet period, but they have. Leaving us with
the question, are predators more important than habitat in controlling
deer numbers? That is why we have switched our emphasis to the habitat
quality and deer nutritional condition. Certainly, there must be an interaction
between habitat quality, deer nutritional condition, predation and fawn
survival. Hopefully our large outdoor laboratory will help illuminate what
conditions desert mule deer fawns need for optimal survival.
This project was conducted under the direction of
Stan Cunningham, Arizona Game and Fish Department, 5000 W. Carefree Highway Phoenix, AZ 85086-5000