| Helping
a Remnant Bighorn Sheep Population Survive |
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 Background:
During the last 50 years, nearly all of the endemic bighorn sheep populations
in the interior of Arizona have been extirpated. The Silver Bell Mountains
near Tucson maintain the last endemic bighorn sheep population from
what was no doubt a population complex that included the Santa Rita,
Catalina, and Rincon mountains. As a result, it is important to ensure
that this unique population is maintained and to evaluate the potential
management options to sustain long-term viability in this remnant population.
Our research objectives are to: (1) evaluate habitat conditions in
the Silver Bell complex to assess potential for natural expansion or
supplemental transplants; (2) identify travel corridors between mountainous
habitats; (3) use frequent GPS-equipped collar locations to evaluate
adequacy of previous habitat evaluation models currently applied to
sheep management; (4) monitor a recent disease outbreak in this population,
treat infected individuals, and identify potential implications; and
(5) seek funding to examine genetic variability of this population
relative to other desert bighorn sheep populations and/or evaluate
survey techniques that provide statistically valid population estimates
to enhance monitoring of bighorn sheep populations in the future.
Location:
Currently occupied sheep habitat in the Silver
Bell Complex includes the Silver Bell Mountains,
West Silver Bell Mountains, Ragged Top Mountain,
and the Waterman Mountains, which are part
of a series of low desert mountain ranges in
south-central Arizona about 20 miles west of
Marana. This area supports about 75-100 bighorn
sheep.
Approach:
The recent advent of Global Positioning System
(GPS) technology incorporated into radiocollars
provides opportunities to evaluate detailed
habitat use patterns, where multiple locations
are collected throughout each 24-hour period
over several seasons. GPS locations will be
attempted every 5 hours to assess habitat use
and movement patterns from at least 18 bighorn
sheep of various sex and age classes. Habitat
use patterns from collared bighorn sheep will
be used to evaluate habitat conditions of adjacent
areas to assess potential for natural expansion
or supplemental transplants if warranted. In
addition, habitat use patterns of bighorn sheep
from fine-scaled analyses in the Silver Bell
Mountains will be used to evaluate previously
developed habitat use models applied at larger
scales.
Disease has been widely documented as a potential factor significantly
limiting bighorn populations. Recently, blindness has been observed in
several bighorn sheep in the Silver Bell Mountains. This condition has
been diagnosed as a bacterial caused form of pinkeye, which may have been
contracted from a domestic goat herd that escaped a nearby allotment. A
similar disease resulted in a 60% mortality rate in a Yellowstone bighorn
population during the early 1980s. It is unlikely the Silver Bell population
could withstand a similar mortality rate due to its small size and degree
of isolation. Identifying and treating infected sheep is underway, and
survival and reproduction will be monitored to assess potential implications
to the population.
Once populations become isolated at relatively low densities, extinction
risk increases due to reduced resiliency of that population to other factors
such as predation, disease, annual forage and moisture fluctuations, and
reduced genetic variability. Ability to cope with limiting factors is likely
reduced as populations become bottlenecked and individuals in the population
become more closely related. If adequate funding is obtained, genetic variability
of the Silver Bell bighorn population will be compared to other, more robust
populations in southern Arizona to determine if this population is becoming
genetically inhibited.
Ability to estimate the size of wildlife populations is a desirable and
often difficult aspect of wildlife management. Statistically valid population
estimates become increasingly important when managing isolated populations
at low density, typical of desert bighorn sheep populations. The Silver
Bell bighorn project will result in about 25 to 35% of the population being
radiocollared to accomplish other objectives and will provide an excellent
opportunity to evaluate survey techniques for estimating population size.
Funding will be solicited to examine multiple methods to estimate size
of bighorn sheep populations.
Benefits:
The primary benefit from this project will
be a set of recommendations that resource managers
can use to ensure continued existence of this
endemic bighorn population including: (1) identification
of important use areas such as movement corridors,
lambing sites, watering areas, and foraging
sites; (2) development of a detailed habitat
use model for evaluating habitat quality of
adjacent, unoccupied ranges and to evaluate
adequacy of previous habitat evaluation methods;
and (3) identify prevalence of a recent disease
outbreak and implement measures to reduce impacts.
In addition, funding will be solicited to evaluate
genetic variation of the Silver Bell bighorn
sheep population and to investigate survey
methods that provide statistically valid population
estimates for bighorn sheep management.
For more information
contact:
Richard Ockenfels, Arizona Game and Fish Department, 5000 W. Carefree Highway Phoenix, AZ 85086-5000
.
Phone: (623) 236-7221. E-mail: rockenfels@gf.state.az.us |
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