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Sierra Nature Notes, Volume 8, May 2008
Winter
Carnivore Survey Finds that Wolverines (Gulo gulo)are Likely
Extirpated from Sequoia-Kings Canyon National Parks
Brian R. Hudgens
David K. Garcelon
Institute for Wildlife Studies
PO Box 1104
Arcata, CA 95518
hudgens@iws.org
Editor's Note: This article is an excerpt from the original. The full paper can be found here in PDF. Also, this was written before a bait station in the Lake Tahoe area photographed a visit from a wolverine (follow-up article here). DNA tests of that wolverine's scat showed it to be genetically related to populations found throughout the Rocky Mountains, Alaska and Canada and not from the last known samples of wolverine populations endemic to California. The conclusions of this paper would not likely be changed by these recent sightings.
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| Wolverine
at Lake Tahoe, CA bait station: winter, 2008. (Credit: Katie Moriarty) |
One wolverine in Wyoming fitted with a tracking collar covered some 543 miles over 42 days before its collar fell off. |
Abstract
Wolverines
have disappeared from almost half of their range in North America. In
California, no verified wolverine sightings have been reported in the last
50 years, although unconfirmed sightings in the southern Sierra Nevada, especially
in Sequoia-Kings Canyon National Parks, provide doubt about the species’ status
in the southernmost part of its range. We determined that four animals
persisting in the parks corresponded to the minimum viable density necessary
to have a persistent population since the last physical evidence of wolverines
was recorded 25 years ago. To determine if a wolverine population persists
in Sequoia-Kings Canyon National Parks, we used baited camera traps from
January through May at high elevation sites throughout the parks. Our
cameras took 602 pictures of animals over 1482 effective trap days. No
wolverines were detected during our survey, although we did record other
mesocarnivore species at 17 of the 18 bait stations. We conducted power
analyses both to determine the effort required to have a high probability
of detecting wolverines before setting up the survey and to interpret the
absence of wolverine detections after the survey. Power analyses were
based on trap efficiency estimates from studies in extant wolverine populations
and movement simulations for a hypothetical wolverine population in our study
area. We estimated that
our survey had an 85% to >98% chance of detecting wolverines if as few
as four animals persist in the park, which population models suggest would
correspond to the minimum viable density. We conclude that it is highly
unlikely that a viable population of wolverines persists in the southern
Sierra Nevada, and that reintroduction would be the most appropriate conservation
strategy for the species in this part of its range.
Introduction
The wolverine (Gulo gulo) is the largest terrestrial mustelid and
one of the rarest mammals in North America. Prior to European settlement
of North America, the geographic range of the wolverine extended from the
north slope of Alaska and Canada through the montane ecoregions of southern
California, Arizona and New Mexico (Hash 1987). Wolverines have disappeared
from almost half of their former range (Paquet and Hackman 1995). At
the southern limit of their historic range, the distribution is limited to
montane regions and distinct gaps occur between subpopulations (Wilson 1982).
Wolverines
historically occurred in the remote and high altitude areas of California,
ranging from the northwestern part of the state to the southern Sierra Nevada
(Grinnell et al. 1937, Schempf and White 1977). Wolverines were probably
never numerous in California, due to their extensive home range size and
the relatively small amount of suitable habitat in the state. Pressure
from fur trappers may have further reduced wolverines to as few as 15 pairs
by the 1930s (Grinnell et al. 1937).
There has
been no specimen or photograph collected of a California wolverine for over
50 years, although there have been several unconfirmed sightings and reports
of wolverines. It was believed that California’s wolverine population
may have increased during the 1960s and 1970s (Yocum 1973, Schempf and White
1977). However, this conclusion is based primarily on increased numbers of
unconfirmed, and therefore potentially unreliable, sightings over this period. It
is unclear whether greater numbers of sightings actually reflect a rebound
of the California wolverine population during this time or simply reflect
an increase in the numbers of people accessing the backcountry for recreation. Tracks
photographed by Andrews (1980) during winter surveys in 1979 and 1980 represent
the most recent physical evidence for the species persisting in California. Reported
sightings of wolverines have dropped off sharply from the 1980s to the present.
The current
status of the California wolverine is unknown. The California Department
of Fish and Game lists the wolverine as present but threatened in the state. Reports
of sightings in California have led some state and federal biologists to
conclude that the California wolverine continues to persist (Graber 2006). On
the other hand, the lack of physical or photographic evidence for the presence
of wolverines has led other biologists to conclude that the species has been
extirpated from California (Aubry and McKelvy 2005, Zielinski et al. 2005,
T. Kucera pers. comm.). Several recent forest carnivore surveys conducted
in California have failed to document any evidence of wolverines (Kucera & Barrett
1993, Zielinski et al. 2005, Green 2006). However, these surveys have
been conducted during summer months when wolverines are less likely to be
attracted to bait stations (Zielinski and Kucera 1995). The lack of
physical evidence of wolverines from California is widely perceived as evidence
that the species is extinct in the state (Zielinski et al. 2005, T. Kucera
pers. comm.), although unsubstantiated wolverine sightings, mostly from Sequoia-Kings
Canyon National Parks, conflict with this conclusion (Graber 1996, CDFG 2002). The
concentration of unsubstantiated wolverine reports from Sequoia-Kings Canyon
National Parks corresponds to the historic belief that the southern Sierra
Nevada represents the last stronghold of wolverines in California (Grinnell
et al. 1937, Zielinski et al. 2005). We believe that a winter survey
is much more likely to detect wolverines than summer surveys, and that a
concentrated effort in Sequoia-Kings Canyon National Parks would have a high
probability of detecting wolverines if a population persists in the parks. In
this paper we present the results of a wolverine survey conducted during
winter 2006 in Sequoia-Kings Canyon National Parks designed to detect animals
that may persist at a very low density.
The most
difficult challenge of presence-absence surveys is interpreting negative
results. An
absence of wolverine detections could result from a real absence of wolverines
in the study area, a failure of the survey mechanism to record the presence
of wolverines, or insufficient power to detect wolverines that are present
in the survey area. Our survey method of a baited camera station has
been successfully used to detect wolverines in areas where they are known
to persist (Copeland and Kucera 1997, Fisher et al. 2004, 2005, LeFroth et
al. 2005) and other rare mesocarnivores in the Sierra Nevada (Zielinski et
al. 2005, Green 2006). Nonetheless, we will look at whether our stations
were successful at attracting and recording the presence of other mesocarnivores
as evidence of their effectiveness.
A more difficult challenge is determining the power of our survey to detect
wolverines, as they would occur at much lower densities than other mesocarnivores
if they do persist in the park. The key to this analysis is information
about the probability of detecting the species of interest (Pollock et al.
2004). Most statistical tools available to analyze survey power rely
on applying detection probabilities estimated from surveys conducted in areas
of known occupancy (e.g., MacKenzie et al. 2004, Peterson and Bayley 2004). In
the absence of reliable information from surveys in known occupied habitats,
detection probability can be estimated from simulation (Choquenot 2001 et
al., Conn et al. 2004).
Detection probability may depend on a number of factors that vary between
sites or through time. The most commonly considered variable is population
density (e.g., MacKenzie et al. 2004, Peterson and Bayley 2004). Survey
power may also be heavily influenced by the spatial distribution of the target
species relative to survey effort (Choquenot et al. 2001, Pollock et al.
2004) and the attraction of survey stations within an animals home range
(Choquenot et al. 2001). In our analyses we use a combination of previously
reported detection probabilities and simulations to conduct a thorough power
analysis incorporating the influence of wolverine density, spatial distribution
and effective trap area.
Methods
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| Figure 1. Map of baited survey sites with overlaying 20 km X 20 km grid. Labeled black dots indicate locations of baited survey stations, red stars indicate locations of unverified wolverine sightings reported to Sequoia-Kings Canyon National Parks (courtesy NPS) since 1980 (two northernmost stars indicate tracks reported by Andrews 1980). (Click photo to enlarge). |
Study species
Wolverines live in remote areas receiving
large amounts of winter snowfall (Grinnell et al. 1937, Aubry and McKelvey
2005). They consume a wide variety of foods, including berries, mushrooms,
amphibians, small mammals and carrion (Fry 1923, Grinnell et al. 1937). Small
mammals and carrion comprise the most important parts of their winter diets
(Hornocker and Hash 1981, Gustavsen et al. 2005, van Dijk et al. 2005). Adult
wolverines maintain large home ranges (104 km2-526 km2 for females, 382
km2-1522 km2 for males; Magoun 1985, Banci 1987, Copeland 1996, Dawson
et al. 2005). Wolverines typically have little or no home range overlap
with other adults of the same sex, although male home ranges typically
overlap with 2-4 female home ranges (Magoun 1985, Copeland 1996). Females
begin breeding after age 2-3, producing litters of about 2 kits every 1-3
years (Persson 2003). Adult females with young reduce their winter
home ranges, concentrating their activity around one or more den sites
(Banci 1987). Wolverines dig dens into the snow, taking advantage
of rock piles and fallen logs in open habitats (Pulliainien 1968, Raush
and Pearson 1972, Magoun 1985). Young are weaned at 9-10 weeks and
begin to travel with their mother by late May. Vangen et al. (2001)
provide the best information on juvenile dispersal. Young males
disperse from their mother’s territory at approximately one year
of age, but remain in their father’s territory until they approach
two years old. Young females typically disperse from their mother’s
territory after their second year (Persson 2003).
Study site
We conducted this survey in Sequoia-Kings Canyon National Parks, California,
USA. These parks represent the southern most extent of the wolverine’s
historic range in the Sierra Nevada. The parks are characterized by
rugged mountains with 3076 km2 of habitat above 2100 m and 1933 km2 above
3000 m in the park boundaries, including Mt. Whitney, the tallest peak in
the Continental United States at 4417 m. High elevation habitats in
Sequoia-Kings Canyon National Parks include upper montane habitats dominated
by lodgepole pine (Pinus contorta) and red fir (Abies magnifica),
sub-alpine habitats dominated by whitebark pine (Pinus albicaulis),
foxtail pine (Pinus balfouriana) and mountain hemlock(Tsuga
mertensiana), and alpine habitat which is predominantly covered by
tundra or barren rock. Winters at high elevations are characterized by heavy
snowfall. During the winter of 2005-2006, snow survey stations within
the park recorded maximum snow depths of 300 cm at 3250 m elevation. The
greatest snowfall typically occurs in March, and heavy snowstorms are not
uncommon in April. In 2006, snow accumulation was greater than snowmelt
through May.
Results
Camera performance
Overall, the PIR triggered cameras worked
very well under a wide range of conditions. We collected 2939 pictures
during our survey in addition to pictures taken while surveyors set up,
maintained and took down the stations. Pictures were taken during
all hours of the day throughout the entire survey period. Most images
were “empty”, probably triggered by rapid increases in ambient
temperature. Animals were visible in 602 pictures.
Survey
results
No wolverines were recorded visiting any bait station. We
did identify the nine species of mammals and three species of birds visiting
survey stations. Martens (Martes americana) were the most common
visitor, recorded in over 400 pictures from fourteen stations. Other
mesocarnivore species recorded at survey stations included coyote (Canis
latrans), fisher (Martes pennanti) , and black bear (Ursus americanus). In
addition, one photograph was taken of an animal just outside the range of the
camera flash that we believe to be a bobcat (Lynx rufus), although
we cannot rule out the possibility that it was a coyote.
Sciurids
were also common visitors to survey stations. Twenty-three pictures from
three stations were taken of northern flying squirrel (Glaucomys sabrinus). Ten
pictures of Douglas squirrel (Tamiasciurus douglasii) were taken from
two stations and one picture of a golden-mantle ground squirrel (Spermophilus
lateralis). Other mammals photographed included deer mouse (Peromyscus
maniculatus) and white-tailed hare (Lepus townsendii)
We also
recorded Clark’s nutcracker (Nucifraga columbiana) and dark-eyed
junco (Junco hyemalis) at survey stations. One camera (MK) knocked
down by black bear recorded pictures of a green-tailed towhee (Pipilo chlorurus)
and a white crowned sparrow (Zonotrichia leucophrys) bird species
at the base of the tree between April 29 and May 10.
Preliminary Power analysis
Extinction probability was highly
sensitive to both female reproductive rates and initial starting size. Populations beginning with only 5 animals had a 50%->99% chance
of extinction and are clearly not viable. Populations starting with 10
animals had a 30%-80% chance of extinction, while populations starting with
25 animals had a 0-50% chance of extinction. These results suggest that
a minimal viable population is between 10 and 25 animals. We conservatively
chose 10 interacting animals as the minimum viable population. Given
a mean dispersal distance of 51 km (Vangen et al. 2001) wolverine density in
the Sierra Nevada would have to be at least 1/817 km2 (hereafter “target
density”) for 10 animals to be within the dispersal range of each other. This
density corresponds to 4-5 animals living within the parks’ boundaries,
with other animals living in the wilderness areas north of the parks.
Based on the average trap efficiency from baited camera surveys in populations
with known wolverine densities, we calculated that we would need an effort
of at least 960 effective trap days. This effort corresponds to 16
stations set for two months.
Final Power Analyse
Our survey had a greater than 95% chance
of detecting wolverines if they persist in the park at densities greater than
1 animal/1300 square kilometers.
Discussion
The key results of this study are that our survey was able to detect and
record the presence of several species of mesocarnivores and that we did
not detect the presence of wolverine. Although the power of this survey
to detect wolverines depends on both wolverine density and the attraction
radius of our baited survey stations, we had a low chance of missing wolverines
under a wide range of conditions. The power of our survey under a variety
of detection criteria, in conjunction with negative results from other surveys,
lead us to believe that there is not a viable populations of wolverines within
Sequoia-Kings Canyon National Parks, and that the species has likely been
effectively extirpated from the southern Sierra Nevada.
The conclusion from this study that no viable population of wolverines persists
in Sequoia-Kings Canyon National Parks is bolstered by negative findings
from other recent carnivore surveys in the southern Sierra Nevada. Although
the probability of wolverine attraction to baited survey stations is likely
to be much lower in summer months, intensive mesocarnivore surveys by Zielinski
et al. (2005) and Green (2006) had over 2000 trap-days of effort within the
historic wolverine range with no detections. In addition, Kucera and
Barrett failed to detect wolverines from 12 baited camera stations spread
throughout the Sierra Nevada (T. Kucera unpublished report). A
California Department of Fish and Game wolverine survey conducted in winter
2006 to the north and east of our survey, including one site in Kings Canyon
National Park, also failed to detect any wolverines (C. Cotter, pers. comm.).
As with the studies mentioned above, we did record evidence of other mesocarnivore
species in the southern Sierra Nevada. Our records include the highest
known occurrence of coyotes in California. While removing camera stations
we observed a coyote traveling across a 3500 m pass out of Dusy Basin. The
abundance of mesocarnivores detected at high elevations in this study indicates
that prey populations are also available at high elevations during winter
months. This conclusion is bolstered by records of white-tailed hare,
Douglas squirrels and flying squirrels at multiple stations.
This survey, in conjunction with other recent survey efforts, presents compelling
evidence that wolverines have been effectively extirpated from the southern
Sierra Nevada. We therefore believe the most appropriate management
action for wolverines in California would be to re-introduce the species
to the Sierra Nevada. A successful reintroduction would depend on
sufficient, appropriate habitat being available for a population to persist. While
a full evaluation of the potential for a successful reintroduction into the
Sierra Nevada is beyond the scope of this paper, we note that the region
includes an extensive, contiguous block of high-elevation protected habitat
that supports a rich, if currently incomplete, native mesocarnivore community.
Acknowledgements
We would
like to thank several people for their invaluable assistance in completing
this project. First, we thank Sequoia-Kings Canyon National Parks staff
for their logistical support and cooperation, particularly D. Graber, H. Werner
and R. Mazur. C. Rall headed IWS monitoring efforts, with help from C.
Miles and B. Czibesz, J. Nelson and K. Thompson. J. King coordinated
California Snow Survey monitoring efforts related to this project and we are
grateful to all the snow surveyors who checked stations for us. Substantial
logistical support was also provided by G. Schmidt, R. Rall and D. Johnson.
For Further Reading
Search
for Rare Furbearers Leads CSERC Staff into Remote Corners of the Forest: Andy
Hatch, Biologist, Central Sierra Environmental Resource Center
John Buckley, Director, CSERC
Maps of wolverine sightings in Yosemite and Sequoia and Kings Canyon National Parks. Records were submitted by visiting public, park staff and a literature search of park reports. Without photos, study skins or other definitive evidence, they cannot be otherwise confirmed. Each sighting is labeled with year of observation.
Winter
Carnivore Survey Finds that Wolverines (Gulo gulo)are Likely
Extirpated from Sequoia-Kings Canyon National Parks
(Full Report, PDF)
Brian R. Hudgens
David K. Garcelon
Institute for Wildlife
Studies
PO Box 1104
Arcata, CA 95518
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