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Development and Evaluation of a Monitoring Program for Pygmy Rabbits

Background

The recent petition for Endangered Species Act (ESA) listing for pygmy rabbits was, in part, based on a perceived decline in the species, however, data to evaluate this supposition are not available. Efforts during the past 2-3 years have documented numerous new occurrences of the species in Idaho, which have helped to fill out the statewide distribution of pygmy rabbits. However, it is not known if populations of pygmy rabbits fluctuate or cycle, as documented in other lagomorphs, and some observations suggest that populations may shift across a landscape over time. Therefore, an understanding of population trends over time requires information on changes in both abundance and distribution. This work addresses the first of these population criteria.

Monitoring burrow systems over the past six years in the Lemhi Valley has documented marked fluctuations in density of active burrows, which likely reflect fluctuations in population density of rabbits. Although burrow entrance counts are commonly used to estimate population abundance for semi-fossorial mammals, this relationship has not been evaluated for pygmy rabbits. Therefore this work will investigate the link between density of burrow systems and density of rabbits, and this information will be used to evaluate an index of rabbit abundance that could be employed by wildlife biologists to monitor changes in abundance of pygmy rabbit populations over time.

Objectives

The purpose of this research is to develop a standardized method to monitor abundance of pygmy rabbits and to gain an understanding of how pygmy rabbits affect their habitat. Specific objectives are to:

  • Calibrate an index of abundance based on burrow systems by correlating the index with estimates of population density;
  • Evaluate factors that affect the probability of detection during mark-resight exercises;
  • Design standardized protocols for monitoring abundance; and,
  • Evaluate the effect of pygmy rabbits on sagebrush shrubs around burrow systems

Accomplishments Through 2007

Project Design. Three sites were delineated for 2007 field work: two sites in the Lemhi Valley and one site at the INL Site. A census of all burrow systems and mark-resight exercises were completed at all three study sites. Census of burrow systems provide an evaluation of the density and activity status of rabbit burrows, and mark-resight exercises provide and estimate of abundance of rabbits. A second method for estimating abundance of rabbits based on observations of tracks at burrow systems immediately following snow fall will continue through the winter. Mark-resight and snow-track techniques will be used to evaluate and calibrate an index of abundance based on burrow systems.

Burrow Censuses. A complete census of burrow systems was conducted at two sites in the Lemhi Valley (Cedar Gulch and Rocky Canyon) and on one site on the INL Site (Atomic City). For each burrow system, global positioning system (GPS) locations and the number of burrow entrances were recorded, pellets were collected at a random selection of active burrow systems for species confirmation, and each system classified based on sign/activity as described by Roberts (2001).

As expected, systematic censuses in the Lemhi Valley and on the INL Site indicate a difference in the number of burrow systems in each activity class and the density of burrow systems on each of the 3 study areas. Cedar Gulch had the lowest total number of burrow systems (131) and the lowest density of burrow systems, while Rocky Canyon had the largest number of burrow systems (505) and the highest density burrow systems. Atomic City fell in the middle with 449 total burrow systems (Figure 9-6).

Trapping and Radio-collaring. Trapping was conducted from 4-14 days on the 3 study sites. At sites in the Lemhi Valley a visual search and chase technique was the sole method used to capture animals; however, due to low success of this technique on the INL Site other methods were employed. Additional techniques used were: drift fences, spotlighting, and placing traps in active locations during daylight hours. Captured animals were fitted with 4.2 g radio transmitters (Holohil Inc., Toronto), PIT tags were implanted, and standard mammalian measurements were collected (weight, hind foot, ear length).

Trapping in the Lemhi Valley was conducted for approximately 12 days between the two study areas. On Cedar Gulch, 13 animals were fitted with radio-collars (5 males, 8 females) and on Rocky Canyon 14 animals were collared (6 males, 8 females). The day after capture we located rabbits to visually check collar fit.

Trapping effort at Atomic City yielded only one animal captured. The visual technique used in the Lemhi Valley proved to be inefficient, and therefore other techniques were attempted. We set Tomahawk and Havahart traps at sites of active sign. Traps were set at sunrise and baited with apples, carrots or green beans, and checked at sunset. After several days without success, we constructed four drift fences (Burak 2006, Faulhaber et al. 2005) and placed them in areas where we had spotted rabbits previously. Traps were placed along the fences. Drift fences proved to be unsuccessful, but with some improvement might be a useful method in the future. The one rabbit
that was successfully trapped (male) was caught by the visual technique, and remained in the same complex of burrows it was captured.

Mark-resight Work. Upon completion of trapping events, mark-resight surveys commenced. Animals were resighted by using maps and GPS to navigate to all burrow systems categorized as “active” or “recently active” during the previous burrow censuses. Using this technique allowed us to maximize resight probabilities of all animals. In the Lemhi Valley resight occasions were rotated every other day between Rocky Canyon and Cedar Gulch, for five resight days on Rocky Canyon and six resight days on Cedar Gulch. On the INL Site, resight events were conducted every day for five resight occasions.

Mark-resight surveys in the Lemhi Valley yielded a total of 25 collared and 44 uncollared rabbit sightings on Cedar Gulch over 6 days, and 25 collared, 22 uncollared rabbits on Rocky Canyon over five days. On several occasions one or two rabbits were documented offsite at both Rocky Canyon and Cedar Gulch, and thus were not available for resight (Table 9-1).

Mark-resight surveys at Atomic City yielded 10 sightings of rabbits over five occasions. The collared rabbit was observed three out of five occasions and a total of seven uncollared rabbits were recorded (Table 9-1). This information represents a preliminary summary. The mark-resight data will be analyzed using Program MARK to estimate numbers of rabbits using each site. Observations at Atomic City suggest that more burrow systems are used per rabbit at Atomic City where sagebrush appears much more continuous than in the Lemhi Valley. An understanding of factors that influence the relationship between rabbits and burrows is essential for developing an index based on burrow systems. To this end, we will be conducting vegetation measurements at each site.

 

Plans for Continuation

In 2008, we plan to conduct snow- track surveys as weather permits on the three established study areas. We also plan to establish further study sites on the INL Site, in the Lemhi Valley, and possibly Camas Prairie. Burrow censuses, mark- resight, and snow-track surveys will continue to be used in order to develop an index of rabbit abundance based on burrow systems. Additionally, vegetation analysis will be conducted over the summer months at the Lemhi sites to gain an understanding of pygmy rabbit use and impact on vegetation around their burrow systems.


Investigators and Affiliations

Amanda J. Price, Masters Candidate, Department of Conservation of Natural Resources, University of Idaho, Moscow, Idaho

Janet Rachlow, Professor, Department of Conservation of Natural Resources, University of Idaho, Moscow, Idaho

Christopher Jenkins, Conservation Scientist, North American Program, Wildlife Conservation Society, Bozeman, Montana

Funding Sources

United States Department of Energy, Idaho Operations Office

Idaho Bureau of Land Management Challenge Cost Share Program (with Idaho Department of Fish and Game)

References

Burak, G. S. 2006. Home ranges, movements, and multi-scale habitat use of pygmy rabbits (Brachlagus idahoensis) in southwestern Idaho [Masters Thesis]. Boise, Idaho: Boise State University. 106 p.

Faulhaber, C. A., Silvy, N. J., Lopez, R. R., Porter, B. A., Frank, P. A., and M. J. Peterson. 2005. From the field: Use of drift fences to capture Lower Keys marsh rabbits. Wildlife Society Bulletin33(3): 1160-1163.

Roberts, H. B. 2001. Survey of Pygmy rabbit distribution, numbers and habitat use in Lemhi and Custer counties, Idaho. Boise, Idaho: Idaho Bureau of Land Management. Report nr 01-11.
 


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