Pygmy Rabbit Survey

 

Survey of Pygmy Rabbit (Brachylagus idahoensis) Occurrence on the Idaho National Laboratory Site

 

     The pygmy rabbit (Brachylagus idahoensis) is a sagebrush obligate, meaning that it is dependent on sagebrush for food and shelter.  In fact, nearly 100 percent of its diet in winter is composed of sage-brush.  Currently, the U.S. Fish and Wildlife Service is reviewing the status of the pygmy rabbit to determine if it needs to be listed as threatened or endangered under the Endangered Species Act of 1973.  As a result, there is increased interest among managers to document pygmy rabbit occurrence and determine its status across much of the west where it is expected to occur.  Unfortunately, little is known about this species, and techniques for monitoring populations and quantifying abundance are in their infancy (Price 2009). 

     Pygmy rabbits have been documented on the INL Site in the past (e.g., Wilde 1978), but the species’ current distribution is unknown.  Because the pygmy rabbit is considered a sensitive species in Idaho, and because there is a potential for pygmy rabbits to be listed as threatened or endangered in the near future, it is important to conduct pygmy rabbit surveys on the INL Site not only to verify the presence and population abundance, but to characterize critical habitat associated with active burrows.  

    

Objectives

The purpose of this research is to determine where pygmy rabbits occur on the INL Site so that when DOE-ID needs to construct new infrastructure or make improvements to existing structures, it can make informed decisions that will minimize impacts to pygmy rabbits.  Our specific objective was to conduct surveys on randomly selected 16-ha plots to determine if active burrows were present.  

 

Accomplishments through 2009

Since winter 2006, we have surveyed 551 16-ha plots that were selected based on a stratified random design (i.e., more random points were selected in “good” habitat than in poor habitat or in areas that have recently burned).  We documented 1,141 burrow systems since 2006, and found at least one active pygmy rabbit burrow system on 31 percent of surveyed plots (n = 170 plots, range = 1 – 38 burrow systems per plot; see Table 2, Figure 5).  During the winters of 2006 and 2007, our surveys were restricted to the southeast corner of the Site, and, on average, 52 plots were surveyed each season.  Fall 2007 marked the beginning of Site-wide sampling, and so data collected from that point on represent a more accurate description of pygmy distribution across the entire Site.  (No data were collected for this project during 2008 because our efforts were directed towards another pygmy rabbit project led by researchers at the University of Idaho - see Annual Report 2008).  In fall 2007, pygmy rabbits occurred on 37 percent of surveyed plots (n=244).  In contrast, in winter 2009 only 12 percent of plots had active burrows (n=178).  During fall 2009, plots with active burrows comprised 44 percent of sampled plots (n=25).  Because analysis of these data is pending, we cannot yet offer an explanation for why the number plots with recent pygmy rabbit activity is so much lower in winter 2009 than in other seasons.  It is possible the population experienced a sharp decrease in numbers in winter 2009 similar to what happens in other rabbit species.  Further analyses are required, however, before such statements can be verified.  

Pygmy rabbit plots surveyed for active burrows between 2006 and 2009. Each dot represents the southeast corner of a 16-ha square plot.


Seasonal data for pygmy rabbit plot surveys, 2006-2009.  Note that there are no data from 2008 because survey efforts were directed towards a different project (see Annual Report 2008).

 

 

 

Plots surveyed

 

Plots occupied ( percent)

Mean # burrow systems on active plots (range)

Winter 2006

48

24 (50 percent)

3.04 (1-14)

Winter 2007

56

23 (41 percent)

4.48 (1-30)

Fall 2007

244

90 (37 percent)

6.40 (1-38)

Winter 2009

178

22 (12 percent)

Unavailable

Fall 2009

25

11 (44 percent)

Unavailable

 

 

 

 

 

 

 

 

 

 

Plans for Continuation

  • Organize the pygmy rabbit data into a database so that it is ready for future analysis

  • Draft a CCA for pygmy rabbit between DOE-ID and USFWS.

  •  Develop a model that predicts pygmy rabbit occurrence across the INL Site based on an analysis of pygmy rabbit occupancy data coupled with the vegetation map that is currently being prepared by Stoller.  This will be an important outcome from the past 4 years of field research because it will provide Site planners with a useful tool for project and infrastructure planning.