Periodic Status Review for the Pygmy Rabbit in Washington (2018)

Pygmy rabbits (Brachylagus idahoensis) occur mainly in the Great Basin and some of the adjacent intermountain areas of the western United States, including Washington, Oregon, California, Idaho, Montana, Wyoming, Colorado, Nevada, and Utah. Columbia Basin pygmy rabbits are geographically and genetically discrete from the remainder of the taxon and this population is significant due to the unique ecological setting (i.e., geologic, climate, soil, and vegetation community) in which it occurs. For these reasons, the Columbia Basin pygmy rabbit was designated as a Distinct Population Segment (DPS) for protection under the U.S. Endangered Species Act.

Pygmy rabbits are sagebrush obligates. Within their broad geographic range, pygmy rabbits have a patchy distribution and are found where sagebrush occurs in tall, dense clusters and soils are sufficiently deep and friable to allow for burrowing. Dense stands of sagebrush provide pygmy rabbits with year round food and shelter; native, perennial grasses and forbs provide an important food source beginning in spring and especially in summer and fall and deep, friable soils allow them to construct burrows for shelter and to give birth to their young.

Historic documented locations of pygmy rabbits indicate a past distribution that included portions of five counties but nothing is known about the extent of area they occupied in these locations. By 1997, pygmy rabbits were known to occur at only six isolated populations in pockets of suitable habitat in Douglas County (5 sites) and northern Grant county (1 site) and three of these sites had fewer than 30 active burrows. By March 2001, five of the six populations had disappeared and pygmy rabbits were known to occur only at Sagebrush Flat Wildlife Area. Captive breeding of purebred Columbia Basin pygmy rabbits began in 2002 from a small number of rabbits remaining in the wild, but from the outset reproductive output was very poor. Beginning in 2003, intercrossing purebred Columbia Basin pygmy rabbits with Idaho pygmy rabbits increased genetic diversity and improved reproduction of captive rabbits over the years (2003-2010), but mortality of young was high, especially pre-emergence, and disease accounted for over one-half of known mortalities of juveniles and adults. High mortality from disease and limited space in breeding facilities hampered efforts to increase the size of the off-site captive population. As a result, the off-site captive breeding program was de-emphasized in 2011 and transitioned to on-site semi-wild breeding in enclosures located in a more natural setting. From 2011 to 2013, wild adult pygmy rabbits were translocated from other states and added to the breeding enclosures to bolster population size and genetic diversity. Semi-wild breeding within enclosures on release sites and subsequent capture and release of suitable numbers of kits for release into the wild (2012-present) has been more successful.

Large-scale loss, degradation, and fragmentation of native shrub-steppe habitat likely played a primary role in the long-term decline of the Columbia Basin pygmy rabbit. By the mid-1900s, large portions of shrub-steppe habitat within the Columbia Basin were converted to agricultural crops or urban and rural developments. Remaining stands of sagebrush are affected by other, often interacting, factors including historical overgrazing by livestock, invasion by non-native plant species, and altered fire frequency. Predation is the main cause of pygmy rabbit mortality in the wild. Disease has been a significant threat to pygmy rabbits in the captive and semi-wild population.

The Columbia Basin pygmy rabbit population has not met population or secure habitat criteria for downlisting from its current state endangered classification. The population remains small and its distribution in the wild is extremely limited. It is therefore recommended that the pygmy rabbit remain a state endangered species in Washington.