Published: January 2020
Author(s): Jon Gallie and Gerald Hayes
This document summarizes pygmy rabbit (Brachylagus idahoensis) reintroduction activities in central Washington since 2011, outlines fundamental principles guiding decision-making, and presents the Washington Department of Fish and Wildlife’s reintroduction strategy for the next several years. It is a consolidated update of the 2011 Reintroduction and Genetic Management Plan (Becker et al. 2011). A comprehensive outline of strategies and tasks needed to attain population viability of the Columbia Basin (CB) pygmy rabbit are provided in the Washington State Recovery Plan (WDFW 1995). A periodic status review of the CB pygmy rabbit and the factors affecting its status was recently completed (Hayes 2018). Based on the recommendation in that document, the Washington Fish and Wildlife Commission maintained the state endangered classification.
In 2011, the reintroduction strategy transitioned to breeding pygmy rabbits in on-site breeding enclosures in shrub-steppe habitat and augmenting captive-bred CB rabbits with wild rabbits translocated from beyond the Columbia Basin in order to provide adequate numbers of rabbits for releases and to improve genetic diversity. Sagebrush Flat (Douglas County) was identified as the highest priority site for reintroductions. Two permanent breeding enclosures were established on WDFW’s SBF Wildlife Area within the Sagebrush Flat Recovery Area. From 2011 to 2013, 112 adult pygmy rabbits were captured from out of state populations in Oregon, Nevada, Utah, and Wyoming and translocated to semi-wild breeding enclosures at SBF and mixed with captive-bred CB pygmy rabbits. A third and fourth permanent breeding enclosure were established in the Beezley Hills Recovery Area and Burton Draw Recovery Area. From 2011 to 2019, 2,246 kits were produced in semi-wild breeding enclosures and 1,518 rabbits were released at Sagebrush Flat. In the Beezley Hills Recovery Area, a release of 479 rabbits failed to establish a population in 2015 but reintroductions were later resumed in this recovery area with the release of 50 rabbits in 2017-2019. In the Burton Draw Recovery Area, 38 rabbits were released in 2018-2019.
Non-invasive genetic monitoring of fecal DNA was used to evaluate demographic and genetic status of reintroduced populations (DeMay et al. 2016, DeMay et al. 2017). For rabbits released from 2012 to 2014, apparent survival (winter detections) was 39%, 13% and 10%, respectively (DeMay et al. 2017). In the Sagebrush Flat Recovery Area, the percentage of wild-born rabbits increased from 9% in the early years of releases to between 88 and 100% in later years. Heterozygosity, a measure of genetic diversity, averaged 0.747 (range 0.701 – 0.789) from 2012-2019, remaining stable within this period. Currently, the genetic diversity of both the enclosure and wild populations in the Columbia Basin are similar to values observed in Great Basin populations. Average percent composition of Columbia Basin ancestry within individual rabbits declined from 70% in the fully captive pygmy rabbits (USFWS 2012) to 22-24% (range 20.8% to 28.7%) in the semi-wild population in 2019. The remaining genetic composition averages about 67% from the collective Great Basin population (NV/OR/ID), 10% from the WY/ UT (northern) population, and <1% from the UT population (southern). The transition to on-site, semi-permanent, large breeding enclosures to support reintroduction efforts was successful in producing adequate numbers of kits to support large-scale releases through 2016.
While the pygmy rabbit recovery effort has had many successes, including early signs indicating establishment of a wild population at SBF, the program experienced some setbacks beginning in 2015. In 2015, a disease outbreak of coccidia occurred in all four breeding enclosures and resulted in an associated decline in adult survival, kit production, and survival of released kits. Habitat conditions within breeding enclosures also declined. Both high rabbit densities and long durations of occupancy of rabbits in enclosures likely contributed to degraded habitat conditions by decreasing natural forage and increasing invasive weeds as a result of soil disturbance, soil compaction, and seeds introduced from supplemental food. Further, in 2017 the largest and most productive breeding enclosure at Beezley Hills, along with nearly half the semi-wild breeding population, was lost from a wildfire and resulted in suspension of additional releases that year. Over the next two years, overwinter mortality was high and productivity was low which resulted in low numbers of kits available for releases. To address degraded habitat conditions and disease prevalence in enclosures the project transitioned to a smaller, mobile breeding enclosure design that would allow relocating the enclosures to new locations in shrub-steppe every 2-3 years. On-going risk to the SBF population from wildfire prompted resuming reintroductions to establish new populations in other recovery areas.
Given that kit production in the permanent breeding enclosures was insufficient to support releases or sustain rabbit populations within the enclosures, the Pygmy Rabbit Science Team supported the capture of a small number (20-30) of rabbits (kits/adults) from the wild SBF population for translocation to mobile breeding enclosures and/or temporary release pens in BH and BD Recovery Areas. In the summer of 2018, 19 kits were captured from the wild SBF population and translocated to the single mobile breeding enclosure at BH and to temporary release pens at BH and BD Recovery Areas. Results from implementing the new mobile breeding enclosures were positive with increased adult survival and kit production, declining coccidia and parasite levels, and significantly reduced labor required for maintenance. However, record snowfall in February and snowmelt in late March of 2019 prevented access to field sites to construct additional mobile breeding enclosures at BH and BD Recovery Areas, or the capture of 20-30 adults from the wild SBF population for translocation to the two mobile breeding enclosures before the breeding season began in March. Attempts to capture kits from the wild SBF population in the summer of 2019 resulted in low trapping success. Two additional mobile breeding enclosures were completed in the fall of 2019 and constructed on-site at BH and BD Recovery Areas. Additional attempts to capture a small number of kits from the wild SBF population in the fall of 2019 for translocation to BH and BD Recovery Areas resulted in no rabbits captured. Findings from the summer and fall trapping in 2019 at the wild SBF population indicated that this population had experienced reduced survival or productivity and therefore was unsuitable as a source of rabbits for in-state translocation in 2020.
Reintroductions necessary to support establishment of new populations at BH and BD require obtaining pygmy rabbits from out-of-state sources in 2020. The Department is in communication with other state agencies on the availability of rabbits for translocation to central Washington in March of 2020. Pygmy rabbit populations in the Great Basin (California, Nevada, Oregon, Idaho, and Montana) are the highest priority region for genetic or demographic rescue to the CB population, followed by northern Utah/Wyoming region, and lastly the southern Utah region.