Ongoing Efforts to Preserve the Riparian Brush Rabbit
Restoring populations of California's most endangered mammal in the San Joaquin River Corridor
What are Riparian Brush Rabbits?
Like many riparian species in California, much of the historic range of the riparian brush rabbit (Sylvilagus bachmani riparius) has been lost due to the conversion of the San Joaquin floodplain to agriculture. Once a common feature in the San Joaquin River corridor, the species is now only found in small, fragmented populations prompting its placement onto the California Endangered Species Act in 1994, and the Federal Endangered Species Act in 2000.
A riparian brush rabbit sits beside a California quail (Callipepla californica) for size comparison. Note the small tail, gray-brown fur, and lack of black ear tips.
Riparian brush rabbits are one of thirteen subspecies of brush rabbit, and uniquely suited to life in the riparian corridor. They can be distinguished from desert cottontails (Sylvilagus audubonii) by their darker coloration, uniformly colored ears, small, inconspicuous tails, and their diminutive size. An adult riparian brush rabbit generally weighs between 500-800g, or 1-2lbs.
These rabbits are highly dependent on riparian forests for survival, less than 1% of which remains in California's Central Valley. The dense shrubby cover provided by species like California rose (Rosa californica), Baccharis, and California blackberry (Rubus ursinus) create a habitat in which these small mammals are ideally suited to tunnel through.
A variety of stakeholders, including private, state, and federal agencies dedicated to preserving the riparian brush make up the Riparian Brush Rabbit Working Group.
A "bunny's eye view" of riparian forest habitat. The dense black willow, gumplant, and mugwort stand to the left provides shelter and room for tunnels, seen on the right.
The establishment of a new riparian brush rabbit population through adaptive management
Looking to the Future
With an established population of riparian brush rabbits at Dos Rios Ranch, River Partners will continue to study and manage this population. The camera traps initially installed to assess the presence of the rabbits are now being monitored to better study this elusive species. Currently, the cameras are being used to monitor the age, type and density of vegetation the rabbits are found in most often. This information can be used in future restoration events to select vegetation designs suited for the establishment and survival of the species
Click below to see additional camera trap photos and learn more about the goals of the study.
As the working group has seen success in managing swift and increasingly common flooding events, a new challenge has emerged which threatens the rabbits survival.
Although there have been outbreaks in North America before, this year the first outbreaks of rabbit hemorrhagic disease (commonly known as RHDV2) began in Southwestern US. While the source of the outbreak is unknown, it has already affected several native California species like the desert cottontail and black-tailed jackrabbit (Lepus californicus) in southern California.
The current spread of RHDV2 in California, as of October 1, 2020. Map courtest of the USDA. To receive ongoing developments and updates, refer to their website.
The working group has already stepped up to take on the challenge of ensuring the survival of this species against habitat loss, flooding, and natural disaster, and its next step is to tackle the long term preservation of the species against this invasive zoonotic disease. As we learned from our previous work, these are tough little bunnies, and we're not afraid to try, learn, and try again.
A vaccinated riparian brush rabbit at SJRNWR. The metal ear tags indicate that the rabbit has been captured, vaccinated and released. Researchers can use these tags to assess population size in capture-recapture studies.
Current efforts are ongoing at the SJRNWR and Dos Rios Ranch to vaccinate riparian brush rabbits against RHDV2 ahead of its arrival in Stanislaus County. In accordance with their mission of conservation, the Oakland Zoo has stepped in to house twenty resident riparian brush rabbits as a safeguarded population against the disease. These animals will remain at the zoo as an emergency breeding colony, and they are actively pursuing funding to install the rabbits as a permanent exhibit to educate Californians about the plight and resilience of our smallest rabbit subspecies.
As we look towards the future, River Partners and the rest of the working group are working toward the long-term goal of the removal of this species from the state and federal Endangered Species Act. As part of the species recovery plan for the riparian brush rabbit, the establishment of new populations within the historic range is pivotal to delisting. In the coming years, we look forward to using our acquired knowledge of rabbit management, habitat suitability, and bunny berms to aid in the establishment of future riparian brush rabbit populations.
Thank you to all the funders, collaborators, and members of the Riparian Brush Rabbit Working Group, without whom this work would not have been possible.
