Results Are In: Endangered Rabbits are Living in our Planted Habitats!
By Julie Rentner, Restoration Ecologist
With a captive breeding program producing hundreds of healthy baby bunnies (more than 1,000 have been released to date), the ESRP needed additional suitable habitat in the region to release the rabbits. Much of the dense shrub thicket habitat that the rabbits require had been lost to clearing for farming or grazing and for flood conveyance concerns. Even in conservation areas that had not been cleared, the physical processes of regular flooding and occasional bank and floodplain scour had been eliminated by the large water projects in the region. River Partners was called onto the project to develop habitat restoration strategies that targeted brush rabbit habitat in spite of the lack of river physical processes required to establish it naturally.
River Partners brainstormed with the RMTG about cost-effective ways to create new high ground areas for bunnies. Historic floodplain habitat features such as seasonal wetland basins and oxbow lakes at the Refuge had been leveled to facilitate farming. Clearly, restoring the topography of these natural features for the benefit of waterfowl would create a brush rabbit opportunity. Spoils from wetland excavation were piled high enough to stand out of the flood depths, creating high-elevation flood refuge for rabbits. We went to work planning and fundraising for projects that construct and vegetate these and other high-ground areas of the Refuge (levees and fields) using drip irrigation and hand labor – the plantings would be too dense to allow for mechanized vegetation management.
A shrub community of dense rose and blackberry was planted on the sides of elevated mounds, while the top was vegetated with rose, blackberry, elderberry, coyote brush, mulefat and golden currant. The vegetation grew wildly, with shrubs reaching near 100 percent cover after just two to three growing seasons.
This map of riparian habitat restoration on the West Unit of the San Joaquin River National Wildlife Refuge was created by River Partners in May 2010 from a basemap by the National Agriculture Aerial Imagery Program (NAIP 2009). Vegetated levees (purple) provide riparian brush rabbit migration routes between remnant and restored areas. Unvegetated levees will be targeted for restoration in the coming years. Basemap provided by USGS.
In February and March of 2010, ESRP scientists performed “standardized” live trapping on River Partners’ vegetated levees and mounds to assess for the first time the extent these “refugia” were being used by the rabbits. Of those trapped, over 60% were Refuge-born indicating the natural population is taking off. This year’s preliminary census data for riparian brush rabbits in natural habitats at the Refuge are also the highest than ever recorded (NB: ESRP began the rabbit surveys in 2005).
More rabbits were trapped on vegetated levees adjacent to remnant habitat (and original release sites) than at distances away from remnant habitat, but the trap results on the levees show that the dense vegetation in a linear arrangement on the levees is providing rabbit “highways” across the Refuge, possibly facilitating dispersal into other restored areas and to other high-elevation portions of the Refuge. These results suggest that rabbits and other terrestrial species will use vegetated levees to evade rising floodwaters as well as to repopulate drying floodplain areas following the next “big one.” This will increase population resilience and genetic mixing of brush rabbit populations across the Refuge, ensuring the long-term viability of the species.
Other species observed on the levees during the live trapping efforts include: desert cottontail, California vole, deer mouse, black rat, striped skunk, and opossum. A few long-tailed weasels were trapped on the constructed mounds in the Vierra Unit. Weasels are thought to be a problematic predator of riparian brush rabbits, and a new challenge will be researching interactions between vegetation and and weasels to find strategies to discourage weasel populations while enhancing rabbit populations in future restoration efforts.
River Partners continues to raise funds for levee revegetation. In 2010, under a generous grant from the Bureau of Reclamation’s Central Valley Project Habitat Restoration Program and Conservation Program, we will begin work on both sides of a 1.8 mile stretch of levee in the Lara Unit of the Refuge. This expansion of the project presents a great opportunity to facilitate rabbit repopulation of restored areas to the south. Levees on the east side of the San Joaquin River and Dos Rios Ranch will be targeted in the coming years to provide multidirectional evacuation and migration routes. Coordinated levee breaching and floodplain wetland inundation projects in this area will work cooperatively with rabbit recovery efforts to ensure the drastic extinction scares are a thing of the past.
The above article originally appeared in the June 2010 issue of the River Partners Journal.