Flooding: What's a Rabbit to Do?
By Chris Stevenson, Restoration Biologist, San Joaquin Valley
Vegetated Bunny Mounds as part of the restored landscape with vegetated levees in the foreground. Photo by Chris Stevenson, River Partners.
The riparian brush rabbit (Sylvilagus bachmani riparius), which was once numerous in riparian areas along the San Joaquin and Stanislaus Rivers, is now considered one of the most endangered species in California. As with many other endangered species, habitat loss is the major culprit in the decline of the brush rabbit. By the late 20th century, the only known population of riparian brush rabbits was located in remnant riparian forest at Caswell Memorial State Park, along the Stanislaus River. At one point, following catastrophic flooding and the threat of wild fires, the species was considered on the brink of extinction. However, brush rabbits were subsequently captured during post-flood surveys at Caswell. In 1998, the second known population was discovered in degraded habitat in the Sacramento-San Joaquin Delta.
In 2001, the Endangered Species Recovery Program (ESRP) initiated a captive propagation and reintroduction program using Delta breeding stock, and releases began the following year on the San Joaquin River National Wildlife Refuge (Refuge). These releases coincided with the initiation of River Partners and the U.S. Fish and Wildlife Service (USFWS) joint efforts to restore and enhance riparian habitat on the Refuge. As we learned more about the ecology of the brush rabbit, River Partners began incorporating suitable habitat features into our restoration projects. Riparian brush rabbits favor brushy habitat characterized by dense thickets of California rose (Rosa californica), California blackberry (Rubus ursinus), willows (Salix spp.), and coyote brush (Baccharis pilularis).
Kim Forrest, USFWS with riparian brush rabbit. Photo by Heather Bell, USFWS.
Although brush rabbits live in riparian areas, they also need access to higher ground in times of flooding. The 2006 flood on the Refuge was critical in demonstrating the need for integrating high ground refugia into restoration design. Post-flood surveys by the ESRP found high mortality of riparian brush rabbits and other wildlife. These surveys also included flood depth monitoring and a GIS analysis to identify critical habitat areas which had low impacts from flooding combined with good brush rabbit habitat. These data were used by USFWS to identify areas that would be suitable for the placement of high-ground refugia ("bunny mounds"). This led to the construction of a network of protected flood refugia consisting of bunny mounds' and vegetated levees for the brush rabbit and other wildlife.
To date, River Partners and USFWS personnel have constructed 32 bunny mounds of which 27 have been planted with native riparian vegetation favored by riparian brush rabbits (the remaining five mounds will be planted in 2009, pending available funds).
The bunny mounds are planted with dense, low growing vegetation to provide the rabbits with cover while they're seeking escape from the flood waters. Typical plantings include California rose, California blackberry, mulefat (Baccharis salicifolia), coyote brush, and blue elderberry (Sambucus mexicana). A dense band of sandbar willow (Salix exigua) was planted around the base of the mounds as natural rip-rap to provide protection from scouring during floods.
Riparian Brush Rabbit Release at the Buffington Unit. Photo by River Partners Staff.
In addition to the construction of bunny mounds, River Partners has vegetated 23,000 linear feet of levees within the Refuge to provide additional protected flood refugia. Funding is pending for an additional 6,400 linear feet of levee revegetation, which would extend the levee plantings to a total of 5.5 miles. Vegetating levees to create high water refugia, while practical from a wildlife management perspective, can have substantial obstacles in implementation. The primary issue is that many levee districts do not allow trees and shrubs on levees, as it can make them more difficult to survey for maintenance issues, and some argue that plant growth compromises levee strength and reduces its ability to withstand the energy of floods. Studies surveying post-flood effects on levees however, have shown that vegetation can protect levees by slowing floodwater velocities, reducing scouring by dissipating flood energy and increasing soil shear strength.
In some cases, areas of natural high ground can be incorporated into riparian restoration projects to provide cover for wildlife that require refugia during flood events. In 2009, River Partners began restoration on a parcel in the southeastern part of the Refuge known as the Arambel Unit. This unit contains areas of natural high ground that did not flood during the 2006 flood. River Partner's planting design for the Arambel Unit has integrated small thickets comprised of high density plantings, twice that of the surrounding fields within the higher elevation zones.
The development of riparian brush rabbit refugia within River Partners' riparian restoration projects is just one example of how River Partners develops restoration plantings targeting specific wildlife objectives. Bunny mounds are integrated into a larger planting mosaic, which has been designed to provide multiple niches for wildlife. Although it is not possible to integrate the needs of all species into a restoration planting, developing multi-purpose plantings can ensure that the needs of many species are met. In the next major flood event you can bet that our bunny mounds will be providing refuge for this endangered species.
Acknowledgements: The author would like to thank Dr Patrick Kelly, and Mathew Lloyd at the Endangered Species Recovery Program and Kim Forrest at the US Fish and Wildlife Service for their comments, as well as the staff at River Partners.