Reintroduction of the riparian brush rabbit in the San Joaquin Valley, California, USA

Summary

The riparian brush rabbit is an endangered subspecies confined to riparian habitat in California's northern San Joaquin Valley, reduced to under 1 percent of its historical extent by irrigated farming, river damming and channelization, with the only known population long thought to be the one inside the small, farmland-ringed Caswell Memorial State Park. The 1998 discovery of a second population scattered across private land in the South Delta gave researchers a source for a captive-breeding and reintroduction program launched in 2001, centred on the San Joaquin River National Wildlife Refuge. Rabbits bred in large outdoor pens were released from July 2002 onward, first through soft-release enclosures and, from 2007, directly, with many fitted with radio collars to track survival and dispersal; by the program's suspension in December 2013, 1,496 captive-bred rabbits had been released across the refuge and two additional protected properties that together created more than 32 continuous river-kilometres of connected habitat. Flooding proved the greatest threat: a 2006 flood killed nearly all refuge rabbits and took two to three years to recover from even with continued releases, prompting construction of 32 vegetated flood-refuge mounds and two berms plus revegetation of 19 kilometres of levees, measures credited with letting the population rebound within months, unaided, after further floods in 2011 and 2017. The project rates itself highly successful, crediting a multi-agency technical group, sustained funding and the discovery of that second source population as the foundations of its recovery.