Evidence of the use of brackish marsh habitat by the endangered salt marsh harvest mouse in South San Francisco Bay

The federally endangered salt marsh harvest mouse (Reithrodontomys raviventris) is a primary focal species for efforts to preserve and restore the marshes of the San Francisco Bay.  These conservation efforts are thereby aided by a precise understanding of the habitat requirements of this species.  Prime habitat for the salt marsh harvest mouse (SMHM) has been defined as mature salt marsh with tall, dense pickleweed and associated vegetation.  Brackish marsh has traditionally been considered to possess little to no habitat value for the southern subspecies of SMHM, and for this reason areas of South San Francisco Bay that contain brackish marsh have not been considered SMHM habitat to date.  Recent surveys utilizing a modified live-trapping method in the North San Francisco Bay have shown that the northern subspecies of SMHM may utilize brackish marshes.  We performed replicated SMHM surveys with both the traditional and the modified live-trapping method in the summer of 2006 across a range of marsh types in the South San Francisco Bay at plots that were previously surveyed in 1990.  Our results verified that densities of SMHM have significantly increased in brackish marshes between 1990 and 2006.  In addition, the modified live-trapping method detects greater numbers of SMHM than does the traditional method.  These results suggest that brackish marsh may be of greater conservation value to this endangered species than previously thought.  Furthermore, microscale analysis of habitat use by the small mammal assemblage suggests that SMHM may avoid competition with the larger, more dominant California vole (Microtus californicus) in brackish environments by preferentially utilizing the bulrush habitat during high tides when the marsh plain is flooded and the voles have moved to higher elevation portions of the marsh.