The Ecology of the Salt-Marsh Harvest Mouse (Reithrodontomys raviventris) in a Diked Salt Marsh

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The salt-marsh harvest mouse (Reithrodontomys raviventris) was a fugitive species in this diked marsh. These mice were able to use more open and saltier pickleweed (Salicornia virginica) when numbers of meadow mice (Microtus californicus) were high and they moved into deeper and less salty pickleweed as the population of meadow mice declined. In this first extensive study of a diked marsh, salt-marsh harvest mice were shown to swim and run across larger open areas than they had been thought to cross previously. The salt-marsh harvest mouse (Reithrodontomys raviventris) endemic to the marshes of San Francisco Bay (Fisler, 1965), has become endangered primarily because of destruction and modification of its habitat. Approximately 80% of the historic tidal marshes have been destroyed and most of those that remain support few to no mice because of backfilling, subsidence, or vegetation changes (Shellhammer, 1982). Approximately 30% of the historic tidal marshes of San Francisco Bay remain as diked marshes. Eighty percent of the diked marshes are located in Suisun Bay, the northeasterly component of the San Francisco Bay system. The habitat is primarily managed for waterfowl use; mouse populations are small and separated by large areas of inappropriate habitat. Approximately 13 km2 of diked marshes remain in the South San Francisco Bay and constitute potential refuges for the more endangered of the two harvest mouse subspecies, R. r. raviventris, within a ring of large cities and extensive bayland development. Little, however, is known of the biology of the mouse in diked marshes subject to great changes in moisture and salinity throughout the year. Some are flooded periodically by rain waters or by bay water after breaks in outboard levees. Most surface water evaporates each summer. This study was conducted to ascertain mouse numbers, distribution, and interactions with other species during the summer when moisture is low and salinities are high; the microhabitat requirements of these mice in diked marshes; and how barriers such as sloughs and open areas affect movements of the mice.