The role of succulent halophytes in the water balance of salt marsh rodents.

The role of succulent halophytes in the water balance and ecology of salt marsh rodents is dependent upon an evaluation of the composition of the available sources and the physiological properties of their potential consumers. Studies of the osmotic properties of succulent halophytes from southern California coastal salt marshes are presented, together with experiments regarding the utilization of Common Pickleweed (Salicornia virginica L.) by indigenous populations of cricetid rodents (harvest mouse Reithrodontomys megalotis limicola Von Bloecker, and meadow-mouse Microtus californicus stephensi Von Bloecker). These data are discussed in relation to other available information concerning the ecology of coastal salt marshes, particularly in western North America.Extruded sap of Common Pickleweed was found to have a mean total osmotic pressure (TOP) of 1,450 mOsm/liter, with an average chloride ion content of 876 mEq/liter (about 70% of the TOP). A related species, Salicornia subterminale, had a slightly lower TOP (1,300 mOsm/liter), of which about 29% was accounted for by chloride ion concentration. Sea Blight (Suaeda fruticosa) was the only species in which the TOP correlated with the distance from the tide level; sap TOP increased away from the lagoon's edge. In both Sea Blight and Common Pickle weed, TOP was not directly related to chloride content, indicating the importance of other osmotically active solutes.Harvest mice were placed on three experimental regimes: 1) millet seeds only, 2) pickleweed only, and 3) pickleweed and millet seed. Meadow mice were tested on the last regime only. Harvest mice survived best on a strict millet seed diet; when Salicornia was consumed to a detectable extent, the mice did not survive. Meadow mice, however, could survive using Salicornia as a dietary source in conjunction with seeds. Kidney electrolyte concentrating abilities indicated that harvest mice should be able to utilize pickleweed; this was not confirmed in my experiments. It is suggested that cathartic ions (possibly magnesium, sulphate, or oxalate) prohibit the utilization of certain halophytes. The mechanisms that enable meadow mice to utilize Salicornia are not clearly understood.Measurements of harvest mouse evaporative water loss are among the highest reported for small mammals (1.35 mg H2O/cc O2 consumed). On the basis of these data and other information in the literature, a water budget was constructed. The results suggest that harvest mice may enter daily torpor in response to osmotic stress or water deprivation.The role of dew and fog precipitation in the ecology of small rodents inhabiting coastal marshes is discussed. Apparently a sufficient amount of free water is available to meet the requirements of the salt marsh populations, although the quality of the available water may be influenced by salt-excretion activity of certain halophytes. Less halophytic succulents are available in the coastal marshes; these species may be more readily utilized than Salicornia by small vertebrates.