Influence of Meadow Mouse Populations on California Grassland

The influence of meadow mouse (Microtus californicus) populations on grass—land ecosystems was studied at two sites with similar densities of Microtus, but in different types of grassland–a coastal prairie and an inland valley grassland. Height of vegetation, volume of standing crop, and percentage cover were greater at the dry inland site where the annual grass Bromus rigidus was dominant. At the milder coastal site the annual grass Lolium multiflorum and the perennial bunchgrass Danthonia californica were dominant; forbs and other perennial grasses were more important. During a year with high rainfall and moderate densities of Microtus (1967) total standing crop, average height of vegetation, and weight of seed crop at the coastal site increased. Although 1966 and 1968 were both dry years, standing crop, seed production, and volume and cover of vegetation were less in 1968, when the meadow mouse population was high, than in 1966, when it was low. Exclosure studies demonstrated that these vegetation changes resulted from changes in Microtus density. Major food plants for the mice contributed 85% less volume to the vegetation on unexclosed areas after a high density of 395 Microtus per hectare (160/acre). Seed fall from preferred grasses was diminished 70% on grazed areas.