Population Fluctuations of Small Mammals and Sooty Grouse in California

DOCTORATE DISSERTATION:Studies were carried out in Tilden Park, Contra Costa County, and Sagehen Creek, Nevada County, California., from 1951 to 1955. Population trends of Microtus montanus at Sagehen Creek and Microtus californicus at Tilden Park showed no interspecific synchrony. A local population of sooty grouse (Dendragapus fuliginosus) at Sagehen Creek also showed little indication of synchrony between the long grouse-hare cycle and the short microtine cycle. Both M. montanus and M. californicus exhibit an inverse relationship between population density and natality, as measured by ovulation rate and litter size. The age at which reproductive maturity is reached is apparently quite constant and independent of the stage of the population cycle. In M. montanus the length of the breeding season and seasonal litter production changed only slightly, declining as population density increased up to the cyclic peak. In contrast, length of the breeding season and litter production varied considerably in M. californicus. The conclusion reached is that variable reproduction is not a cause of the cyclic changes observed in population density. Thus, mortality must be the variable that produces the differences in populations from year to year. The population of adult voles immediately prior to the beginning of the breeding season is about the same each year. The level that the population reaches at the end of each breeding season depends upon the survival of individuals born during that breeding season. Changes in the survival rates of young voles are considered to be the main cause of the population differences found in the summer and autumn of the different years. Incidental to the work on population cycles, certain basic aspects of comparative reproduction have been uncovered. In M. californicus, a vole with a long breeding season, litter size is small and precocious breeding is rare. M. montanus, with a short breeding season, has large litters, and precocious breeding and polyovuly are common. Another difference between the species is that litter size is highest at the start of the breeding season in M. montanus, and then progressively declines. In M. californicus, litter size is initially low, rises to a peak in the middle of the breeding season, and then declines again. This pattern correlates with changes in the quality of forage available to the voles. A possible nutritional basis for the population fluctuations was investigated by applying commercial nitrogen and phosphorus fertilizer to several of the meadows at Sagehen Creek in 1954. The M. montanus population apparently responded to the fertilization in that the decline in numbers during the summer of 1954 was not as severe in the fertilized meadows as in the unfertilized meadows, and breeding was prolonged. The response was, however, temporary.
A side study on Sooty Grouse was undertaken in Sagehen Creek Basin. The major objectives were to devise some was of determining the relative abundance of the grouse from year to year, and to get some measure of the annual reproduction.