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.博士学位论文：本研究于1951年至1955年间，在加利福尼亚州康特拉科斯塔县的蒂尔登公园（Tilden Park）以及内华达县的塞奇恩溪（Sagehen Creek）开展。针对栖息于塞奇恩溪的山间田鼠（Microtus montanus）与蒂尔登公园的加州田鼠（Microtus californicus）的种群动态研究显示，二者不存在种群周期同步性。栖息于塞奇恩溪的当地乌松鸡（Dendragapus fuliginosus）种群，也未表现出与长周期的松鸡-野兔周期、短周期田鼠周期之间存在同步性的迹象。 两种田鼠均呈现出种群密度与出生率（以排卵率和每胎产仔数衡量）呈负相关的关系。个体达到生殖成熟的年龄相对恒定，且不受种群周期阶段的影响。就山间田鼠而言，其繁殖季长度与季节性产胎数仅发生轻微变化，随种群密度上升至周期峰值时逐渐降低。与之形成鲜明对比的是，加州田鼠的繁殖季长度与每胎产仔数波动幅度较大。 本研究得出的核心结论为：繁殖可塑性并非导致种群密度周期性变化的诱因。因此，死亡率应为造成不同年份种群差异的关键调控变量。繁殖季开始前的成年田鼠种群规模，每年大致保持一致。每个繁殖季结束时的种群规模，取决于该繁殖季内新生个体的存活率。幼田鼠存活率的变化，被认为是造成不同年份夏季与秋季种群差异的主要原因。 本研究在种群周期相关工作之外，还揭示了两种田鼠在比较繁殖生物学方面的若干基础特征。繁殖季较长的加州田鼠，每胎产仔数较少，且早熟繁殖现象较为罕见；而繁殖季较短的山间田鼠，每胎产仔数较多，早熟繁殖与多排卵现象则较为普遍。两物种间的另一差异在于：山间田鼠的每胎产仔数在繁殖季初期达到峰值，随后逐步下降；加州田鼠的每胎产仔数则初始较低，在繁殖季中期达到峰值后再度回落。这一产仔数变化模式，与田鼠可获取的饲草质量变化存在显著关联。 为探讨种群波动可能存在的营养基础，本研究于1954年向塞奇恩溪区域的多个草甸施加商用氮肥与磷肥。山间田鼠种群对施肥处理表现出明显响应：1954年夏季，施肥草甸内的种群数量下降幅度远小于未施肥草甸，且繁殖期得以延长。不过这种种群响应仅为暂时性的。 本研究还在塞奇恩溪流域开展了一项关于乌松鸡的辅助研究，核心目标为探索可用于逐年评估松鸡相对种群丰度的方法，并获取年度繁殖情况的量化数据。