暮鼠越冬种群结构的年间变化

自1984年4月至1999年8月,我们沿铁路轨道检测了农田中暮鼠(Calomys venustus)的多度,研究了种群密度、越冬种群结构和繁殖期长度之间的关系,发现种群具有增长、高数量和下降年份。将动物按照同生群(种群内在相同时间内出生的一些个体)分开,利用2×2列联表卡方检验对不同性别的越冬种群结构与数量(高数量和种群下降)间的关联性进行了分析。就高密度与种群密度下降年份中的同生群组成而言,不同密度的年份显示种群结构有显著差异。在数量增长和高密度的年份里,在繁殖季节的起始期,前一年秋季出生的个体比同年夏季出生的个体在数量上要占优势,而在同生群数量下降年份则相反,特别是雌性。高密度年份具有繁殖期缩短的特征,而种群增长年份繁殖期延长。结果表明,这些变化不能只归结于繁殖期长或繁殖率引起密度变化。我们的结论是:解释C·venustus种群消长不能仅考虑动物同生群本身,需要考虑其社会和环境因子的作用。

Inter-annual changes in structure of overwintering population in Calomys venustus

We examined the relationship between population density and overwintering population structure and length of the breeding periods ofCalomys venustus. Abundance ofC.venustus was monitored from April 1984 to August 1999 on railway banks in a rural area. Years of increasing density, high, and declining density were recognized. An index of population change from spring to autumn in each year was calculated. Animals were assigned to a cohort and a 2×2 contingency table by a Chi-square statistic was calculated to test for dependence between the overwintering population structure and density years (high and decline) by sex. Years of different density showed significant difference in population structure, in respect to composition of the overwintering cohort during high density and declining density years. At the onset of breeding periods, during increase and high density years, animals born the previous autumn (cohort 3) was more representative than cohort 2 (animals born the previous summer), whereas during declining density years cohort 2 was more prevalent, especially females. Years of high density were characterized by a shortened breeding period and years of increasing density by an extended breeding period. Our results indicated that the shift could not be attributed only to density-dependent variation in the length of breeding period or to reproductive rates. In conclusion, in explanation of fluctuations in population density ofC.venustus, not only the cohort, but the social and environmental factors to which the animals are exposed should be considered.