Observations on the Nature of the Decline in Numbers of Two Microtus Populations

Abstract

Microtus populations are known to be subject to marked fluctuations in numbers, with periods of peak abundance alternating with periods of scarcity, and recurring at approximately four-year intervals. In 1952 Chitty advanced an hypothesis to account for the ‘crash’ of the voles at the end of each period of abundance, explaining the severe mortality at this stage of the cycle as resulting from conditions to which the parents had been exposed during the previous (i.e. peak) breeding season, and not from abnormal environmental conditions affecting the progeny at that time or during the decrease. This view is developed more fully by Chitty (1954). The hypothesis is based on evidence obtained during a study of vole populations in Wales where severe mortality occurred among the first-born young during the peak breeding season. Young born later in the season had a higher survival rate at first, but decreased rapidly during the winter, and on some areas the survivors did not breed again in the following year. Even where breeding did occur, the population still experienced a crash. He postulates that intraspecific strife occurs during the peak breeding season, causing the death of the first-born young and a permanent derangement of the physiological condition of the adult females. Defects in the females are believed to be transmitted in a more severe form to the next generation. Some information that appears relevant to this hypothesis was obtained during a study of voles ( Microtus agrestis ) on two isolated areas of natural grassland in the Oxford University Estate at Wytham, Berkshire, between 1949 and 1952. Intensive live-trapping showed that the voles on the smaller area (the Dell = 2 acres) reached a peak in 1950 and crashed in 1951, while those on the larger area (Rough Common = 4½ acres) reached a peak in 1951 …