Mortality among voles (Microtus agrestis) at Lake Vyrnwy, Montgomeryshire in 1936–9

Abstract

Lake Vyrnwy is a reservoir surrounded by many types of habitat including newly afforested areas suitable for field voles. Other areas form ecological barriers between these populations. The purpose of the present inquiry was to discover the causes of the periodic fluctuations in numbers which occur, often simultaneously, among such vole populations. Several separate vole populations reached peak densities in 1937 and declined in 1938, in which year numbers were at a peak in most of the younger plantations. During the peak breeding season of 1937 there was an excessive juvenile mortality before August. The survival rate of the young then improved and the old animals became scarce, as they usually do at this time of year. All age groups ceased breeding in August 1937 and the survivors did not breed next spring. By contrast, populations which were at peak densities in 1938 continued breeding from May to October and the survivors bred again in 1939, though no recovery in numbers was brought about. Both in May 1937 and May 1938 high numbers were associated with unusually high body weights. A sample population was studied by marking and recapturing the animals. Most spent their lives within an area of 10 to 15 yd. diameter. Both in September 1936 and 1937 there were about 300 young voles per acre. During 1938 survival was very poor from January onwards, particularly among the males. During the time of their almost complete disappearance in 1938 or 1939 voles were not subjected to any known environmental conditions likely to have caused excessive mortality. The hypothesis is therefore advanced that death was primarily due to adverse conditions to which the parents were subjected in the previous breeding season. It is inferred (a) that the high juvenile mortality at that time was a symptom of severe intraspecific strife, (b) that intraspecific strife had deleterious effects, probably upon the hormone balance of the females, so that (c) their young suffered defects which reduced the longevity of those which survived the breeding season, and also (d) impaired their ability to produce a normal generation. Peak numbers were reached simultaneously over wider areas than seems likely by chance alone. It is argued that this must have occurred through some interaction of the independent biological units with their common weather system. Competition for space, the process believed to control excessive increase, is shown to be a possible means through which environmental changes may cause further variations in mortality rates. A sufficiently variable physical environment should thus tend to eliminate differences in density between populations and so to effect a regional synchrony of their cycles. It seems likely that periodic mortality in the North American snowshoe hare is also a delayed result of intraspecific strife. This extension of the hypothesis has implications which are at variance with some of the existing views about cycles in birds and mammals.