Vole Population Dynamics: Influence of Weather Extremes on Stoppage of Population Growth

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ABSTRACT.-

Effects of 74 episodes of extreme weather on stoppage of population growth and resulting amplitudes of annual population fluctuation of 39 fluctuations of Microtus ochrogaster and 20 fluctuations of M. pennsylvanicus were studied over a 25 y period in east-central Illinois. Episodes of extreme weather may have stopped population growth of only six M. ochrogaster fluctuations and of two M. pennsylvanicus fluctuations. Cessation of growth of only one population fluctuation (M. pennsylvanicus) could be attributable solely to an episode of extreme weather. Episodes of extreme weather occurred during 62% of the increase phases of M. ochrogaster population fluctuations and 75% of those of M. pennsylvanicus, with no associated cessation of population growth. We conclude that episodes of extreme weather were not a primary factor responsible for cessation of population growth or variation in amplitudes of population fluctuations of either M. ochrogaster or M. pennsylvanicus.

INTRODUCTION

Populations of many arvicoline rodents undergo large fluctuations in numbers. Some population fluctuations are short-term, occurring over a few months (Krebs and Myers, 1974; Taitt and Krebs, 1985), whereas others may take 2-3 y to run their course (Oksanen and Henttonen, 1996). Population fluctuations may be annual or erratic or may occur periodically at 2-5 y intervals (Krebs et al., 1969; Krebs and Myers, 1974; Taitt and Krebs, 1985; Krebs, 1996; Bjornstad et al, 1998). An understanding of factors responsible for high-amplitude population fluctuations, irrespective of their periodicity, is necessary to evaluate patterns of fluctuation of arvicoline populations.

Populations of small mammals in temperate and high latitude regions often display annual fluctuations in population density. Population densities increase rapidly during the growing season (typically spring-early autumn) when food availability is high and decline during late autumn-winter because of reduced reproduction and survival in response to lesser food quality and quantity (Batzli, 1992). Episodes of unusually extreme weather conditions (i.e., high and low temperatures and high or low precipitation) may result in sudden high mortality or reduced reproduction, producing multiannual fluctuations in population density that obscure annual fluctuations (Batzli, 1992). Although it has been shown that weather conditions may influence population densities of small mammals, mainly through effects on food supplies (Drabeck, 1977; Lewellen and Vessey, 1998a, b; White, 2004), there is little evidence for episodes of adverse weather having direct effects on survival and reproduction (Pinter, 1988) or resulting in cessation of population growth (Getz et al., 1987; Batzli, 1992).

Getz et al. (2006) concluded that increased mortality was the main factor involved in cessation of population growth of the prairie vole, Microtus ochrogaster. The time at which such increased mortality occurred appeared to be the primary determinant of peak densities and amplitudes of population fluctuations. Predation from generalist predators was presumed to be the most likely source of mortality. Reduced survival also was most commonly associated with cessation of population growth of the meadow vole, M. pennsylvanims (Getz et al., in press) ; however, a winter decline in reproduction was associated with cessation of growth of population fluctuations that peaked in late autumn-winter. Neither of these papers addressed the potential for episodes of extreme weather to stop population growth through effects on survival and reproduction and to influence the peak densities and amplitudes of population fluctuation. In the present paper we address the question of whether episodes of extreme weather may be involved in cessation of population growth of arvicoline rodents.

We used the results of a 25-y study (Getz et al., 2001) of sympatric populations of Microtus ochrogaster and M. …