Intrinsic and Climatic Determinants of Population Demography: The Winter Dynamics of Tundra Voles

Abstract

The relative impacts of intrinsic factors (e.g., density dependence) and extrinsic factors (e.g., climate) on winter demography may be critical for the generation of different population dynamic patterns (including cyclicity) in northern vole and lemming populations. However, little is known about winter demography because studies with temporal and spatial replication at the population level and an adequate sample of individuals with known fates within each population are rare. In this study, we monitored the winter demography of 48 local tundra vole populations introduced to experimentally enclosed plots the preceding spring for four years in Norway. The rate of population change over the winter (November–May) was density dependent due to recruitment. However, the large variation in the rate of change between the different winters was due to a density‐independent, and most likely a climatically driven, variation in survival rate. In particular, mild weather that led to the formation of ice on the ground seemed to be detrimental for winter survival. We predict that if increased frequency of such events arose, due to climate change, normal cyclic dynamics of northern small rodent populations would be disrupted. We found support for the hypothesis that voles adjusted their body mass toward a certain mean during the winter so as to maximize winter survival. The survival rate of males was lower than that of females, possibly due to their larger body mass, and this resulted in female‐biased population sex ratios in the spring. This result suggests a link between sexual selection (responsible for the sexual size dimorphism) and natural selection (operating though size‐dependent winter survival) with implications for the demographic structure of the population.

Determinants of lemming outbreaks

“…Population increase during winter in lemmings results from winter reproduction (4, 16)-a demographic trait virtually absent in the gray-sided vole (14). The recruitment com- ponent of lemming winter growth can also explain why only lemmings exhibited density-dependent growth rate in winter, as recruitment in northern rodents appears to be more sensitive to density than adult survival (17). In essence, our analysis suggests that the contrasting topologies of the population peaks in lemmings and voles are more likely caused by different intrinsic demographic tactics than different trophic interactions.…”

Timing of arrival at breeding grounds determines spatial patterns of productivity within the population of white stork (Ciconia ciconia)

“…Winter plays a crucial role in shaping population dynamics of small vertebrates across the northern regions (Hanson and Henttonen 1988). Such environmental events as formation of ice on the ground have been reported to exert detrimental effects on the survival of rodents (Aars and Ims 2002), an important diet component of white stork (Antczak et al 2002). In consequence, late arrival of storks at breeding grounds may coincide with low prey abundance throughout the entire reproductive period, under which circumstances the intensity of intra-specific competition is likely to increase and high-quality pairs may try to monopolize most of available resources at the expense of their low-quality neighbours.…”

Effect of photoperiod on body mass, food intake and body composition in the field vole,Microtus agrestis

“…Iverson and Turner, 1974;Hansson, 1992;Aars and Ims, 2002). This reduction affects all three major carcass components -lean tissue, fat and body water (Dark et al, 1983).…”