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No abstract

Most small rodent populations in the world have fascinating population dynamics. In the northern hemisphere, voles and lemmings tend to show population cycles with regular fluctuations in numbers. In the southern hemisphere, small rodents tend to have large amplitude outbreaks with less regular intervals. In the light of vast research and debate over almost a century, we here discuss the driving forces of these different rodent population dynamics. We highlight ten questions directly related to the various characteristics of relevant populations and ecosystems that still need to be answered. This overview is not intended as a complete list of questions but rather focuses on the most important issues that are essential for understanding the generality of small rodent population dynamics.

We studied habitat selection and foraging behaviour of the house mouse (Mus domesticus) related to increasing mouse densities and depleting food resources over the breeding season. The study was conducted during the increase phase of an incipient outbreak of mice in a grain‐growing area of southeastern Australia. A 3‐year rotation created a mosaic of large paddocks of grain crop, pasture, and fallow. The narrow fence lines between paddocks provide an important stable habitat for the mice. We monitored population densities with live‐trapping and habitat preference by measuring giving‐up densities (GUD) using artificial food patches. Food patches were established in crop fields, fence lines, and pasture. Avian predation risk was assessed by daily counts of raptors. Before harvest most mice were found in the crop fields. Fewer mice were found along the fence lines and no mice were found in the pasture. After harvest, the number of mice increased along the fence line and in the pasture. Mice started to exploit pasture but only a few trays were visited. As the population densities of mice increased, they inhabited all habitats. Feeding activity in the pasture remained low due to high predation risk. Along fence lines feeding activity increased and mice exploited open and covered patches similarly. We conclude that during high densities mice experience a strong trade‐off between food and safety. Vegetation along fence lines offered cover but little food. However, soon after harvest this habitat was favoured by mice. Pasture offered little food and was an extremely risky habitat because of the lack of cover. We suggest that during high densities of mice, habitat use became more opportunistic. Mice took greater risks in all habitats and within each habitat at the microhabitat level. The “Stalingrad effect” is a good descriptor of foraging decisions of mice during the progression of an incipient plague in the cereal‐growing regions of southeastern Australia.

According to the kin selection theory the degree of genetic relatedness affects the nature of intraspecific competition and it might promote cooperative and/or altruistic behavior between individuals. We examined kinship effects on reproductive success of territorial females and survival of juveniles in the bank vole Clethrionomys glareolus. Four experimental populations were founded in large enclosures (0.5 ha): two consisted of five related (Related, R) and two of five mutually unrelated females (Unrelated, UR). Each population had five males of heterogeneous origin.Our earlier results showed that populations of the R grew twice as fast as the UR. Here we show that recruitment and offspring survival is connected to spacing behavior of mothers. Unrelated females who had their home ranges close to each other produced significantly fewer recruits than the related ones. Their home ranges overlapped significantly less and ranges were smaller if they were situated close to each other. Furthermore, the survival of juveniles declined with dispersal distance from the natal territory in populations of the UR. Trappabilities of juveniles were also lower among the young whose home ranges were close to unrelated females, indicating avoidance of mature females.Our results indicate that competition for space is more intense among unrelated neighbors, decreasing their reproductive success. Infanticide towards unrelated nestlings and/or juveniles is one possible mechanism decreasing survival and causing a lower recruitment of young. On the contrary, sharing of space among related neighbors did not seem to cause notable costs on their reproduction. Mature females allowed related young females to use their territory, but still, the young did not acquire an area for their own reproduction, probably due to a mother-daughter conflict. The basic social structure of Clethrionomys populations, i.e., exclusive female territoriality, remained the same regardless of a high degree of relatedness.