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We have synthesized the data on population dynamics and densities of rodents in seven biomes of the Palearctic (mainly western part), and related them to the data on standing crop of biomass and net productivity of ground vegetation (as rough indicators of food availability to rodents). Analysis of 44 long-term (greater than or equal to 5 years) series of rodent trapping showed that there was a continuum from highly cyclic to non-cyclic populations. Rodents inhabiting tundra, taiga, steppe, and farmlands (wintercrops) in the temperate zone have highest cyclicity indices. Definitely non-cyclic are rodents in the temperate forests (mixed and deciduous forests, steppe woodland) and desert. Standing crop of biomass of ground vegetation (analysis of 63 data points) correlated positively with latitude; it was highest in the northern tundra and decreased towards South. Variation within biomes was most pronounced in the temperate zone, with forests having ground vegetation biomass as low as that in deserts, whereas farmlands in that zone - as high as that in tundra. In various habitats (natural open, farmland, and forested) located in seven biomes, the mean index of rodent cyclicity was significantly positively correlated to the mean standing crop of ground vegetation. Net productivity of ground vegetation (30 data points) did not show latitudinal trends. It was lowest in desert, tundra, and all types of forests, and highest in open habitats of the temperate zone and steppes. Mean densities of rodents (calculated as averaged spring and autumn estimates) were lowest in tundra, desert, and all types of forests (8-29 rodents/ha). The highest average densities were recorded in the farmlands of temperate zone and steppe (143-490 rodents/ha). Mean and maximum densities of rodents were strongly positively correlated with the mean productivity of ground vegetation. Dichotomy between seasonal (non-cyclic) and multiannual (cyclic) fluctuations in rodent numbers was not found. The magnitude of seasonal changes in rodent densities (from spring to autumn) was a continuous variable related to the propitiousness of climate for plant growth. Irrespectively of the type of population dynamics, seasonal changes in rodent densities were small in the coolest and the hottest biomes (tundra, desert) and big in the temperate zone. Results of our long-term study on predation on rodents in the temperate deciduous forests did not support the hypothesis on the role of specialist and generalist predators in shaping rodent dynamics. We found no qualitative difference between predatory impacts by generalist and specialist predators. All predators exerted the heaviest impact at low or moderate densities of rodents (inversely density-dependent predation). Predation may be an important factor of rodent mortality but it does not shape the pattern of rodent population dynamics. Based on the observed vegetation-rodent correlations, we have proposed an interpretation of the mechanisms of rodent population dynamics in the Palearctic biomes. A prerequisite for rodent cycles to occur is abundant winter food, which enables rodents to continue an increase phase beyond one growing season (by winter breeding). Habitats with mean standing crop of ground vegetation of over 4000 kg dry weight/ha in summer are expected to harbour cyclic populations of rodents.

Rodent cycles in relation to biomass and productivity of ground vegetation and predation in the Palearctic