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Northern vole populations exhibit large-scale, spatially synchronous population dynamics. Such cases of population synchrony provide excellent opportunities for distinguishing between local intrinsic and regional extrinsic mechanisms of population regulation. Analyses of large-scale survey data and theoretical modelling have indicated several plausible synchronizing mechanisms. It is difficult, however, to determine the most important one without detailed data on local demographic processes. Here we combine results from two field studies in southeastern Norway--one identifies local demographic mechanisms and landscape-level annual synchrony among 28 enclosed experimental populations and the other examines region-level multi-annual synchrony in open natural populations. Despite fences eliminating predatory mammals and vole dispersal, the growth rates of the experimental populations were synchronized and moreover, perfectly linked with vole abundance in the region. The fates of 481 radio-marked voles showed that bird predation was the synchronizing mechanism. A higher frequency of risky dispersal movements in slowly growing populations appeared to accelerate predation rate. Thus, dispersal may induce a feedback-loop between predation and population growth that enhances synchrony.

We studied dispersal movements in 12 enclosed, patchy populations of root voles (Microtus oeconomus) during the breeding season. The habitat was manipulated experimentally so that there were four replicates of three types of habitat patch configuration: two large patches, six small isolated patches, and six small patches of which two patch triplets were connected by corridors. The total habitat area (1350 m 2 ) and the distance to nearest neighboring patch (15 m) were the same in the three different configurations. The matrix area between the patches was kept open and uninhabitable for voles by weekly mowing. Dispersal was defined as shifts between patches. Movements across the open matrix habitat to traps located along the edge of the enclosures were also recorded.The frequency of shifts between patches was higher among small than among large patches. Corridors channeled dispersal between corridor-connected patches but did not enhance the frequency of shifts between patches at the population level. Dispersal was strongly density dependent, and most so for subadult animals. High-density patches had low emigration rates. Root voles immigrated onto patches with a smaller number of individuals, especially of their own sex and reproductive state, than were present in the patch they left. Thus most shifts between patches took place from patches with relatively low density to patches with even lower density. Small patches had higher spatiotemporal variability in density and demographic composition than large patches, and this probably caused the higher dispersal rate among small patches. Emigration and immigration contributed most to the demographic turnover in small patches. In particular, emigration was the main demographic parameter behind declining numbers and patch extinction in small patches with few individuals.Our study highlights the importance of taking patch-specific conditions such as patch size, demographic composition, and spatiotemporal demographic variability into account when studying dispersal rates. Moreover, the kind of density-dependent emigration-immigration dynamics found in our study does not match the common perception that dispersal works primarily to reduce extinction probabilities through rescue effects. In particular, the impact of emigration as a factor that may increase the extinction probability of small, isolated patches with few individuals needs to be considered in future studies.

Mice, rats, and other rodents threaten food production and act as reservoirs for disease throughout the world. In Asia alone, the rice loss every year caused by rodents could feed about 200 million people. Damage to crops in Africa and South America is equally dramatic. Rodent control often comes too late, is inefficient, or is considered too expensive. Using the multimammate mouse (Mastomys natalensis) in Tanzania and the house mouse (Mus domesticus) in southeastern Australia as primary case studies, we demonstrate how ecology and economics can be combined to identify management strategies to make rodent control work more efficiently than it does today. Three more rodent–pest systems – including two from Asia, the rice‐field rat (Rattus argentiventer) and Brandt's vole (Microtus brandti), and one from South America, the leaf‐eared mouse (Phyllotis darwini) – are presented within the same bio‐economic perspective. For all these species, the ability to relate outbreaks to interannual climatic variability creates the potential to assess the economic benefits of forecasting rodent outbreaks.

R odents are among the most important global pests (Prakash 1988; Singleton et al. 1999a), and are often featured in fiction, as in Albert Camus' The Plague. The black rat (Rattus rattus) played a key role in the plague during the Middle Ages (Scott and Duncan 2001). Farmers in many parts of the world, particularly those in developing countries, tend to view economic losses due to rats and mice as unavoidable (Posamentier 1997; Singleton et al. 1999a). In fact, the impact of rodents has been greatly underestimated and largely ignored in the general scientific literature, with a small number of exceptions (Elton 1942; Singleton et al. 1999a). Competing with rodents for food Worldwide, there are about 1700 species of rodents, but only 5-10% are major pest species in agricultural and urban environments, and even fewer cause problems over larger geographic areas. Some of these consume substantial amounts of agricultural produce (Table 1), and in many developing countries, farmers consider rodents the main impediment to higher yields (Makundi et al. 1999). Every year, rats in Asia consume food crops that could feed 200 million people for an entire year (Singleton 2003). In Indonesia, rodents are the most important pre-harvest pests in economic terms, causing on average at least 15% annual losses of rice (Geddes 1992). In Africa, the numbers are similar. Damage due to rodents in Tanzania causes an estimated annual yield loss of 5-15% of maize (corn), corresponding to about $45 million, and food which could feed about 2 million people (Leirs 2003; Table 1). In parts of South America, native rodents cause crop damage varying between 5-90% of total production (Rodriquez 1993). Obviously, we need better pest control strategies than we have today. The design of rodent control strategies has both an ecological dimension, relating to the interaction of the pest population and its resources and enemies (Singleton et al. 1999a), and an economic dimension, relating to crop damage, which affects yield, and the use of pesticides, which in turn affects production costs (Carlson and 367

Models of source–sink population dynamics have to make assumptions about whether, and eventually how, demographic parameters in source habitats are dependent on the demography in sink habitats. However, the empirical basis for making such assumptions has been weak. Here we report a study on experimental root vole populations, where estimates of demographic parameters were contrasted between source patches in source–sink (treatment) and source–source systems (control). In the presence of a sink patch (simulated by a pulsed removal of immigrants), source‐patch populations failed to increase over the breeding season, mainly due to a high spatially density‐dependent dispersal rate from source to sink patches. The per capita recruitment rate was almost two times higher in source–sink than in the source–source systems, but this did not compensate for the loss rate due to dispersal from source to sink patches. Sex ratio in the source–sink systems became less female biased, probably as a result of an enhanced frequency of dispersal movements in females. Good knowledge of the degree of density‐and habitat‐dependent dispersal is critical for predicting the dynamics of source–sink populations.

The synchronization of the dynamics of spatially subdivided populations is of both fundamental and applied interest in population biology. Based on theoretical studies, dispersal movements have been inferred to be one of the most general causes of population synchrony, yet no empirical study has mapped distance-dependent estimates of movement rates on the actual pattern of synchrony in species that are known to exhibit population synchrony. Northern vole and lemming species are particularly well-known for their spatially synchronized population dynamics. Here, we use results from an experimental study to demonstrate that tundra vole dispersal movements did not act to synchronize population dynamics in fragmented habitats. In contrast to the constant dispersal rate assumed in earlier theoretical studies, the tundra vole, and many other species, exhibit negative density-dependent dispersal. Simulations of a simple mathematical model, parametrized on the basis of our experimental data, verify the empirical results, namely that the observed negative density-dependent dispersal did not have a significant synchronizing effect.

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