| Summary: | Man shares crops and a number of parasites with rodents. Every environmental modification and change in human activities can lead to an increased risk of rodent outbreaks. Some biological processes that determine rodent populations and outbreaks are now better known (Figure 1). This concerns three kinds of environmental modification: (i) those resulting from the extension of habitats favourable to some species, (ii) those resulting from landscape homogenization (with or without extension of favourable habitats), (iii) those resulting from the creation of new habitats favourable to species already present or to species with a high adaptive capacity and high colonization potential. Several examples of rodent outbreaks due to such modifications are reported. Outbreaks of grassland rodents in France illustrate the effects of land composition at different scales. This provides a framework for assessing outbreak risks at the regional scale (Figure 2). At a more local scale, e.g. the commune, heterogeneity due to hedgerow networks or woody mosaics significantly decreases the extent of density variations (Figure 3). Furthermore, landscape homogeneization induces greater extension of outbreaks (Figures 4, 5 and 6). Some rodent species are cyclical. In grassland landscapes of Franche-Comté, in eastern France, these cycles last 3-4 years for the common vole, and 5-6 years for the fossorial water vole. Density variations for forest rodents of similar size in this region show a synchronicity trend with respect to grassland rodent cycles (Figure 7). In northeastern France, outbreaks of forest rodents induce regular epidemics of haemorrhagic fever with renal syndrome (Figure 8). Suitable studies should be carried out to check whether these epidemic outbreaks are linked with grassland rodent dynamics, as already demonstrated in the Franche-Comté grassland landscape. Moreover, the time-lag between the cycles of common vole and of fossorial water vole (two species of different size) provides favourable conditions for maintaining ...
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