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In order to map the spatial distribution of twenty tree species groups over Europe at 1 km 9 1 km resolution, the ICP-Forest Level-I plot data were extended with the National Forest Inventory (NFI) plot data of eighteen countries. The NFI grids have a much smaller spacing than the ICP grid. In areas with NFI plot data, the proportions of the land area covered by the tree species were mapped by compositional kriging. Outside these areas, these proportions were mapped with a multinomial multiple logistic regression model. A soil map, a biogeographical map and bioindicators derived from temperature and precipitation data were used as predictors. Both methods ensure that the predicted proportions are in the interval [0,1] and sum to 1. The regression predictions were iteratively scaled to the National Forest Inventory statistics and the Forest map of Europe. The predicted proportions for the twenty tree species were validated by the Bhattacharryya distance between predicted and observed proportions at 230 plot data separated from the calibration data. Besides, the map with the predicted dominant species was validated by computing the error matrix. The median Bhattacharryya distance in the subarea with NFI plot data was 1.712, whereas in the subarea with ICP-Level-I data, this was 2.131. The scaling did not significantly decrease the Bhattacharryya distance. The estimated overall accuracy of this map was 43%. In areas with NFI plot data, overall accuracy was 57%, outside these areas 33%. This gain was mainly attributable to the much denser plot data, less to the prediction method.

Eutrophication due to high anthropogenic nutrient loading has greatly impacted ecological processes in marine coastal waters and, therefore, much effort has been put into reducing nitrogen and phosphorus discharges into European and North-American waters. Nutrient enrichment usually resulted in increase of biomass and production of phytoplankton and microphytobenthos, often coinciding with shifts in species composition within the primary producer community. Consequences of increasing eutrophication for higher trophic levels are still being disputed, and even less is known about the consequences of nutrient reduction on coastal food webs. Here, we present 30-year concurrent field observations on phytoplankton, macrozoobenthos and estuarine birds in the Dutch Wadden Sea, which has been subject to decades of nutrient enrichment and subsequent nutrient reduction. We demonstrate that long-term varia-

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Genetic dierentiation among populations of the moor frog (Rana arvalis) was tested on a spatial scale where some dispersal between populations is expected to occur, in a landscape in The Netherlands that has become fragmented fairly recently, in the 1930s. Five microsatellite loci were used, with 2±8 alleles per locus. F IS was 0.049 across loci, and most populations were in HW equilibrium. The degree of population subdivision was low (F ST 0.052). A signi®cant positive correlation between genetic distance and geographical distance was found, indicating a limitation in dispersal among populations due to distance. To test the impact of the landscape mosaic on the connectivity between patches, distance measures were corrected for relative amounts of habitat types with known positive or negative in¯uence on moor frog dispersal. Notably, the resistance variable for the fraction of negative linear elements (roads and railways) gave a higher explanatory value than geographical distance itself. Therefore, it is particularly the number of barriers (roads and railways) between populations that emerges as a factor that reduces exchange between populations. It is concluded that genetic techniques show promise in determining the in¯uence of landscape connectivity on animal dispersal.Keywords: dispersal, gene¯ow, landscape resistance, microsatellite marker, metapopulation, Rana arvalis. IntroductionFragmentation is the result of the destruction of habitat suitable for a species, resulting in small fragments, or patches, that are separated by unsuitable habitat or other barriers (Hanski & Gilpin, 1991; Opdam et al., 1993; Hanski & Simberlo, 1997). Many species have a naturally disjunct distribution pattern, associated with the heterogeneity of the landscape. For instance, the moor frog (Rana arvalis) requires both terrestrial and aquatic habitats during its life cycle, and so migrations between spatially separated habitats occur naturally in its life history. However, if habitat fragmentation is the result of man-made changes in land use, we want to know to what extent species are coping with this fragmentation of their habitat. An important prerequisite for regional survival is that connectivity within the habitat network (or metapopulation, Levins, 1970) is sucient to enable individuals to disperse between patches (Opdam, 1990; Wiens, 1997). For grounddwelling species with low dispersal capacity, such as frogs, connectivity depends on the distance between suitable habitat patches and on the relative resistance of the landscape mosaic between patches (Forman & Godron, 1986; Wiens, 1997; Bennett, 1999). Hartung (1991) demonstrated that the movement patterns of moor frogs are in¯uenced by the landscape mosaic: moor frogs prefer ditches and hedgerows and avoid dry and open areas. A negative eect of road density on the distribution pattern of the moor frog was demonstrated in an earlier study (Vos & Chardon, 1998).Although quantifying dispersal distances has a high research priority, progress is slow, as these studies tend to...