This study postulates that the invasive black cherry has originated from east of the Appalachian Mountains (mainly the Allegheny plateau) and its invasiveness in north-western Europe is mainly due to multiple introductions containing high numbers of individuals.

Several bee species are experiencing significant population declines. As bees exclusively rely on pollen for development and survival, such declines could be partly related to changes in their host plant abundance and quality. Here, we investigate whether generalist bumblebee species, with stable population trends over the past years, adapted their diets in response to changes in the distribution and chemical quality of their pollen resources. We selected five common species of bumblebee in NW Europe for which we had a precise description of their pollen diet through two time periods ('prior to 1950' and '2004-2005'). For each species, we assessed whether the shift in their pollen diet was related with the changes in the suitable area of their pollen resources. Concurrently, we evaluated whether the chemical composition of pollen resources changed over time and experimentally tested the impact of new major pollen species on the development of B. terrestris microcolonies. Only one species (i.e. B. lapidarius) significantly included more pollen from resources whose suitable area expanded. This opportunist pattern could partly explain the expansion of B. lapidarius in Europe. Regarding the temporal variation in the chemical composition of the pollen diet, total and essential amino acid contents did not differ significantly between the two time periods while we found significant differences among plant species. This result is driven by the great diversity of resources used by bumblebee species in both periods. Our bioassay revealed that the shift to new major pollen resources allowed microcolonies to develop, bringing new evidence on the opportunist feature of bumblebee in their diets. Overall, this study shows that the response to pollen resource drift varies among closely related pollinators, and a species-rich plant community ensures generalist species to select a nutrient-rich pollen diet.

We investigated the spatial genetic structure within and between two plots of Calluna vulgaris and the extent to which the soil seed bank di ered genetically from adults at seven allozyme loci. Averaged over the two plots, the seed bank and adult populations contained very similar levels of genetic diversity. Moreover, seeds contained in a single soil core (100 cm 3 ) exhibited similar mean allozyme diversity to the surrounding adult population, indicating that the seed bank preserves genetic diversity at a very local scale. Few di erences in allelic frequencies were found between the seed bank and its surrounding adult population in each plot. Mean G ST indicated a lack of di erentiation between the two plots at adult (G ST 0.008) and seed bank (G ST 0.002) stages. Low interplot di erentiation is consistent with the outcrossing mating system of the population (t m 0.91 in one plot) and its history of human disturbance. In contrast, spatial autocorrelation analysis of adults indicated a genetic structure at a very local scale, with positive autocorrelation for all alleles below 2 m in one plot and with a pattern of positive autocorrelation below 8 m in the two plots. Current limitation to seed dispersal rather than spatial extension of clones is thought to be responsible for local genetic structure.Keywords: allozymes, clonality, gene¯ow, genetic structure, seed bank, spatial autocorrelation. IntroductionGenetic structure is the nonrandom distribution of alleles or genotypes in space or time. It is expected that the spatial distribution of genetic variation within plant populations is not random, owing to the e ects of mating system, seed and pollen dispersal, clonality and selection (Levin & Kerster, 1974). Spatial genetic structure within a population is expected to be far more extensive in sel®ng than in preferentially outcrossing species, but spatial genetic structure has been detected even in outcrossing species (Loiselle et al., 1995). Recent empirical data suggest that entomophilous plants may show high levels of pollen¯ow even among populations (Hamrick et al., 1995). In contrast, seed dispersal is generally more limited (Levin & Kerster, 1974) resulting in localized gene¯ow that, alone, can cause spatial structuring through the action of genetic drift. Vegetative reproduction, in extreme cases, would result in clusters of genetically identical ramets (Sokal & Oden, 1978), and isolated clonally reproducing populations may diverge by di erential loss of alleles through drift and by ®xation of unique mutations. Restricted genē ow also allows di erentiation of subpopulations in response to local natural selection (Slatkin, 1987).Besides the spatial genetic structure, species with a seed bank may exhibit temporal genetic structure in the form of genetic di erences between the seed bank and the adult populations (Templeton & Levin, 1979). As a result, the seed bank could profoundly a ect the evolutionary potential of plant populations. Genetic variants lost from the standing population when adults die or f...

Although specialist pollination systems are often thought to be the outcome of selection, wide evidence exists for generalization within plant-pollinator interactions. We studied the pollination system and reproductive success of Calluna vulgaris (L.) Hull over 3 years in an Upper Ardennes site in Belgium. A wide variety of insect visitors was recorded including Hymenoptera, Diptera, and Lepidoptera. Relative abundance of families and species of visitors varied markedly among years except for bees. Bumblebees and honeybees were the most efficient pollinators at the study site. Syrphid flies, mainly large ones, acted as important co-pollinators because of their high relative frequency. Calluna pollen was also dispersed by wind, and insect-exclusion experiments demonstrated that wind pollination contributed to reproductive success. Following open pollination, fruit set was high, ranging from 82.0 to 97.2% and independent of sampling location. Mean seed number per fruit was less than maximum potential seed number and varied strongly among sampling locations. We discuss this pollination system (generalist insect and wind) in terms of the ecological and geographical success of the species.Key words: Calluna vulgaris, pollinator visitation, insects, wind pollination, reproductive success.

SUMMARYGenetic diversity, population genetic structure and gene flow in Calluna vulgaris (L.) Hull were assessed by means of seven allozyme loci scored in 18 populations from the South-Western area of the species' range. Genetic diversity was lower (H.^ = 0-20) than reported for long-lived widespread species but was characterized by a high number of alleles per locus (5'60at the species level) of which more than 70'-\) were rare. More than 95 "r, of genetic variation vi'as found at the intrapopulation level (G^T -0'047). High levels of past gene flow-were inferred, based on the allozyme data (A'^ = 5-2 from G^T, A''^, = 10-2 from the 'private allele' method). Calluna vulgaris exhibited several geographic patterns of genetic variation. Both cluster analysis, constructed with various genetic distances and principal components analysis showed that Spanish and Pyrenean populations were clearly different from those collected in the Massif Central and Belgium. Also, a trend for decreasing genetic diversity towards Northern populations was detected. These patterns might be related to the post-glacial history' of Calluna. In addition, it is shown that isolation by distance has played a role in the geographic shaping of genetic \'ariation in this species.