Quantitative scenarios are coming of age as a tool for evaluating the impact of future socioeconomic development pathways on biodiversity and ecosystem services. We analyze global terrestrial, freshwater, and marine biodiversity scenarios using a range of measures including extinctions, changes in species abundance, habitat loss, and distribution shifts, as well as comparing model projections to observations. Scenarios consistently indicate that biodiversity will continue to decline over the 21st century. However, the range of projected changes is much broader than most studies suggest, partly because there are major opportunities to intervene through better policies, but also because of large uncertainties in projections.

Wild cherry (Prunus avium L.), a partially asexual self-incompatible forest tree, shows heterozygote excess, which is a poorly studied phenomenon. In three natural populations, we found significant heterozygote excess at almost all investigated loci (eight microsatellites and markers for the self-incompatibility locus). We examined four hypotheses to account for this observed heterozygote excess. First, negative F(IS) can result from a lack of selfed progeny in small populations of outcrossing species. A second explanation for negative F(IS) is selection during the life cycle of the most heterozygous individuals. A third explanation is negative assortative mating when reproduction occurs between individuals bearing phenotypes more dissimilar than by chance. The last explanation for negative F(IS) relies on asexual reproduction. Expectations for each hypothesis were tested using empirical data. Patterns of F(IS) differed among loci. Nevertheless, our experimental results did not confirm the small sample size hypothesis. Although one locus is probably under a hitch-hiking effect from the SI locus, we rejected the effect of the self-incompatibility locus for the genome as a whole. Similarly, although one locus showed a clear pattern consistent with the selection of heterozygous individuals, the heterosis effect over the whole genome was rejected. Finally, our results revealed that clonality probably explains significant negative F(IS) in wild cherry populations when considering all individuals. More theoretical effort is needed to develop expectations and hypotheses, and test them in the case of species combining self-incompatibility and partially asexual reproduction.

Tree species with wide distributions often exhibit different levels of genetic structuring correlated to their environment. However, understanding how environmental heterogeneity influences genetic variation is difficult because the effects of gene flow, drift and selection are confounded. We investigated the genetic variation and its ecological correlates in a wind-pollinated Mediterranean tree species, Fraxinus angustifolia Vahl, within a recognised glacial refugium in Croatia. We sampled 11 populations from environmentally divergent habitats within the Continental and Mediterranean biogeographical regions. We combined genetic data analyses based on nuclear microsatellite loci, multivariate statistics on environmental data and ecological niche modelling (ENM). We identified a geographic structure with a high genetic diversity and low differentiation in the Continental region, which contrasted with the significantly lower genetic diversity and higher population divergence in the Mediterranean region. The positive and significant correlation between environmental and genetic distances after controlling for geographic distance suggests an important influence of ecological divergence of the sites in shaping genetic variation. The ENM provided support for niche differentiation between the populations from the Continental and Mediterranean regions, suggesting that contemporary populations may represent two divergent ecotypes. Ecotype differentiation was also supported by multivariate environmental and genetic distance analyses. Our results suggest that despite extensive gene flow in continental areas, long-term stability of heterogeneous environments have likely promoted genetic divergence of ashes in this region and can explain the present-day genetic variation patterns of these ancient populations.

We examined large-scale patterns of morphology, genetic structure and ecological correlates of Fraxinus excelsior and the closely related species Fraxinus angustifolia in France, in order to determine the degree of hybridization between them. We sampled 24 populations in two putative hybrid zones (Loire and Saône), and five control populations of each species. We measured foliar characteristics of adult trees and used five nuclear microsatellites as molecular markers. Canonical discriminant analysis indicated that the two species differ in morphology, but that intermediate types are common in the Loire region but less frequent in the Saône region. Bayesian population assignment identified one F. angustifolia and two F. excelsior gene pools. Most Loire individuals clustered genetically with the F. angustifolia gene pool. In contrast, the Saône region presented individuals belonging mostly to F. excelsior pools, although the F. angustifolia type was frequent in certain populations. The lowest F(ST) values were found between the Loire and F. angustifolia controls that also exhibited no significant isolation by distance. The proportion of the F. angustifolia gene pool in each locality was negatively correlated with winter temperatures, suggesting that a cold climate may be limiting. Hybridization is probably favoured by the intermediate climatic conditions in the Loire region that allow both species to occur, but is somewhat hampered by the harsher winters in the Saône area where morphological introgression has apparently not yet occurred.

Aim The aim was to examine whether recent mortality can be explained by hydraulic failure linked to water deficit. Location Western Europe. Time period 1986–2014. Major taxa studied Forty‐four tree species. Methods We modelled the hydraulic safety margin (HSM) across the ranges of 44 tree species at their driest margin (n = 193,261 plots), defined as the difference between the estimated minimal soil water potential of each plot and the species water stress threshold, which corresponds to the hydraulic failure of the vascular system. Soil water potential was estimated by applying Campbell's equations on the minimal and maximal soil water contents estimated from 1979 to 2010 in the top 289 cm of soil and five soil textures across the species ranges. For each species, we modelled the amount of average mortality derived from plots of the Spanish and French National Forest Inventories to the variation in modelled hydraulic safety margin and environmental drivers across the species ranges using hurdle models. Results We did not identify any global convergence of modelled HSM within the species distribution ranges, finding instead a rather large variability in modelled HSM for most of the studied species. Fifteen species, out of 25 for which the models were practicable, showed significantly higher mortality in populations with negative HSM in comparison to those showing positive HSM, with positive and negative interaction along the aridity index. Main conclusions The combination of competition, average climate and modelled HSM explained average tree mortality. Most of the species presented at least one population that had already experienced a negative HSM and many other populations a positive but narrow HSM, suggesting that climate change is likely to push some populations towards a higher risk of hydraulic failure in the drier conditions projected for Western Europe.

The two closely related ash species Fraxinus excelsior L. (common ash) and Fraxinus angustifolia Vahl (narrow-leaved ash) have a broad contact zone in France where they hybridize. However, little is known about the local structure of hybrid zone populations and the isolation mechanisms. We assessed the potential effect of floral phenology on the structure of a riparian ash hybrid zone population in central France. The distribution of flowering times was unimodal and lay between the flowering periods of the two species. Using microsatellite markers, we detected isolation by time, which has possibly originated from assortative mating. Multivariate analyses indicated that morphological variation is not distributed at random with respect to flowering times. Spatial autocorrelation analyses showed that temporal and spatial patterns were tightly linked. Interestingly, despite the fact that the population shows isolation by time, neighbourhood size and historical dispersal variance (sigma = 63 m) are similar to those detected in pure stands of F. excelsior where individuals flower rather synchronously and hermaphrodites are not the most frequent sexual type. Trees flowering at intermediate dates, which comprised the majority of the population, produced on average more flowers and fruits. We detected no significant differences in floral parasite infections relative to reproductive timing, although there was a tendency for late flowering trees to suffer from more gall attack. We discuss the impact of temporal variation in fitness traits and their possible role in the maintenance of the hybrid zone.

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Ancient managed landscapes provide ideal opportunities to assess the consequences of habitat fragmentation on the patterns of genetic diversity and gene flow in long-lived plant species. Using amplified fragment length polymorphism (AFLP) and allozyme markers, we quantified seed-mediated gene flow and population genetic diversity and structure in 14 populations of Myrtus communis (myrtle), a common endozoochorous shrub species of forest patches in lowland agricultural Mediterranean areas. Overall, allozyme diversity for myrtle was low (P(95) = 25%; A = 1.411; H(e) = 0.085) compared to other known populations, and a significant portion of populations (57%) had lower levels of allelic diversity and/or heterozygosity than expected at random, as shown by simulated resampling of the whole diversity of the landscape. We found significant correlations between allozyme variability and population size and patch isolation, but no significant inbreeding in any population. Genetic differentiation among populations for both allozyme and AFLP markers was significant (Phi(ST) = 0.144 and Phi(ST) = 0.142, respectively) but an isolation-by-distance pattern was not detected. Assignment tests on AFLP data indicated a high immigration rate in the populations (ca. 20-22%), likely through effective seed dispersal across the landscape by birds and mammals. Our results suggest that genetic isolation is not the automatic outcome of habitat destruction since substantial levels of seed-mediated gene flow are currently detectable. However, even moderate rates of gene flow seem insufficient in this long-lived species to counteract the genetic erosion and differentiation imposed by chronic habitat destruction.