Summary 1Relationships between plant population size, fitness and within-population genetic diversity are fundamental for plant ecology, evolution and conservation. We conducted meta-analyses of studies published between 1987 and 2005 to test whether these relationships are generally positive, whether they are sensitive to methodological differences among studies, whether they differ between species of different life span, mating system or rarity and whether they depend on the size ranges of the studied populations. 2 Mean correlations between population size, fitness and genetic variation were all significantly positive. The positive correlation between population size and female fitness tended to be stronger in field studies than in common garden studies, and the positive correlation between genetic variation and fitness was significantly stronger in DNA than in isoenzyme studies. 3 The strength and direction of correlations between population size, fitness and genetic variation were independent of plant life span and the size range of the studied populations. The mean correlations tended to be stronger for the rare species than for common species. 4 Expected heterozygosity, the number of alleles and the number or proportion of polymorphic loci significantly increased with population size, but the level of inbreeding F IS was independent of population size. The positive relationship between population size and the number of alleles and the number or proportion of polymorphic loci was stronger in self-incompatible than in self-compatible species. Furthermore, fitness and genetic variation were positively correlated in self-incompatible species, but independent of each other in self-compatible species. 5 The close relationships between population size, genetic variation and fitness suggest that population size should always be taken into account in multipopulation studies of plant fitness or genetic variation. 6 The observed generality of the positive relationships between population size, plant fitness and genetic diversity implies that the negative effects of habitat fragmentation on plant fitness and genetic variation are common. Moreover, the stronger positive associations observed in self-incompatible species and to some degree in rare species, suggest that these species are most prone to the negative effects of habitat fragmentation.

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Defense against herbivores often changes dramatically as plants develop. Hypotheses based on allocation theory and herbivore selection patterns predict that defense should increase or decrease, respectively, across ontogeny, and previous research partly supports both predictions. Thus, it remains unclear which pattern is more common and what factors contribute to variability among studies. We conducted a meta-analysis of 116 published studies reporting ontogenetic patterns in plant defense traits and herbivory. Patterns varied depending on plant life form (woody, herbaceous, grass), type of herbivore (insect, mollusk, mammal), and type of defense trait (secondary chemistry, physical defense, tolerance). In woody plants, chemical defense increased during the seedling stage, followed by an increase in physical defenses during the vegetative juvenile stage. Mammalian herbivores showed a strong preference for mature compared to juvenile tissues in woody plants. Herbs experienced a significant increase in secondary chemistry across the entire ontogenetic trajectory, although the magnitude of increase was greatest during the seedling stage. Correspondingly, mollusks preferred young compared to older herbs. Future research investigating growth/defense trade-offs, allometry, herbivore selection patterns, and ecological costs would shed light on the mechanisms driving the ontogenetic patterns observed.

Ecosystem effects of biodiversity manipulations in European grasslands AbstractWe present a multisite analysis of the relationship between plant diversity and ecosystem functioning within the European BIODEPTH network of plant-diversity manipulation experiments. We report results of the analysis of 11 variables addressing several aspects of key ecosystem processes like biomass production, resource use (space, light, and nitrogen), and decomposition, measured across three years in plots of varying plant species richness at eight different European grassland field sites. Differences among sites explained substantial and significant amounts of the variation of most of the ecosystem processes examined. However, against this background of geographic variation, all the aspects of plant diversity and composition we examined (i.e., both numbers and types of species and functional groups) produced significant, mostly positive impacts on ecosystem processes.Analyses using the additive partitioning method revealed that complementarity effects (greater net yields than predicted from monocultures due to resource partitioning, positive interactions, etc.) were stronger and more consistent than selection effects (the covariance between monoculture yield and change in yield in mixtures) caused by dominance of species with particular traits. In general, communities with a higher diversity of species and functional groups were more productive and utilized resources more completely by intercepting more light, taking up more nitrogen, and occupying more of the available space. Diversity had significant effects through both increased vegetation cover and greater nitrogen retention by plants when this resource was more abundant through N2 fixation by legumes. However, additional positive diversity effects remained even after controlling for differences in vegetation cover and for the presence of legumes in communities. Diversity effects were stronger on above-than belowground processes. In particular, clear diversity effects on decomposition were only observed at one of the eight sites.The ecosystem effects of plant diversity also varied between sites and years. In general, diversity effects were lowest in the first year and stronger later in the experiment, indicating that they were not transitional due to community establishment. These analyses of our complete ecosystem process data set largely reinforce our previous results, and those from comparable biodiversity experiments, and extend the generality of diversity-ecosystem functioning relationships to multiple sites, years, and processes. Abstract. We present a multisite analysis of the relationship between plant diversity and ecosystem functioning within the European BIODEPTH network of plant-diversity manipulation experiments. We report results of the analysis of 11 variables addressing several aspects of key ecosystem processes like biomass production, resource use (space, light, and nitrogen), and decomposition, measured across three years in plots of varying plant species ri...

In this review, we test the hypothesis that abiotic stress increases the suitability of plants as food for herbivores. We conducted a meta-analysis that included 70 experimental studies in which insect performance was measured on woody plants subjected to water stress, pollution, and/or shading. Overall, plant stress had no significant effect on insect growth rate, fecundity, survival, or colonization density. We found great variation, however, in the magnitude and direction of insect responses among studies, most of which was related to insect feeding guild. In general, boring and sucking insects performed better on stressed plants, whereas plant stress adversely affected gall-makers and chewing insects. Reduction in performance of chewers was greater on stressed slow-growing plants than on stressed fast growers. Reproductive potential of sucking insects was increased by pollution but reduced by water stress. In some cases where sample sizes were small or the treatment periods short, apparent differences in insect responses to stress were probably artifacts due to inappropriate experimental design.