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To predict the threat of biological invasions to native species, it is critical that we understand how increasing abundance of invasive alien species (IAS) affects native populations and communities. The form of this relationship across taxa and ecosystems is unknown, but is expected to depend strongly on the trophic position of the IAS relative to the native species. Using a global metaanalysis based on 1,258 empirical studies presented in 201 scientific publications, we assessed the shape, direction, and strength of native responses to increasing invader abundance. We also tested how native responses varied with relative trophic position and for responses at the population vs. community levels. As IAS abundance increased, native populations declined nonlinearly by 20%, on average, and community metrics declined linearly by 25%. When at higher trophic levels, invaders tended to cause a strong, nonlinear decline in native populations and communities, with the greatest impacts occurring at low invader abundance. In contrast, invaders at the same trophic level tended to cause a linear decline in native populations and communities, while invaders at lower trophic levels had no consistent impacts. At the community level, increasing invader abundance had significantly larger effects on species evenness and diversity than on species richness. Our results show that native responses to invasion depend critically on invasive species' abundance and trophic position. Further, these general abundance-impact relationships reveal how IAS impacts are likely to develop during the invasion process and when to best manage them. [ABSTRACT FROM AUTHOR]

Traits have been widely used in plant ecology to understand the rules governing community assembly, and to characterize primary strategies that define community structure and ecosystem properties. Relatively little is known as to whether the traits that are ecologically important at macroecological scales are either variable, or of consequence at fine scales within species. We measured trait variation within populations of the grassland plants Festuca ovina L. and Koeleria macrantha (Ledeb.) Schult., to test the hypothesis that fine-scale intraspecific trait variation drives local community structure. Both species showed large genotypic variation for all traits. Size-related traits of genotypes of K. macrantha observed under monoculture predicted their performance in model grassland communities that possessed both genetic and species diversity. The same correspondence was much weaker for the experimental population of F. ovina. A trade-off in allocation between shoot mass and relative allocation to reproduction was evident in the experimental population of F. ovina. Furthermore, both species showed evidence of a positive relationship between specific leaf area (SLA) and allocation to culm mass. This trait covariation indicates the existence of intraspecific trade-offs in life-history and growth strategies similar to those used to define primary plant strategies, which potentially buffers both species abundance and species coexistence against environmental challenges. En écologie végétale, on a largement utilisé les caractères génétiques pour comprendre les règles régissant l'assemblage des communautés, et pour caractériser les stratégies primaires définissant la structure des communautés et les propriétés des écosystèmes. On ne sait pas vraiment si les caractères écologiquement importants aux échelles macro-écologiques sont variables ou encore une conséquence aux fines échelles dans l'espèce. Les auteurs ont mesuré la variation de caractères á l'intérieur de populations de plantes des prairies, les Festuca ovina L. et Koeleria macrantha (Ledeb.) Schult., pour vérifier l'hypothèse à savoir que la variation intraspécifique des caractères aux fines échelles guide la structure de la communauté locale. Les deux espèces montrent une importante variation génotypique pour tous les caractères. Les caractères liés à la grandeur chez les génotypes du K. macrantha observés en monoculture permettent de prédire leur performance dans des communautés herbacées modèles possédant à la fois une diversité génétique et spécifique. La même correspondance s'avère beaucoup plus faible chez la population expérimentale du F. ovina. De plus, les deux espèces montrent l'existence d'une relation positive entre la surface foliaire spécifique (SLA) et l'allocation à la masse des chaumes. La co-variation de ces caractères indique l'existence de compromis intraspécifiques dans le cycle vital et des stratégies de croissance, semblables à celles utilisées pour définir les stratégies primaires des plantes, lesquelles tamponnent potentiellement l'abondance des deux espèces et leur coexistence devant des défis environnementaux. [ABSTRACT FROM AUTHOR]

It is becoming apparent that genetic diversity can influence the species diversity and structure of ecological communities. Here, we investigated the intraspecific trait variation responsible for this relationship We grew 10 genotypes of the sedge Carex caryophyllea, as monocultures, under standardized conditions and measured traits related to morphology, growth, and life history. The same genotypes had been prominent in determining the structure of multispecies experimental communities, equivalent in species diversity, in which the genetic diversity of the constituent plant species had been varied in parallel. The trait measurements revealed substantial phenotypic variation among Carex genotypes, related predominantly to differences in physical size and to the spatial deployment of above- and belowground tissue. Genotypes successful in experimental communities were larger in size and tended to adopt a "guerrilla" clonal growth strategy. In general, multivariate trait summaries of genotype size (and to a lesser extent, variation along a linear discriminant axis) predicted genotype and species abundance in experimental communities. However, one genotype exhibited a large disparity in this respect. The performance of this genotype lay closer to prediction when it was growing with a highly competitive grass genotype. The strength of the relationship between genotype size and performance within communities decreased with decreasing community genetic diversity. These results indicate that intraspecific trait measurements are useful for predicting and understanding community structure. They also imply that competitive interactions between the genotypes of different species play an increased role in determining phenotype in genetically impoverished communities. [ABSTRACT FROM AUTHOR]