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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.

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 Koeieria 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. Key words: community ecology, genotypic diversity, perennial grasses, primary plant strategies, species coexistence, trait trade-offs. En ecologie vegetale, on a largement utilise les caracteres genetiques pour comprendre les regles regissant l'assemblage des communaules, el pour caracteriser les strategies primaires definissant la structure des communautes el les proprietes des ecosystemes. On ne sait pas vraiment si les caracteres ecologiquement importants aux echelles macroecologiques sont variables ou encore une consequence aux fines echelles dans l'espece. Les auteurs ont mesure la variation de caracteres a l'interieur de populations de plantes des prairies, les Festuca ovina L. et Koeleria macrantha (Ledeb.) Schult., pour verifier l'hypothese a savoir que la variation intraspecifique des caracteres aux fines echelles guide la structure de la communaute locale. Les deux especes montrent une importante variation genotypique pour tous les caracteres. Les caracteres lies a la grandeur chez les genotypes du K. macrantha observes en monoculture permettent de predire leur performance dans des communautes herbacees modeles possedant a la fois une diversite genetique et specifique. La meme correspondance s'avere beaucoup plus faible chez la population experimentale du F. ovina. De plus, les deux especes montrent l'existence d'une relation positive entre la surface foliaire specifique (SLA) et l'allocation a la masse des chaumes. La co-variatiou de ces caracteres indique l'existence de compromis intraspecifiques dans le cycle vital et des strategies de croissance, semblables a celles utilisees pour definir les strategies primaires des plantes, lesquelles tamponnent potentiellement l'abondance des deux especes et leur coexistence devant des defis environnemeniaux. Mots-cles : ecologie des communautes, diversite genotypique, herbes perennes, strategie primaire des plantes, coexistence des especes, compromis entre caracteres. [Traduit par la Redaction], Introduction Life-history theory predicts that species growing in specific environments will share similarity in certain key traits or combinations of traits (MacLeod 1894; MacArthur and Wilson 1963; Caswell et al. [...]

To purchase or authenticate to the full-text of this article, please visit this link: http://dx.doi.org/10.1111/j.1365-2745.2007.01275.x Byline: RAJ WHITLOCK (*), J. PHIL GRIME ([dagger]), ROSEMARY BOOTH ([dagger]), TERRY BURKE Keywords: community ecology; community genetics; competition and coexistence; genetic diversity; genotype-environment interactions; grasslands; species diversity Abstract: Summary A recent experiment varied the genetic diversity of model grassland communities under standardized soil and management conditions and at constant initial species diversity. After 5 years' growth, genetically diverse communities retained more species diversity and became more similar in species composition than genetically impoverished communities. Here we present the results of further investigation within this experimental system. We proposed that two mechanisms - the first invoking genetically determined and constant differences in plant phenotypes and the second invoking genotype-environment interactions - could each underpin these results. This mechanistic framework was used as a tool to interpret our findings. We used inter-simple sequence repeat (ISSR) DNA markers to confirm which of the individuals of six study species initially included in the model communities were unique genotypes. We then used the molecular markers to assess the survival and abundance of each genotype at the end of the 5-year experimental period. The DNA marker data were used to create, for the first time, a genotype abundance hierarchy describing the structure of a community at the level of genotypes. This abundance hierarchy revealed wide variation in the abundance of genotypes within species, and large overlaps in the performance of the genotypes of different species. Each genotype achieved a consistent level of abundance within genetically diverse communities, which differed from that attained by other genotypes of the same species. The abundance hierarchy of genotypes within species also showed consistency across communities differing in their initial level of genetic diversity, such that species abundance in genetically impoverished communities could be predicted, in part, by genotypic identity. Three species (including two canopy-dominants) experienced shifts in their community-level genotype abundance hierarchies that were consistent with an increased influence of genotype-environment interactions in genetically impoverished communities. Our results indicate that under relatively constant environmental conditions the species abundance structure of plant communities can in part be predicted from the genotypic composition of their component populations. Genotype-environment interactions also appear to shape the structure of communities under such conditions, although further experiments are needed to clarify the magnitude and mechanism of these effects. Author Affiliation: ([dagger])Unit of Comparative Plant Ecology, Department of Animal and Plant Sciences, University of Sheffield, Sheffield S10 2TN, UK Article History: Received 2 March 2007; accepted 21 May 2007Handling Editor: Honor C. Prentice Article note: (*) Author to whom correspondence should be addressed: R. Whitlock. E-mail: r.whitlock@sheffield.ac.uk.

Many endemic plant species belong to taxonomically complex groups. These endemics have often arisen as a consequence of recent and rapid evolutionary divergence facilitated by processes such as hybridization, polyploidy and/or breeding system transitions. The rapid and dynamic nature of divergence in taxonomically complex groups leads to problems in the implementation of traditional species-based approaches for the conservation of the biodiversity that they contain. Firstly, the taxa of interest can be difficult to define and identify, leading to practical difficulties in implementing conservation measures. Secondly, a species-based approach often fails to capture the complexity of diversity present in the taxonomically complex group. To accommodate these challenges, we have developed a Process-Based Species Action Plan approach. This is designed to conserve the processes leading to the generation of biodiversity, rather than focusing on the preservation of individual named taxa. We illustrate the approach using a group of endemic tree species ( Sorbus) on the Scottish island of Arran that have originated via a combination of multiple recent hybridization events and apomixis. The plan focuses on the optimization of habitat management to ensure the reproduction and regeneration of Sorbus in the zone in which these evolutionary processes operate, and to facilitate hybridization that will ensure the continued generation of diversity in this group. © 2011 The Linnean Society of London, Botanical Journal of the Linnean Society, 2012, 168, 194-203. [ABSTRACT FROM AUTHOR]

Recent evidence indicates that grassland community structure and species diversity are influenced by genetic variation within species. We review what is known regarding the impact of intraspecific diversity on grassland community structure, using an ancient limestone pasture as a focal example. Two genotype-dependent effects appear to modify community structure in this system. First, the abundance of individual constituent species can depend upon the combined influence of direct genetic effects stemming from individuals within the population. Second, the outcome of localized interspecific interactions occurring within the community can depend on the genotypes of participating individuals (indicating indirect genetic effects). Only genotypic interactions are thought to be capable of allowing the long-term coexistence of both genotypes and species. We discuss the implications of these effects for the maintenance of diversity in grasslands. Next, we present new observations indicating that losses of genotypic diversity from each of two species can be predicted by the abundance of other coexisting species within experimental grassland communities. These results suggest genotype-specific responses to abundance in other coexisting species. We conclude that both direct and indirect genetic effects are likely to shape community structure and species coexistence in grasslands, implying tight linkage between fine-scale genetic and community structure.