Invasions des corridors fluviaux du Sud-Ouest par des espèces végétales exotiques

L'étude de 245 sites répartis le long de huit rivières du Sud-Ouest de la France, depuis leur source jusqu'à leur embouchure, a permis d'identifier 420 espèces végétales exotiques au sein du corridor riverain, la rivière la plus colonisée étant l'Adour. Ces plantes présentent une vaste palette d'attributs biogéographiques, historiques, morphologiques, biologiques et écologiques. Dans leur grande majorité, elles ne sont pas particulièrement spécialisées vis-à-vis des conditions riveraines. La structure du corridor, le fonctionnement hydrologique, le degré d'anthropisation et le climat contrôlent l'arrivée et le maintien des espèces exotiques dans les zones riveraines des rivières. Etudié sur 32 sites de l'Adour, le pourcentage en espèces exotiques (par rapport à la richesse totale en espèces) n'est pas constant d'une station à l'autre. Il évolue selon les gradients environnementaux qui participent à la sectorisation fonctionnelle du fleuve. Ce pourcentage en exotiques reflète la sensibilité des systèmes riverains aux invasions et peut être utilisé comme un marqueur du dysfonctionnement de ces systèmes

Light interception principally drives the understory response to boxelder invasion in riparian forests

Since several decades, American boxelder (Acer negundo) is replacing white willow (Salix alba) riparian forests along southern European rivers. This study aims to evaluate the consequences of boxelder invasion on understory community in riparian areas. We determined the understory species richness, composition and biomass in boxelder and white willow stands located in three riparian forests, representative of three rivers with distinct hydrological regimes. We investigated correlation of these variables to soil moisture and particle size, main soil nutrient stocks, potential nitrification and denitrification, tree canopy cover and photosynthetic active radiation (PAR) at the ground level. A greenhouse experiment was then conducted to identify the causal factors responsible for changes in the understory. The effect of soil type, PAR level and water level on the growth and the biomass production of Urtica dioica were examined. A lower plant species richness and biomass, and a modification of community composition were observed for boxelder understory in all sites, regardless of their environmental characteristics. The strongest modification that follows boxelder invasion was the decline in U. dioica, the dominant species of the white willow forest understory. These differences were mainly correlated with a lower incident PAR under boxelder canopy. The greenhouse experiment identified PAR level as the main factor responsible for the changes in U. dioica stem number and biomass. Our results indicate that adult boxelder acts as an ecosystem engineer that decreases light availability. The opportunistic invasion by boxelder leads to important understory changes, which could alter riparian ecosystem functioning

Naturalised Vitis Rootstocks in Europe and Consequences to Native Wild Grapevine

The genus Vitis is represented by several coexisting species in Europe. Our study focuses on naturalised rootstocks that originate in viticulture. The consequences of their presence to the landscape and to native European species (Vitis vinifera ssp. silvestris) are evaluated. This study compares ecological traits (seven qualitative and quantitative descriptors) and the genetic diversity (10 SSR markers) of populations of naturalised rootstocks and native wild grapevines. 18 large naturalised rootstock populations were studied in the Rhône watershed. Wild European grapevines are present in four main habitats (screes, alluvial forests, hedges, and streamside hedges). In contrast, naturalised rootstock populations are mainly located in alluvial forests, but they clearly take advantage of alluvial system dynamics and connectivity at the landscape level. These latter populations appear to reproduce sexually, and show a higher genetic diversity than Vitis vinifera ssp. silvestris. The regrouping of naturalised rootstocks in interconnected populations tends to create active hybrid swarms of rootstocks. The rootstocks show characters of invasive plants. The spread of naturalised rootstocks in the environment, the acceleration of the decline of the European wild grapevine, and the propagation of genes of viticultural interest in natural populations are potential consequences that should be kept in mind when undertaking appropriate management measures

Patterns of ash (Fraxinus excelsior L.) colonization in mountain grasslands: the importance of management practices

International audienceWoody colonization of grasslands is often associated with changes in abiotic or biotic conditions or a combination of both. Widely used as fodder and litter in the past traditional agro-pastoral system, ash (Fraxinus excelsior L.) has now become a colonizing species of mountain grasslands in the French Pyrenees. Its present distribution is dependent on past human activities and it is locally controlled by propagule pressure and abiotic conditions. However, even when all favourable conditions are met, all the potentially colonizable grasslands are not invaded. We hypothesize that management practices should play a crucial role in the control of ash colonization. From empirical field surveys we have compared the botanical composition of a set of grasslands (present and former) differing in management practices and level of ash colonization. We have displayed a kind of successional gradient positively linked to both ash cover and height but not to the age of trees. We have tested the relationships between ash presence in grassland and management types i.e. cutting and/or grazing, management intensity and some grassland communities' features i.e. total and local specific richness and species heterogeneity. Mixed use (cutting and grazing) is negatively linked to ash presence in grassland whereas grazing alone positively. Mixed use and high grazing intensity are directly preventing ash seedlings establishment, when low grazing intensity is allowing ash seedlings establishment indirectly through herbaceous vegetation neglected by livestock. Our results show the existence of a limit between grasslands with and without established ashes corresponding to a threshold in the intensity of use. Under this threshold, when ash is established, the colonization process seems to become irreversible. Ash possesses the ability of compensatory growth and therefore under a high grazing intensity develops a subterranean vegetative reproduction. However the question remains at which stage of seedling development and grazing intensity these strategies could occur

Intermediate predator naïveté and sex-skewed vulnerability predict the impact of an invasive higher predator

The spread of invasive species continues to reduce biodiversity across all regions and habitat types globally. However, invader impact prediction can be nebulous, and approaches often fail to integrate coupled direct and indirect invader effects. Here, we examine the ecological impacts of an invasive higher predator on lower trophic groups, further developing methodologies to more holistically quantify invader impact. We employ functional response (FR, resource use under different densities) and prey switching experiments to examine the trait- and density-mediated impacts of the invasive mosquitofish Gambusia affinis on an endemic intermediate predator Lovenula raynerae (Copepoda). Lovenula raynerae effectively consumed larval mosquitoes, but was naïve to mosquitofish cues, with attack rates and handling times of the intermediate predator unaffected by mosquitofish cue-treated water. Mosquitofish did not switch between male and female prey, consistently displaying a strong preference for female copepods. We thus demonstrate a lack of risk-reduction activity in the presence of invasive fish by L. raynerae and, in turn, high susceptibility of such intermediate trophic groups to invader impact. Further, we show that mosquitofish demonstrate sex-skewed predator selectivity towards intermediate predators of mosquito larvae, which may affect predator population demographics and, perversely, increase disease vector proliferations. We advocate the utility of FRs and prey switching combined to holistically quantify invasive species impact potential on native organisms at multiple trophic levels

Invasions des corridors fluviaux du Sud-Ouest par des espèces végétales exotiques

L'étude de 245 sites répartis le long de huit rivières du Sud-Ouest de la France, depuis leur source jusqu'à leur embouchure, a permis d'identifier 420 espèces végétales exotiques au sein du corridor riverain, la rivière la plus colonisée étant l'Adour. Ces plantes présentent une vaste palette d'attributs biogéographiques, historiques, morphologiques, biologiques et écologiques. Dans leur grande majorité, elles ne sont pas particulièrement spécialisées vis-à-vis des conditions riveraines. La structure du corridor, le fonctionnement hydrologique, le degré d'anthropisation et le climat contrôlent l'arrivée et le maintien des espèces exotiques dans les zones riveraines des rivières. Etudié sur 32 sites de l'Adour, le pourcentage en espèces exotiques (par rapport à la richesse totale en espèces) n'est pas constant d'une station à l'autre. Il évolue selon les gradients environnementaux qui participent à la sectorisation fonctionnelle du fleuve. Ce pourcentage en exotiques reflète la sensibilité des systèmes riverains aux invasions et peut être utilisé comme un marqueur du dysfonctionnement de ces systèmes

Introductions de macrophytes aquatiques et riverains dans les hydrosystèmes français métropolitains : essai de bilan

Les introductions de macrophytes aquatiques et riverains sur le territoire métropolitain restent difficiles à évaluer, dans la mesure où les agents d'introduction sont multiples. Le devenir de ces introductions reste extrêmement variable, entre une régression rapide et une expansion génératrice de nuisances. Un premier bilan est néanmoins présenté pour les espèces aquatiques. Tous les types de milieux aquatiques sont concernés par ces introductions. La plupart des types biologiques végétaux, des algues aux arbres, sont susceptibles d'être introduits. Les facteurs potentiellement responsables lors d'expansions importantes de macrophytes sont discutés en regard de leurs caractéristiques biologiques et des hydrosystèmes récepteurs. Dans les cas d'expansions importantes, la gestion des milieux touchés est complexe. L'absence de réglementation adaptée aux risques environnementaux encourus est patente. Elle s'ajoute à un manque d'analyse globale des systèmes, à la méconnaissance et à l'imperfection des moyens adéquats permettant de limiter les risques environnementaux induits par ces introductions