Audrey Muratet

Does the invasive species Reynoutria japonica have an impact on soil and flora in urban wastelands

Invasive plants are recognised as a major threat to biodiversity. Although they are well-established in natural areas, the supposed negative impacts of invasive plants upon communities and ecosystems have so far been poorly investigated in urban areas, where invasions are a main issue for ecologists and for urban planners and managers. We propose to assess the effects of an invasive species along an invasion gradient in a typical urban habitat. We focused on the Japanese knotweed (Reynoutria japonica Houtt.), a widespread invasive species in Europe and North America. We considered eight urban wastelands invaded by this species in the heart of the Greater Paris Area, France. On each site, we ran four transects from the centre of the Japanese knotweed patch towards the uninvaded peripheral vegetation. We recorded the flora using the line intercept method, and several soil parameters (thickness of A horizon, abundance of earthworm casts, topsoil Munsell value, pH) every metre along each transect. The A horizon was thicker and the topsoil darker under R. japonica canopy. Thus, this invasive plant species seemed to influence soil organic matter pool. However, our results also steadily showed that R. japonica locally excluded and/or severely reduced the cover of many plant species through competition. Our study clarified the local effects of R. japonica: an influence on the soil organic matter, and a severe negative impact on wasteland plant communities. We suggest implications in both conservation and restoration ecology.

Evaluation of floristic diversity in urban areas as a basis for habitat management

ABSTRACT Questions: How can floristic diversity be evaluated in conservation plans to identify sites of highest interest for biodiversity? What are the mechanisms influencing the distribution of species in human-dominated environments? What are the best criteria to identify sites where active urban management is most likely to enhance floristic diversity? Location: The Hauts-de-Seine district bordering Paris, France. Methods: We described the floristic diversity in one of the most urbanized French districts through the inventory of ca. 1000 sites located in 23 habitats. We built a new index of floristic interest (IFI), integrating information on richness, indigeneity, typicality and rarity of species, to identify sites and habitats of highest interest for conservation. Finally, we explored the relationship between site IFI and land use patterns (LUP). Results: We observed a total of 626 vascular plant species. Habitats with highest IFI were typically situated in semi-natural environments or environments with moderate human impact. We also showed that neighbouring (urban) structures had a significant influence on the floristic interest of sites: for example, the presence of collective dwellings around a site had a strong negative impact on IFI. Conclusions: Our approach can be used to optimize management in urban zones; we illustrate such possibilities by defining a ‘Site Potential Value’, which was then compared with the observed IFI, to identify areas (e.g. river banks) where better management could improve the districts biodiversity. Nomenclature: Kerguélen (2003).

Biogeographic comparisons of herbivore attack, growth and impact of Japanese knotweed between Japan and France

To shed light on the process of how exotic species become invasive, it is necessary to study them both in their native and non-native ranges. Our intent was to measure differences in herbivory, plant growth and the impact on other species in Fallopia japonica in its native and non-native ranges. We performed a cross-range full descriptive, field study in Japan (native range) and France (non-native range). We assessed DNA ploidy levels, the presence of phytophagous enemies, the amount of leaf damage, several growth parameters and the co-occurrence of Fallopia japonica with other plant species of herbaceous communities. Invasive Fallopia japonica plants were all octoploid, a ploidy level we did not encounter in the native range, where plants were all tetraploid. Octoploids in France harboured far less phytophagous enemies, suffered much lower levels of herbivory, grew larger and had a much stronger impact on plant communities than tetraploid conspecifics in the native range in Japan. Our data confirm that Fallopia japonica performs better-plant vigour and dominance in the her-baceous community-in its non-native than its native range. Because we could not find octoploids in the native range, we cannot separate the effects of differences in ploidy from other biogeographic factors. To go further, common garden experiments would now be needed to disentangle the proper role of each factor, taking into account the ploidy levels of plants in their native and non-native ranges. Synthesis. As the process by which invasive plants successfully invade ecosystems in their non-native range is probably multifactorial in most cases, examining several components-plant growth, herbivory load, impact on recipient systems-of plant invasions through biogeographic comparisons is important. Our study contributes towards filling this gap in the research, and it is hoped that this method will spread in invasion ecology, making such an approach more common.

Use of extensive habitat inventories in biodiversity studies.

Large monitoring programs exist in many countries and are necessary to assess present and past biodiversity status and to evaluate the consequences of habitat degradation or destruction. Using such an extensive data set of the floristic richness in the Paris Ile-de-France region (France), we compared different sampling efforts and protocols in different habitat units to highlight the best methods for assessing the actual plant biodiversity. Our results indicate that existing data can be used for a general understanding of site differences, but analysts should be aware of the limitations of the data due to non-random selection of sites, inconsistent observer knowledge, and inconsistent sampling period. The average species diversity recorded in a specific habitat does not necessarily reflect its actual diversity, unless the monitoring effort was very strong. Overall, increasing the sampling effort in a given region allows improvement of the (1) number of habitats visited, (2) the total sampled area for a given habitat type, (3) the number of seasons investigated. Our results indicate that the sampling effort should be planned with respect to these functional, spatial and temporal heterogeneities, and to the question examined. While the effort should be applied to as many habitats as possible for the purpose of capturing a large proportion of regional diversity, or comparing different regions, inventories should be conducted in different seasons for the purpose of comparing species richness in different habitats.