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Abstract Background Linear transportation infrastructures (roads, railways, oil and gas pipelines, powerlines and waterways) are recognized as important contributors to the fragmentation of species habitats. On the other hand, verges of linear transportation infrastructures (road and railway embankments, strips of grass under power lines or above buried pipelines, or waterway banks) form vast networks of continuous habitats. While the loss of natural habitats still poses a significant threat to biodiversity, verges can provide habitats or corridors in anthropogenic areas, although this potential for conservation remains controversial. The current paper is the first synthesis of evidence addressing this topic for vascular plants (except strictly aquatic species) in temperate ecosystems. We asked the following question: can linear transportation infrastructure verges constitute habitats and/or corridors for vascular plants in temperate ecosystems? Methods We conducted a systematic literature survey using two online bibliographic databases, three search engines, specialist websites, and by sending a call for literature to subject experts. We also integrated studies from a previous systematic review with an overlapping scope. We successively screened the articles for relevance on titles, abstracts and full texts using criteria detailed in an a priori protocol. We then used six specific questions to categorize the selected studies and critically assess them. These questions encompassed the potential of verges as habitats and corridors for vascular plants, and the effects of landscape and management on these potentialities. We created a database of the studies with low and medium risk of bias. We synthesized results for specific questions in narrative syntheses. Finally, studies about the habitat role of verges that met the criteria for a meta-analysis were used for quantitative syntheses. Review findings Our systematic literature survey yielded 101,524 search results. After critical appraisal, we included in our systematic review 294 articles that reported 316 studies. Most studies were conducted along road verges or waterway banks, with only a handful of studies involving powerlines, railways or pipelines. We were not able to draw conclusions on the role of verges as corridors for vascular plants as too few relevant studies were obtained. Regarding the habitat function of verges however, meta-analyses were conducted based on 205 cases from 47 primary studies that compared abundance and/or species richness in verges vs habitats away from transportation infrastructure for exotic, native or all species together. For non-highway road verges, both the abundance and richness of exotic species were higher on non-highway road verges, but we found no significant differences among species in general, or for native species specifically, which implies that alien species would often add but not subtract species. A wide variety of management practices were also represented in the evidence base. Overall, systematic impacts on species richness or abundance rarely emerged, but human interventions were seldom neutral and usually altered, at least temporarily, the balance between the native and exotic flora or among various functional groups. Conclusions We identified a major knowledge gap regarding the potential of linear transportation infrastructure verges as corridors for vascular plants. Thus, we call for more research on this particular topic, especially as the evidence synthesis underlined the potential of verges as habitat for exotic and invasive flora.

Abstract Background Linear transportation infrastructures (roads, railways, oil and gas pipelines, powerlines and waterways) generate well documented fragmenting effects on species habitats. However, the potential of verges of linear transportation infrastructures (road and railway embankments, strips of grass under power lines or above buried pipelines, or waterway banks) as habitat or corridor for biodiversity, remains controversial. In a context of constant loss of natural habitats, the opportunities of anthropogenic areas for compensating the loss of biodiversity they generated have to be considered. This paper is the first synthesis of evidence addressing this topic for vertebrates (mammals, birds, amphibians and reptiles) in temperate ecosystems. Methods We conducted a systematic literature survey using two online publication databases, three search engines, specialist websites, and by sending a call for literature to subject experts. We successively screened the articles for relevance on titles, abstracts and full texts using criteria detailed in an a priori protocol. We then used six specific questions to categorize the retained studies and to critically appraise them. These questions encompassed the potential of verges as habitats and corridors for vertebrates, and the effects of landscape and management on these potentialities. We critically appraised all studies to assess their risk of bias and created a database of the studies with low and medium risk of bias. We synthesized results for each specific question in narrative syntheses. Finally, studies that met meta-analysis requirements were used for quantitative syntheses. Results Our initial searches identified 83,565 documents. After critical appraisal, we retained 119 documents that reported 128 studies. Most studies were conducted in Europe (49%) and in the United States of America (22%), and were about mammals (61%) and birds (20%). Results from the narrative synthesis and meta-analyses converged and revealed that the potential of linear transportation infrastructures verges to constitute a habitat for vertebrate species varies according to the infrastructure and the biological group considered. Especially, highway verges may be a refuge for small mammals but seems detrimental to birds. The potential also varied depending on the landscape considered, with urbanisation being related to lower biodiversity hosted by verges. We found a wide variety of verge management practices with few studies on each practice, which prevented us from drawing general conclusions. Likewise, we found too few studies assessing the corridor potential of verges to be able to fully conclude although this potential seems to exist. We did not find any study assessing the effect of landscape context or management on the role of corridor of verges. Conclusions We identified a major knowledge gap regarding the potential of linear transportation infrastructure verges as corridors for vertebrates, and when they exist studies rarely directly measured movements on verges. We thus encourage more research on this topic and the development of protocols that enable direct measures of vertebrate movements. The effect of management practices on the role of habitat of verges also deserves further investigations, and research efforts should be coordinated to focus on one specific practice (e.g. vegetation management).

Abstract Background The role of linear transportation infrastructures (roads, railways, oil and gas pipelines, power lines, rivers and canals) in fragmenting natural habitats is fully acknowledged. Up to now, the potential of linear transportation infrastructures verges (road and railway embankments, strips of grass under power lines or above buried pipelines, or waterway banks) as habitat or corridor for biodiversity, remains controversial. In a context of decreasing natural habitats, the opportunities of anthropogenic areas for contributing to wildlife conservation have to be considered. The present paper is the first synthesis of evidence about the potential of linear transportation infrastructure verges as corridor and/or habitat for insects in temperate landscapes. Methods A systematic literature survey was made using two online publication databases, a search engine and by sending a call for literature to subject experts. Identified articles were successively screened for relevance on titles, abstracts and full texts using criteria detailed in an a priori protocol. We then used six specific questions to categorize and to critically appraise the retained studies. These questions encompassed the potential of verges as habitats and corridors for insects, and the effects of management and landscape context on these potentialities. A user-friendly database was created to sort the studies with low and medium susceptibility to bias. We used these studies to synthesize results of each specific question in a narrative synthesis. Finally, studies that met the meta-analysis requirements were used for a quantitative synthesis. Results Our searches identified 64,206 articles. After critical appraisal, 91 articles that reported 104 studies were included in our review. Almost all of them had “control-impact” design, only two studies used “before-after-control-impact” design, and one study used “before-after” design. In some cases, artificialization of transportation infrastructures lowered insect biodiversity while vegetation restoration had a moderate positive effect; the trend remained unclear for mowing/grazing practices. Urbanization and agriculture in the surroundings tended to lower the biodiversity hosted by verges, while natural and forested areas tended to promote it. No study dealt with the influence of management or surrounding landscape on insect dispersal along the verge. The small number of studies that compared the dispersal along verges and in habitats away from transportation infrastructures, together with the inconsistencies of their results, prevented us from drawing conclusions. Meta-analyses were performed on 709 cases from 34 primary studies that compared biodiversity in verges vs. other habitats. Overall insect species richness did not differ between LTI verges and compared habitats. Globally, insect abundance seemed higher on LTI verges than in compared habitats, a result driven by the higher abundance of pollinators and primary consumers on non-highway road verges than in habitats away from roads. Conclusions A major knowledge gap regarding the potential of linear transportation infrastructure verges as corridors for insects has been identified. Thus, we encourage more research on this topic. Infrastructure practitioners could benefit from our results about linear transportation infrastructure verges as habitat for certain taxa and about the impact of their management practices on insect abundance and species richness.