The Pollinear project is part of the current context of the decline of pollinating insects and its consequences on the pollination of wild and cultivated plants. The main reasons for this decline are the intensification of agricultural practices and the loss and fragmentation of habitats, of which linear transport infrastructure (LTI) is one of the important causes. However, the green dependencies (DV) of the LTIs are areas relatively undisturbed by human activities, often comparable to grassland and extended to all territories crossed. Pollinear aimed to evaluate, explain, and even make operational proposals to develop the potential of ILT DVs as habitats and sources of wild pollinators (apiform hymenoptera or "wild bees" and lepidoptera rhopalocères or "butterflies"). The issues addressed were:
- analysis of the potential of DV as habitat for wild pollinators (wild bees and butterflies) - study of Habitat function;
- analysis of the ability of DVs to act as a source of pollinators for the surrounding landscape mosaic - study of the Source function;
- the analysis of the connectivity allowed to insects within the DV- study of the Conduit function.
The LTIs considered to provide these case studies were those of the unpaved (auto) road network, the rail network, the waterways and the electricity network (high and medium voltage). The area of investigation covered the regions of Brittany, Pays-de-Loire, Poitou-Charentes, Limousin, Aquitaine, Midi-Pyrénées.
In order to highlight the cross-cutting exchanges, the Habitat-Source (HS) study sites had to contain an attractive food resource at the edge of the right-of-way. The fields of rapeseed (Brassica napus) were chosen. In order to avoid the lateral dispersal of insects, the Habitat-Conduit (HC) study sites had to be lined with unattractive environments. The forest massifs have been chosen, constituting green dependencies in forest trenches.
The selected HS sites were non-conceded road network DVs located in Brittany, Pays-de-Loire and Poitou (6 sites studied). The selected HC sites were DVs in the high-voltage electricity grid located in Limousin and Aquitaine (31 sites studied + 25 semi-natural reference sites: meadows, heaths, lawns).
A taxonomic analysis was carried out on the HS and HC sites and observations were collected on the quality of the habitats offered by road DV and LHT in forest trenches. HC sites are used to analyze the similarities / dissimilarities between their insect communities and the influence of environmental variables related to the habitat offered by forest trenches in particular. Conclusions are drawn as to the relationships that can be established between remote sites within forest trenches and the maintenance measures that would make it possible to develop these habitats.
Green road dependence presenting a great floristic richness.
Photo credit © Violette Le Féon
Contributions and resultsSpecies richness (butterflies and bees) and bee abundances are comparable between forest trench sites and reference sites, while a small proportion of bee species (35%) are common to both types of sites. (74% for butterflies). Forest trench sites offer less plant diversity but more nesting resources for bees (bare soil, dead wood, hollow stems) than reference sites. Parasitic bees, which are significantly more numerous (abundance and species richness) in forest trench sites, are good indicators of the host populations.
DVs host common species, species for which data are scarce, endangered species under the Habitats Directive and some identified outside their known distribution area. The data set indicates that DVs can contribute to the regional pool of wild bee species and butterflies.
Similarities between bee communities established in remote sites suggest the possible existence of longitudinal exchanges within forest trenches. Similarities are also observed between communities of bees in forest trench sites and those of neighboring reference sites, suggesting possible cross-cutting exchanges with the landscape mosaic.
Recommendations for action
Because of the contribution they can make to the regional pool of bee species, forest trenches are carefully maintained habitats. In forest-dominated landscapes, they offer the opportunity for pollinator-friendly habitats and interconnection between distant open habitats. In places where the forest extends spontaneously, the maintenance of the DV of power lines allows to maintain in the landscape relics of open environments (eg dry lawns).
The good habitat conditions in the forest trenches are always fortuitous and show the margins of improvement easily accessible in the maintenance of DV to make them more suitable for wild pollinators: regulation of ferns, promotion of the floristic richness, preservation of nesting resources (bare soil, stumps and branches, hollow-stemmed plants).
The findings on road DVs show the expected progress of the floristic diversification of herbaceous plants by simple mowing, and the maintenance of a spontaneous shrubby vegetation mainly entomogamous and often useful to cavicultural bees (brambles, brooms). Roadside DVs are marked by the absence of bare soil, linked to the predominance of grasses which goes hand in hand with the nutrient richness of the soil due to the lack of export of mowing products.
Given their contribution to the knowledge on the distribution of wild bee species, the data collected will be able to feed current inventories (regional and national, even European). In order to support conclusions about the potential of DVAs as habitat, managers may in some places implement appropriate maintenance that would result in assessments of increased abundance and diversity of butterflies and bees. The stakes of the Source function (integration of DVs with the green frame and pollination service) would deserve that its specific study be renewed.
As part of the IENE 2016 conference:
- The habitat value of power line of beefs and butterflies in agricultural landscapes. Abstract: http://iene-conferences.info/index.php/conferences/2016/paper/view/323
- Assessing the potential of linear infrastructure for conservation and dispersal of wild pollinators in the landscape - The PolLinear approach. Abstract: http://iene-conferences.info/index.php/conferences/2016/paper/view/394
As part of the Sfécologie 2016 conference:
- The habitat value of power line rights-of-way in rural woody landscapes. Abstract: https://sfecologie2016.sciencesconf.org/107428
- In the 2016-2020 National Action Plan for wild bees and pollinators: http://www.developpement-durable.gouv.fr/IMG/pdf/PNA_pollinisateurs-web.pdf