We knew the toaddrome, the toad, the ecuroduct… But not yet the butterfly ecobridge. This hole in the racket, among all the road facilities that exist for animals, will soon be filled. The Gironde departmental council is going to experiment with an eco-bridge to the Azure of the Sanguisorbea species of butterfly affiliated with the plant of the same name, on the occasion of the inauguration on November 26 of the Taillan-Médoc road diversion, near Bordeaux.
This bridge with a flared profile, the construction of which amounts to 1.6 million euros out of an overall budget of 47 million euros for the detour, should make it possible to “connect” this butterfly to the ants and the bloodworm with which it interacts. “The design of the structure has been considered with an environmental office”, specifies the departmental council, which organized a site visit on Tuesday less than two months before the opening to the public of this new road axis.
Channel the butterfly to the eco-bridge through a funnel effect
“We had never made an ecobridge for a butterfly before,” says Marc Magendie, head of the northern design office in the infrastructure and mobility service of the departmental council, showing us the sanguisorbe which has been moved from the route of the deviation. , to replant it on the aisles. Walls three meters high, over a distance of 300 meters on one side and a little over 1,000 meters on the other, will also be installed, “where the presence of the butterfly has been identified, to prevent it from ‘go on the roadway, and channel it towards the overpass, in a sort of funnel effect,’ he continues.
But why take so much care, and spend so much money, on a butterfly? It should first be remembered that road improvements kill millions of animals and insects every year in France. Above all, the route of this road diversion simply passes through the middle of the Lesqueblanque site in Saint-Aubin-de-Médoc, a perimeter of nearly 3 hectares, which represents the last known spot in New Aquitaine of the Azuré, a protected species. This butterfly with its specific biology (eggs are laid in flowers and larvae are raised by a single type of ant) is particularly rare, and threatened with the gradual disappearance of its habitat.
This is why the presence of this butterfly, which is the subject of a National Action Plan (PNA) aimed at ensuring its protection, had led to the cancellation of the site in July 2015. Suffice to say that the community does not now skimps more to pamper it… Even more so after the bitter legal battle led by several environmental associations, which mobilized to have the project for this 8 km road canceled, threatening, according to them, the very existence of the Azuré.
“Worst possible route” according to the associations
The road diversion project for the town of Taillan-Médoc, located between the Médoc and Bordeaux, dates back to forty years ago. The objective is to offer a relief route to the 20,000 vehicles, including approximately 10% heavy goods vehicles, which cross the central artery of Taillan every day, creating monster traffic jams. In 2014, the first clearing work finally began. Before being stopped in 2015, after a first cancellation of the project by the administrative court, seized of an appeal by the associations France Nature Environnement and Natur’Jalles.
These then underline that “the worst possible route” was chosen for this deviation, since over 8 kilometers, “we do not encounter less than three species of protected butterflies, nine of amphibians, seven of reptiles, 36 of birds , 23 from mammals and 19 from bats. And in particular the Azuré de la Sanguisorbe.
The department must therefore review its copy, and is working on a new project, with more ambitious environmental compensation, and the construction of more substantial ecological corridors. A favorable opinion was given in 2019, before the project was attacked again in 2020, the year during which two association appeals were this time rejected.
End of the tunnel
After a final round before the administrative court last April, the department therefore sees the end of the tunnel in this case. But the community wanted to shield itself on the environmental aspect, which it regularly highlights. “On this project, nearly 20% of the budget was devoted to environmental defence”, insists Jean Galand, vice-president in charge of mobility and infrastructure. That is around nine million euros.
This diversion between Arsac and Saint-Aubin-de-Médoc has thus gone from 2×2 lanes to 2×1 lane, except for a short section, to reduce its grip. “We were at the start on a 70-meter-wide spindle, which we reduced to 46 meters in the wetlands, and even 25 meters to the right of the Azuré de la Sanguisorbe station”, continues Marc Magendie. . Some 164 hectares around the perimeter of the diversion are subject to compensatory measures, 80% of which have been bought by the department to “sanctuarize” these spaces over time.
We will have to measure the effect on the butterfly
About ten underground hydraulic structures should allow all the fauna to cross without incident, supplemented by two others, specific to bats. In the continuity of the walls, a hedge and a fence are also supposed to protect the natural spaces from deviation. “Everything is done to allow the animals to pass above or below, the infrastructure must not constitute a barrier” explains the Environment project manager Nicolas Joubert.
The ecobridge will be used for all wildlife as well. It now remains to measure its effect on the butterflies… The moth “lives mainly on the flower, therefore rather on the ground, for a period that hardly exceeds three or four days” assures Marc Magendie, confident in the ability of the device to capture specimens. .
“We have to follow up the population over three years, warns the head of the design office, because it is possible that the butterfly, for weather or other conditions, will not come out for a year, which does not bode well. what will happen next year. “A capture-marking operation will be carried out from 2023, “to see if the butterfly has used the ecobridge or not. »