Impacts of pairing transport infrastructures on biodiversity plants butterfly and reptiles inside interstitial zones between infrastructures

Major transport infrastructures (i.e. high-speed railways, highways) are known to have major impacts on landscapes and biodiversity. These impacts can be direct (i.e. mortality by collision) or indirect (i.e. pollutions, loss of habitat quality and quantity). The developments of such infrastructures in past and future decades make this phenomenon worse. In order to reduce the negative effects of such constructions (pollutions, landscape fragmentation etc), the pairing of transport infrastructures has been proposed. Two paired infrastructures are next to the other, just separated by an interstitial zone (variable width from few meters to 10-100m or more). We focused this study on these understudied interstitial zones with the objective to compare biodiversity inside these particular zones with biodiversity outside (along transport infrastructures). The hypothesis is that if connectivity between interstitial zones and landscape around paired infrastructures is deteriorated (or totally broken), then biodiversity can be different inside interstitial zones and outside (probably lower than outside).

In that way, two case studies of paired transport infrastructures were studied in France, both in a context of farming landscapes. In the two cases the pairing is asynchronous: for one of the study cases the railway was first constructed and few decades later the highway was paired. For the other case the highway was first constructed and few decades later the railway was paired. In all studied situations, interstitial zones were always herbaceous habitats. We chose to study biodiversity through several different taxa, with different dispersion capabilities: plants, butterflies, reptiles, with appropriate inventory methodologies (quadrat method for plants; transect method for butterfly; shelter method for reptiles). The inventories were carried out on the one hand in different kind of interstitial zone width (few meters to few dozen of meters and more), and on the other hand outside the infrastructures, along railway or highway (control plots).

Our results show that studied biodiversity is not significantly different inside interstitial zones and outside infrastructures. Indeed, butterflies are not impacted (both when considering species richness, abundance or diversity indices). The very low number of observed reptiles make the conclusions difficult to do for this taxa. Nevertheless, we can affirm that interstitial zones are not deprived of reptiles. Plans communities are characteristics of ruderal habitats and are very similar in the different configurations (both for species richness and diversity indices).

This study was only conducted on two study cases, in one year. It emphasizes the necessity of long term inventories, in order to conclude on population fitness in interstitial zones. If populations are stabilized, we can think that functional connectivity between interstitial zones and outside is efficient and that habitat quality is suitable. Otherwise, we could suspect a great inbreeding due to functional isolation, leading to population shrinking and weakness.

Road Ecology, Biodiversity, Pairing Transport Infrastructure, Interstitial Zones