Vulnérabilité des habitats subalpins face aux changements climatiques : suivi des syrphidés (Diptera) dans la Réserve naturelle nationale des Hauts de Chartreuse (France, Isère) entre 2008 et 2020

As part of the management plan for the Hauts de Chartreuse National Nature Reserve (Savoie-Isère, France), surveillance of the syrphid fauna (Diptera) was initiated in 2008, with the sampling programme repeated in 2019-2020. Sampling was conducted using four Malaise traps, on two remarkable sites in high-altitude habitats (subalpine grassland/heath and mountain pine forest). A significant reduction in species richness and diversity was observed at one site (Mont Granier) over the decade between surveys, while at the other (Dent de Crolles) both parameters were more stable. But at both sites there were clear changes in species composition between the two surveys, the changes exhibiting similar characteristics on each site. These similarities suggest the changes are a consequence of a factor operating at regional level, impacting both sites. The natural character of the Chartreuse Massif is karstic savannah in a sub-alpine climate. This is shown by the absence of almost all the detritivorous species associated with the accumulation of litter and humus. This observation is supported by the unexpected presence of several species associated with acidic (decalcified) variants of sub-alpine grasslands and montane/subalpine heath. However, none of these species were recorded during the 2019-20 survey. On both sites all phytophagous (–18%) and zoophagous (–23%) groups showed a sharp decline between 2008 and 2019-20. These declines are particularly noticeable on Mont Granier. Analysis of species assemblages, using the Syrph the Net database, identifies deterioration of habitat functionality (ratio between species present and species predicted) on both of the studies sites, between 2008 and 2019-20. At Dent-de-Crolles the decrease is –8% and at Mont Granier –27%. Since 2008 there has also been a marked increase in unexpected species from lower-altitude, montane zone forest, associated with tree species (beech, spruce, fir) which are not represented at the altitudes sampled. A further change is in the proportion of species from cool, damp subalpine habitats, which are also found on the outskirts of the study sites. These are species not predicted to occur on the sampled sites, because their habitats are absent there. Representation of these species dropped sharply over the decade, from 37% to 24% at Dent de Crolles and from 34% to 15% at Mont Granier. Overall, the sample data show a decrease in the diversity of species “indigenous to” the subalpine habitats represented on the sample sites and other subalpine habitats in the vicinity, accompanied by an increased catch of species from lower altitudes that are associated with habitats not represented on the sample sites. Lack of equivalent data sets from other subalpine sites, for the period 2008 – 2019-20 limit the conclusions which can be drawn from our data. However, the similarity between the decreases in specific components of the subalpine syrphid fauna recorded on the two sites and the accompanying increases in representation of components of the fauna from lower altitudes (and habitats not represented on the sample sites), are cause for concern. It is difficult to attribute them to site management or site use by Man – neither site has been subject to any form of exploitation for decades, and neither is subject to “hands-on” management. But everywhere is now subject to the influences of climate change which has been increasing in amplitude during the last decades. Our results imply significant levels of loss of sub-alpine fauna may already be occurring, at a regional level. They signal the need for more comprehensive sampling of the subalpine syrphid fauna in the Alps, as well as demonstrating the value of continuing its surveillance on the Dent-de-Crolles and Mont Granier sites.