Hoverflies, the family Syrphidae, are among the most species-rich groups of alpine insects and include many endemic and threatened species. They are also pollinators whose larvae consume aphids or decompose organic matter, giving them outsized ecological importance. Across Europe, their abundance and species richness have declined over the past 50 years as a result of habitat loss, pollution and global warming. The Alps may be especially susceptible: fragmented mountain habitats combined with steep geographic and climatic gradients make both the landscapes and the insects that inhabit them vulnerable to warming and extreme weather events. Yet establishing population trends in alpine hoverflies has remained a formidable challenge, because many species are small, scarce or hard to identify, and surveying remote slopes above the tree line demands significant physical effort and careful timing around altitude- and season-specific flight periods.

The standard toolkit for hoverfly monitoring has long relied on fixed installations. Malaise traps set at defined locations, or hand-net sampling along well-defined transects during a specified time, produce standardized data that can be compared year after year. But in mountainous terrain, covering all habitats and flight periods between 500 and 2500 metres with such methods can require enormous sampling effort. An all-inclusive study in the Italian Dolomites used traps at ten different altitudes; comparable schemes in the French Alps proved effective but laborious. Large sampling effort, the authors note, could reduce the practical repeatability of a study, since scientific insect surveys often operate under economic and time constraints. Meanwhile, centralized databases of hoverfly records for the Swiss Alps remain too sparse and unevenly distributed in time and space to assess population trends.

The Bergün survey took a third path: hand-net sampling along haphazardly selected routes, the explorative method commonly used by syrphidologists in mountain areas but rarely with future repeatability in mind. Four surveyors chose their own routes, methods and sampling strategies individually, photographing some flies in the field with a macro lens and collecting others for identification under a binocular microscope using published keys. Identification of some genera, such as Cheilosia with more than 70 species in the Swiss Alps, demands high expertise, and difficult specimens were sent to specialists in Europe. All pinned specimens are stored in private and museum collections in Bonn and Lausanne, and record data were transmitted to the Swiss national fauna database info fauna. One Malaise trap, set along a river at 1130 metres, added a fixed-location component to the otherwise variable scheme.

The survey area spans roughly 34 square kilometres, about 18 percent of the municipality, with sampling locations ranging from 1100 to 2300 metres across two valleys. Below the tree line, spruce forests and pine clusters intermix with cultivated meadows; Apiaceae, the umbellifers especially attractive to hoverflies, bloom abundantly from mid-June at lower altitudes and through July and August higher up. Two high-alpine sites above the tree line, one around 2100 metres west of the Albulapass and another between 1800 and 1900 metres at Tuors Chants, were particularly well sampled. In total, 1438 individuals comprising 173 species were caught or photographed, representing 84 percent of the 206 species known from the municipality and 51 percent of the 341 species known from Graubünden.

The statistical heart of the study lies in rarefaction analysis, a technique that estimates species richness for a standardized number of samples, smoothing out differences in individual sampling intensity. The authors compared only surveys between 15 and 30 June with estimated durations of 4.5 hours or more: six surveys totalling roughly 40 field hours in each year. The rarefaction curves for 2023 and 2024 proved strikingly similar, reaching 108 and 111 species respectively after six surveys, nearly crossing or overlapping. Crucially, this similarity did not stem from recording the same species twice: 71 species were recorded in both years, while 37 and 40 species were unique to 2023 and 2024 respectively. The matching curves follow from sampling a matching proportion of species with the same detection probability, a statistical index for the overall population size of the hoverfly community averaged over many species.

That statistical averaging is what makes the method robust. A rarefaction curve expressing species richness against the number of surveys does not depend on the number of individuals sampled per species, does not depend on recording any particular species, and tracks the average trend of the entire community, making it relatively insensitive to sampling biases or deviating trends shown by single species. The authors found that when four common species were omitted from the analysis, all four surveyors collected a remarkably similar 53 to 59 species per 100 sampled individuals, suggesting that among experienced syrphidologists the averaged sampled species richness is not strongly influenced by personal choices of method, route or intuition. If a scarcely sampled species declined by 50 percent in population size, its probability of detection would fall accordingly for a given sampling effort, so a population decline across the community would show up as reduced recorded species richness.

The survey yielded notable conservation findings. Three species were recorded as new for the Canton of Graubünden: Platycheirus transfugus, a female collected at 1600 metres in June 2024; Sphaerophoria estebani, a male from Tuors Chants at 1837 metres; and Psilota exilistyla, a male photographed at 1560 metres in June 2023. Fourteen species recorded in the Bergün surveys are on the European Red List of Hoverflies, 11 categorized as endangered and 3 as vulnerable. The Graubünden checklist as a whole includes one critically endangered species, Platycheirus altomontis, 32 endangered species, 6 vulnerable species and 18 near-threatened species at European level, with another seven categorized as Data Deficient.

Interannual variation remains a caveat. Alpine hoverfly phenology may be linked to the flowering phenology of the plants they visit, which in turn depends on the timing of snowmelt, and simultaneous impacts on many species could shift rarefaction curves. Around 80 species in each year were represented by only one to three sampled individuals, and 135 of the municipality’s 206 species fall below the annual detection threshold for the 15 to 30 June window. The authors argue that each year’s different subset of these low-detectability species likely reflects sampling probability rather than genuine absence. Regular repetition is the only way to clarify the extent of such yearly fluctuations, which is precisely why the method’s low sampling effort matters: feasibility of repeating a survey decreases with increasing effort, especially under the constraints that funded science projects face.

To make replication straightforward, the team proposes a practical framework: at least six surveys per year, spread over at least three days, totalling roughly 40 field hours with each survey lasting at least 4.5 hours, carried out by one to four experienced syrphidologists within the 15 to 30 June timeframe. Routes are free to choose but should cover many habitats and altitudes, including at least one trip above the tree line in sunny weather, using hand-netting, field observation or macrophotography. The annual measurement can be achieved by a single surveyor over six days or by two surveyors over three days. The 2023 and 2024 rarefaction curves now stand as a baseline reference, and the authors hope they will motivate similar surveys elsewhere in the central Alps, turning haphazard mountain hikes into a long-term early-warning system for one of Europe’s most threatened insect communities.

Subject of Research: Ecology and conservation biology

Article Title: A hoverfly survey in the inner alpine municipality of Bergün-Filisur (Switzerland) by means of variable route sampling ( Diptera , Syrphidae )

Article References: Bisschop, J., Fisler, L., Pétremand, G., & Mengual, X. (2025). A hoverfly survey in the inner alpine municipality of Bergün-Filisur (Switzerland) by means of variable route sampling (Diptera, Syrphidae). Alpine Entomology, 9, 79-90. https://doi.org/10.3897/alpento.9.170334

Image Credits: AI Generated

DOI: 10.3897/alpento.9.170334

Keywords: hoverfly, survey, inner, alpine, municipality, Berg, n-Filisur, Switzerland, means, variable, route, sampling