Glaciers are retreating worldwide at an ever-increasing rate, exposing new ice-free areas to ecological succession. This process leads to changes in biodiversity and potentially to novel species interactions. However, we still have a limited understanding of how glacier retreat influences species interaction networks, particularly the structure and robustness of mutualistic networks. After reconstructing plant-pollinator networks along a 140-year chronosequence on a glacier foreland, we address the effects of glacier retreat on network structure and robustness. Our results show that the prevalence of different network motifs changes over spacetime, leading to a decrease in network robustness. With glacier retreat, mutualistic networks shift from highly connected with diverse specialist interactions to loosely connected with few generalist interactions. Furthermore, despite the turnover of plant species, we find that species' structural roles remain constant over spacetime while depending on species identity. Our findings suggest that glacier retreat reshuffles mutualistic networks with motifs posing low robustness, leading to increased fragility. Understanding the assembly and breaking down of species interaction networks provides novel insights into the development and stability of novel, post-glacial ecological systems facing glacier extinction.

This case study was carried out at Mont Miné glacier foreland, Val d'Hérens, Switzerland (46°3'33.646’N/7°32'54.550’E). Based on existing geochronological records of the Mont Miné glacier (Lambiel et al. 2016, Nicolussi et al. 2022) and our reconstruction of historical cartography (‘Journey through time’ tool at https://map.geo.admin.ch), we reconstructed terrain age given the year when glacier retreated and divided the chronosequences into four stages: Stage 1, Stage 2, Stage 3 and Stage 4, representing terrain deglaciated since at least 1989, 1925, 1900 and 1864, respectively. In each stage, we randomly selected 4 sampling sites, for a total of 16 sites.

Sampling took place during the flowering season of summer 2023, from June the 15^th to July the 15^th. Within each transect, we recorded plant–pollinator visitations. Flower visitors getting in direct contact with flower anthers or stigmas were considered as pollination interactions (Gibson et al. 2011). We recorded interactions by collecting flower-visiting insects using an entomological aspirator or by sweep netting, depending on insect size. Collected pollinators were stored in 70% ethanol for subsequent identification. For conservation and ethical reasons, we limited the collection of specimens to those necessary for species identification, and proceeded with recording visitations by direct observation. To standardise sampling time, we walked transects at a slow and steady pace for 15 minutes per branch, for a total of 30 minutes per transect (Martínez-Núñez et al. 2022). Sampling sessions were carried out on sunny and low-windy days from 8:30 a.m. to 5:30 p.m. We sampled each site eight times, for a total of n=128 temporal replicates. To minimise the impact of sampling time on pollinator activity, we randomised the sampling order of sites within a day and within a round of replicates such that every site was sampled at all possible times of the day (Martínez-Núñez et al. 2022).

Plant species were identified according to Flora Helvetica (Lauber and Wagner 1998, updated version on https://www.infoflora.ch). We observed a total of 108 blooming plant species belonging to 28 families. Pollinator specimens were identified at species or morphospecies level whenever possible using taxonomic keys (Rognes 1991, Cerretti et al. 2012, Gregor et al. 2016, Cappellari et al. 2018, Falk 2019, Michez et al. 2019, Rasmont et al. 2021). We identified 65 families belonging to 8 orders; 29.0% of specimens are identified at species level, 42.3% as morphospecies, 1,7% as genus, 27% at family or higher level. Specimens were photographed using a stereomicroscope and updated on iNaturalist platform to gather further taxonomic information, for data sharing and for public outreach (https://www.inaturalist.org/projects/pollinator-diversity-at-ferpecle-glacier-ecosystems).