Primary successions of glacier forelands are unique model systems to investigate community dynamics and assembly processes. However, successional changes of plant and insect communities have been mainly analysed separately. Therefore, changes in plant-insect interactions along successional gradients on glacier forelands remain unknown, despite their relevance to ecosystem functioning. This study assessed how successional changes of the vegetation influenced the composition of the flower-visiting insect assemblages of two plant species, Leucanthemopsis alpina (L.) Heyw. and Saxifraga bryoides L., selected as the only two insect-pollinated species occurring along the whole succession. In addition, we investigated the links between reproductive output of these plants and pollinator abundance through experimental exclusion of pollinators. Plant community structure changed along the succession, affecting the distribution and the abundance of insects via idiosyncratic responses of different insect functional groups. L. alpina interacted with ubiquitously distributed pollinators, while S. bryoides pollinators were positively associated with insect-pollinated plant species density and S. bryoides abundance. With succession proceeding, insect assemblages became more functionally diverse, with the abundance of parasitoids, predators and opportunists positively related to an increase in plant cover and diversity. The reproductive output of both plant species varied among successional stages. Contrary to our expectation, the obligate insect-pollinated L. alpina showed a reproductive output rather independent from pollinator abundance, while the reproductive output of the self-fertile S. bryoides seemed linked to pollinator abundance. Observing ecological interactions and using functional traits, we provided a mechanistic understanding of community assembly processes along a successional gradient. Plant community diversity and cover likely influenced insect community assembly through bottom-up effects. In turn, pollinators regulate plant reproductive output through top-down control. We emphasise that dynamics of alpine plant and insect communities may be structured by biotic interactions and feedback processes, rather than only be influenced by harsh abiotic conditions and stochastic events. bryoides seemed linked to pollinator abundance. Observing ecological interactions and using 45 functional traits we provided a mechanistic understanding of community assembly processes 46 along a successional gradient. Plant community diversity and cover likely influenced insect 47 community assembly through bottom-up effects. In turn, pollinators regulate plant 48 3 reproductive output through top-down control. We emphasize that dynamics of alpine plant 49 and insect communities may be structured by biotic interactions and feedback processes, 50 rather than only be influenced by harsh abiotic conditions and stochastic events. 51 52

In mountain ecosystems, climate change can cause spatiotemporal shifts, impacting the composition of communities and altering fundamental biotic interactions, such as those involving flower-visiting arthropods. On of the main problems in assessing the effects of climate change on arthropods in these environments is the lack of baseline data. In particular, the arthropod communities on early flowering high-altitude plants are poorly investigated, although the early season is a critical moment for possible mismatches. In this study, we characterised the flower-visiting arthropod community on the early flowering high-altitude Alpine plant, Androsace brevis (Primulaceae). In addition, we tested the effect of abiotic factors (temperature and wind speed) and other variables (time, i.e., hour of the day, and number of flowers per plant) on the occurrence, abundance, and diversity of this community. A. brevis is a vulnerable endemic species growing in the Central Alps above 2000 m asl and flowering for a very short period immediately after snowmelt, thus representing a possible focal plant for arthropods in this particular moment of the season. Diptera and Hymenoptera were the main flower visitors, and three major features of the community emerged: an evident predominance of anthomyiid flies among Diptera, a rare presence of bees, and a relevant share of parasitoid wasps. Temperature and time (hour of the day), but not wind speed and number of flowers per plant, affected the flower visitors’ activity. Our study contributes to (1) defining the composition of high-altitude Alpine flower-visiting arthropod communities in the early season, (2) establishing how these communities are affected by environmental variables, and (3) setting the stage for future evaluation of climate change effects on flower-visiting arthropods in high-altitude environments in the early season.

In the present paper we provide the first contribution to the knowledge of the flower-visiting insect assemblages of the alpine plant species Callianthemum kernerianum Freyn ex A. Kerner (Ranunculaceae). This focal plant species was selected since it is a steno-endemic and critically endangered species belonging to the IUCN red-list. Fifteen taxa were recorded, among which very few are true pollinators, whereas all the others can be considered only indirect pollinators. The peculiar phenology of the plant and the harsh habitat conditions in which it grows probably affect the richness and abundance of flower-visiting insects as well as of true pollinators. This could be the reason for this plant to be a self-compatible species.