Rare plants sustain communities of visitors with similar diversity but different composition across populations of different size

Interactions are fundamental pillars of ecosystem functions, and plants often host a multitude of organisms both above- and belowground. However, plant populations of the same species may be visited by different communities depending on their size or local environment. Here we thoroughly surveyed the aboveground arthropod and belowground bacteria communities interacting with six Pyrenean rare plant species. We compared the patterns of alpha and beta diversity between patches of similar size located in small and large plant populations, and explored the relationship between aboveground arthropod and belowground bacterial diversities. Alpha diversity of arthropod visitors did not differ between plant patches, but belowground bacterial diversity was higher in small populations. In addition, significant differences were observed in the composition of arthropod communities, with patches in small populations having communities that differed from those in large populations. Finally, above- and belowground patterns of beta, but not alpha diversity were positively correlated. Our results suggest that isolated small plant populations may sustain communities of similar diversity as similar sized patches located within large populations, although with different taxonomic composition. This highlights the importance of small plant populations, often regarded as having lower ecological value, as hosts to a wide range of species both above- and belowground. • We surveyed arthropod and bacterial communities in six rare Pyrenean plants • Patches in plant populations of different size had similar alpha diversity • Arthropod visitor composition was distinct in small populations compared to large ones • Above and belowground beta diversity showed positive correlation across plant species • Small plant populations host distinct communities, highlighting their ecological value