Abstract
Climate change may not only disrupt plant-pollinator interactions by eliminating either partner, but also disturb the simultaneous alignment of phenology, spatial overlap, signal perception, behavior, and morphological fit necessary for successful pollen transfer. However, the ecological dimensions that make specialized pollination systems vulnerable have rarely been comprehensively assessed within the same system. Here, we combined more than a decade of field observations with floral volatile analysis, antennal electrophysiology, behavioral assays, seasonal population monitoring, quantitative morphology, and in situ introduction experiments to examine the rewardless alpine orchid Cypripedium bardolphianum. Female winter-morph Drosophila immigrans dominated effective pollination during the orchid's short flowering season. The orchid attracted drosophilid flies with ethyl tiglate (ET), a multifunctional volatile associated with aggregation, feeding, and oviposition, but responsiveness to this compound extended across several drosophilid lineages. Temporal and spatial overlap, seasonal phenotype, sex-specific preference, and morphological fit progressively narrowed this broad response pool to the realized pollination niche. Two species that are geographically isolated but are attracted to ET, D. robusta and D. virilis, visited flowers and removed pollinia after in situ introduction, revealing compensatory potential beyond the locally realized interaction. These findings show that specialization can emerge from a narrow multidimensional interaction window even when signal-response breadth is comparatively wide. Therefore, environmental change may disrupt current partners through mismatch, while exposing latent functional compatibility through community reassembly. To predict the persistence of specialized pollination, it is necessary to measure the breadth of signal response together with the number and climate sensitivity of the filters that convert response capacity into pollen transfer.