A plant–pollinator invasional meltdown: the Japanese pagoda tree Styphnolobium japonicum facilitates the invasion of the giant resin bee Megachile sculpturalis

An invasional meltdown can occur when a non‐native species facilitates the spread of another non‐native species in the invaded area, increasing its likelihood of survival or its ecological impact. Plant–pollinator systems pose an exceptional study system to test this concept, which remains poorly known. In this study, we aim to evaluate a potential invasion meltdown between Megachile sculpturalis , a solitary bee native to East Asia that has been introduced to Europe and has widely expanded its range, and the non‐native tree Styphnolobium japonicum , an important pollen source for the bee in its non‐native range. We collected 150 M. sculpturalis occurrences and 21 383 S. japonicum occurrences in the study area (Catalonia, NE Spain). We then modelled how the occurrence of the bee could be explained by climatic suitability, availability of S. japonicum , and geographical position, and calculated the relative importance of these factors by fitting generalised linear models. Our results support the existence of an invasional meltdown between both species, since the availability of S. japonicum was at least as important as climatic suitability at shaping the distribution of M. sculpturalis . Moreover, there was a trend suggesting that the effect of suitability was conditioned on the presence of S. japonicum . The introduction route of M. sculpturalis may also play a crucial role, leading to a higher probability of presence in the eastern part of the study area, whereas S. japonicum may be facilitating the expansion of M. sculpturalis at the invasion front in the west. These findings highlight the role that non‐native ornamental plants can play in the spread of alien species and emphasise the importance of reconsidering native species in urban plantings to reduce such risks.