Alien species introduction is a global phenomenon involving different invasion patterns and is characterized by niche conservatism or shift. We describe the spatial distribution ofCacyreus marshalliButler, [1898] (Lepidoptera: Lycaenidae) in its native (southern Africa) and invaded (Europe) ranges.C. marshalliis the only alien butterfly in Europe, introduced by the trade of ornamentalPelargoniumplants, and might threaten native lycaenids because of the chance of its naturalization on indigenousGeraniumspp. In Europe,C. marshalliis widespread in the Mediterranean basin, but absent in northern countries. We investigate invasion patterns and their temporal dynamics in Italy, the most extensively invaded country, identifying three phases and different rates of spread resulting from multiple introductions and human-mediated movements. We also characterize and compare the native and invasive ecological niches ofC. marshalliwith a multivariate approach based on bioclimatic, ecological and human demographic variables. The little overlap between the native and invaded niches (12.6%) indicates a shift in the realized niche ofC. marshalli. While the expansion potential ofC. marshalliin Europe remains constrained by the distribution of suitable host plants, our niche comparison analysis suggests the species has already invaded new ecological and climatic spaces. This includes colder areas than would be suggested by its native distribution in Africa.

Cacyreus marshalli is the only alien butterfly in Europe. It has recently spread in the Gran Paradiso National Park (GPNP), where it could potentially compete with native geranium-consuming butterflies. Our study aimed to (1) assess the main drivers of its distribution, (2) evaluate the potential species distribution in GPNP and (3) predict different scenarios to understand the impact of climate warming and the effect of possible mitigations. Considering different sampling designs (opportunistic and standardised) and different statistical approaches (MaxEnt and N-mixture models), we built up models predicting habitat suitability and egg abundance for the alien species, testing covariates as bioclimatic variables, food plant (Pelargonium spp.) distribution and land cover. A standardised approach resulted in more informative data collection due to the survey design adopted. Opportunistic data could be potentially informative but a major investment in citizen science projects would be needed. Both approaches showed that C. marshalli is associated with its host plant distribution and therefore confined in urban areas. Its expansion is controlled by cold temperatures which, even if the host plant is abundant, constrain the number of eggs. Rising temperatures could lead to an increase in the number of eggs laid, but the halving of Pelargonium spp. populations would mostly mitigate the trend, with a slight countertrend at high elevations.