Aim  Invasive alien species (IAS) are recognized as major drivers of biodiversity loss, but few causal relationships between IAS and species declines have been documented. In this study, we compare the distribution (Belgium and Britain) and abundance (Belgium, Britain and Switzerland) of formerly common and widespread native ladybirds before and after the arrival of Harmonia axyridis, a globally rapidly expanding IAS.Location  EuropeMethods  We used generalized linear mixed‐effects models (GLMMs) to assess the distribution trends of eight conspicuous and historically widespread and common species of ladybird within Belgium and Britain before and after the arrival of H. axyridis. The distribution data were collated largely through public participatory surveys but verified by a recognized expert. We also used GLMMs to model trends in the abundance of ladybirds using data collated through systematic surveys of deciduous trees in Belgium, Britain and Switzerland.Results  Five (Belgium) and seven (Britain) of eight species studied show substantial declines attributable to the arrival of H. axyridis. Indeed, the two‐spot ladybird, Adalia bipunctata, declined by 30% (Belgium) and 44% (Britain) over 5 years after the arrival of H. axyridis. Trends in ladybird abundance revealed similar patterns of declines across three countries.Main conclusion  Together, these analyses show H. axyridis to be displacing native ladybirds with high niche overlap, probably through predation and competition. This finding provides strong evidence of a causal link between the arrival of an IAS and decline in native biodiversity. Rapid biotic homogenization at the continental scale could impact on the resilience of ecosystems and severely diminish the services they deliver.

Summary: After its initial discovery in Macedonia in 1985, during the last 19 years the leafminer mothCameraria ohridellahas invaded most of Central and Western Europe. The species, which causes aesthetic damage to horse chestnuts, is generally observed first in highly populated locations before colonizing the countryside. This pattern is consistent with a stratified dispersal process combining long‐distance movements and local diffusion. Using large‐scale spatial data on damage caused byCameraria ohridellain Germany, three stochastic spatial models of spread are compared: a diffusion model, a leptokurtic dispersal model and a stratified dispersal model that assumes a two‐scale dispersal process. In addition, the association between human population and moth invasion is tested in this last model by linking long‐distance infestation probability to human population density. Finally, these spatial models constructed with data from Germany are tested at the European scale and compared to historical records of first occurrence. The fat‐tailed dispersal kernel models (leptokurtic and stratified dispersal models) allowing for long‐distance dispersal provide better predictions than the diffusion model. Among these models, the stratified dispersal model incorporating the effect of human population density provides the best description of the spread ofCameraria ohridellain Germany in predictive (lowest sum of squared errors) and qualitative (similar fractal dimension) terms. The roles of short‐distance and long‐distance dispersal inCameraria ohridellainvasion ecology in relation to human population are discussed, together with the models’ scale‐dependence and limitations.