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.

Aim: Species distribution models built with geographically restricted data often fail to capture the full range of conditions experienced by species across their entire distribution area. Using such models to predict distribution shifts under future environmental change may, therefore, produce biased projections. However, restricted‐scale models have the potential to include a larger sample of taxa for which distribution data are available and to provide finer‐resolution projections that are better applied to conservation planning than the forecasts of broad‐scale models. We examine the circumstances under which the projected shifts in species richness patterns derived from restricted‐scale and broad‐scale models are most likely to be similar.Location: Europe.Methods: The distribution of butterflies in Finland, Belgium/Netherlands and Spain was modelled based on restricted‐scale (local) and broad‐scale (continental) distribution and climate data. Both types of models were projected under future climate change scenarios to assess potential changes in species richness.Results: In Finland, species richness was projected to increase strongly based on restricted‐scale models and to decrease slightly with broad‐scale models. In Belgium/Netherlands, restricted‐scale models projected a larger decrease in richness than broad‐scale models. In Spain, both models projected a slight decrease in richness. We obtained similar projections based on restricted‐scale and broad‐scale models only in Spain because the climatic conditions available here covered the warm part of the distributions of butterflies better than in Finland and Belgium/Netherlands.Main conclusions: Restricted‐scale models that fail to capture the warm part of species distributions produce biased estimates of future changes in species richness when projected under climatic conditions with no modern analogue in the study area. We recommend the use of distribution data beyond the boundaries of the study area to capture the part of the species response curves reflecting the climatic conditions that will prevail within that area in the future.