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.

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Myrmica sabuleti forms mature colonies of about one thousand workers and a few queens. Although living in dry and sunny biotopes, this species avoids foraging during the warmer hours of the day which correspond to the period of the maximum foraging activity of two sympatric species, Formica fusca and F. cunicularia. M. sabuleti is the most frequently observed species on the extrafloral nectaries of Vicia sativa, even during the warmer hours of the day, whereas F. fusca is the most frequent species during those hours at tubes of 1 M sucrose solutions placed on the ground. In spite of this temporal segregation, the foraging activity of M. sabuleti and F. fusca overlaps two times a day (10:00 -11: 00 am and 5 :00 -8 : 00 pm). Newly discovered large food sources are exploited by M. sabuleti through an explosively increasing recruitment, whereas workers of F. fusca forage mainly individually. Interference between M. sabuleti and F. fusca was studied when offering sucrose solutions, large dead insects unretrievable by individual foragers of either species (cockroaches), medium-sized insects retrievable by single foragers of F. fusca but not by single M. sabuleti (larvae of Calliphora erythrocephala) and small insects (Drosophila) retrievable by individual foragers of both species. Owing to its rapid recruitment, M. sabuleti is able to displace F. fusca from sucrose solution and large dead prey during the overlaping of the activity cycles of both species. When medium-sized corpses are offered, the issue depends on the speed of the events. F. fusca is able to take the prey away only if it discovers it before M. sabuleti has recruited nestmates. Small dead prey do not lead to interference, but is taken by the first worker that finds it.