The benefits of bottlenecks

The last successful invasion of Britain began in the summer of 2004. Like all evils, it stemmed from the continent, and quickly the invaders were rumoured to be smelly, to squat in houses by the thousands and to damage the livelihood of their autochthonous relatives. The invaders go by the harmlesssounding name of Harmonia axyridis or harlequin ladybird, but there is nothing harmonious or lady-like about them. These beetles spread at a fierce pace and are more ferocious eaters and breeders than other ladybird species. In autumn, they can form spectacular mass aggregates, which may have contributed to their notoriety. Like other famous examples of biological invasions — grey squirrels in Britain, rabbits in Australia or giant hogweed in central Europe — their success is staggering, which for biologists begs the question why this should be so. Biological invaders do suffer one obvious disadvantage, namely that at the beginning of the invasion their numbers are low. Such so-called population bottlenecks mean that the invader populations can go extinct quite easily, and that, compared with larger native populations, their genetic variability will be lower. But this need not always be a disadvantage, as a paper by Benoit Facon and colleagues on harlequin ladybirds in this issue of Current Biology illustrates. In fact, a bottleneck of the right size can have quite the opposite effect — it can purge a population of deleterious alleles and thus increase their fitness rather than reduce it though inbreeding depression. Harlequin ladybirds stem naturally from Central and Eastern Asia. Unlike many other species of the ladybird family (Coccinellidae) that can be identified by the number of spots on their hardened wings, these beetles’ colour patterns are highly variable. Harlequin ladybirds are also bigger than most of their relatives and are voracious aphid eaters. Their appetite and their prolificacy make them a potential threat to resident plant and animal species. Although the actual impact is as yet unclear, Peter Brown from the UK ladybird survey notes that “there is growing evidence that native ladybirds — and some other Feature