NewScientist 02 April 2014] — WE HAVE all heard a lot of bad stuff about introduced species: they run rampant through our ecosystems, costing billions to control each year. They are also accused of driving native species extinct. Indeed, alien species are often cited as one of the big threats to biodiversity. Not so fast. In Where Do Camels Belong? The story and science of invasive species, plant biologist Ken Thompson argues that most alien species – even some topping the eco-horror lists – cause little or no lasting damage and aren’t worth the angst, effort or money we devote to controlling them. Purple loosestrife, for example, is often viewed as one of the worst invasive weeds in North America because it forms dense stands of tall, conspicuous flowering heads. But when ecologists looked closer, reports Thompson, there was little evidence of actual harm. Even in Hawaii – poster child for the noxious effects of alien species – invaders tend to make ecosystems more diverse, not less. Nor are introduced species the financial burden they are often made out to be. For one thing, says Thompson, hardly anyone bothers to count the economic benefits of “aliens” such as wheat and cows – a sum that runs to $800 billion per year in the US alone. Moreover, much of the cost of the invaders turns out to be the money spent controlling them. There’s a deeper problem, too, in our attitude towards aliens. Viewed over millions of years, plants and animals are constantly shifting their distributions over Earth. Just a few thousand years ago, North America was full of camels. Indeed, they evolved there and reached their greatest diversity on that continent. So should camels be regarded as native or alien there today? Then there is the small-flowered tongue orchid, native to mainland Europe, that first turned up in England in 1989. No one knows whether it arrived by seeds that blew across the English Channel – in which case it’s an endangered native, worthy of nurture – or arrived stuck to someone’s trouser cuff, in which case, says Thompson, “it’s just another bloody weed, to be ruthlessly exterminated”. Should how it arrived in the country really make that much difference? Thompson makes his case in a lively, readable style, spiced with a healthy dose of sarcasm towards “aliens = bad” fundamentalists. Better yet, he bolsters his argument with plenty of citations from the scientific literature, which adds welcome heft.