Not weeds

TO JUDGE by some of the headlines, you might think they were triffids. Even the term employed for them, “invasive”, has pejorative military overtones. But do interloping plant species from other lands actually cause environmental damage by outcompeting the locals?

That was a question Chris Thomas and Georgina Palmer of the University of York, in England, asked themselves. The answer, just published in the Proceedings of the National Academy of Sciences, is that they do not. Where aliens thrive, locals thrive too. Where they don’t, neither do the locals. No one, it seems, is being driven towards extinction by introductions.

Dr Thomas and Dr Palmer drew their data from the Countryside Survey, a study of Britain’s wildlife that is conducted every six or seven years. They were able both to track the changes that happened between 1990 and 2007 at 479 sites around Britain, looking for evidence of introduced species outcompeting native ones, and to examine the effect of newcomers over much longer periods of time.

What counts as native and what as foreign is not as clear-cut as it might be. The two researchers actually recognised three categories. True natives were those present from time immemorial. In practice, most of these would have arrived after the end of the last ice age, since before that the small part of Britain not buried under glaciers would have been tundra. Then, there were archaeophytes. These were known introductions that happened before 1500—that is before plants started arriving from the New World. Neophytes, by contrast, were introduced after 1500.

There was no suggestion of the newcomers running amok. Though almost a fifth of species recorded by the survey of 2007 were aliens, all 50 of the most widespread species were native. Even extending the count as far as 100 introduced only seven outsiders (four archaeophytes and three neophytes). In terms of area covered rather than geographical range aliens did a bit better. Even so, only 11 of the 100 most abundant species, counted this way, were alien. And, though there were changes between 1990 and 2007 in the abundance of many species, natives were as likely as either archaeophytes or neophytes to increase or to decrease.

The upshot, Dr Thomas and Dr Palmer suggest, is that British members of the vegetable kingdom are more than capable of holding their own against newcomers—whether those newcomers have been around for several centuries or are more recent immigrants. This does not, they observe, mean that all introduced species are without problems. Japanese knotweed, for example, is immensely destructive of the foundations of buildings. But buildings are not plants. And, if inconvenience to human beings is the criterion, some natives are just as bad. Ragwort, for instance, is toxic to horses and tends to grow in pastureland of the sort used to graze them.

The situation is different for invasive animals. There is no doubt that the introduction into Britain of North American grey squirrels was bad for the native red variety, which is extinct in all but a handful of places. And, farther afield, the introduction of rats, cats and pigs onto small islands has often had a disastrous effect on local fauna. But plants seem benign. In this context, Dr Thomas and Dr Palmer think it odd that only 820 of the species listed in the Global Invasive Species Database (a register kept by the International Union for the Conservation of Nature) are animals, whereas 3,163 are plants. Their research, albeit drawn from one smallish country, suggests the database’s threatening-plant list could do with pruning.

Some conservationists object to introduced species just because they are alien. Their view seems to be that the way things are today (or, more accurately, were in some prelapsarian moment in the recent but pre-industrial past) represents a “natural” state of affairs, deviations from which are to be regretted. But given the continual flux of nature even without human intervention, stasis is probably as unnatural as the modern reality of introductions from afar. And if those introductions are increasing biodiversity, what’s not to like?