「经济学人」Sleeper species

The sooner they can be weeded out, the better

THE MISSION is simple. Arrive on foreign soil, blend in and wait. Then, when the time is right, emerge from the shadows and take over. This procedure, familiar from spy fiction as the preserve of sleeper agents, is also the modus operandi of certain invasive plant species.

When botanists warn against invasive plants, they are usually referring to those that colonise in plain sight—a group that includes Japanese knotweed. But not all invasive species are created equal. Though some spread like wildfire from the moment they arrive, others seem content to stay put. New work suggests that conservationists ignore them at their peril.

Invasive species that stay put, known as sleeper species, are not hard-wired to do so. Their natural tendency to expand is, instead, held back by some external factor. With the climate rapidly changing, Bethany Bradley at the University of Massachusetts wondered whether expected alterations in temperature and precipitation had the potential to activate some sleeper species. As she reports in a paper in Biological Invasions, a journal, the answer is a definitive yes.

Dr Bradley and her collaborators came to their conclusions by studying 1,795 plant species in north-eastern America identifiable as foreign but not yet locally listed as invasive. Of these, 169 were unquestionably demonstrating invasive behaviour in other parts of the world and causing damage to their environment. That suggested they had the capacity to wreak havoc in America too.

To determine whether the conditions coming to the region would activate any of these 169, the researchers first identified the conditions under which these species would thrive. A number of the plants on their list, including Japanese wisteria, were constrained by the cold winters that are less and less common in the region. They also found that other species, like kudzu, a climbing vine, required more rainfall than is typical.

According to the team’s findings, climate change is on track to awaken 18 sleeper species that could cause serious environmental and economic problems. The risks include anything from choking rivers to outcompeting plants that pollinators and livestock depend upon. Dr Bradley estimates that they will start to make themselves felt between 2040 and 2060 if something is not done soon. Botanists in other rapidly warming regions should take heed.

The most practical course of action, in her view, is to seek these sleeper species out before they are able to spread. Removing them today may be expensive, but it will be far cheaper than trying to weed them out tomorrow.(Aug 14th 2024)

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