Plant ‘time bombs’ highlight how sneaky invasive species can be

A stealthy, destructive weed — the sycamore maple — began its “don’t worry, just love me” phase of invading Great Britain so long ago that the tree didn’t have what we’d call a scientific name.

The tree had arrived from Central Europe by 1613, and Carl Linnaeus, who set up modern Latin naming, wouldn’t be born for almost another century. Altogether, 320 years passed before biologists found the tree crowding out native plants, researchers report in the March Nature Ecology & Evolution.

The sycamore maple’s lag between charmer and menace is extreme, but about a third of the roughly 3,500 plant species examined in the study seemed harmless when they first showed up in a new region, warns weed ecologist Mohsen Mesgaran of the University of California, Davis. The charades lasted for at least five years (the definition he chose for this study). “It’s just some of them, they need time,” he says. “Then we have this storm — explosion! — of this species rapidly growing.” 

Mesgaran and colleagues analyzed more than a million data points from herbarium records showing when and where plants were collected across nine regions around the world. In six of the regions, some destructive plants lagged for more than 100 years. After that century-plus of what looked like plants just meekly getting by, their populations skyrocketed. They started choking out native species and disrupting the creatures that relied on those plants.

The first place that hitchhiking plants land in a new ecosystem may be survivable, but not great. A new climate niche often doesn’t kill off the newcomer, but also doesn’t let it flourish, Mesgaran and colleagues propose. Humans may dismiss the new greenery as harmless when, in fact, it’s merely stuck in a dump and in need of a ride.

What’s more, temperature changes play a role in when and where the plant time bombs finally explode, the team found. That’s an unsettling thought as the planet warms and temperature patterns shift.

Even taking the rosy view that laggards aren’t the majority of weeds in the study, the finding that 35 percent lagged deceptively is “still bad,” says invasion ecologist Shaun Coutts of the University of Lincoln in England. That portion is “thousands of potentially damaging introductions all over the world,” he says.

The findings, Mesgaran says, are a flashing-red warning against moving plants out of their native range.  “Any judgment that we make on a species based on its past and present is not going to be a good predictor of what it’s going to do in the future,” he says. “Don’t think, ‘Oh yeah, this species has been around — nothing has happened.’”