Sometimes one needs patience to answer a research question. Lots of it. The Mountain Invasion Research Network (MIREN) already asked itself this important question back in 2006: how fast are non-native species travelling uphill along mountain roads? Now it’s 2023, and for the first time we have an answer to this, thanks to long-term vegetation monitoring by a team of researchers across the globe, and the work of MIREN master-turned-PhD candidate Evelin Iseli, whose work now got published in Nature Ecology & Evolution (!). What follows is the English translation of the press release by our university. The paper itself can be found here!

Suppose you put your hiking boots in your backpack, fly to another continent and go hiking in the mountains. Chances are that you may find that seeds of local vegetation travelled the whole way with you in the mud on the soles of your shoes. If these fall off your shoe abroad, an alien plant may sprout where that seed has fallen. Thus, without realising it, you contribute to the spread of non-native plant species.

Moving uphill

Ecologists from the global MIREN-network found that the number of non-native plant species in mountain areas around the world has significantly increased in the past decade. Until now, alien plant species were advancing less in mountains compared to their increase at lower elevations. But due to climate warming on the one hand and increasing human influence at high elevations on the other, it now appears that non-native plants in mountains are also advancing steadily uphill.

“Although the presence of native species in mountains is relatively well documented, long-term studies of alien species in mountain areas are very rare,” says Jonas Lembrechts, biologist at UAntwerpen. Lembrechts and his colleagues have therefore been monitoring non-native species along mountain roads since 2007, in 11 mountain areas around the world: in Norway, Switzerland, the Canary Islands, New South Wales, Victoria (Australia), central and southern Chile, India, Hawaii, Montana and Oregon.

“There are big differences in the speed of the invasion, but the general increase is unmistakable,” says Lembrechts. The study of the 11 mountain regions worldwide shows an increase in the number of alien species by as much as 16% over the past decade.

Mainly European plant species

Moreover, in 10 of the 11 regions studied, the scientists found the alien species at significantly higher altitudes than ten or even just five years ago. Moving to higher altitudes to follow their preferred climate is a well-known strategy for plants to defend themselves against climate change. Only, the rate at which alien species are climbing upwards is higher than if climate alone were the culprit.

“Europe is the largest exporter of exotic plants to mountainous regions worldwide, says Jonas Lembrechts. Left: Trifolium pratense from Western Europe in the Norwegian Scandes. Right: the European Taraxacum officinale in the Chilean mountains.

“Exotic species often enter new regions through the lowlands, where most of the people are,” says Lembrechts. “From there, they find their way to the mountains. But with a little help from humans, their spread can sometimes be lightning-fast, especially along mountain roads and trails.”

Off balance

Often these are then also European plant species, research shows. “Europe is currently by far the biggest exporter of exotic plants to mountain areas worldwide,” Lembrechts explains. “Not so strange, when you consider that in recent centuries it was also mainly Europeans who started visiting mountain areas on other continents, for tourist and commercial purposes.”

However, this is not good news. “Mountain nature is often very fragile,” says Lembrechts. “Alien plant species can threaten or drive out native species, which are important in the local ecosystem, throwing the system out of balance.”