Running off the road
article written by MRI
13.07.18 | 09:07

Mountain roads – and the cars and people on them – facilitate non-native species movement up to high elevations. Here: Davos, Switzerland

It is a familiar pattern by now, confirmed over and over in virtually all mountain regions we study: roads are facilitating the introduction of non-native plant species into mountains. Humans introduce – on purpose or by accident – new species in the valleys, and from there they start spreading uphill. On their way towards high elevations, mountain roads serve as a great highway. But with increasing elevation, fewer and fewer non-natives will be found, as they progressively drop out the higher you get. The few that make it all the way to the top by road could possibly spread from there into the natural mountain vegetation, but even fewer species manage that.

All of that we knew, indeed, but a crucial question remains: who wins this race to the top? Which traits make a non-native species good at this quest for the high elevations?

We aimed to answer these questions in our latest paper in the journal Biological Invasions. Using our multi-regional MIREN database (, we looked at all the species that are travelling uphill, and hunted down the global patterns.

You’re eager now to know what turns these non-native plants into winners, right? Well, it’s a lot tougher than you might think. The magical words describing the problem these plants face: the double filter.

Some non-native plant species are better at reaching the top of a mountain road than others. Especially those that are good at handling the open, disturbed conditions of mountain roadsides and the cold of high elevations. Here: happy tourist on a mountain top in Yellowstone National Park, USA.

First of all, the colonizing non-native species need to be able to handle roadside conditions: highly disturbed environments, with open vegetation and a peculiar microclimate. Moreover, they should be able to handle these conditions along the whole elevational gradient, from warm all the way to cold conditions. That is the first filter, which slashes out a lot of species. Annual species are progressively filtered out like this, for example, as it gets increasingly hard to perform your life cycle within one growing season. Species adapted to warm conditions slowly disappear as well (although not as fast as we assumed).

Then there is a second filter, one that decides whether a plant species can run off the road and into the natural vegetation. This filter selects for totally different traits than the first one: moist- and shade-adapted species do better in this case, for example, as they’ll need to colonize an environment that’s already covered with plants.

It is unlikely that many species have traits that help them pass both those filters at the same time. And we still haven’t mentioned the specificities of the receiving habitat: conditions there should also promote non-native species colonization, through the availability of bare ground for example. These results thus show that a lot of things need to be ‘just right’ for a non-native species to succeed in high elevation natural environments, which explains why so few non-native species are currently present there: passing the double filter test is just really hard.

Achillea millefolium (common yarrow, here in Montana, USA) seems to be a species with the potential to pass the double filter, as it is successful in mountain regions worldwide.

So do we not have to worry about non-native species invasions in mountains? Well, not quite. There is another common pathway of introduction that helps non-native species get around this double filter issue. Indeed, if humans introduce mountain plants directly at high elevations, for example in ski resort gardens, invasion becomes much more likely.

Want to know more?
McDougall, K. L., Lembrechts, J., Rew, L. J., Haider, S., Cavieres, L. A., Kueffer, C., … & Seipel, T. Running off the road: roadside non-native plants invading mountain vegetation. Biological Invasions, 1-13. 
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