It is a known pattern by now, as it is confirmed over and over in virtually all mountain regions we study: roads are facilitating non-native plant species introduction into mountains. Humans introduce – on purpose or by accident – new species in the valleys and from there, they start spreading uphill. On their way up to high elevations, mountain roads serve as a great highway. Yet with increasing elevation, less and less 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 less species manage that.

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

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

These questions we aimed to answer in our latest paper in the journal Biological Invasions, using our multiregional MIREN database (www.mountaininvasions.org). We looked at all these species that are travelling uphill, and hunted down the global patterns.

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

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. Warm-adapted species slowly disappear as well (yet not as fast as we assumed).

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. 

Then there is a second filter, one that decides if you can run off the road, 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.

Colonizing the native mountain vegetation is easier if you can handle the shade underneath their leaves. Here: meadow vegetation in Dischma, Switzerland.

It is unlikely that many species have traits that help them pass both those filters at the same time. And then we haven’t mentioned the specificities of the receiving habitat yet: conditions there should also promote non-native species colonization, for example through the availability of bare ground. 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. Further studies will have to figure out what exactly makes it stand out. 

So we do not have to worry about non-native species invasions in mountains? Well, not quiet. There is another common pathway of introduction that helps non-native species to 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. Check it here or contact me for the full paper