Hitchhiking mountain roads and trails

The Mountain Invasion Research Network (MIREN) has made itself ‘world famous’ through its protocol for the long-term monitoring of vegetation along mountain roads. We even got a recent publication dedicated in its entirety to that protocol! However, mountains – and the rest of our increasingly connected world, for that matter – are crisscrossed by a whole bunch of other ‘linear disturbances’. An obvious question thus arose: are the vegetation patterns we see along roads mirrored along these other linear disturbances, such as trails?

How problematic to nature are mountain trails – linear disturbances that protrude deep into our most precious protected nature.

To answer that question, we set up a second global vegetation monitoring protocol, yet now focussed on mountain trailsides. While our roadside protocol has been adopted in more than twenty regions worldwide, the tally for the trail protocol is currently at six. So a tad more humble in its scope, yet still more than enough to satisfactorily answer a first important general question – at the core of our MIREN business: are patterns in non-native species richness along mountain trails a mirror of those found along mountain roads?

Hiking along mountain trails can bring non-native species to surprisingly high elevations. In this stony desert above the treeline in the Chilean Andes, we found some European Taraxacum flowers!

The answer can be found in a recent publication – part of a new book on the role of tourism and recreation as drivers of biological invasions.

So how do our trails compare? Observed levels of plant invasion were substantially lower along trails than along roads, adding up to only about 20% of the species numbers. Nevertheless, the same decline in richness with elevation is found that is so characteristic of mountain roadsides across the globe, and many of the most common non-native plant species along roads are also found along trails.

Most striking, however, is the lack of decline in non-native species richness with distance to the disturbance. Along roads, we see worldwide the same characteristic pattern, with high numbers of non-natives in roadsides and then a more or less steep drop in species close to the road in the interior vegetation. Along trails, this pattern is virtually not there, which implies that levels of invasion are largely the same in trailsides and the interior vegetation.

Non-native species richness along mountain trails (red line) and in the adjacent natural vegetation (turquoise line) across six mountain regions (see map). Yellow-to-brown ‘smudges’ on the map depict the global mountain regions.

The latter has some worrisome implications, as it shows that the resistance to invasion of the interior vegetation close to trails is much lower than along roads. This could be due to the sparser vegetation (most studied trails were at higher elevations in the mountains than the roads), but most likely also to the differences in disturbance dynamics: while roadsides are often highly disturbed close to the road, trailsides are less disturbed in general, yet the ‘disturbers’ often wander off the trail more, spreading its impact further into the interior vegetation.

European dandelion in the alpine zone of the Chilean Andes

The results of our first global comparative analysis of roads and trails are thus both encouraging and worrying. Encouraging for our trailside managers is the fact that levels of invasion are lower here than along roads. Nevertheless, the lower resistance against invasion of the surroundings asks for strong preventive action to limit the impact of invasions along mountain trails, especially those in vulnerable protected areas.


Barros et al. (2022). The Role of Roads and Trails for Facilitating Mountain Plant Invasions. https://doi.org/10.1079/9781800620544.000. Full text here!

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