In 2016, we published the results from a much-needed experiment. We had sown seeds in two extreme environments: the north of Scandinavia, and the very south of Chile, to answer a fundamental research question in invasion ecology: what is the most important driver of plant invasions in mountains?
Results of that experiment were overwhelming: whatever the temperature – both regions have a harsh climate that should limit plant invasions – human disturbance always came out on top: if humans disturbed the vegetation, plant invasions soared. No disturbance made successful establishment of non-native species virtually impossible; and that all along the elevation gradients in both Chile and Scandinavia.
However convincing, those results were ‘only’ experimental. Question remained if that dominance of anthropogenic disturbance would hold up in a real setting. To settle that argument, we went to the Chilean Andes, where we monitored non-native plant richness and abundance along three mountain roads using the trusted survey design of the Mountain Invasion Research Network (MIREN, www.mountaininvasions.org). Along these roads, we assessed the relative importance of anthropogenic (human disturbance), abiotic (e.g. the climate, but also soil nutrients), and biotic (interactions with the native vegetation) factors as drivers of plant invasions.
And indeed, as hypothesized based on our experiment from half a decade ago, anthropogenic drivers here again came out on top. While low elevation areas were the most invaded – suggesting perhaps that cold climate at high elevations would be limiting invasion – patterns in non-native species distributions were driven mainly by anthropogenic factors, which explained between 20 and 50% of the variation along the three roads.
At the regional scale, the abundance of non-native species was again explained best by anthropogenic factors (24% of the variance), yet non-native richness was driven most strongly by abiotic factors such as soil nitrogen content and pH (15% of the variance).
These results thus elegantly confirm the conclusions from our 2016 experiment that anthropogenic factors largely override abiotic factors as drivers of plant invasions in mountains, both at the local and the regional scale. Importantly, these results also imply that non-native plant invasion in mountains is currently not strongly limited by climate, suggesting that with further increases in disturbance in the Andes, increased plant invasion will most likely quickly follow.
These results should thus serve as an important warning: we urgently need better biosafety protocols and control of tourism and agricultural activities in the Chilean Andes, where human influence keeps expanding into natural areas. Only with coordinated efforts like these that keep disturbance at bay, we can limit the risks of further spreading of invasive plant species in the vulnerable Andean landscape.