For MIREN, we are working on an awesome new blog series summarizing our scientific findings from the last 15 years for conservation, policy makers and the global public. This is chapter 2 in the series, follow the whole story on www.mountaininvasions.org.

Non-native species are species living outside their natural area, arriving there by following humans in their trace. Recent global change, most notoriously the exponential increase in global trade and travel, have rapidly turned the invasion by non-native species into a global issue. However, there have up till recently always been a few pristine environments that could be considered little affected by invasions. Mountains, with their harsh climate and their low accessibility, were a perfect example ¹. These low levels of invasions in mountains are indeed reported in the first MIREN-publications, mentioning only few non-native plant species in the alpine zone in Switzerland ² as well as in the Northwest of North America ³ more than a decade ago.

The harsh climate in mountains (here the Cañadas del Teide on Tenerife) has always been seen as an adequate barrier against non-native plant invasion.

This expected increase in non-native plant species introductions into mountainous areas is in fact happening under our very eyes. Over a thousand non-native species have now become established in natural areas at high elevations worldwide, and although many of these are not invasive, some may pose a considerable threat to native mountain ecosystems ⁴. In total nearly 200 non-native plant species have even been recorded from alpine environments (above the tree line) around the world ⁶. Islands turn out especially vulnerable for these invasions ⁷, with over a 150 naturalized plant species already present above 2000 meters in Hawaii as early as 2005 ⁸.

Non-native plant species are increasingly found at high elevations in mountains across the globe. Here a dandelion (Taraxacum officinale), a common invader in mountain regions everywhere.

Non-native species richness is highest in the New World regions, reflecting the effects of colonization from Europe ⁹. Surprisingly, similarity among regions is low and due mainly to certain Eurasian species, mostly native to temperate Europe, occurring in all New World regions ^8,9. Most of these non-native plant species appear to have been introduced unintentionally (e.g. as seeds attached to vehicles, animals and humans), but a few are introduced to assist with revegetation of disturbed soils and for amenity plantings in ski resorts ¹⁰. Most non-native species are lowland forb species, yet alpine non-native species pose a much greater risk, as they are already pre-adapted to the harsh mountain climate ^4,11. We nevertheless identified only three species as specifically cold-adapted, with the overwhelming majority having their centers of distribution under warmer environments ⁶.

Differences in elevational distributions between non-native species mostly result from differences in biogeographical affinities and climatic tolerances (Arévalo et al. 2005). Range limits of non-native species at high elevation are associated with high population turnover, which results in a transition zone characterized by source-sink dynamics ¹². Populations within this zone exhibit reduced probability of occurrence, and smaller patch size. Yet they are there, undeniably.

We thus observe a rapid increase in non-native species presence in high elevation areas around the world, triggering questions on what is driving these introductions, and how to deal with them. In the next episode of this series, we will first discuss the elevational distribution patterns of non-native species in mountains, and how these species deal with the harsh mountain climate.

References

1. McDougall, K. L. et al. (2011). Plant invasions in mountains: global lessons for better management. Mountain Research and Development 31, 380-387.
2. Becker, T. et al. (2005). Altitudinal distribution of alien plant species in the Swiss Alps. Perspectives in Plant Ecology Evolution and Systematics 7, 173-183.
3. Parks, C. G. et al. (2005). Natural and land-use history of the Northwest mountain ecoregions (USA) in relation to patterns of plant invasions. Perspectives in Plant Ecology, Evolution and Systematics 7, 137-158.
4. Pauchard, A. et al. (2009). Ain’t no mountain high enough: plant invasions reaching new elevations. Frontiers in Ecology and the Environment 7, 479-486.
5. Kueffer, C. et al. in Plant invasions in protected areas 89-113 (Springer, 2013).
6. Alexander, J. M. et al. (2016). Plant invasions into mountains and alpine ecosystems: current status and future challenges. Alpine Botany 126, 89-103.
7. Arteaga, M. A. et al. (2009). How do alien plants distribute along roads on oceanic islands? A case study in Tenerife, Canary Islands. Biological Invasions 11, 1071-1086.
8. Daehler, C. C. (2005). Upper-montane plant invasions in the Hawaiian Islands: Patterns and opportunities. Perspectives in Plant Ecology Evolution and Systematics 7, 203-216.
9. Seipel, T. et al. (2012). Processes at multiple scales affect richness and similarity of non-native plant species in mountains around the world. Global Ecology and Biogeography 21, 236-246.
10. McDougall, K. L. et al. (2005). Plant invasions in treeless vegetation of the Australian Alps. Perspectives in Plant Ecology Evolution and Systematics 7, 159-171.
11. Alexander, J. M. et al. (2011). Assembly of nonnative floras along elevational gradients explained by directional ecological filtering. Proceedings of the National Academy of Sciences of the United States of America 108, 656-661.
12. Seipel, T. et al. (2016). Range limits and population dynamics of non-native plants spreading along elevation gradients. Perspectives in plant ecology, evolution and systematics 20, 46-55.