A road winds through a mountain landscape covered in red mossy vegetation and shrubs.
Invasive plants are creeping up mountain roads, threating native species that had a niche in high-altitude environments. Credit: Tubbn, Pixabay

Mountain ranges are home to numerous plant and animal species, many of which are specifically adapted to life at high altitudes and occur nowhere else on Earth. But new research suggests mountain ecosystems are under threat as non-native plants spread and displace endemic species.

The study is the culmination of a 10-year effort by an international group of researchers belonging to the Mountain Invasion Research Network (MIREN). From 2007 to 2017, members monitored non-native plants along transport routes in 11 mountainous regions, including the Australian and Swiss Alps, Chile, Hawaii, and the mountain ranges of Kashmir in India.

Invaders Threaten Alpine Areas

Over that time period, the team found that the number of non-native plant species, or neophytes, living at high altitude increased by an average of 16%. This increase is concerning because alpine plants are already stressed by global warming. As mountain climates become milder, fair-weather plants spread to higher altitudes and outcompete plants that are specialized for survival in harsh, mountainous conditions.

“To see significant effects within only 5–10 years was surprising.”

Until recently, high-altitude competitors were largely other native species, but that has changed with increasing human presence in the mountains. Neophytes might be planted for decorative purposes in mountain resorts or gardens, but seeds from lowlands can also reach high elevations when embedded in car tires or visitors’ boots. Once dispersed, neophytes often proliferate along the roadside, where the native flora is already weakened by pollution and human interference.

According to the new study, non-native species are advancing more rapidly than expected. “Ecological processes such as the establishment of new species often take a long time, so to see significant effects within only 5–10 years was surprising,” explained Evelin Iseli, a plant ecologist at ETH Zurich and coauthor of the study.

Neophytes mainly spread along roads, offering hope that more pristine, undisturbed areas are better able to resist the invaders, she added. But the researchers point out that under the warming climate, that could change.

A Collaborative Approach to Mountain Monitoring

Sonja Wipf, a plant ecologist with Swiss National Parks who was not involved with the research, praised the global scope of the study. It shows “the relevance of non-native species spread for all mountain ecosystems,” she said, “even those that lack such observations.”

The results reflect a heroic effort on the part of the MIREN members, who collected almost 15,000 observations from hundreds of sites, Iseli said. After driving for hours or trekking along mountain trails, the researchers defined sampling plots along the road and meticulously logged the plants in each plot. The sites were marked with posts or tags, enabling MIREN members to return and repeat their observations later.

The researchers—who participated voluntarily and funded the work using their own resources—faced bad weather, sites lost to construction, and the challenge of combining the data (with all their regional species names, spellings, and classifications) into a global database.

Despite those hurdles, “the collaboration between so many regions from different parts of the world is rewarding,” Iseli said.

The team’s large-scale approach was crucial to the study, she explained, because the proliferation of non-native species could be detected only by pooling observations from multiple regions. Thus, “large projects like MIREN enable us to detect global trends which could potentially be missed if a study is limited to a specific region.”

Struggling to Protect Mountain Habitats

“Roads can provide an early warning for those species.”

Understanding the causes and timescales of environmental change can help to protect endangered habitats. Because non-native species spread along transport routes, “roads can provide an early warning for those species,” Iseli said, “and might be valuable for predicting emerging threats to native species and ecosystems.” Continued monitoring is essential for recognizing and managing problematic species, she added.

It is almost impossible to eliminate neophytes once they have taken root, Wipf said. But it would help to not make the problem worse, she added. “Prevent any further spread of new species, plant natives in gardens, and most importantly, fight climate change by reducing CO2 output.” If humans succeed in limiting the extent of global warming, the climate on the world’s mountaintops might remain harsh enough to be inhospitable to non-native species.

If current trends continue, “mountain ecosystems may experience some homogenization,” Wipf said. As lower-elevation species colonize higher areas and come to dominate their environments, people driving from the lowlands to the mountains will notice fewer changes in vegetation. This homogenization can be global as well as local: The common dandelion, for instance, is native to Europe but has invaded mountain ranges from Alaska to the Andes.

The decline of native high-altitude plants will also have far-reaching effects on interconnected mountain wildlife: Many native insects, birds, and other primary consumers in mountain food webs will lose a staple of their diets, in turn reducing the number and diversity of prey species available for regional predators.

In the future, MIREN researchers will continue their observations and implement monitoring in more regions. Iseli said that despite the challenges involved, she hopes that “the growing numbers of participating mountain regions across the world will help us to better document and protect plant biodiversity in mountains globally, especially in parts of the world where biodiversity data are currently lacking.”

—Caroline Hasler (@carbonbasedcary), Science Writer

Citation: Hasler, C. (2023), Native plants are hiding up high, but invaders are catching up, Eos, 104, https://doi.org/10.1029/2023EO230093. Published on 9 March 2023.
Text © 2023. The authors. CC BY-NC-ND 3.0
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