Life above the treeline: a global perspective on alpine biodiversity

Across the world’s mountains, alpine zones (located between the treeline and permanent snow or glaciers) have been shaped by hundreds of thousands of years of climatic fluctuations. As climates cooled and warmed, treelines repeatedly shifted up and down, glaciers advanced and retreated, and alpine zones became connected, fragmented and isolated again. With these changes species moved, disappeared from some places and colonized others what repeatedly reshuffled alpine species communities.

Our Mountains in Motion research group led by Suzette Flantua investigates different aspects of this dynamic alpine history. Augusto Lima and Sjur Brandon study past glacier dynamics, Eline Rentier reconstructs shifts in treeline elevation and the resulting changes in alpine biome connectivity and with my PhD research I try to answer how these dynamics have shaped present day alpine biodiversity.

But before I could dive into this research, we first needed to answer something more fundamental: What does alpine biodiversity look like today in mountains around the world? How many alpine species are there for plants, birds, mammals and reptiles in different mountain regions? And are alpine biodiversity patterns comparable across mountains? These questions became the starting point for my first PhD chapter and our recently published study, in which we compare alpine biodiversity across taxonomic groups and mountain regions worldwide.

Mountains of data

When I started my PhD, I thought these questions sound rather straightforward to answer.  Well, turns out, our endeavor got a bit more complex than initially thought. To compare alpine biodiversity in mammals, birds, reptiles and plants, we first needed a dataset that was assembled consistently across mountains. This was easier said than done. Information on alpine species is fragmented, biased towards certain regions and taxonomic groups, and comes in very different data formats (e.g., plot data, checklists, range shapefiles,..). We therefore combined information from global open-access databases, published literature and spatial analyses to create a first set of species lists for mountain regions around the world. But we also knew that particularly global databases come with biases and limitations. Therefore, through the Global Mountain Biodiversity Assessment (GMBA), we reached out to experts in the field and asked whether they would help validate our species lists. After months of emailing and online meetings, researchers from 22 different countries helped verify and correct data for not all but many of the mountains we analysed. Although this effort took time, it for me became a very rewarding part of our project. It showed me how engaged researchers in the field are to share their knowledge and how important local expertise is – not only for improving large-scale biodiversity datasets but also for understanding the patterns they reveal.

What is an alpine species?

With these species lists at hand, we entered a new discussion with our team. How do we actually define an alpine species? Are they alpine only if they occur above the treeline or also if they live also in life zones further down in the mountain? After long meetings discussing this question, we eventually classified species according to their elevational ranges relative to the treeline in each mountain ranging from species that occur across the entire elevational gradient of a mountain to those that are restricted to the alpine zone (i.e., alpine specialists) (for details see methods in our paper). Of course, any classification like this simplifies ecological reality, but one thing I also had to learn throughout this project is that working at such a large scale inevitably requires compromises. No dataset or definition can ever fully capture the complixity of life in mountains, and being transparent about the limitations inherent in global datasets and analyses is just as important as describing the broader patterns that emerge despite them.

So, what does alpine biodiversity look like across the world’s mountains?

Again, there was no simple answer. Alpine biodiversity varied a lot among mountains and among plants, birds, reptiles and mammals. The Ethiopian Highlands for example support diverse alpine mammal and bird communities, while the East African Rift is less diverse in species of these groups and supports particularly few reptiles. In North America, the western cordillera is more diverse with far more alpine specialists than the eastern cordillera. The Hindu Kush shows relatively high richness in birds, mammals and plants but less so for reptiles. Yet, from all this variation our study confirmed what many regional studies have already shown for individual taxonomic groups. The Neotropical Andes stood out as the global hotspot of alpine biodiversity (Fig 2). With a “hotspot” we mean in our study alpine ecosystems that have very high overall species richness across all taxonomic groups and the greatest number of species restricted to the alpine zone. Some temperate mountain regions in contrast, such as the European Alps, support many species that occur across several life zones, but comparatively fewer species restricted to the alpine zone.

Are these differences simply because some mountains have more alpine area?

Larger alpine area can, in theory, provide more space and habitat for more species. But even after accounting for alpine area size across mountains in our analyses, the Neotropical Andes still contained far more species than expected. So did some small and fragmented alpine areas in East Africa while other large alpine areas, such as the Tibetan Plateau, support fewer species than its area alone would predict.

And what about latitude?

Across many ecosystems, species richness peaks towards the tropics and declines towards the poles. This is thought to be driven largely by climatic gradients, such as temperature and energy availability. But because alpine environments share largely similar climatic conditions across mountains globally, we wondered whether alpine biodiversity would decouple from this classical gradient.

It does. Tropical mountain ranges certainly contain some of the greatest alpine biodiversity hotspots, but across all mountains in our study we found no simple consistent decline of alpine biodiversity with latitude.

But what then explains these patterns?

Our study shows the immense variation of alpine biodiversity patterns across mountain, but it also opens the door to the question why these patterns occur? Understanding the processes behind these patterns may also help us understand how alpine species will respond to ongoing global change.

Part of the answer may lie in the different biogeographic histories of different mountain regions. The Northern Andes for instance experienced repeated changes in alpine connectivity and fragmentation during past climate fluctuations (Flantua et al. 2019) while the alpine zone of the European Alps was displaced and fragmented by glaciers for a vast amount of time (Schönswetter et al. 2005). Together with the unique topographies and environmental conditions of each mountain, these histories must have left an imprint on the biodiversity in alpine zones we see today.

Together with all the research done by our team on glaciers, treelines and alpine biome connectivity, we now hope to link these histories to the alpine biodiversity patterns we found in our study and to better understand how mountains in motion have shaped alpine biodiversity around the world