Mountains tell stories. Their geology and topography tell the story of how each mountain range was uplifted over millions of years. Their life zones from mountain forest to alpine grasslands and permanent snow tell how changing climate and topography shaped diverse habitats along elevational gradients. And the plants and animals living in mountains tell stories of adaptation to life at high elevations.
One of the highest of these life zones is the alpine zone. It is located between the treeline and the permanent snow line, or glaciers in some mountains. Around the world, alpine zones are known by different names. In the Andes for example they are called the páramo and the puna, or bugyal in parts of the Himalaya, and in South and East Africa the Afro-alpine zone. For simplicity, I use the term alpine throughout this article to refer to all ecosystems above the treeline.
Across the world’s mountains, alpine zones 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. These changes repeatedly reshuffled alpine species communities. Today, alpine ecosystems are changing again. Temperatures are rising and humans are increasingly transforming mountain landscapes globally. To understand what these changes mean for alpine biodiversity, however, we first need to dive deeper into one important question: how does alpine biodiversity look like across the world’s mountains?
Fragmented data to answer a global question
At first, this sounded like a rather straightforward question for the first chapter of my PhD. But after screening hundreds of papers, searching through datasets and talking to researchers in the field, I started to realize that this question was not as straightforward as initially thought.
Information on mountain- and particularly alpine species is surprisingly fragmented. For instance, some mountain regions have been studied in detail, while others have hardly any research at all. Also, existing data on alpine species are scattered across the literature and come in many different data formats that are not really compatible such as sampling plots, checklists and distribution maps. I also realized that research on life in the alpine zone tends to be biased towards certain taxonomic groups such as plants. Unfortunately, groups like reptiles or amphibians are much less studied in many mountain regions.
To create a global comparison of alpine biodiversity, we needed a dataset that could be assembled consistently across mountain regions. We therefore relied on global open access databases. For plants we used GIFT (Global Inventory of Flora and Traits), a database that integrates regional curated checklists with species respective elevational ranges (the lowest and highest elevation at which a species occurs). For mammals, birds and reptiles, we combined global distribution maps with published elevation limits from the literature. We knew that using these sources would introduce biases and limitations inherent to heterogenous global databases, but they provided a consistent starting point for our study.
Global data needs local expertise
Yet, if we wanted to better understand how alpine biodiversity looks like in the world’s mountains, we needed people who know these mountains best. Through the GMBA (Global Mountain Biodiversity Assessment) network, which connects mountain researchers around the world, we contacted experts and asked whether they would help validate our species lists. Many experts replied to our emails, others didn’t, others suggested colleagues and, in the end, researchers from 22 different countries helped verify and correct data for many of the mountain regions we analysed. Coordinating this effort took time, but for me it became one of the most rewarding parts of the 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.
How to define an alpine species
Then, with species lists for plants, birds, mammals and reptiles across 32 mountain regions at hand, we reached the next challenge: What is an “alpine” species? Does it have to live exclusively in the alpine zone, or can it still be considered alpine if it also lives in the life zones below? The answer is again not as straightforward as it may seem. Where a species occurs within a mountain is shaped by local climates, topography and ecological interactions. Within our team we spent hours discussing how an “alpine species” could be defined in a way that would be meaningful and comparable across the globe.
We eventually classified species according to their elevational ranges relative to the treeline of each mountain region. We opted for five categories, 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, and no single definition can ever fully capture the complexity of life in mountains. But global scale analyses inevitably require compromises and recognising their limitations is just as important as reporting the patterns they reveal. With this framework in place, we could now finally return to our main question: What does alpine biodiversity look like across the world’s mountains?
Every mountain tells its own story of alpine diversity
The answer was, once again, more complex than I had expected. Alpine biodiversity varied a lot among mountains and taxonomic groups. The Ethiopian Highlands for example are home to 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, one clear pattern did emerge from all this variation.
Confirming 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). Here I mean a “hotspot” because they 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.
Bigger alpine area does not always mean more diverse
But were these differences simply because some mountains contain much larger alpine areas than others? Ecological theory predicts that larger areas generally support more species because they provide more space and thus habitat. Yet, even after accounting for alpine area size across mountains in our analyses, the Neotropical Andes still contained far more species than expected! And so did the small and fragmented alpine areas in East Africa. Surprisingly, other large alpine areas, such as the Tibetan Plateau, support fewer species than one would think from their size alone (Fig. 2).
Alpine biodiversity does not follow a latitudinal gradient
Clearly, area alone could not explain the differences in alpine biodiversity we found across mountains. But was there still a broader ecological pattern underlying them?
In many of the world’s biomes, biodiversity follows a latitudinal gradient, where species richness generally peaks near the equator and declines towards the poles. This pattern 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.
Although tropical mountain ranges consistently emerged as biodiversity hotspots we found no simple consistent decline of alpine biodiversity with latitude. And when comparing plants, birds, mammals and reptiles there are no consistent patterns at all! So…. the global distribution of alpine biodiversity must be shaped by much more than area AND climate alone.
Why?
We do not yet have the full answer. But we do know from many regional studies how different the biogeographic stories of different mountain regions have been. Other research has shown that 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 local environments, these histories must have left imprints on the patterns of alpine biodiversity we observed in our study.
Perhaps this is the biggest lesson from this project.
What began as a seemingly simple question became a reminder of how complex mountain ecosystems are. Understanding life in alpine zones and how vulnerable they may be to ongoing global change depends on understanding the individual geological, climatic and biogeographic history of each mountain system.
Our study revealed the diversity of biodiversity patterns within and among mountains and taxonomic groups. And, more importantly, it offers a glimpse of the many stories in the world’s mountains that are still waiting to be discovered.