Mountaintops, especially in the balmier regions of the world, are reservoirs of astonishing levels of butterfly diversity; but, as the climate continues to warm with no end in sight, these high elevation hotspots might be turning into extinction traps for butterflies. 

So warns a new paper led by Dr. Stefan Pinkert, former postdoc at the BGC Center and current staff scientist at the Universität Marburg, and colleagues recently published in Nature Ecology and Evolution. In the paper, Pinkert and colleagues compile data on geographic ranges and phylogenetics for more than 12,000 species of butterflies to map global patterns of diversity along with looming climatic threats – the first study of its kind for an insect taxon. Their research revealed unexpected and alarming results, highlighting the urgent need for both targeted conservation action to protect threatened butterfly hotspots and enhanced efforts to study and assess other insect taxa. 

“Insects are a crucial and omnipresent component of the web of life, supporting key ecosystem functions,” said Pinkert, whose research focuses on butterflies and odonates, the group of insects that includes dragonflies. “Among the estimated 5.5 million insects on our planet, butterflies are probably among the most well-documented and certainly most prominent groups of insects. Yet they are also very numerous and particularly in tropical regions often elusive, which translates into a comparatively sparse availability of information on their presences – and has long hampered their integration into global scale conservation efforts.” 

Pinkert and colleagues needed to confront the lack of geographic range map data among butterflies to begin their study. The researchers combined multiple approaches to fill out this space: first, they assembled existing expert range maps from the literature; where those were not available, they developed species distribution models from occurrence data; and where occurrence data was insufficient for modeling, they generated ecoregional range maps, which estimate species ranges based on occurrence data and mapped biogeographical regions. The latter approach, initially developed and evaluated for this effort by Pinkert et al. 2022, offers high potential for improving the taxonomic coverage of many other data-poor groups, according to the researchers. In total, their efforts resulted in geographic range maps for 65% of all known butterfly species. 

Previous work in data integration and taxonomic harmonization carried out in the Center, as well as by other colleagues in the field, were critical in supporting this monumental task. “Imagine more than a hundred thousand names of species needed to be compiled to build a reference for data integration with a butterfly going by, on average, 3.5 different names from different sources,” said Pinkert. “This babylonian name confusion is, I think, exceptional and yet so fundamentally important for global synthesis. Sandall et al. (2023), a paper led by former BGC Center postdocs, developed a globally integrated structure of taxonomy that greatly assisted Pinkert and coauthors in integrating and harmonizing the butterfly data from a multitude of sources. 

The researchers generated measures of global diversity patterns along three key dimensions: species richness, species range rarity, and phylogenetic distinctiveness. Richness deals with the overall number of species and rarity with the relative restrictedness of species ranges. According to Pinkert, “Phylogenetic diversity reflects the evolutionary history of a taxon, much of its functional characteristics and also species’ capacity to cope with environmental changes.” This multifaceted analysis allowed the researchers to consider a broader picture of butterfly diversity, whereas focusing on just one measure of diversity could mean many areas important for preserving butterflies would be overlooked. 

To assess these patterns, the researchers identified hotspots around the world for each measure of diversity and compared their overlap, or lack thereof. Richness was found to be highest at the equator and at elevations above 2,500 meters, while rarity was highest in the subtropics and above 3,500 meters. Only 10% of global hotspots for richness and rarity overlapped. Adding phylogenetic diversity into the picture complicates things even more, with extremely weak correlations revealed with the other two diversity measures. These results confirmed the researchers' suspicions that different measures of butterfly diversity would highlight different areas in the world for conservation importance.

On top of the weak correlations among butterfly diversity metrics, the researchers also found that the overall patterns of butterfly richness and rarity correlated moderately with other species groups, including mammals, birds, amphibians, reptiles, ants, and plants, but hotspots between different species correlated weakly Nearly half of all butterfly rarity hotspots had no overlap with any known vertebrate rarity hotspots. What this means is that focusing conservation efforts on areas deemed important to one group of species will not necessarily preserve the highest concentrations of diversity in other species groups – an incorrect assumption that has guided many past conservation efforts. 

The researchers also evaluated the extent to which butterfly hotspots occurred in mountains, whose unique geography has long been known to nurture species diversification and provide refuge in a changing climate, using a global inventory of mountains developed by Snethlage et al. (2022) alongside Center collaborators. While mountains cover only about a third of the Earth’s land surface, three-fourths of all butterfly richness and rarity hotspots and over half of phylogenetic diversity hotspots occurred in mountainous regions, the researchers found. 

“Butterfly caterpillars are typically highly specialized to certain plants, and our results indeed demonstrate that the richness of both plants and butterflies, different that of the other considered groups, uniquely strongly increases with elevation,” said Pinkert about the revealed diversity patterns. “Also, thermal adaptations further support butterfly activity in the narrow windows of opportunity that the short vegetation periods in these harsh climates leave for butterflies to grow and reproduce.”

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Finally, the researchers investigated how the ongoing climate crisis may affect these butterfly hotspots. Using models of projected emissions and subsequent temperature increases to 2070, they analyzed how the change in the temperature niche of butterfly hotspots (defined as the extent to which the current average temperature across a hotspot is still within the range of projected future temperatures) compares to the projected change in non-hotspots. This comparison allowed Pinkert and colleagues to reveal that butterfly hotspots – those tropical and subtropical mountainous areas – face a higher burden of projected temperature niche loss than non-hotspots, pointing to the troubling conclusion that these currently highly biodiverse mountaintops may actually be deads ends for butterflies in a warming world. 

“Our alarming results about the fate of butterfly diversity, but also implications for the broader, unassessed insect diversity, urge for targeted and immediate action,” said Pinkert. Two current efforts to map biodiversity, the 30-by-30 project, in which a BGC Center team has mapped species richness and rarity at 1-kilometer resolution across the USA and Canada, and the Half-Earth Project Map, which features global species diversity patterns, present unprecedented opportunities for filling in species data gaps and making this biodiversity data publicly available.

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