A global analysis shows that many butterfly species are gaining new territories, losing others, or changing altitude. In the majority of documented cases, these shifts are associated with climate change and extreme weather events.
To draw up this map, scientists gathered 6,182 records of range shifts from 567 studies and 68 assessments carried out by specialists. Their database covers 1,758 species in 105 countries and territories, or about 10% of known butterfly species. It also brings together publications in 15 languages, in order to reduce the usual weight of English-language studies.
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The range of a species is simply the geographic area where the species is present. It can expand when a butterfly colonizes new territories. It can also shrink if the species disappears from certain areas. Finally, its presence can shift to different altitudes, especially along mountains.
Range expansions are the most frequently reported changes. They concern 1,427 species in 97 countries. Reductions have been documented for 480 species in 27 countries, and altitude changes for 381 species in 18 countries. The same species can experience several types of shift depending on the region.
Climate very often appears behind these movements. In total, 79% of recorded shifts have been associated with climate change or extreme weather events. A rise in temperatures can make a previously too-cold region more favorable. Conversely, some populations can lose suitable conditions in the areas they previously occupied.
These shifts are therefore not just a general advance toward the north or toward the peaks.
The study also reveals a major bias in what we know. Europe has many long-term monitoring programs, which makes it easier to detect changes. In many tropical countries, the lack of data can give the impression that butterflies move less. The addition of non-English-language studies and specialist observations has in particular brought to light more range expansions in the tropics.
This difference in monitoring matters for species protection. A butterfly may find a more favorable climate elsewhere, but it still needs to be able to reach that area. Roads, cities, crops, or the disappearance of certain plants can limit its movement. In tropical regions, land-use changes thus add an additional pressure to that of the climate.
The authors now consider it necessary to improve monitoring in the least documented regions, particularly in the tropics. More regular observations would make it possible to distinguish true absences of movement from simple data gaps. They would also help to identify earlier species whose range is shrinking, a change more difficult to detect than an arrival in a new territory.