Butterfly range shifts reveal changes across the world

Butterflies are reshaping the map of their habitats. A worldwide review of butterfly range shifts has identified movement on every continent where the insects live, on a scale no previous survey had captured.

The implications extend well beyond butterflies themselves. They pollinate plants, provide food for birds and serve as an early warning. When butterfly ranges shift so quickly, entire food webs are changing beneath them.

The researchers brought together 6,182 records of range shifts covering 1,758 species in 105 countries. That represents approximately one in ten butterfly species known to science.

Butterfly ranges shift worldwide

The study was led by Dr Shawan Chowdhury, head of the Global Change Ecology Lab at Monash University, and co-authored by 68 butterfly experts from 49 countries.

“We found butterfly species shifting their ranges on every continent where they occur. The scale is astonishing, and it’s happening far beyond the well-studied regions of Europe and North America,” says Chowdhury.

Across the dataset, 80% of species had expanded their ranges. A further 27% had experienced contractions, while 22% had moved either up or down mountain slopes. Individual species could appear in more than one category.

Climate change and severe weather were associated with 79% of the recorded shifts. Agriculture and human disturbance followed as causes, and were the dominant pressures where ranges contracted.

Butterfly range expansion is not necessarily good news

An expanding range might sound positive, but Chowdhury cautions against assuming that it is.

Speaking to Earth.com, Chowdhury said, “Range expansion should not be treated as good news by default. A species may expand in one region while declining in another, and movement can signal that former habitats are becoming unsuitable.”

That distinction is important for planning: a map showing a species reaching a new place does not reveal what it has lost elsewhere in its range.

The figures also conceal a problem with the routes available to butterflies, helping to explain why so many attempts to move fail.

“In many cases, climate change may push species to move, while land-use change blocks the route,” says Chowdhury.

The paper echoes that finding. Even if an area's climate becomes suitable, butterflies cannot successfully colonise it without host plants, appropriate microclimates or connected habitat.

Tropical butterfly data remain scarce

The available evidence is highly uneven. Five European countries – the Czech Republic, Finland, Luxembourg, Spain and Sweden – have records covering more than half of their butterfly species.

In most other countries, shifts have been documented for fewer than 10% of their butterfly species. Data are particularly sparse in Central Africa, Southeast Asia, New Guinea and the Amazon Basin.

These regions face the greatest risk. Tropical butterflies already live close to their upper heat limits, often within narrow ranges and with few cooler refuges they can reach.

“The tropics surprised me most. By including non-English studies and expert knowledge, we found more tropical evidence than would usually appear in global reviews,” says Chowdhury.

Movement between elevations is the most conspicuous gap. It was seldom recorded in tropical countries, despite roughly two-thirds of butterfly species living mainly on mountain slopes.

Climate drives most butterfly range shifts

The team classified reported causes using threat categories established by the IUCN. Nine categories appeared in the records, led by climate change and severe weather, which were linked to 278 of the 352 species for which threat data were available.

The effects differed by pressure. Climate was the leading factor in range expansions and movements between elevations, whereas farming and human intrusion were the main factors in range contractions.

Examples span several continents. In Germany, bog drainage deprived Boloria aquilonaris of much of its habitat in the country.

In the United States, the arrival of Pieris rapae caused a sharp retreat in the range of Pieris oleracea.

Patterns also differed between regions. Development and invasive species were the leading factors in Oceania, while human intrusion and disturbance predominated in South America.

Long-running counts in Britain illustrate the cumulative effects of such pressures. A hot summer no longer raises butterfly numbers as it once did.

Filling global data gaps

The evidence available to researchers was itself part of the finding. Papers published only in English tilted the picture towards expansions and understated contractions.

The results changed when the team added studies in 15 languages and 68 expert assessments.

“When we included non-English studies and expert knowledge, a completely different global picture emerged,” says Chowdhury.

“We need long-term, locally led monitoring supported by proper funding. That means working with local researchers, natural history societies, museums, citizen scientists and conservation groups,” says Chowdhury.

He adds that simply collecting more records is not the whole answer. “The key is not only collecting more data, but supporting the people and institutions already working in these regions,” says Chowdhury.

The authors caution that the percentages indicate how well butterfly range shifts are documented, rather than how frequently they occur. Bias may also make the proportion of expansions appear higher.

Conservation must follow butterflies

Protected areas were designed for a world in which species stayed more settled. Around 76% of insect species are poorly represented in the existing network, rising to 85% for migratory butterflies.

“Conservation planning needs to move beyond static maps and consider where species are moving, where they are being lost, and whether landscapes allow them to move safely,” says Chowdhury.

In practice, this requires corridors, host plants, cooler refuges and restored habitat, as well as attention to nectar quality, which warming is quietly eroding.

National plans under the Kunming-Montreal Global Biodiversity Framework will need to incorporate butterfly range shift data into spatial planning and reporting.

“Butterflies are early-warning indicators. If their ranges are shifting this dramatically, it signals profound changes across ecosystems,” says Chowdhury.

Image credit: Shawan Chowdhury