Butterflies forced to flee; A tragedy written by human hands

By Shawan Choudhary, Aletta Bonn, Jonathan Lenoir for The Conversation

Imagine spotting a butterfly species in your garden that you have never seen before. You might take a photo and upload it to a citizen science app, or tell a local nature group about your new encounter. It feels like a small moment of joy.

But if that butterfly has appeared in your backyard because its old home is becoming too hot, too dry or too damaged, the sighting is more worrying.

Around the world, ongoing environmental changes are prompting many butterfly species to relocate and search for new homes, a process referred to as species range shifts or species redistribution. As temperatures, rainfall and habitats change, butterflies can relocate over long distances, or even move vertically up and down mountain ranges.

Our new global study found that one in ten known butterfly species has already shifted in range. So, where are they going – and why?

How do we know butterflies are on the move?

The data used in our global study were comprehensive: more than 6,180 reports of butterfly range shift from 565 studies and 68 expert assessments, which covered 1,758 species across 105 countries.

These showed 80 per cent of the documented butterfly species had expanded their ranges latitudinally, meaning north to south, or longitudinally, meaning east to west. We found 27 per cent had reduced their range and 22 per cent had shifted up or down slopes on hills or mountains. Many species showed a combination of these range changes.

The public narrative is often simplistic: as the world warms, species are shifting towards cooler temperatures at higher latitudes or moving upwards in mountains. This is happening, but it is only one piece of the puzzle.

In fact, butterflies need more than to find new homes at the right temperature. Caterpillars often depend on specific host plants; adults need nectar, shelter and safe places to breed; and populations also require connected habitats so individuals can move easily through the landscape, meet, reproduce and breed.

Change can come quickly

Some species show how fast changes can happen. The tawny coster (Acraea terpsicore), native to the Indian subcontinent, has expanded through Southeast Asia and into Australia. In Brazil, Godartiana byses has expanded from Rio de Janeiro and Bahia into São Paulo, a shift linked to climate warming.

Other species show the opposite pattern. The small blue (Cupido minimus) has lost 40 per cent of its habitat in the United Kingdom over the past 150 years. In Germany, the cranberry fritillary (Boloria aquilonaris) has lost much of its original habitat because of bog drainage. In Romania, an increase in farming has been linked to a severe range contraction of the Danube clouded yellow (Colias myrmidone).

Even range shifts along mountain slopes are not always straightforward. In Mexico, the painted lady (Vanessa cardui) shifted upslope at a rate of about 83 metres per year between 1988 and 2011. Over the same period, the queen butterfly (Danaus gilippus) shifted downslope at a rate of about 32m per year.

Together, these examples show butterfly species range shifts are not simply a story of animals moving towards the poles.

What’s causing this change?

In our study, climate change and extreme weather events were the most common reasons why butterflies shifted their range. But climate change is not the only factor. Agriculture, forestry, urban development and habitat destruction are also reshaping where butterflies can live and reproduce. These disturbances destroy host plants and breeding sites, and fragment large habitats into patches too small to sustain a healthy population.

Europe and North America have many long-term butterfly monitoring programs, run by expert volunteers and citizen scientists. We found that in some European countries, range shifts have been documented for more than half of the national butterfly species. But in most countries, records exist for fewer than 10 per cent of known butterfly species.

We know the least about butterflies in Central Africa, Southeast Asia, New Guinea and the Amazon Basin, some of the richest places on Earth for biodiversity. They are also places where many species may already live close to their upper thermal limits. Even small increases in temperature can make parts of their habitat unsuitable.

Supporting experts and volunteers to develop citizen science monitoring programs would better protect butterflies in these megadiverse countries.

Why this matters

Why should people care? Butterflies are part of the living fabric around us. Adult butterflies contribute to pollination. Caterpillars feed birds, spiders and other insects. When butterflies disappear, it can signal that the plants, grasslands or woodlands once visited by butterflies are also changing.

This matters for conservation. Many protected areas are based on maps of where species lived in the past. But if butterflies and other species are moving, those maps quickly become outdated.

The message is simple. Conservation cannot only protect habitats where species once lived – it must include the places where they will relocate, and the wildlife corridors they will use to get there. If not, we may keep protecting yesterday’s nature while tomorrow’s nature has nowhere left to go.

(Shawan Chowdhury is the Head of Global Change Ecology Lab, Monash University; Aletta Bonn is the Head of Department Biodiversity and People, Professor of Ecosystem Services, Helmholtz Centre for Environmental Research-UFZ; German Centre for Integrative Biodiversity Research; Friedrich-Schiller-Universität Jena; Jonathan Lenoir is a Senior Researcher in Ecology & Biostatistics (CNRS), Université de Picardie Jules Verne (UPJV))