Until a few years back, the butterfly species known as the southern small white was rarely seen north of the Alps. This changed due to a widespread invasion across Europe, which led to a significant increase in the butterfly’s distribution while simultaneously causing a rapid decline in the species’ genetic diversity.
Until a few years back, the butterfly species known as the southern small white was rarely seen north of the Alps. This changed due to a widespread invasion across Europe, which led to a significant increase in the butterfly’s distribution while simultaneously causing a rapid decline in the species’ genetic diversity.
Initially, zoologist Daniel Berner took some time to realize that a butterfly species not native to his region had settled in his garden. Soon enough, he began spotting it everywhere: Pieris mannii, commonly referred to as the southern small white, which has a wingspan of about four centimeters and features white wings with large black spots.
Historically, only a few small, localized populations of this primarily Mediterranean butterfly existed in Switzerland, specifically in Valais and Ticino. However, around 2005, the butterfly started to migrate north and east, and it has now been reported near the North Sea and in the Czech Republic.
Examining Museum Specimens
With its northward and eastward spread came a notable decline in genetic diversity. “Our research indicated that as it invaded new regions, the southern small white homogenized local populations within the species,” explains Dr. Daniel Berner from the University of Basel. Together with colleagues from the University of Greifswald and the Senckenberg German Entomological Institute, Berner conducted a study featured in Current Biology, exploring the impact of the butterfly’s spread on its genetic diversity.
The team compared the genetics of newly captured butterflies with older specimens stored in museums — specifically, specimens collected before the invasion started. This approach revealed that the genetic profile of local populations has changed significantly, with a considerable portion of the original genetic makeup replaced by that of the expanding population.
“Without comparing with the museum specimens, we might not have detected this genetic shift,” states Berner. The researchers successfully sequenced butterflies from the Natural History Museum of Bern’s collection, allowing them to analyze the genetic characteristics of these insects. They were fortunate that Heiner Ziegler, a lepidopterist, had built an extensive collection of southern small whites over the years, which proved invaluable for their research.
Preferred Garden Plants
The rise of urbanization has accelerated the butterfly’s spread. The southern small white typically does not fly long distances; it spends its short three-week life flying within a small radius of its birth area, where its caterpillar food sources thrive — mainly arugula and candytuft, the latter being common in residential gardens. Thus, the growth of human settlements has allowed this butterfly to spread extensively.
Additionally, there are about five or six generations of southern small whites each year, as opposed to just one. “This enables the species to quickly establish large populations in newly colonized areas, facilitating their movement over great distances,” Berner elaborates. He believes it’s likely that the butterfly will continue its expansion as long as food sources remain available. “Butterfly researchers in England, for instance, are eagerly waiting to encounter the first one,” he adds.
Expansion and Genetic Mixing — A Benefit or a Drawback?
From a conservation standpoint, the expansion of the southern small white presents a mixed bag. Since the species primarily thrives in human-altered habitats in newly occupied areas, it is not expected to compete with native butterflies. Furthermore, this expansion has led to a larger overall population, which reduces its extinction risk. However, this positive aspect is countered by the loss of genetic diversity that has accumulated over millennia: “While it’s natural for some local populations to become extinct, what’s concerning about the southern small white is that the loss of its original genetic diversity coincides with the expansion of human development, and is thus driven by human actions.”
At present, the researchers are unclear why the southern small white has experienced such significant expansion, or where this process began. “Presumably, there’s nothing fundamentally new occurring with the butterfly itself. So far, we haven’t detected any significant genetic changes in the expanding population, and climate change does not seem to be a crucial factor,” Berner explains. The team intends to investigate these queries further, but they suspect that the expansion may have started in eastern France.
