Butterfly's rapid expansion led to lower genetic diversity 

Until a few years ago, the butterfly species known as the southern small white (Pieris mannii) was seldom seen north of the Alps. That was before the butterfly underwent a Europe-wide expansion, significantly increasing its range. 

However, this rapid spread has come at the cost of a marked decline in genetic diversity within the species.

Expansion of the small southern white butterfly 

Zoologist Daniel Berner from the University of Basel said it took some time before he realized a non-native butterfly species had established itself in his garden. Then, all of a sudden, Pieris mannii – with its four-centimeter wingspan and white wings marked with large black spots – was seemingly everywhere.

Until recently, the southern small white – which is mainly native to Mediterranean areas – had small, isolated populations in Switzerland, particularly in Valais and Ticino.

However, around 2005, the butterfly began migrating north and east, and it has now been observed as far as the North Sea and the Czech Republic.

The experts report that the butterfly’s expansion has resulted in a considerable reduction in genetic diversity. 

“We found that, as it invaded, the southern small white standardized local populations of its own species,” Berner explained. 

Butterfly expansion and genetic diversity

Working with researchers from the University of Greifswald and the Senckenberg German Entomological Institute, Berner explored the impact of the butterfly’s spread on genetic diversity within its species.

The researchers compared the genetic makeup of newly caught butterflies with older museum specimens, which were collected before the species’ expansion began. 

The analysis revealed that the genetic composition of the studied local populations had significantly changed, with much of the original genetic variation being replaced by the genes of the expanding population.

“If we hadn’t made the comparison with the museum specimens, we wouldn’t have spotted this genetic change,” Berner said. 

What is causing the butterfly expansion?

To conduct their analysis, the team used specimens from the collection of the Natural History Museum of Bern, which included an extensive assortment of Pieris mannii collected by lepidopterist Heiner Ziegler over the years.

Urbanization has played a crucial role in the butterfly’s rapid spread. Interestingly, the southern small white does not typically travel long distances. 

This butterfly tends to remain within a small radius of its birthplace, where its caterpillars’ food plants, such as arugula and especially candytuft, thrive. These plants are commonly found in gardens in urban areas, which has facilitated the butterfly’s expansion.

Building up populations quickly?

Additionally, the southern small white produces five or six generations each year, unlike many other butterfly species that only have one. 

“This species can therefore quickly build up large populations in a newly settled area, favoring the settlement of new land over large distances,” Berner explained. 

He added that the species is likely to continue spreading as long as its food plants remain available. “In any case, butterfly researchers in England are just waiting to spot the first one.”

Implications for conservation 

The southern small white’s expansion brings both positive and negative implications for conservation. 

On the one hand, since the butterfly thrives primarily in human-modified environments, it is not expected to compete heavily with native species. Additionally, the increase in the butterfly’s population reduces its risk of extinction. 

However, this comes at the cost of losing genetic diversity that had developed over millennia.

“Although it’s the fate of living things that some local groups can die out, what’s special about the situation facing the southern small white is that the loss of original population diversity accompanies expansion of the built environment – and is therefore caused by humans,” Berner remarked.

Many questions remain

Researchers are still unsure why the southern small white has expanded so rapidly, or where the expansion began. 

“Presumably, nothing fundamentally new has happened on the side of the butterfly. So far, we haven’t found signs of major genetic change in the expansive population, and climate change doesn’t appear to play a key role in the situation,” explained Berner. 

Although the exact reasons for this expansion remain unclear, the team suspects that the process may have originated in eastern France.

—–

Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates.

Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.

—–