Non-native host plant availability and habitat preference promote butterfly expansion across urbanized environments

Cities are often seen as concrete jungles where nature struggles to survive, yet they have become unexpected launchpads for biological invasions. When species move into new territories where they did not evolve, they are called invasive, and this process is a major driver of global biodiversity loss. While climate change and human transport are well-known engines of this spread, the specific role of city life itself remains a puzzle. Do cities simply act as accidental stepping stones, or do they offer something unique that actively encourages invaders to thrive? To answer this, scientists must look beyond the broad strokes of climate and trade and examine the fine details of how a specific animal interacts with the streets, gardens, and buildings of a new home.

A recent study in northern Switzerland investigates exactly this dynamic through the eyes of the Southern Small White butterfly. Historically, this insect was a resident of southern Europe, but in the last two decades, it has swept across Central Europe, colonizing new territories at a rapid pace. What makes this expansion unusual is that the butterfly is almost exclusively found in towns and cities within its new range, rarely venturing into the surrounding countryside. Researchers wanted to understand how a creature that was once thought to be sedentary and local managed to spread so quickly and so far, and whether the urban environment itself was the key to its success.

The investigation began by looking at the butterfly's food supply. The caterpillars of this species are picky eaters, feeding only on a few specific types of plants. In the new urban range, their primary food source is a non-native ornamental plant called Evergreen Candytuft, which is commonly planted in private gardens and public flower beds. The researchers walked through four villages, systematically mapping every instance of this plant. They found that the food was not just present, but abundant and close together. In the areas they surveyed, the distance between individual plants was often just a few meters, with most stands separated by less than forty meters. This meant that a caterpillar did not need to travel far to find a meal, and the plant's evergreen nature ensured food was available year-round for the butterfly's multiple generations.

To understand how the butterflies moved through this landscape, the team conducted mark-release-recapture experiments. They caught individual butterflies, marked them with a unique dot pattern on their wings, and released them back into the same spot. Over several weeks, they returned to the villages to see how many marked butterflies they could find again and where they had gone. They compared the Southern Small White with its close relative, the Small Cabbage White, a species known for being more mobile and widespread. The results showed that the Southern Small White was much more likely to be caught again in the same village, suggesting it tends to stay within a specific local area rather than wandering off. When they did move, the distance between their captures was generally shorter than that of their more restless relatives.

However, staying local does not explain how the species crossed hundreds of kilometers of new territory. The data revealed that while these butterflies are sedentary within a single town, they occasionally make the jump to neighboring settlements. In one set of observations, a small number of marked individuals were found in villages just over a kilometer away. Because the females can lay hundreds of eggs and the species produces up to six generations in a single year, even rare movements between towns are enough to establish new populations. A single female arriving in a new village can found a colony that grows rapidly, provided the local environment supports her offspring.

The study also probed the butterfly's instincts to see if it actively chooses city life. Researchers captured wild females, raised their offspring in cages, and then released these butterflies with no prior exposure to the urban environment at the edge of a town, right where the paved streets met open farmland. They watched to see which way the butterflies flew. The results were striking: about eighty percent of the Southern Small Whites flew into the urban area, while their relatives showed no such preference. This suggests that the butterfly is not just stuck in the city by accident; it is drawn there. The researchers suspect that the visual cues of the city—walls, hedges, and paved surfaces—mimic the rocky habitats the species prefers in its native southern European range, tricking it into staying put.

The findings paint a clear picture of how this invasion succeeded. It was not a case of the butterfly conquering the wild, but rather of it finding a perfect match between its needs and the human-made world. The abundance of a specific garden plant provided the fuel for rapid population growth, while the butterfly's natural tendency to stay close to home was overcome by occasional, long-distance hops between towns. The urban landscape, with its dense network of gardens and structural features, acted as a series of stepping stones, allowing the species to expand its range without needing to cross vast, empty stretches of countryside. This success highlights how human-altered environments can create unexpected opportunities for certain species, turning our cities into powerful engines of biological change.