A bird amidst discarded plastic bottles. Image credit: Corey T. Callaghan (CC BY-4.0)
Urbanization is often assumed to be harmful to plants and animals. And while many species disappear from cities, others survive or even thrive. Scientists have spent decades trying to understand why. One idea is that body size plays an important role because it influences how species move, find food, compete and cope with changing environments.
For example, a large bird can easily move in and out of urban areas, whereas a small beetle may be effectively trapped within the city. However, previous studies have focused on only a few groups of organisms or a handful of cities, making it difficult to identify broad patterns. By examining more than 30,000 species from around the world, Callaghan et al. asked whether body size consistently helps explain which species succeed in urban environments.
The researchers found that most species avoid cities, while only a relatively small number thrive in them. Body size mattered, but not in the same way for every group. Larger plant species were generally more likely to tolerate urban environments, whereas the relationship in animals was much more variable. Some animal groups showed the opposite pattern, while many showed little relationship at all. This suggests there is no single recipe for urban success. To help others explore these patterns, Callaghan et al. created an interactive that allows users to examine results for their favourite groups, from moths and birds to orchids.
As cities continue to expand worldwide, understanding which species are likely to persist is becoming increasingly important. The results of Callaghan et al. show that body size can help explain urban affinity in some groups, but it is only one piece of the puzzle. Explaining why species succeed or fail in cities will require considering many different traits and ecological processes. To support future research, the researchers also provided openly accessible, region-specific urban affinity scores for more than 30,000 species. Together, these resources provide a framework for developing and testing new ideas about how urbanization shapes biodiversity.
