Cities Supercharge Biological Invasions as Global Study Exposes Hidden Drivers

Cities have long been described as gateways for foreign species, but a sweeping new analysis suggests they do far more than simply open the door. A research team led by Zhining Wang and Xuan Liu of the Chinese Academy of Sciences reports in the journal Nature Cities that urbanization itself actively promotes biological invasions, independent of the sheer volume of trade and travel that funnels non-native animals into the world’s metropolitan areas. Drawing on records of 436 non-native terrestrial vertebrate species established across 1,029 cities worldwide, the study provides the most detailed picture to date of why some urban centers teem with alien birds, mammals, amphibians and reptiles while others host comparatively few.

The scale of the undertaking reflects the growing urgency of the problem. Biological invasions are now recognized as one of the leading challenges to global sustainability, contributing to biodiversity decline, substantial economic losses and the transmission of diseases. International assessments have repeatedly flagged the spread of invasive alien species as a primary driver of ecological degradation, yet the mechanisms that concentrate these species in particular cities have remained surprisingly poorly understood. Without that understanding, managers have struggled to design effective interventions. The new study set out to close that gap by assembling a global dataset linking where alien vertebrates have established populations with the environmental, economic and social characteristics of the cities they inhabit.

The resulting maps reveal a striking geographic asymmetry. Hotspots of urban biological invasion are concentrated mainly in developed countries, with island cities emerging as especially heavily invaded. Urban centers such as those in North America, Europe, Oceania and wealthy Asian island territories consistently show elevated richness of established non-native vertebrates. Yet when the researchers traced the native ranges of those same alien species, a different picture emerged: the overwhelming majority of the introduced animals originated in developing countries. Species are, in effect, being extracted from biodiversity-rich, less-developed regions and deposited in wealthy, highly urbanized ones, a pattern that echoes earlier findings on how colonial history and global trade routes continue to shape modern invasion flows.

This disparity between origins and destinations carries important practical implications. The trade in live animals, whether for pets, food, traditional medicine or religious release ceremonies, moves vast numbers of vertebrates out of source regions in the tropics and subtropics. Once those animals reach affluent cities with busy ports and airports, the probability that some individuals will escape or be deliberately released, and then survive and reproduce, rises sharply. Prior research has shown that the more individuals of a species are introduced to a location, the higher the colonization pressure and the greater the chance of establishment, a foundational principle of invasion ecology that the new analysis confirms at a global urban scale.

Crucially, however, the study shows that propagule pressure alone cannot explain the distribution of alien vertebrates across cities. After statistically accounting for the physical area of each city and for uneven sampling effort, the researchers found that cities with more non-native species tended to share three characteristics indicative of greater urbanization: higher levels of habitat modification, warmer temperatures and lower native biodiversity. In other words, once an alien animal arrives in a city, whether it manages to establish a self-sustaining population depends heavily on how thoroughly that city has been transformed by human activity.

Each of these three factors points to a distinct biological mechanism. Habitat modification, measured through indices of human alteration of terrestrial systems, fragments natural landscapes, degrades the ecosystems that native species depend on and creates the kind of disturbed, simplified environments in which many generalist invaders thrive. Roads, construction and continuous expansion of the built footprint open dispersal corridors for adaptable species while removing the ecological barriers that once kept them in check. Previous work has documented how cane toads exploit roads as dispersal highways and how urban sprawl facilitates the spread of exotic plants across multiple spatial scales, and the new vertebrate analysis suggests similar dynamics operate in cities everywhere.

Warmer temperatures add a second layer of opportunity. The urban heat island effect raises temperatures within cities relative to surrounding countryside, and the analysis found that cities with warmer climates harbor more non-native vertebrates. Many of the alien species now established in temperate cities originated in warmer native ranges, so heat-retreating urban environments effectively pre-adapt those species for survival far beyond their historical climatic limits. Studies of ectotherms such as lizards have shown that thermal spikes from urban heat islands can radically alter habitat suitability, and a recent global meta-analysis by members of the same team found that non-native animals are generally less sensitive to extreme weather than their native counterparts. Cities thus warm the stage and recruit performers already primed for the conditions.

The third factor, reduced native biodiversity, connects the invasion story to one of ecology’s oldest debates. Eroded native communities can leave functional gaps that invaders fill, weaken the biotic resistance that intact ecosystems mount against newcomers, and signal that a city’s environment has already been pushed past thresholds of disturbance. Prior global analyses have shown that native diversity buffers ecosystems against the severity of non-native tree invasions, and the new study extends that principle to urban vertebrates. Biotic homogenization, the process by which urbanization replaces distinctive local assemblages with a cosmopolitan set of hardy winners, appears to feed on itself: as native species vanish, cities become easier to invade, and as invasions proceed, native species come under further pressure.

The authors emphasize that these findings translate directly into management priorities. Because invasion risk rises with habitat modification, reducing anthropogenic disturbance to natural habitats within and around cities should curb the establishment of new alien populations. Because degraded native communities offer less resistance, conserving and restoring native species in urban areas strengthens ecological defenses. And because the flow of species from developing source regions into wealthy destination cities persists, strengthening quarantine measures along trade and transport pathways remains essential for intercepting introductions before they begin. None of these measures is a substitute for the others; the study’s central message is that invasion risk is generated at multiple points in the introduction-establishment-spread continuum, and effective control requires intervention at each of them.

The implications reach well beyond city limits. Cities do not merely accumulate alien species; they act as bridgeheads from which invaders can radiate into surrounding landscapes, and modeling work has shown that complex landscape geometry and stepping-stone dispersal can dramatically accelerate spread. With more than half of humanity now living in urban areas and urban land cover expanding rapidly, the number of invasion platform cities will only grow. Projections of alien species accumulation through 2050 suggest no saturation in the global rate of new establishments, meaning the problem will intensify without deliberate policy action. By disentangling propagule pressure from the ecological conditions that convert arrivals into populations, this global analysis of more than a thousand cities gives policymakers a clearer roadmap: slow the flow of species across borders, protect and restore the natural fabric of urban ecosystems, and treat thriving native biodiversity not as an amenity but as a critical line of defense against one of the defining environmental challenges of the century.

Subject of Research: Drivers of non-native terrestrial vertebrate establishment in cities worldwide

Article Title: Urbanization promotes biological invasions in global cities

Article References: Wang, Z., Wang, Y., Du, Y., Gu, S., Li, W., Tu, W., & Liu, X. (2026). Urbanization promotes biological invasions in global cities. Nature Cities. https://doi.org/10.1038/s44284-026-00517-w

Image Credits: AI Generated

DOI: 10.1038/s44284-026-00517-w

Keywords: biological invasions, urbanization, invasive species, urban ecology, biodiversity, terrestrial vertebrates, propagule pressure, island cities, habitat modification, quarantine, biodiversity conservation, Nature Cities