Island Biogeography Theory-inspired predictions reveal that urban non-native plant richness is source dependent and defies classical isolation predictions

A recent study employing the island biogeography theory reveals that factors influencing urban non-native plant richness are more complex than previously thought. The global dataset of urban alien plants was utilized to model alien richness, demonstrating that city size and geographic isolation do not independently affect the richness of alien plants.

Instead, the interaction between the two factors consistently predicts alien richness. This finding aligns with the fundamental prediction of island biogeography theory, which posits that island size and isolation together determine species richness. However, the strength of this interaction between city size and isolation is influenced by how city size is quantified.

Socio-economic dimensions of city size show a stronger positive interaction with geographic isolation compared to physical measures. Additionally, the concept of introduction hub identity further modifies these relationships. The study found that North America, as a hub for alien plants, weakens the interaction between city size and isolation.

This suggests that donor regions of alien plants may not be ecologically equivalent. Interestingly, simulations revealed that larger, more connected cities tend to accumulate more alien plants, even as they become geographically more isolated. However, the magnitude and direction of these responses vary depending on the specific urban "hub" being studied.

These findings suggest that classical island biogeography theory can provide a mechanistic explanation for the global variation in urban alien plant richness in an increasingly urbanized and globally connected world.