Urban size effect on biodiversity: The need for a conceptual framework for the implementation of urban policy for small cities

Answers are still being sought to the question of how to plan cities to ensure good living conditions for humans while also protecting urban biodiversity. We asked what is the minimum city size that causes the depletion of wildlife biodiversity, and how quickly does this process develop? We summarized data for wild ground-dwelling small mammals (Rodentia and Soricomorpha) living in green areas in cities/settlements of sizes from 1.3 thousand to 1.7 million inhabitants (Poland, Central Europe) and used various statistical tools to explore this issue. Our models indicate that the explicit transformation and depletion of ground-dwelling fauna occurs in cities with a density higher than 1000 people/km(2) (20,000 inhabitants, area of 20 km(2)). We estimated species richness and the rate of diversity decline as a city grows. Our findings indicate that in order to preserve the relatively high biodiversity of native wildlife in cities (even in central areas) appropriate actions should be taken in the early stages of city development. This knowledge may facilitate the design of more sustainable cities and could be particularly useful for the promotion of the implementation of appropriate urban policies for smaller cities. Nowadays, the theory of sustainable design of small cities is still insufficient.