As the world’s population increasingly inhabits cities, urbanized areas have become the most rapidly expanding habitat type worldwide. To understand the patterns of biodiversity changes of terrestrial animals in cities, we reviewed studies of urbanization effects on abundance, diversity, and species richness. We asked if urbanization effects vary among climatic zones and among various animal taxa (birds, arthropods, reptiles, mammals, nematodes and amphibians). We studied 93 articles that reported species richness, abundance or biodiversity data of terrestrial animals along a gradient of urbanization.
Studies spanned urban environments across all major climatic zones. However, most of the studies have been conducted in temperate regions (54 of total 92) and involve arthropods (44) or birds (39). The majority of studies indicate that urbanization decreased diversity, abundance and species richness although there is variation among animal groups. The vast majority of bird studies indicate lowered species richness with urbanization. In contrast, about half of studies involving arthropods indicate decreased or no effect on species richness with urbanization. Approximately half of the 39 conducted studies involving birds show increased in abundances whereas the other half showed decreased abundances with increasing urbanization. In contrast, for arthropods, only six out of 26 studies showed increased abundances, 15 showed decreased abundances, and five show no change in abundance. Most studies in temperate cities show general declines in species richness (27 of 46) but fewer reductions in abundances (19 of 38). In tropical cities, the majority of studies also show declines in richness (6 of 11) and abundances (7 of 12). But in cities with arid climates, the majority of studies (3 of 5) show increases in abundances and equal number of studies where richness increases (5) or decreases (5). Our results support previous studies showing urbanization may cause reductions in diversity, abundance and species richness with increasing urbanization. However, our results indicate that urbanization has widely varying effects on richness and abundances depending on taxonomic group and climatic zone of the city. Furthermore, our study shows that urban biodiversity research has been strongly biased towards temperate regions, and birds and arthropods.