Urbanization has complex effects on species richness, with outcomes varying based on factors such as taxonomic group, spatial scale, and urbanization intensity. While many studies on birds show a decrease in species richness with increasing urbanization, other studies show no change or even increases. This review expands on bird studies by examining 105 studies on non-avian species, including mammals, reptiles, amphibians, invertebrates, and plants. For all groups, species richness tends to decrease in areas of extreme urbanization, such as central urban cores. However, moderate urbanization (e.g., suburban areas) has varying effects across groups. Most plant studies indicate increased species richness with moderate urbanization, while only a minority of invertebrate and non-avian vertebrate studies show similar results. Possible explanations include nonnative species importation, spatial heterogeneity, intermediate disturbance, and scale. Urban areas can have high spatial heterogeneity due to diverse land uses and plant choices, which can increase beta diversity and species richness, especially for species requiring small areas. Urban habitats often have higher primary productivity due to imported resources, which can enhance biodiversity. Additionally, high species dispersal rates, especially for nonnative species, can increase urban biodiversity. However, the net effect of urbanization on species richness depends on whether nonnative species introductions outweigh native species losses. Temporal studies show that in some cities, plant species richness has increased due to introductions outpacing extinctions, while mammal species have decreased. Urban-rural gradient studies show that urban areas often have lower species richness than rural areas, though some studies report increases or no change. The complex interplay of factors suggests that urbanization can both reduce and increase species richness, depending on the context.Urbanization has complex effects on species richness, with outcomes varying based on factors such as taxonomic group, spatial scale, and urbanization intensity. While many studies on birds show a decrease in species richness with increasing urbanization, other studies show no change or even increases. This review expands on bird studies by examining 105 studies on non-avian species, including mammals, reptiles, amphibians, invertebrates, and plants. For all groups, species richness tends to decrease in areas of extreme urbanization, such as central urban cores. However, moderate urbanization (e.g., suburban areas) has varying effects across groups. Most plant studies indicate increased species richness with moderate urbanization, while only a minority of invertebrate and non-avian vertebrate studies show similar results. Possible explanations include nonnative species importation, spatial heterogeneity, intermediate disturbance, and scale. Urban areas can have high spatial heterogeneity due to diverse land uses and plant choices, which can increase beta diversity and species richness, especially for species requiring small areas. Urban habitats often have higher primary productivity due to imported resources, which can enhance biodiversity. Additionally, high species dispersal rates, especially for nonnative species, can increase urban biodiversity. However, the net effect of urbanization on species richness depends on whether nonnative species introductions outweigh native species losses. Temporal studies show that in some cities, plant species richness has increased due to introductions outpacing extinctions, while mammal species have decreased. Urban-rural gradient studies show that urban areas often have lower species richness than rural areas, though some studies report increases or no change. The complex interplay of factors suggests that urbanization can both reduce and increase species richness, depending on the context.