Urbanization decreases species richness, and increases abundance in dry climates whereas decreases in wet climates: A global meta-analysis

Soil invertebrates have an essential role in decomposition, nutrient turnover and soil structure formation, all of which are strongly threatened by urbanization. Sealing, compaction by trampling and pollution destroy and degrade city soils and potentially damage soil-living invertebrates. The existing literature on how urbanization affects soil invertebrates is inconsistent, presenting both negative and positive effects. Therefore, here we aimed to synthesize the effects of urbanization on soil invertebrates considering their taxonomic (Acari, Annelida, Carabidae, Collembola, Gastropoda, Isopoda, Myriapoda, Nematoda) and functional (soil living vs. soil-related; mobility) identities, as well as to examine how the overall effect is modulated by climatic conditions (total annual precipitation, annual mean ambient temperature), urban heat island effect (based on ambient temperature differences between urban and rural areas) and city population. In a systematic review using hierarchical and categorical meta-analyses, we extracted 158 effect sizes from 75 studies on abundance and 125 effect sizes from 84 studies on species richness. Invertebrate abundance showed an increase, whereas species richness significantly decreased with increasing urbanization. The reason behind this could be that a few generalist species can adapt well to the urban environment and achieve strongly elevated densities. The species richness of annelids, springtails, and snails decreased with advancing urbanization, most probably because these animals are sensitive to soil compaction and pollution, both of which are common consequences of urbanization. The temperature did not modify the effects of urbanization, but precipitation modified the effects on abundance. Abundance increased with advancing urbanization in drier climates, probably because irrigation increased soil moisture, whereas it decreased in wet climates, as urban areas were drier than their surroundings. Making future cities more climate-neutral could better sustain soil biodiversity.