Biodiversity-friendly practices to support urban nature across ecosystem levels in green areas at different scales

Biodiversity is pivotal for delivering ecosystem services to the human society, but lack of nesting, shortages of trophic resources and disharmonic biological communities are common problems in urban areas. In this review, we aimed to understand how to transform urban green areas into biodiversity-friendly spaces. We surveyed studies by targeting several trophic levels: from plants and their soil symbionts to pollinators, birds and mammals in order to find ways of reactivating ecosystem functioning and redundancy. Specifically, here we focused on three key ecological pillars: establishment (i.e., planting/nesting), resources acquisition (i.e., feeding) and the multiplicity of ecosystem levels. We also propose to integrate the actions used on broad surfaces and big parks with micro-injections for local scales and small green areas to increase habitat suitability, and we detailed a set of best and bad practices to streamline the enhancement of multi-taxa urban biodiversity by applying a combination of appropriate management of existing urban features and targeted installation of supporting elements. We also plea for a systematic incorporation of post-operam monitoring to test action efficacy and highlight the crucial role of a cooperative attitude among society participants, and we also highlight three main topics for urgent future research. In this review, we show that restoring urban nature could be based on an ensemble of simple, yet effective, supporting actions targeting different ecological levels to sustain ecosystem functioning and services at different spatial scales.