Urban ecosystems and the functional role of vegetation in ecological dynamics

Urban ecosystems differ from the natural ecosystems in their extended input and output environments, dysfunctional matter cycle and large flows of auxiliary energy. These peculiarities in ecosystem processes reflect on, and are determined by, reduced biodiversity and connectivity among habitat patches, exogenous control of ecological successions and high pollutant loads, collectively producing simplified heterotrophic ecosystems with impaired system dynamic stability. Indeed, microclimate and hydrogeological conditions tend to be more severe in urban ecosystems, with heat island effects, higher precipitations and hydrogeological instability in respect to the natural periurban ecosystems. Most of these issues can be traced back to the substantial reduction of soil surfaces and vegetation, i.e. the total assemblage of herbaceous plants, shrubs and trees, and to the critical control they exert on ecosystem processes. Indeed, the interaction between soil and vegetation improves the internal cycling of matter, thereby shrinking the input and output matter flows from and to the periurban ecosystems. It forms diversified microhabitats, promoting their colonization by rich communities, thus enhancing biodiversity and increasing system complexity, with direct and indirect promotion of milder climatic and hydrological conditions. Moreover, urban vegetation may be effectively used in biomonitoring and bioremediation, evaluating the presence of anthropogenic pressures and removing inorganic and organic pollutants from contaminated air, water and soil. Overall, the establishment of green areas in urban ecosystems helps thus providing the functions characteristic of natural ecosystems, enhancing human well-being and, ultimately, improving urban sustainability. To reach these goals, however, the understanding of green areas ecology, evolutionary ecology and landscape ecology is crucial in allowing their proper design and management, as well as ensuring the balance among ecological, economic and social requirements. Ultimately, the ecological value of urban vegetation contributes to reach the sustainable development goals included in the United Nations 2030 Agenda, aimed at improving life quality and at maintaining the ecosystem ecological functions on which life depends.