A global research project was recently launched to examine the impact of urban policies on human health and wellbeing. They demonstrated strong evidence linking biodiversity with production of ecosystem services and between nature exposure and human health.
The study has been published in the journal Ecosystem Services: http://www.sciencedirect.com/science/article/pii/S2212041614001648
And an article about is available on BBC news: http://www.bbc.co.uk/news/science-environment-30989105