Mental health conditions are placing an increasing burden on individuals and health systems worldwide. In 2010, poor mental health was estimated to cost the global economy around US$ 2.5 trillion in health and productivity losses. Two of the most common mental disorders, anxiety and depression, alone accounted for about US$ 1 trillion. Globally, effective interventions to reduce the burden of mental disorders remain limited, with significant gaps in service availability, funding for scale-up, and access to care. Therefore, there is growing interest in identifying interventions that are both scalable and cost-effective, among which nature-based mental health support is gaining momentum.
Urban green spaces and urban parks have been seen as buffers against mental health conditions like anxiety and depression. However, we noticed that, people seem to have a more profound experience when immersed in “real nature”, for example, biodiverse protected areas or forests. With our sustained interest in biodiversity, we have increasingly considered its role as a potential ‘nature dose’ influencing mental wellbeing. We can’t help wondering: Could richer biodiversity mean stronger mental health benefits? Can biodiverse, less-managed natural areas more effectively alleviate anxiety and depression? And more importantly, can people in cities access such areas on a regular basis? Is the benefit worth the cost?
Our study, recently published in Nature Cities, has explored these questions by assessing how accessible biodiversity-rich recreational areas are to residents of 9,034 cities worldwide and evaluating their cost-effectiveness as potential mental health treatments. We estimated three types of biodiversity-rich recreational areas, including recreation-permitted protected areas, key biodiversity areas, and intact forest landscapes, as they are widely recognized as having distinct and high biodiversity features. Using a global friction surface (a map of travel speed), we calculated the minimum travel time from each city to the nearest biodiversity-rich recreational area and estimated the affordability of this travel relative to per capita income.
The results are promising. Over 96% of cities worldwide have at least one biodiversity-rich recreational area within two hours. Accessibility and affordability are high in Europe, Oceania, and North America. Globally, biodiversity-rich recreational areas near cities receive an estimated 12.2 billion visits annually, based on per capita visiting rates under the estimated travel costs. Drawing on prior research linking biodiversity dose to reduced anxiety and depression, we estimated that nature experiences in these areas may prevent 137,299 (90% uncertainty range: 10,368–598,509) disability-adjusted life years (DALYs) related to anxiety and depression. In many high-income countries, these benefits can be achieved at a favorable cost.
Our findings revealed that access to biodiversity-rich recreational areas is not equal globally. Urban residents in wealthier countries typically enjoy better accessibility and lower travel costs, enabling more frequent visits and making such nature-based interventions a viable public health strategy. In contrast, for cities in some developing countries, high travel costs remain a significant barrier even when biodiversity-rich recreational areas are nearby. As a result, in these regions, biodiversity-rich recreational areas become a less cost-effective option for addressing depression and anxiety.
This insight holds valuable implications for protected area planning. Biodiversity conservation is not only about protecting nature itself, it also belongs in the public health agenda. Strategically placing biodiversity-rich recreational areas near cities in protected area planning could significantly enhance the cost-effectiveness of their mental health benefits for urban residents. In our study, we made an estimation that if every city had access to a nearby protected area requiring no travel cost, the mental health benefits in global cities could rise to 205,095 DALYs—achieving an increase of nearly 50%. This highlights the substantial potential of making biodiversity-rich recreational areas more accessible.
Despite increasing recognition of the importance of biodiversity, current fundings for biodiversity conservation remain far from sufficient to achieve global goals. Our study highlights that recognizing the contribution of biodiversity to mental health offers a promising way to bridge this gap. We hope this study sparks deeper collaboration between conservation and public health and finally helps correct the currently highly insufficient allocation of resources for conservation.
(Poster image source: https://www.pexels.com/zh-cn/photo/2009-17435995/)