As an ecologist and life-long admirer of nature, being outside is where I feel happiest. Now that I live in urban Ottawa (Ontario, Canada), often surrounded by concrete, finding pockets of nature has become central to my well-being. Using data to show that bird and tree diversity improves mental health has underscored the importance of protecting natural spaces in cities for everyone’s mental wellbeing.

The impacts are considerable, especially given that more than half of the population in middle- and high-income countries will experience at least one mental health disorder at some point in their lives [1]. The rise in mental illness is outpacing the capacity to provide patients with primary care. At the same time, the rate of biodiversity loss is accelerating [2], with decreasing opportunities for people to interact with nature. There is a wealth of research demonstrating the relationship between exposure to nature and health outcomes. However, the role of biodiversity and different components of the natural environment, things at the heart of my research program, in mental health research is unresolved [3].

In a new study published in Nature Communications Earth & Environment we combined Canadian health data with biological data to explore the association between mental-health and species diversity. We used self-rated mental health and stress from adult respondents of the Canadian Community Health Survey, where approximately 65,000 Canadians are interviewed each year on various aspects of their health and lifestyle [4]. We then matched survey responses with the species diversity of birds and trees in urban postal codes. Estimates of bird diversity came from a large network of community scientists. Birdwatchers across the globe log their bird checklists on the eBird app (https://ebird.org/home), resulting in a particularly rich source of bird data in cities, where there are millions of eBird users. We also gathered many other factors known to influence mental health, including socio-demographic factors such as age, income, and education; health behaviours, such as alcohol consumption; and distance to and amount of greenspace and bluespace (oceans, rivers, and lakes). Because socio-economic status is known to be a major driver of mental health, we also stratified our analysis by neighborhood marginalization.

After controlling for other important factors, we found that bird and tree species diversity were significantly positively related to good self-rated mental health. Living in a residential area with higher bird diversity than average increased good mental health by about 7%, and living in a residential area with higher tree species richness than average increased good mental health by about 5%. These results add to the growing body of literature that shows the benefits of biodiversity to human health. For example, in a smaller scale experimental study, listening to recordings with a higher diversity of bird species song was associated with lower depression [5]. We found only weak evidence for an association between bird diversity and self-rated stress and no association between tree diversity and self-rated stress. This could mean that bird and tree diversity are affecting mental health through avenues other than stress, such as positive emotion, happiness, or fascination [6].

The size of the association between self-rated mental health and bird and tree species diversity was similar to that of daily servings of fruit and vegetables. Similar to Canada’s food guide on daily fruit and vegetable intake, [7], standards or recommendations could be developed for urban planners to foster bird and tree diversity in urban areas. There are a growing number of Park Prescription programs rolling out across Canada, where physicians prescribe patients time spent in nature [8]. By working with ecologists, physicians could develop recommendations for patients facing mental health issues to spend more time in areas with higher bird and tree diversity.

Contrary to other research, we found little evidence that distance to or amount of greenness in postal codes was related to self-rated mental health. We suspect that this association was weak because we included both bird and tree diversity in addition to greenspace and bluespace variables. In public health research, greenspace is generally treated as one type of ‘natural’ environment. However, urban greenspace can range from forests, lawns, parks, gardens, yards, street trees, sports fields, and vacant lots of invasive vegetation, each with varying ability to support biodiversity and human health [9]. Also, contrary to other research, we found that socio-economic status of a neighborhood did not affect the association between bird diversity and mental health. Although other research shows a stronger benefit of nature in lower socio-economic neighborhoods [10], we found little evidence of a relationship between tree diversity and mental health.

We are at a key juncture - just as we are beginning to appreciate the health benefits from nature and biodiversity, we are experiencing rapid rates of biodiversity loss. Just as nature protects our health, we too must protect the health of nature. Improving biodiversity can be an important tool for mental health planning and policy in Canadian cities.

References

1. Trautmann, S., J. Rehm, and H.-U. Wittchen, The economic costs of mental disorders: Do our societies react appropriately to the burden of mental disorders? EMBO reports, 2016. 17(9): p. 1245-1249.
2. IPBES, Global assessment report of the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services, S. Díaz, et al., Editors. 2019, IPBES Secretariat: Bonn, Germany. DOI: 10.5281/zenodo.3831674 p. 1-1753.
3. Hough, R.L., Biodiversity and human health: evidence for causality? Biodiversity and Conservation, 2014. 23(2): p. 267-288.
4. Gravel, R. and Y. Béland, The Canadian Community Health Survey: mental health and well-being. Can J Psychiatry, 2005. 50(10): p. 573-9.
5. Stobbe, E., et al., Birdsongs alleviate anxiety and paranoia in healthy participants. Scientific Reports, 2022. 12(1): p. 16414.
6. MacKerron, G. and S. Mourato, Happiness is greater in natural environments. Global Environmental Change, 2013. 23(5): p. 992-1000.
7. Health Canada, Canada's dietary guidelines for health professionals and policy makers. 2019, Health Canada: Ottawa, ON.
8. Lemieux, C.J., Groulx, M.W., Buxton, R.T., Blye, C.J., Reining, C.E., Hassen, N., Harding, S.L.P., Halpenny, E.A., Lem, M., Jakubec, S.L., Wright, P., Makletzoff, T., Kerry, M., Keenleyside, K., Salah van der Leest, P., Bueddefeld, J., Lemelin, R.H., Carruthers Den Hoed, D., Steinberg, B., Moon, R., Scott, J., Grant, J., Khan, Z., Carr, D., McLaughlin, L., Krehbiel, R., The ‘healthy parks–healthy people’ movement in Canada: progress, challenges, and an emerging knowledge and action agenda. Parks, 2022. 28(1): p. 7-21.
9. Aronson, M.F., et al., Biodiversity in the city: key challenges for urban green space management. Frontiers in Ecology and the Environment, 2017. 15(4): p. 189-196.
10. Maas, J., et al., Green space, urbanity, and health: How strong is the relation? Journal of epidemiology and community health, 2006. 60: p. 587-92.