Mental health is positively associated with biodiversity in Canadian cities

Urbanization, while offering numerous benefits, poses significant challenges to both human well-being and biodiversity. A substantial portion of the global population resides in urban areas, where exposure to nature is primarily limited to urban biodiversity. However, urban environments often contribute to stress and restrict access to restorative natural settings. Considering the well-established link between nature exposure and improved health outcomes, understanding the relationship between biodiversity and mental health within cities is crucial for effective urban planning and public health initiatives. Mental health disorders represent a significant global burden, with a considerable economic and social impact. In Canada, a substantial portion of the population is affected, emphasizing the need to explore and address contributing factors, particularly those relating to the urban environment. While research has demonstrated the mental health benefits of access to nature, the role of biodiversity itself remains unclear. Existing research often focuses on the amount of greenspace, neglecting the potential benefits of biodiversity—the variety of life at all levels, from genes to ecosystems. Two key psychological theories—attention restoration theory and stress recovery theory—suggest potential mechanisms through which nature improves mental health. However, the specific contributions of different components of biodiversity to mental health require further investigation. The Biophilia Hypothesis suggests humans possess an innate affinity for nature, leading to restorative effects through diverse natural environments. While some studies show positive associations between species richness and mental well-being, others report inconsistencies or a lack of association. Furthermore, the relationship between nature and health may vary based on socioeconomic status (SES), with the "luxury effect" suggesting that benefits are greater in wealthier neighborhoods, while also noting that social policy and other factors are also at play. Given the current mental health and environmental crises, nature-based solutions—which protect, restore, and create biodiversity while addressing societal challenges—are becoming increasingly relevant. Understanding the relationship between biodiversity and mental health can inform the development and implementation of effective nature-based solutions. Previous research indicates a link between neighborhood greenness and mental health, but the role of biodiversity within this relationship remains unknown. This study aims to quantify the relationship between objectively measured bird and tree species diversity in Canadian cities and mental health indicators, while controlling for various confounding factors including socio-economic factors and health behaviors.

The literature surrounding the relationship between nature and mental health is extensive, with substantial evidence indicating a positive association. Attention restoration theory proposes that nature facilitates mental fatigue recovery by engaging involuntary attention processes. Stress recovery theory suggests that natural environments promote physiological stress recovery through autonomic responses. Studies have shown various benefits from access to greenspace, including reduced stress, improved mood, and increased physical activity. However, the role of biodiversity itself remains less well understood. While some studies have demonstrated positive associations between species richness and measures of well-being, such as reduced stress, anxiety, and depression, other studies have found no significant relationship or only a correlation between perceived, rather than objectively measured, biodiversity and well-being. These inconsistencies highlight the need for further research using robust methodologies and objective biodiversity measures. The relationship between access to nature and health outcomes is often discussed in the context of socioeconomic disparities. The "luxury effect" suggests higher-income neighborhoods tend to have greater access to green spaces and associated biodiversity, potentially leading to unequal distribution of health benefits. However, other factors like urban form, social policy, and community preferences can influence biodiversity distribution. The majority of previous studies have primarily focused on affluent populations, making it challenging to generalize findings to diverse communities. The concept of nature-based solutions has emerged as a promising approach to address both environmental and societal challenges. These solutions emphasize the protection, restoration, and creation of biodiversity while simultaneously addressing social and health issues. Examples include urban tree planting, which can provide avian habitats, reduce urban heat, and improve air quality, potentially influencing mental health positively. Despite existing evidence, further investigation is needed to determine how best to integrate the considerations of biodiversity into urban planning and public health policies.

This study employed a cross-sectional ecological design using data from multiple sources. The primary data source was the Canadian Community Health Survey (CCHS), a national survey containing detailed information on various demographic and health-related variables. The CCHS data, georeferenced to six-digit postal codes, provided self-rated mental health and stress scores from respondents aged 18 and older between 2007 and 2021. To focus on urban populations, the analysis was restricted to postal codes within Canadian Census Metropolitan Areas (CMAs). Large postal codes (>16 km²) were excluded to minimize within-postal code variability. Postal codes with insufficient eBird data were also excluded. eBird, a citizen science platform, provided bird species diversity data. Data were filtered to include complete checklists with specific criteria to ensure data quality and reliability. Bird species richness and Shannon diversity indices were calculated. Tree species diversity data were obtained from the National Forest Inventory (NFI), utilizing a combination of ground surveys, photo identification, and remote sensing data to produce tree species volume maps. Tree species richness and Shannon diversity were calculated for each postal code polygon. Other variables, including the Normalized Difference Vegetation Index (NDVI) from MODIS data, distance to and proportion of green and blue spaces from Landsat imagery, and the Canadian Marginalization Index (CMI) were incorporated into the analysis. The CMI, measuring neighborhood socioeconomic status (SES), allowed for stratification of the analysis to examine whether associations between biodiversity and mental health varied across SES levels. Generalized additive models (GAMs) were used to analyze the relationship between mental health indicators (dependent variables: good self-rated mental health and low self-rated stress) and species diversity and greenspace/bluespace metrics (independent variables). Three sets of models were fit: one including only biodiversity and greenspace/bluespace variables, one adding socio-demographic characteristics, and one including both socio-demographic characteristics and health behaviors. The models controlled for unexplained spatial variability and non-linear temporal variability using a random effect of CMA, a continuous smoother for CCHS survey year and postal code area. The analysis involved data stratification by high and low marginalization neighborhoods based on the CMI, to explore the influence of SES on the observed relationships. Missing data were handled using multiple imputation by chained equations.

The analysis revealed significant positive associations between both bird and tree species diversity and self-rated mental health in Canadian cities. These associations persisted even after adjusting for various covariates, including socio-demographic characteristics and health behaviors. Specifically, a one-standard-deviation increase in bird Shannon diversity was associated with a 6.64% (CI = 4.11–9.12%) increase in the likelihood of reporting good self-rated mental health. Similarly, a one-standard-deviation increase in tree species richness was associated with a 5.36% (CI = 2.79–7.86%) increase in the likelihood of reporting good self-rated mental health. The effect sizes of bird and tree diversity on mental health were comparable to the effect of daily fruit and vegetable consumption. These relationships remained consistent across various model specifications and across different socioeconomic strata. The relationship between biodiversity and self-rated mental health is robust across various model specifications that account for multiple covariates, including socio-demographic characteristics and health behaviors. For bird species diversity, the positive relationship with self-rated mental health remained consistent even when data were stratified by socioeconomic status (SES). For tree species richness, this positive relationship was somewhat attenuated among lower-SES groups. In contrast to the findings for mental health, no significant associations were found between species diversity and self-rated stress. The most parsimonious model for self-rated stress included NDVI, tree Shannon diversity, and modeled bird species richness; however, NDVI showed a negative association with low self-rated stress, suggesting higher greenness was linked to higher stress levels. Overall, the findings strongly support a significant and robust association between biodiversity and improved self-reported mental health in urban areas of Canada.

The findings of this study provide strong evidence supporting a positive association between bird and tree species diversity and self-rated mental health in Canadian cities. This association is robust to the inclusion of various socio-demographic and health behavior covariates, suggesting a direct influence of biodiversity on mental well-being. The observed relationships are consistent with previous research suggesting that interaction with wildlife and exposure to biodiversity in urban environments are positively correlated with mental health. The relatively strong associations found between biodiversity and self-rated mental health are similar in magnitude to the impact of dietary factors such as daily fruit and vegetable consumption. The lack of a significant association between biodiversity and self-rated stress, in contrast to its strong relationship with mental health, suggests alternative mechanisms beyond simple stress reduction may be involved. These mechanisms might include increased positive emotions, heightened sense of happiness or fascination, and improved coping mechanisms. These results highlight the importance of considering biodiversity, not just general greenness, in urban planning. The similar effect size of biodiversity and the positive effects of other healthy behaviors, like fruit and vegetable consumption, emphasize that biodiversity could be included in public health recommendations. While this study provides valuable insights, it's important to acknowledge certain limitations. The study's ecological design and cross-sectional nature preclude the establishment of definitive causal relationships. Further research is needed to confirm the causal nature of these relationships and explore the specific mechanisms involved. The use of self-reported mental health measures also warrants consideration, as this assessment relies on subjective self-reporting. The study suggests a critical need to promote urban biodiversity conservation as a nature-based solution to address both environmental and mental health challenges. The findings underline the necessity of integrating ecological considerations, specifically biodiversity, into urban planning, public health policies, and strategies to foster improved community mental health.

This study provides compelling evidence for a significant positive association between bird and tree species diversity and self-reported mental health in Canadian cities. The findings highlight the importance of biodiversity in urban planning and public health initiatives. The robust association, even after controlling for various factors, suggests that promoting urban biodiversity could be a valuable strategy for improving mental well-being. Future research should focus on longitudinal studies, experimental designs, and intervention trials to establish causality and fully elucidate the underlying mechanisms. The inclusion of biodiversity in urban planning and public health policies holds significant potential for creating healthier and more resilient cities.

The cross-sectional nature of this study limits the ability to establish definitive causal relationships between biodiversity and mental health. The ecological design may also be susceptible to ecological fallacy, as analyses were conducted at the postal code level and do not represent individual-level variations. Self-reported mental health measures, although widely used, are inherently subjective and may be influenced by reporting bias. The use of a single year of forest inventory data and potential underestimation of tree species diversity due to focusing on forested areas might also limit the analysis. Finally, the study population may not fully represent the entire Canadian population due to data availability and exclusion criteria. Despite these limitations, this study provides strong correlational evidence that deserves further investigation through more robust and specific research methods.