Chia-Ching Wu
Joy O’Keefe
Yizhen Ding
William C. Sullivan- 1Department of Landscape Architecture, University of Illinois at Urbana-Champaign, Champaign, IL, United States
- 2Department of Natural Resources & Environmental Sciences, University of Illinois at Urbana-Champaign, Urbana, IL, United States
- 3Department of Landscape Architecture & Urban Planning, Texas A & M University, College Station, TX, United States
- 4Smart, Healthy Community Initiative, Office of the Provost, University of Illinois at Urbana-Champaign, Urbana, IL, United States
Although recent studies have explored links between landscape biodiversity and human health, the exact effects of specific types of biodiversity—such as the variety of species or genera—on health outcomes are still uncertain. In this review, we evaluate our current knowledge of how landscape biodiversity influences human health in different cultural contexts. We systematically assessed peer-reviewed literature to: 1) summarize the links between biodiversity variability and human health outcomes; 2) describe the pathways used to examine these links; 3) compare subjective perceptions of biodiversity with objective measures; and 4) describe any known connections between perceived biodiversity and health outcomes. We analyzed 41 studies published between 2018 and 2023, covering research from 15 countries on five continents. A key finding was the lack of significant links between biodiversity variability, like species richness, and health outcomes; only one-third of studies reported positive health effects. We identified two main pathways: restoring health capacities (mental health) and building health capacities (physical health). The public could identify different biodiversity levels, with perceived biodiversity correlating with health. Future research should explore varied mechanisms, physical health outcomes, causal relationships, and effects across diverse populations and regions.
1 Introduction
In urban settings, green spaces are essential for fostering stable habitats and resources for wildlife, thereby enhancing the quality of urban ecosystems (Ives et al., 2016; Oke et al., 2021; Parris and Hazell, 2005). Recent studies have explored the connection between biodiversity and health outcomes for urban residents (Aerts et al., 2018; Houlden et al., 2021; Marselle et al., 2021b). The specific relationships between narrowly defined biodiversity— taxonomic or species richness, which refers to the number of different genera or species within a given area —and health outcomes, however, remain unclear. This measure of biodiversity is particularly relevant as it captures the variety of life forms present, whether counted at the species or genus level. Understanding these interactions is crucial, as they are influenced by individual perceptions of the biodiverse environments and by cultural contexts. This systematic review summarizes recent findings regarding the relationships between biodiversity and human health and reports findings regarding the objectively measured levels of biodiversity and people’s perceptions of biodiversity.
The association between the biodiversity of urban green spaces and health outcomes could unfold in three broad ways. First, biodiverse landscapes may harm human health and well-being. Some studies have found that densely and diversely vegetated urban green spaces evoke negative perceptions and feelings due to concerns over contact with wildlife, safety issues, or crimes within or around these areas (Cariñanos et al., 2017; Sreetheran and van den Bosch, 2014; Qiu et al., 2013). Biodiverse green spaces are associated with a series of negative health outcomes, such as declined mental health, well-being, and a higher prevalence of asthma and allergic reactions (Astell-Burt and Feng, 2019; Jiang et al., 2020; Reid et al., 2017; Zhang et al., 2021).
Second, it is possible that there is no relationship between levels of biodiversity and human health and well-being. Indeed, several recent studies did not find a connection between landscape biodiversity and health or well-being (Dallimer et al., 2012; Marselle et al., 2020; Methorst et al., 2021a). Third, it is possible that exposure to higher levels of biodiversity leads to a range of positive health outcomes. Biodiverse urban nature offers ecosystem services that benefit health or well-being across multiple dimensions, including physiological health, psychological health, life expectancy, resilience, and overall well-being (Sandifer et al., 2015). Due to these mixed results, we examine the extent to which urban green spaces with varying levels of biodiversity are associated with human health or well-being.
Scholars sometimes wonder whether the ability to recognize species or discern the degree of biodiversity is necessary to experience health benefits from a biodiverse green space (Wu, 2024). Previous studies have found both positive and negative correlations between objectively measured biodiversity and subjectively perceived biodiversity, influenced by individuals’ varying levels of knowledge (Dallimer et al., 2012; Fuller et al., 2007). Moreover, individuals who perceived greater biodiversity tended to report greater levels of well-being, regardless of how accurate their perceptions were (Dallimer et al., 2012; Kothencz et al., 2017; Marselle et al., 2016). This discrepancy in people’s ability to perceive biodiversity introduces another level of complexity in the relationship between natural landscapes and health. Thus, in this review, we explore the degree to which subjectively measured biodiversity aligns with objectively measured biodiversity, bridging human perspectives and ecological functionality in research and landscape design.
Several recent review studies have synthesized the growing body of literature examining the links between biodiversity and health and well-being. A systematic review of research from 2006 to 2018 revealed pathways and outcomes that connected biodiversity to human health (Aerts et al., 2018). A narrative review using a broad definition of biodiversity, including factors related to abundance and variability of biodiversity, described the extent to which different biodiversity metrics correlate with mental and physical health outcomes (Marselle et al., 2021b). A recent summary of both empirical research and review studies focused on the relationship between biodiversity and mental health outcomes, concluded there was a need for more empirical studies to obtain results generalizable to a wider range of environments (Houlden et al., 2021).
Although these reviews provide valuable insights, they do not fully address how species richness in biodiversity affects human health. While previous reviews have examined the links between biodiversity and human health, it is crucial to focus on the health impacts of species richness. Species richness has unique ecological functions that differ from abundance factors when considering biodiversity, potentially leading to distinct health outcomes. Additionally, given the surge in publications, we must update and clarify how biodiversity connects to human health and well-being.
To bridge these gaps, we reviewed research published from 2018 to 2023, summarizing the connections between biodiversity and human health in urban settings. Our review differentiated between factors related to biodiversity abundance, like animal counts and plant coverage areas, and those that emphasize biodiversity variability, such as habitat heterogeneity and species richness, focusing on the latter. Our review was guided by four research questions:
1. What are the pathways through which studies examined relationships between biodiversity and human health?
2. To what extent does biodiversity correlate with human health?
3. To what extent does the perceived biodiversity correlate with the objectively measured biodiversity?
4. To what extent does the perceived biodiversity correlate with health outcomes?
2 Methods
2.1 Research scope, keywords, and inclusion criteria
To identify relevant literature for this review, we conducted a search using three groups of keywords related to biodiversity, health, and the environmental scope of the research, summarized in Table 1. Biodiversity, or biological diversity, is defined by the Convention on Biological Diversity (Convention on Biological Diversity, 1994) as “the variability among living organisms from all sources.” Our inclusion criteria spanned multiple levels: within-species, between-species, and between ecosystem diversity. The data sources for biodiversity included primary investigations or secondary databases.
Table 1. Keywords for literature search about biodiversity-health relations.
We drew from the World Health Organization (2020) definition of human health as “a state of complete physical, mental, and social well-being and not merely the absence of disease or infirmity.” Consequently, we included health measures of mental health, physical health, general health, and well-being. The scope of physical environments was limited to built environments and developed settlements in urban or suburban areas. This approach narrowed our review to focus on the impacts of natural landscapes people encounter on a daily basis.
Studies were included in our review if they met the following criteria:
1. The research measured biodiversity, reflecting the richness of organisms (number of different species or genera) within species, between species, or at the ecosystem scale. The data could be continuous, such as investigation outcomes, or categorical, where researchers designated diversity levels.
2. The research measured health outcomes related to mental health, physical health, or well-being. We also include behavioral outcomes closely related to health, such as frequency and intensity of physical activities.
3. The research sites were within suburban or urban areas developed for human settlements.
4. They were quantitative, qualitative, or mixed methods studies, including randomized controlled trials, quasi-experimental studies, natural experiments, and observational studies.
Studies were excluded for any of the following reasons:
1. They measured only indirect biodiversity or used only proxies for biodiversity, such as plant cover (area), abundance, or landscape structure.
2. They exclusively measured indirect outcomes unrelated to human health outcomes or that did not directly reflect the health status of the human body, such as studies reporting the number of human disease vectors present.
3. They focused solely on assessing environmental quality regarding restorativeness, such as perceived restorativeness of the environment or preference.
4. The research site was only in natural areas without human development.
5. They included only one group in the biodiversity variable, such as a case study.
6. They were not written in English.
7. They were not published in peer-reviewed journals.
8. They were not empirical research.
2.2 Search strategies
We conducted multiple searches of the literature between 12 – 13 July 2023. We searched in three electronic databases: ISI Web of Science, Scopus, and PubMed. In the search fields, we used “OR” to connect keywords within each category to cover the most possibilities and used “AND” to connect the three keyword categories to refine the research only exploring relationships between biodiversity and human health in built environments. We searched these keywords in the title, abstract, and author keywords fields, and we identified the publication dates between January 01, 2018, and July 12, 2023.
2.3 Data extraction and analysis
To address the four research questions, we extracted relevant information from each included paper, containing research questions, research environment, spatial scale, study types, independent and dependent variables, covariates, pathways, and findings (see Table 2). Specifically, for the extent to which biodiversity correlates with human health, we categorized the results based on whether they showed positive, negative, or no-relationship effects. We then calculated the total number and percentage of results within each health outcome category. This approach provides an overview of current research trends and findings on the relationship between biodiversity and health outcomes.
Table 2. Summary of the studies exploring the correlations between biodiversity and human health.
In identifying pathways connecting biodiversity and health, we considered their research questions, the theories or mechanisms cited, and the health measures used. To better organize the information, we referenced the framework proposed by Marselle et al. (2021a), categorizing pathways into four groups: restoring capacities, building capacities, reducing harm, and causing harm. In Marselle et al. (2021a), relevant theories and mechanisms were specified in each category. Additionally, we remained open to other pathways that emerged during the extraction process.
We assessed the quality and the risks of biases of these included studies with NIH Quality Assessment Tools (National Institutes of Health, 2021) and modified critical appraisal skills programme (CASP) tool (Long et al., 2020), see Supplementary Materials.
3 Results
Our initial search yielded 1,870 distinct records based on titles and abstracts. After filtering out irrelevant titles and abstracts, we removed 1,818 records, leaving 52 records for full-text screening. Of these, we excluded nine studies due to irrelevant independent variables and six due to unrelated dependent variables, leaving us with 37 identified studies from our search. We found an additional four studies through references and other sources. Consequently, a total of 41 published research articles were included for further analysis (Figure 1, Table 2).
Figure 1. Study selection process.
3.1 Study characteristics
The most frequently examined populations were adults and university students, with 26 studies focusing on these groups, while eight studies focused on children and adolescents (Table 2). Most studies targeting children and adolescents focused on physical health outcomes, including immune system-related health, physical activity, and body weight, with only one examining adolescent mental well-being.
Regarding methodologies, only eight studies implemented experimental or quasi-experimental designs. The experimental studies featured randomized distribution of participants to one or more interventions, while the quasi-experimental studies incorporated interventions without random assignment. Many studies (31) investigated correlational relationships. Additionally, two studies conducted qualitative research, collecting participants’ written or verbal descriptions of their experiences.
The included studies were conducted in 15 different countries across five continents, with a concentration in Europe (Table 3). Studies were conducted in only two Asian countries and one South American country. Given the extensive diversity and vast number of countries in these regions, it is apparent that these areas are underrepresented in the identified studies. Moreover, research on this topic was absent in African countries, highlighting a geographical gap in the literature.
Table 3. Summary of geographic locations and setting types of study sites.
The included articles investigated built environments of various spatial scales as their analysis units, from individual sites like streetscapes and private gardens to larger blue or green spaces, residential neighborhoods, and regional levels such as city-wide, county-wide, and countrywide (Table 3). The site scale of green and blue spaces and the neighborhood scale of the living environment were the most-studied levels, as shown in Figure 2. Studies explored various health effects of plant, animal, and overall biodiversity variables at both scales. Moreover, studies focusing on streetscapes only examined health outcomes related to overall well-being, leaving other health aspects unexamined. Similarly, studies exploring the relationships at the city scale focused only on physical health without evaluating overall well-being.
Figure 2. Number of studies on the relationships between biodiversity and health by the types of built environments and scales of the analysis unit. (Biodiversity measures include the animal diversity, area of green and blue spaces, landscape structure, microbiome diversity, presence of native plants, the overall biodiversity, perceived biodiversity, and plant diversity.).
3.2 Biodiversity measures
As shown in Table 4, most studies assessed biodiversity by measuring species richness (the number of different species) or estimating diversity using the Shannon diversity index or Simpson index. In terms of data sources, secondary data, such as pre-existing data or public records, was often from governmental surveys. With the growth of citizen science databases, one study utilized the eBird database to estimate bird diversity, requiring adjustments using the rarefaction approach (Chen et al., 2023).
Table 4. Summary of the biodiversity measures used as predictor variables.
Several studies used proxy measures for assessing biodiversity; for instance, vegetation diversity was estimated based on landscape structure (Mavoa et al., 2019b), and animal diversity was inferred from habitat diversity (Cavaleiro Rufo et al., 2020, 2021). Studies also estimated the diversity of land cover types or habitats, by measuring the complexity and diversity of the landscape structure (e.g., Mears et al., 2019). Furthermore, subjective methods were also used as proxy measures for biodiversity, such as participants’ perceptions of plant and animal diversity and abundance via questionnaires (e.g., Nghiem et al., 2021).
3.3 Health and well-being measures
The studies included in our review predominantly focused on mental health, investigating diagnosed mental disorders, cognitive functioning, and affective states, as summarized in Table 5. Two studies used objective methods to assess mental health; among those, anti-depression data were used to indicate diagnosed mental disorders (Marselle et al., 2020) and working memory tests were used to assess attention restoration (Douglas and Evans, 2022). Furthermore, most studies (18) relied on self-reported surveys, in which participants were asked about their emotions, feelings of restoration, stress, and depression levels (e.g., Fisher et al., 2021b; Hepburn et al., 2021), providing a subjective perspective on the impact of biodiversity on mental health outcomes.
Table 5. Summary of the health and indirect health outcome variables.
For physical health, we categorized the assessments into biofeedback measures, immune system-related diseases, infectious and respiratory diseases, maternal health, physical activity and weight, and body microbiome diversity (Table 5). There was an emphasis on assessing immune system-related diseases, measured by the prevalence of diseases such as leukemia, asthma, and allergies (e.g., Donovan et al., 2021b; Winnicki et al., 2022), self-reported symptoms like rhinitis, eczema, and wheezing (Cavaleiro Rufo et al., 2021), and T cell counts (Roslund et al., 2020). We also included studies that estimated health through the frequency and intensity of physical activity and body microbial diversity. Although both indicators may not directly reflect health conditions, they are closely linked to mental and physical health (Du Toit, 2019; Pflughoeft and Versalovic, 2012; Rhodes et al., 2017).
A broader perspective on overall health conditions was assessed through mortality rates and life expectancy (Chen et al., 2023; Fernández et al., 2021), providing a macro-view of health conditions related to biodiversity. The research used self-reported methods to reflect well-being, general health, quality of life, life satisfaction, and happiness (Fisher et al., 2022; Hepburn et al., 2021; Methorst et al., 2021b; Taylor et al., 2018). These diverse assessments highlighted the potential health effects of biodiversity from multiple perspectives.
3.4 Pathways connecting biodiversity and human health
Our review included studies that explored how biodiversity impacts human health and well-being through several pathways: restoring capacity, building capacity, reducing harm, and causing harm, following the framework of Marselle et al. (2021a). Additionally, we identified a fifth pathway, One Health, which extends beyond the scope of this structure.
Out of the 41 studies in this review, two did not specify the pathways based on their health measures and research questions, and 13 did not specify the foundational theories that connected biodiversity and human health. Regarding the two that did not specify the pathways based on measures and questions, we classified Schebella et al. (2020) in the restoring capacity pathway, as they linked overall biodiversity to reductions in subjective and physical stress. We classified Zhang et al. (2021) into the pathway of causing harm, in which they measured the risks associated with allergies and respiratory diseases potentially triggered by plants. The findings within each pathway are described below and in Table 6.
Table 6. Summary of pathways and theories or hypotheses used in examining nature-health relationships.
3.4.1 Restoring capacity
We classified 14 articles into this pathway, which explored the connection between biodiversity and health through Attention Restoration Theory, Stress Reduction Theory, and the Biophilia Hypothesis. According to Attention Restoration Theory, contact with everyday natural environments aids in recovering from mental fatigue and replenishes depleted top-down attention (Kaplan & Kaplan, 1982). Stress Reduction Theory suggests that exposure to everyday nature alleviates emotional arousal, thereby reducing stress levels (Ulrich, 1983, 1984). The Biophilia Hypothesis posits that humans possess an innate affinity for nature and other life forms, eliciting positive emotional responses to natural elements (Wilson, 1993).
The empirical evidence from this set of studies indicated positive connections between the diversity of plants and animals, habitat types, and the presence of diverse elements with self-reported health outcomes, such as increased attention restoration, moods and relaxation, and reduced stress levels (Cameron et al., 2020; Fisher et al., 2021b; Randler et al., 2023; Schebella et al., 2019; Wei et al., 2022). One study reported a link between plant diversity and objective measures of blood pressure – higher levels of plant diversity predicted healthier blood pressure measurements (Lindemann-Matthies and Matthies, 2018).
3.4.2 Building capacity
Fifteen articles examined the relationships between biodiversity and health through the lens of the Biodiversity Hypothesis and Hygiene Hypothesis. These hypotheses suggest that physical contact with biodiverse environments boosts immunoregulatory functioning, protecting against allergies and inflammation (Haahtela, 2019; Rook, 2012). The included studies demonstrated that exposure to more diverse plants and animals was associated with higher human microbiome diversity and enhanced immune system functioning (Donovan et al., 2021a; Roslund et al., 2020). Research on how biodiversity supports health capacity has grown from three studies between 2006 and 2018 (Aerts et al., 2018) to eight from 2018 to 2023.
Four other studies linked biodiversity and human health and well-being through physical activities. For instance, areas with diverse bird species were associated with more intense physical activity and lower obesity among adults (Knobel et al., 2021).
3.4.3 Reducing harm
One study assessed the roles of biodiversity in mitigating environmental risks to health. Fisher et al. (2022) explored the effects of vegetation richness on protecting residents’ well-being against traffic-related air and noise pollution but found no significant correlations.
3.4.4 Causing harm
The causing harm pathway considers the potential adverse effects of biodiversity, such as pollen and plant debris triggering allergic reactions and asthma (Lee et al., 2020). One study reported that while higher plant diversity was correlated with a lower incidence of allergic diseases, increased diversity of plant pollens was associated with higher allergic disease rates (Zhang et al., 2021). They explained that greater overall plant diversity may aid the immune system through higher microbiome diversity in the environment; however, the increased pollen diversity can trigger allergies directly.
3.4.5 One Health
One study investigated the relationship between biodiversity and life expectancy through One Health (Chen et al., 2023). One Health emphasizes the interdependent relationships among humans, plants, animals, and their shared environments (Zinsstag et al., 2011). Chen et al. (2023) found dual benefits for both the natural environment and human health arising from the efforts to alleviate threats to biodiversity, emphasizing the interconnection between ecosystem health and human well-being.
3.5 Relationships between biodiversity and health
To what extent does biodiversity correlate with human health and well-being? Among the 39 quantitative studies, we summarized a total of 163 findings. Of these, 44% of results indicated that broadly defined biodiversity was positively associated with health and well-being. On the contrary, 10% of the findings reported negative associations or effects of biodiversity on human health, while 46% reported no significant relationships between these variables (see Supplementary Table S1 in the Supplementary Material).
When focusing specifically on the human health responses to the variability of biodiversity, we examined 82 findings that uncovered a shift in the proportion of positive, insignificant, and negative outcomes. When considering only factors related to the variability of biodiversity, fewer findings demonstrated positive effects (decreased from 44% to 33%), and more studies showed negative effects (increased from 10% to 14%) or insignificant relationships (rose from 46% to 53%). These differences indicate the varied effects of the abundance and the biodiversity variability on health outcomes.
Among the findings related to biodiversity measures on variability factors, affect states, immune system diseases, and well-being were the health outcomes that were examined the most, as depicted in Figure 3. Most studies found no significant association between species richness of all taxa examined and affective states and well-being. Compared to other types of health outcomes, a higher portion (55%) of negative relationships were reported regarding the association between plant and animal diversity on immune system-related diseases. Detailed descriptions are provided below and in Table 7.
Figure 3. Number of health outcomes summarized for biodiversity measures on variability factors, and the effects on health include negative effect (–), no significant relationship (N), and positive effect (+).
Table 7. Summary of relationships between biodiversity factors related to richness and various aspects of human physical and mental health.
3.5.1 Effects of vegetation and habitat diversity
Studies showed that higher plant richness and diversity are associated with mental well-being (Methorst et al., 2021a; Fisher et al., 2022), reduced incidence of children’s acute lymphoblastic leukemia (Donovan et al., 2021a), lowered rates of allergic disease (Zhang et al., 2021), and decreased risks of asthma (Donovan et al., 2021b). Specifically, four studies distinguished the effects of vegetation and habitat richness and vegetation density. These studies found that a higher number of plant species and greater habitat richness are linked to reduced blood pressure (Lindemann-Matthies and Matthies, 2018), better mental well-being (Fisher et al., 2022; Methorst et al., 2021a) and lower mortality rates (Giacinto et al., 2021). These findings reveal the independent health effects of both plants and habitat richness on mental and physical health benefits, even after accounting for vegetation cover.
Conversely, several studies reported no significant relationships between biodiversity and health due to three potential reasons. First, the effect of plant diversity may be too small and overshadowed by the impact of plant cover. For example, Schebella et al. (2019) initially found positive correlations between habitat diversity, structural heterogeneity, and mental well-being, but these relationships became insignificant after adjusting for vegetation cover. Similarly, Vilcins et al. (2021) reported that areas with low vegetation diversity but with high plant cover were associated with heavier (that is, healthier) birth weights. Second, mismatched temporal and spatial scales between biodiversity factors and human experiences could lead to insignificant associations between plant diversity and health. For instance, Southon et al. (2018) may not have allowed participants sufficient time to explore the intervention in the park after the intervention was implemented and before they conducted the survey. Moreover, the planted piece of meadow within the park might not be noticeable in the overall park experience. Finally, people’s inability to perceive and detect vegetation diversity might lead to a disconnect between plant diversity and their health and well-being (Ha and Kim, 2021; Marselle et al., 2020; Methorst et al., 2021a).
Two of the included studies reported that specific aspects of biodiversity may negatively impact health. The greater diversity of vegetation types, plant fibers, and pollens was associated with reduced health status, indicated by reduced well-being and increased allergic diseases (Mavoa et al., 2019b; Zhang et al., 2021).
3.5.2 Effects of animal diversity
The positive effects of animal diversity on human health were particularly evident with birds. Greater avian diversity is associated with more positive moods and restorative perceptions (Douglas and Evans, 2022; Schebella et al., 2019), increased frequency of physical activity, lower obesity rates (Knobel et al., 2021), longer life expectancy (Chen et al., 2023), and improved overall well-being and life satisfaction (Chen et al., 2023; Methorst et al., 2021a; Taylor et al., 2018).
Still, several studies found no significant relationships among species richness, diversity of pollinator, bird, and vertebrate animal species, and various health outcomes, including affective states, cognitive functioning, physical activity levels, weight, immune system function, and well-being (Table 7). These insignificant results may be attributed to the relatively weak impact that these features of biodiversity exert on human health. One study, for instance, suggested that animal diversity had weaker influences on physical activities and weight than the design of the space and the facility (Melo et al., 2021). Another potential explanation for the insignificant results was peoples’ lack of awareness of the surrounding wildlife (Methorst et al., 2021a, 2021).
In contrast, other studies reported negative effects associated with integrated vertebrate species richness. Increased species richness of amphibians, birds, reptiles, and small mammals is correlated with an increased prevalence of asthma and allergic diseases (Cavaleiro Rufo et al., 2020, 2021). Although the factors influencing immune system functioning are complex, one possible reason is the rise in allergens linked to higher animal species richness, which can negatively affect immune system functioning (Cavaleiro Rufo et al., 2020, 2021).
3.5.3 Effects of environmental microbiome diversity
Studies have shown that high levels of environmental microbiome diversity are associated with both direct and indirect positive effects on human health. One research team demonstrated that greater environmental microbiome diversity directly boosted the diversity of children’s body microbiomes and the percentage of T cells, with the exposure duration ranging from several months to a year (Roslund et al., 2020, 2021).
3.5.4 Qualitative research
Qualitative research suggests that enhanced psychological states and perceptions can emerge from experiencing environments of varied biodiversity. Puhakka et al. (2019) discussed how the presence of sod and vegetation in a yard, based on the concept of affordance, stimulated toddlers’ physical movements and provided more opportunities to interact with natural elements, such as taking care of the plants. Children were more relaxed and calmer after playing in such settings. Similarly, Corney and Neave (2019) observed that participants walking in biodiverse riparian areas experienced enhanced perceptions of amenities and greater levels of relaxation and peace of mind during their walks compared to their counterparts. Encounters with wildlife enriched their experiences of excitement and appreciation (Corney and Neave, 2019).
3.5.5 Levels of biodiversity measures
The selection of biodiversity measurements can impact the interpretation of the biodiversity-health relationship. Several studies used dichotomous variables to assess biodiversity, contrasting low versus high levels or the presence versus absence of interventions (e.g., Ha and Kim, 2021; Southon et al., 2018). Other studies, however, revealed that individuals experienced the most benefits at certain levels, and the health effects decreased when the plant diversity exceeded that level (Lindemann-Matthies and Matthies, 2018; Schebella et al., 2020), indicating biodiversity does not impact human health in a linear fashion. Thus, employing dichotomous variables may limit the exploration of the relationship between biodiversity and human health outcomes.
3.6 Alignment of perceived and objective biodiversity
To what extent does a person’s perceived biodiversity align with objectively investigated biodiversity? Among the 41 studies we examined, six demonstrated that people’s perceptions of biodiversity often positively aligned with objective measures of plant and animal diversity. There are positive relationships found between perceived biodiversity and surveyed diversity of birds (e.g., Cameron et al., 2020) as well as the richness of flowering plants, meadows, and trees (e.g., Fisher et al., 2022), as shown in Table 8.
Table 8. Summary of relationships between perceived biodiversity and human physical and mental health.
Additionally, participants were able to differentiate between high and low avian diversity through audio stimuli (Douglas and Evans, 2022) and identify various levels of greenery assessed by a satellite-based measure of greenness, the normalized difference vegetation index (NDVI) (Taylor et al., 2018). Despite the evident positive correlations, only the studies by Cameron et al. (2020); Fisher et al. (2022), and Southon et al. (2018) reported strong or moderate correlations between objective and subjective biodiversity measures, while others reported weak associations. Furthermore, there were instances where people’s perceptions diverged from objective measures (Fisher et al., 2022; Ha and Kim, 2021; Hoyle et al., 2018; Southon et al., 2018), indicating people’s varying abilities to perceive plant and animal diversity.
When asked to estimate richness levels, individuals lacking relevant expertise often depend on visual cues, such as quantity, height, evenness of distribution, and color, which can lead to biased perceptions. Many authors have reported that larger green spaces with taller and more colorful plants are more likely to be perceived as diverse than green spaces with smaller sizes, shorter plants, and less colorful composition, irrespective of their actual diversity levels (Gonçalves et al., 2021; Hoyle et al., 2018; Southon et al., 2018). The uneven frequency of presence of some species, showing low evenness, resulted in an underestimation of richness (Southon et al., 2018). These findings emphasize the challenges and uncertainty in measuring perceived diversity, which might contribute to its weak correlations with objectively measured biodiversity.
3.7 Perceived biodiversity and health outcomes
We examined the extent to which perceived biodiversity correlates with health outcomes. Here again, we uncovered mixed results. Table 8 shows that studies identified positive correlations between perceived biodiversity and psychological aspects, such as emotions, stress, self-esteem, and concentration (Cameron et al., 2020; Fisher et al., 2021b; Schebella et al., 2019). Other studies found no statistically significant associations between perceived biodiversity and restoration, general health, or well-being (Douglas and Evans, 2022; Fisher et al., 2022; Nghiem et al., 2021; Southon et al., 2018). Furthermore, one study reported higher perceived plant species richness was associated with negative emotions, attributing this adverse effect to the incongruent perceptions between objectively and subjectively measured plant richness (Ha and Kim, 2021).
Four studies allowed us to contrast the predictive effectiveness of objective versus subjective measures of biodiversity on health. Two studies reported that compared to objective measures, perceived plant and bird diversity in urban parks was associated with a greater ability to concentrate, positive moods, and lower stress levels (Cameron et al., 2020; Schebella et al., 2019). Conversely, two studies demonstrated that objectively measured biodiversity showed stronger associations for well-being (Fisher et al., 2022; Southon et al., 2018). Based on these limited results, it would be premature to conclude that subjective measures are more powerful predictors of health outcomes than objective measures.
4 Discussion
In this systematic review, we analyzed 41 studies published between 2018 and 2023 that explored the relationship between biodiversity in urban settings and human health. Below, we summarize the findings related to each of our four research questions.
First, through what pathways did the studies examine the relationships between biodiversity and health? We summarized pathways that include restoring capacity, building capacity, reducing harm, and causing harm, fitting the framework from Marselle et al. (2021a). Among these four pathways, restoring and building capacity were the most often examined. Additionally, a fifth pathway was examined: One Health. Moreover, there has been an increase in research demonstrating that exposure to biodiverse natural landscapes enhances immune system functioning.
The second research question asked about the extent to which biodiversity correlated with health outcomes. Our synthesis revealed a range of positive, null, and negative effects on health. Specifically, when focusing on biodiversity variability (e.g., species richness), 33% of studies reported positive correlations with human health, while a small percentage (14%) suggested adverse effects of richness. Over half of the results (53%) showed no relationship between richness and health outcomes. These findings indicate that while biodiversity might not always directly benefit health, it produces little disservice to health.
In addressing the third and fourth research questions, we found that the public could distinguish among varying levels of biodiversity, though more studies reported weak correlations than moderate ones, and some studies found no correlation or negative correlations. This suggests that perceived biodiversity is not a reliable substitute for objective assessments. Solely relying on subjective measures may limit the generalizability of research findings. Moreover, when we examine the correlation between perceived biodiversity and health, accurate perceptions of biodiversity might not be necessary to experience health benefits from exposure to biodiverse urban green spaces. Still, understanding how people perceive biodiversity remains critical as perceptions vary based on backgrounds and knowledge levels, providing insights about personal experiences.
Next, we discuss four insights drawn from findings on the health effect of biodiversity factors focusing on variability and provide suggestions for future research directions.
4.1 The importance of biodiversity variability
Richness in biodiversity plays a critical role in enhancing the ecological quality of the environment. Its health impacts, however, are often overshadowed by the more significant effects of measuring the abundance factors of biodiversity. Our review found that the richness of living organisms (i.e., the number of different species or genera) generally yielded no positive or negative effects on mental or physical health or well-being (Douglas and Evans, 2022; Methorst et al., 2021a, b; Randler et al., 2023; Schebella et al., 2019; Southon et al., 2018) and marginal negative effects on immune system functioning and affective states (Schebella et al., 2019; Vilcins et al., 2021). Nevertheless, a few studies found that plant diversity, after controlling for vegetation density, was positively associated with reduced blood pressure, better mental well-being, and lower mortality (Fisher et al., 2022; Lindemann-Matthies and Matthies, 2018; Methorst et al., 2021a; Schebella et al., 2019). These outcomes highlight the health benefits of plant species richness above and beyond plant cover.
It is crucial to distinguish the health effects of factors related to abundance and variability in biodiversity, as each has distinct implications for ecosystems and environments. These factors can be used in different situations. For instance, while increasing green spaces benefits well-being, it requires space, a scarce resource in urban areas. In contrast, enhancing plant richness without taking more space in the existing green spaces may be more feasible. Moreover, the diversity of species is found to be associated with the quality and stability of other ecosystem services at a local scale (Tilman et al., 2014; Schwarz et al., 2017). For example, habitat diversity is correlated with the species richness of plants and animals in urban green spaces (Matthies et al., 2017), while tree species diversity is associated with more stable air quality in urban areas (Manes et al., 2012). Previous research has shown specific effects of plant and animal species richness on the functioning of green spaces, and our review supports the health effects of richness in the ecosystems. This distinction should encourage designers to create urban settings with higher levels of species richness that will likely benefit the health of the ecosystem, the services it provides, and human health.
4.2 Research methodologies for biodiversity and human health
Our review found a deficiency in studies establishing causal relationships between biodiversity and health with objective measures, aligning with previous work noting this research gap (Dzhambov et al., 2020; Marselle et al., 2021a). Among these studies, four examined affective outcomes and reported mixed positive, negative, and insignificant effects (Douglas and Evans, 2022; Ha and Kim, 2021; Schebella et al., 2020. Southon et al., 2018). Studies using biofeedback measures showed positive impacts on blood pressure (Lindemann-Matthies and Matthies, 2018) and negative effects on heart rate (Schebella et al., 2020).
Our review found that exposure to the increased environmental microbiomes in daycare centers enhanced children’s immune system functioning over several months, using a quasi-experimental method (Roslund et al., 2020, 2021). These results, however, involved the same participant group and locations. Hence, future research should broaden this examination to various living environments, types of green spaces, durations of exposure, and populations to gain a more comprehensive understanding.
Notably, there was much work on attention restoration, but a scant direct connection was made between biodiversity and attentional functioning. Only one study examined attentional capacity with objective measures of cognitive functioning in an experimental design. Furthermore, other studies relied on indirect measures of attention capacity, such as participants’ feelings of restfulness or positive affect. Thus, the direct evidence of the extent to which biodiversity influences attentional functioning assessed under controlled settings with objective measures remains unclear (Celikors and Wells, 2022). Future studies examining causal relationships are needed to provide robust evidence and clarify the links between subjective perceptions and cognitive performances.
Finally, specifying pathways linking biodiversity and health provides valuable insights, especially when exploring long-term health outcomes. Our review revealed that biodiversity benefited well-being, particularly through physical activity and reduced pollution (Fisher et al., 2022; Mavoa et al., 2019a). Conversely, other studies lacked information about specific mechanisms and were limited in providing precise explanations. Exploring these mechanisms between biodiversity and health can help identify approaches for landscape designs to improve both environmental quality and human health.
4.3 Perceptions of avian biodiversity in living environments
Birds are often used as indicators of environmental quality and, for reasons we discuss below, are relevant to human health. Our review found more positive and stronger correlations between the diversity of bird species in a setting and human health compared to other animal taxa. Although the substantial emphasis on birds in research might partly explain the higher numbers of positive findings, other factors likely contribute to their connections with human health. Below, we discuss three characteristics that differentiate birds from other animals in urban areas that can be pertinent to understanding the link between animal diversity and health.
First, birds and humans share similar needs within urban green spaces. It is relevant because people may experience bird habitats more often than they perceive the wildlife per se. Bird habitats encompass various landscape types, potentially reflecting the environmental qualities sought by urban dwellers. Green spaces with higher plant diversity and tree species richness correlate with greater bird diversity (Paker et al., 2014; Stagoll et al., 2012) and are associated with greater perceived restorativeness (Gonçalves et al., 2021; Fuller et al., 2007). Additionally, environments with greater avian diversity were found to be restorative, encouraging outdoor activities and boosting well-being (Chang et al., 2024; Wheeler et al., 2015). These outcomes indicate a shared preference and need between birds and humans for characteristics of urban open spaces.
Second, birds are relatively visible and observable, which may lead to a stronger relationship between birds and human life. Visibility is relevant because the element that can be seen and perceived by users shapes the correlation between green spaces and health, particularly through pathways of attention restoration (Kaplan, 1995) and stress reduction (Ulrich, 1983). Birds, with their high visibility and the ease with which humans distinguish different species, are more noticeable and attractive to people than animals that appear less frequently in urban settings or that are more difficult to perceive (Cameron et al., 2020; Methorst et al., 2021b).
Third, birds are often perceived positively, which may lead to human health benefits. Attitudes toward wildlife vary, and adverse attitudes toward some species can diminish the potential positive effects of nature on health and well-being. Birds, however, are generally perceived less negatively, with their songs being a source of relaxation (Ratcliffe et al., 2013). In contrast, some members of the public are concerned when they observe large mammals and reptiles near their living environments (Methorst et al., 2021b; Lyytimäki, 2014). Thus, birds might be associated with more health benefits than other animal groups due to their positive perceptions.
Birds share environmental requirements with humans, are highly detectable, and are generally perceived more positively than other species, presenting stronger correlations with environmental perceptions and human health than would be true for other animals. This does not imply other animals contribute less to human health and well-being; rather, the relationships are easier to detect and more significantly associated with human health benefits.
4.4 Impact of demographic and environmental contexts on biodiversity perceptions and health
Understanding how participants’ demographic characteristics and broader environmental contexts influence their perceptions of and interactions with biodiversity is critical in studies that include diverse populations. Factors such as personal backgrounds and physical environments can shape these perceptions and interactions. At the individual scale, males, individuals with low financial status, those with high nature connectedness, and people experiencing a stronger sense of social cohesion tended to perceive biodiversity as more closely aligned with the objectively measured biodiversity (Douglas and Evans, 2022; Fisher et al., 2022; Southon et al., 2018).
Demographic factors also directly influence the health outcomes associated with interactions with biodiversity. Our review revealed that women, older adults, and people from lower socioeconomic statuses tended to experience more health benefits from biodiverse environments (Chen et al., 2023; Douglas and Evans, 2022; Marselle et al., 2020; Southon et al., 2018). Additionally, attitudinal and behavioral factors, such as a strong connection to nature and frequent use of green spaces, evoke strong relationships with biodiversity and human health (Samus et al., 2022; Southon et al., 2018). These outcomes emphasize the complex interplay between biodiversity and human health that are influenced by individual characteristics.
At the neighborhood scale, the health impacts of biodiversity vary between urban and rural areas and across levels of development intensity. Larger green spaces in urban areas are associated with lower asthma risks, while in rural areas, more green space correlates with higher asthma risk, likely due to intensive agricultural activities rather than biodiversity per se (Winnicki et al., 2022). In urban areas, the protective effects of vegetation diversity against adult asthma were weaker in areas with greater air pollution compared to environments with lower pollution levels (Donovan et al., 2021b). Additionally, the size of a private garden was a positive predictor of moods for residents with limited access to public green spaces, a relationship diminished for those surrounded by more greenery (Samus et al., 2022). These findings indicate that the health effects of biodiversity depend on broader environmental and developmental contexts. Therefore, research findings should be interpreted and applied cautiously regarding scales and environmental contexts.
4.5 Suggestions for future research
We provide four suggestions for future research that focus on designing research methodologies to enhance understanding of the pathways through which biodiversity might impact human health, exploring the influence of socioeconomic factors, investigating the functional characteristics of natural landscapes and their health benefits, and expanding research to different regions of the world.
4.5.1 Designing research methodologies to enhance understanding
We suggest examining the impact of exposure to biodiverse natural landscapes on well-being through potential pathways. Given that well-being is an overall evaluation influenced by numerous factors, this approach can provide a comprehensive understanding. Researchers should collect data on relevant details, such as demographic characteristics or individual perceptions of natural environments. This approach will help researchers break broad concepts into small, manageable elements. Additionally, considering the complex interactions between biodiversity and human health, future research should adopt multi-level methods to assess biodiversity to effectively illustrate the dynamics between biodiversity and health outcomes.
4.5.2 Exploring the influence of socioeconomic status
Future research can explore the extent to which socioeconomic status interacts with the relationship between biodiversity and human health. Considerable focus has been placed on the inequitable distribution of green spaces and its direct impact on residents’ health and well-being. Communities with lower socioeconomic status often have limited access to professionally designed green spaces (Dai, 2011; Mitchell et al., 2015; Rigolon et al., 2018). The low availability of spaces may result in fewer opportunities for leisure activities and diminished environmental quality, which are associated with lower levels of well-being (Akpinar, 2016).
The inequitable policies governing green spaces also have indirect impacts, such as challenges to gene flow, which reduce genetic diversity and create evolutionary consequences across habitats, ultimately resulting in low overall biodiversity in urban areas (Schell et al., 2020). This condition influences health and well-being through the functionality of environments when coping with hazards. For instance, compared to ecosystems with higher biodiversity, those with low biodiversity are more vulnerable to disturbances from extreme heat and precipitation. In such environments, access to resources like habitat heterogeneity, water, food, and shelter can be easily obstructed by damage (Haight et al., 2023; Rastandeh et al., 2018). Therefore, residents in areas with low biodiversity are put in a more vulnerable condition as those environments are prone to degradation and offer lower environmental quality.
While these scenarios may not significantly generate differences with a city, the inequitable distribution of resources across cities may reinforce the deprived conditions in some areas more than others in the face of climate change. Given the findings discussed in this review study, it is critical for future research to examine how disparities in socioeconomic status, at individual, neighborhood, and cross-city levels, influence the distribution of green spaces, biodiversity, and health outcomes. This is particularly important during the crisis of extreme weather events, as it would shed light on the inequalities in access to nature and its health benefits.
4.5.3 Investigating functional characteristics of natural landscapes
Future research should examine how specific characteristics of natural landscapes, such as the nativeness of species, the behavioral traits of wildlife, and resources needed by animals (animal guilds) influence human interactions with green spaces. Investigating the composition and traits of species can inform the selection of plant species for specific sites, linking micro-ecosystems to broader urban landscapes. Although this level of detail may reduce the generalizability of results to areas with different climates or geographical characteristics, such detailed knowledge is valuable. Generating it will provide precise guidance for practical landscape design, integrating ecological functions with the well-being of urban residents.
4.5.4 Expanding research to different regions of the world
At the global scale, the studies we reviewed reported primarily on research conducted in developed countries of Europe and Oceania, revealing a skewed geographical distribution. Future research should expand to different geographical areas, particularly in Africa, Asia, Central America, and South America, where existing research is scarce. Since social contexts play a key role in shaping interactions between urban green spaces and human health, it is critical to consider the environmental and social context when applying study results from one setting to another. Expanding research to these regions will ensure a more global and comprehensive understanding of the interactions between biodiversity and diverse populations.
5 Conclusion
This systematic review of literature published from 2018 and 2023 reveals the complex connections between biodiversity and human health, emphasizing the need for deeper insight. Despite many studies reporting insignificant associations, it is still crucial to preserve biodiverse urban environments for their broader ecosystem services. Our review demonstrates that even though people frequently misjudge the biodiversity around them, their perceptions are still strongly linked to health outcomes, indicating that perceived biodiversity is crucial for well-being.
Our review of recent literature in this field reveals a significant bias towards Western perspectives, which constrains the general applicability of our insights. It is crucial to extend research to urban areas across the globe, particularly in underrepresented regions. Incorporating a variety of social contexts and environmental conditions will enable future studies to offer a more universal and comprehensive understanding of the relationships between biodiversity and human health. This broader approach will aid planners and landscape architects in designing urban green spaces that enhance the well-being of diverse populations.
Author contributions
C-CW: Conceptualization, Formal analysis, Investigation, Visualization, Writing – original draft, Writing – review & editing. JO’K: Conceptualization, Formal analysis, Writing – review & editing. YD: Investigation, Writing – review & editing. WCS: Conceptualization, Formal analysis, Writing – review & editing.
Funding
The author(s) declare that no financial support was received for the research, authorship, and/or publication of this article.
Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Publisher’s note
All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.
Supplementary material
The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fevo.2024.1467568/full#supplementary-material
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Keywords: urban biodiversity, health and well-being, perceptions of urban green space, urban nature exposure, species richness
Citation: Wu C-C, O’Keefe J, Ding Y and Sullivan WC (2024) Biodiversity of urban green spaces and human health: a systematic review of recent research. Front. Ecol. Evol. 12:1467568. doi: 10.3389/fevo.2024.1467568
Received: 20 July 2024; Accepted: 28 October 2024;
Published: 25 November 2024.
Edited by:
Rui Leandro Maia, Fernando Pessoa Energy, Environment and Health Research Unit (FP-ENAS), PortugalReviewed by:
Leigh Whittinghill, Connecticut Agricultural Experiment Station, United StatesAmelie Y. Davis, United States Air Force Academy, United States
Maria José Dias Curado, University of Porto, Portugal
Copyright © 2024 Wu, O’Keefe, Ding and Sullivan. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Chia-Ching Wu, Y3d1ODdAaWxsaW5vaXMuZWR1