Background: The biodiversity hypothesis and the microbiota–immune–brain (MIB) axis posit that reduced environmental microbial contact in urban settings elevates inflammatory tone and worsens mood. Methods: In a cross-sectional comparison of Filipino adults (n=200; Quezon City [QC] urban=100, Nueva Ecija [NE] rural=100), we quantified standardized air/dust exposure indices (Shannon, Simpson, log10 CFU, EMDE_z), serum cytokines by ELISA (IL-1β, TNF-α; BSL-2), and mood (PHQ-9, PANAS), adjusting for age, BMI, SES quartiles, PSQI, and smoking. Analyses used Welch t tests, Pearson’s r, and OLS with HC3 errors; mediation was tested with 1,000 bias-corrected bootstraps. Results: Microbial diversity was lower in QC than NE (Shannon M=2.59 vs 3.27; t=−14.30, p<.001; g=−2.01), while pro-inflammatory cytokines were higher in QC (lnIL-1β M=1.438 vs 0.793, t=10.87, p<.001, g=1.53; lnTNF-α M=1.933 vs 1.389, t=11.27, p<.001, g=1.59). Diversity inversely correlated with inflammation (e.g., Shannon–lnIL-1β r=−0.719; Shannon–lnTNF-α r=−0.665; all p<.001), and inflammation tracked mood (lnTNF-α with PHQ-9 r=0.776; with PANAS-NA r=0.785; with PANAS-PA r=−0.487; all p<.001). In mediation, Shannon predicted lower lnTNF-α (a=−0.512, p<.001), which predicted higher PHQ-9 (b=4.618, p<.001); the indirect effect was significant (a×b=−2.366, 95% CI −2.945 to −1.800), with a residual direct effect (c′=−0.516, p<.001). Conclusions: Higher environmental microbial diversity is associated with dampened inflammatory activity and lower depressive burden, supporting a bio-ecological pathway linking urbanicity, immunity, and affect. Findings suggest biodiversity-aware urban design as a mental-health–relevant policy lever.

Bacterial communities are poorer at urban park entrances in Finland than Russian Tatarstan – Testing the core presumption of the biodiversity hypothesis

Over the last few decades, the occurrence of chronic inflammatory and allergic disorders has increased exponentially in people and pets. Changes in our environment impact both human and animal health, as One Health recognizes. Several theories have been proposed, and various mechanisms have been considered. The first was the "hygiene hypothesis," which focused on the observation that allergies became more common once hygienic conditions improved. Although the concept that allergies happen because living conditions are too clean has been mostly disproven, parts of it are included in the "old friends hypothesis," which emphasizes the importance of exposure to harmless bacteria through food and the environment that have evolved with us. A further refinement of this idea is incorporated in the "biodiversity hypothesis," which emphasizes the importance of diverse environmental, food, and microbial exposures in educating the immune system and promoting tolerance. The most recent hypothesis is the "epithelial barrier hypothesis," which emphasizes the role of pollutants and chemicals that chronically disrupt epithelia, whether in the skin, gut, or respiratory tract, promoting low-grade inflammation and increasing the risk of allergic sensitization. Each of these theories has focused on the role of urbanization in promoting allergies, and each theory has some supporting evidence. Numerous epidemiological studies, addressed in the companion Currents in One Health by Marsella, JAVMA, forthcoming 2025, demonstrated that urban life increases the risk for allergies in people and dogs. The purpose of this review is to present the current knowledge in dogs and raise awareness regarding these topics.

In modern urbanized societies, the incidence of major immune-mediated diseases is several times higher than before World War II. A potential explanation is that these diseases are triggered by limited possibilities to be exposed to rich environmental microbiota. This requires that the urban environment hosts less and poorer microbiota than the natural environment. The current study was designed to test the assumption that urban man-made environments host less and poorer environmental microbiota, compared to natural habitats. We selected two types of dry environments, natural rocks and playground rubber mats, both of which were used daily and extensively by children. In quantitative PCR and next-generation sequencing, bacterial abundance and richness were higher on the natural rocks than the rubber mats. Altogether, 67 amplicon sequence variants (ASVs) belonging mostly to Actinobacteria and Proteobacteria were indicative of rock microbiota, while three ASVs were indicative of rubber mats. Interestingly, bacteria formed more complex networks on rubber mats than natural rocks. Based on the literature, this indicates that the studied artificial dry environment is more challenging and stressful for bacterial communities than dry natural rocks. The results support the hypothesis that urban man-made environments host poor microbial communities, which is in accordance with the biodiversity hypothesis of immune-mediated diseases.IMPORTANCEThe current study provides new evidence that artificial urban play environments host poor microbial communities and provide a stressful environment for microbes, as compared to dry natural rocks. Through this, the current study underlines the need to enhance microbial diversity in urban areas, especially in outdoor play environments, which have a crucial role in providing essential microbial exposure for the development of children's immune system. This research can potentially offer guidance for urban planning and public health strategies that support planetary health.

Atopic dermatitis (AD) is a prevalent chronic inflammatory disease that significantly burdens individuals and healthcare systems worldwide. The incidence of AD has risen sharply in both developed and emerging economies, necessitating an understanding of its complex etiological factors, including environmental influences and lifestyle changes. Generally, 2 primary preventive strategies for AD have been implemented so far: (1) the "Inside Out" approach that which involves allergen elimination, probiotic supplementation, fish oil supplementation, and vitamin D supplementation aim to regulate the immune system in pregnancy and early childhood and (2) the "Outside In" approach that focuses on improving skin barrier function through emollient use and environmental changes. Although current evidence suggests the potential benefits from these interventions, randomized controlled trials have revealed inconsistencies in their efficacy. It is imperative not only to explore the minute research gaps in existing studies, but also to develop novel interventional studies that consider individual and regional differences based on the epithelial barrier hypothesis, the biodiversity hypothesis, and the 'old friends' hypothesis evolved from the hygiene hypothesis. Ultimately, reversing the rising trend of AD prevalence will most likely require a multifaceted approach that integrates new scientific evidence and promote comprehensive lifestyle changes.

The gradual decrease in the prevalence of serious infectious diseases over the last century has been followed by increase in so called “modern” diseases, including allergies, chronic inflammatory conditions, psychiatric, and metabolic disorders. Between 2019 and 2022, public awareness of the threat of infectious diseases in humans was renewed by the global pandemic of a new type of a coronavirus, the SARS-COV-2. This public interest opened improved possibilities to test hypotheses on the factors associated with inter-individual variation in susceptibility to infectious and “modern” diseases. Based on the Hygiene hypothesis and Biodiversity hypothesis, we predicted that contacts with natural environment and wildlife in childhood and/or in adulthood can improve general health and decrease the risks of severe COVID-19 progression or prevalence of the “modern” diseases, namely the allergies. Here we report the results of an online, self-evaluating questionnaire survey conducted in the Czech Republic, where we contrasted selected health issues, and linked them to the living environment, including the level of contacts with biodiversity. In a sample of 1188 respondents, we revealed a significant association of time spent in nature or contacts with biodiversity with physical and mental health, or incidence of allergies. This is unlike the COVID-19 progression, which was related to age, physical health, smoking, allergies, and interaction of age with smoking, but not to contacts with the natural environmental diversity. Our findings regarding to physical and mental health and allergies are in agreement with the Biodiversity hypothesis of allergy and, linking human and environmental health, they urge for One Health approach application.

The pyrodiversity–biodiversity (P–B) hypothesis posits that spatiotemporally variable fire regimes increase wildlife habitat diversity, and that the fine-grained mosaics resulting from small patchy fires enhance biodiversity. This logic underpins the patch mosaic burning (PMB) paradigm and reinforces the benefits of Indigenous fire management, which tends to promote pyrodiversity. However, tests of the P–B hypothesis and PMB paradigm are few. One of the most comprehensive field evaluations—a snapshot study of pre-existing fire mosaics in south-east Australian semi-arid mallee eucalypt woodlands—found little support. To explore the longer-term effects of fire mosaic grain size on habitat availability and biodiversity, we combined published data from the mallee study with a simple fire simulation. We simulated 500 years of landscape burning under different fire sizes. In the resulting mosaics, we assessed the proportional mixture and patch configuration of successional habitat states, then summarised habitat availability through time using a composite index based on the published fire history responses of 22 vertebrate taxa from the mallee study. Small fires formed fine-grained mosaics with a stable habitat mixture and with habitat diversity occurring at fine scales. Large fires formed coarse-grained mosaics with the opposite properties. The fine-grained mosaics maintained optimal habitat availability for vertebrate diversity over 500 years, while the fluctuating habitat mixture in the coarse-grained mosaics was unlikely to maintain maximum vertebrate diversity. Broadly, our results support the P–B hypothesis and justify further field-testing and evaluation of PMB programs to manage both pyrodiversity and biodiversity in the mallee and other flammable landscapes.

Urban indoor gardening enhances immune regulation and diversifies skin microbiota — A placebo-controlled double-blinded intervention study

Urban development has profoundly reduced human exposure to biodiverse environments, which is linked to a rise in human disease. The ‘biodiversity hypothesis’ proposes that contact with diverse microbial communities (microbiota) benefits human health, as exposure to microbial diversity promotes immune training and regulates immune function. Soils and sandpits in urban childcare centres may provide exposure to diverse microbiota that support immunoregulation at a critical developmental stage in a child's life. However, the influence of outdoor substrate (i.e., sand vs. soil) and surrounding vegetation on these environmental microbiota in urban childcare centres remains poorly understood. Here, we used 16S rRNA amplicon sequencing to examine the variation in bacterial communities in sandpits and soils across 22 childcare centres in Adelaide, Australia, plus the impact of plant species richness and habitat condition on these bacterial communities. We show that sandpits had distinct bacterial communities and lower alpha diversity than soils. In addition, we found that plant species richness in the centres' yards and habitat condition surrounding the centres influenced the bacterial communities in soils but not sandpits. These results demonstrate that the diversity and composition of childcare centre sandpit and soil bacterial communities are shaped by substrate type, and that the soils are also shaped by the vegetation within and surrounding the centres. Accordingly, there is potential to modulate the exposure of children to health-associated bacterial communities by managing substrates and vegetation in and around childcare centres.

Abstract The biodiversity hypothesis suggests that decreased interaction with microbes can lead to an imbalance in the human microbiota (referred to as dysbiosis), which in turn will contribute to the development of immune-mediated diseases including asthma, allergies, and autoimmune diseases such as type 1 diabetes. We have previously utilised a novel approach of bringing humans and mice into contact with microbially rich plant- and soil-based material. In humans, exposure to this material increased commensal microbial diversity in daycare children which also linked to higher levels of anti-inflammatory cytokines TGF-b and IL-10 in plasma. Here, we compared the effects of autoclaved and non-autoclaved, fresh soil powder on the mouse immune system, to understand if soil-derived material contains molecular structures that convey the immunoregulatory signals. These effects were evaluated phenotyping immune populations by flow cytometry, assessing cytokine profiles in serums and microbiome diversity by genetic sequencing. Our results indicate that exposure to non-autoclaved soil did not influence the health of mice but was associated with elevated serum IFN-g expression, and reduction of GATA-3 and Foxp3 expression in mLNs. The observed immunological effects warrant further studies with autoclaved and non-autoclaved soil exposure in immune-related diseases, such as allergy and inflammatory bowel disease models in mice. Academy of Finland, Tampere Tuberculosis Foundation, State Research Funding (for Fimlab Laboratories), Finnish Medical foundation and Northern Finland Laboratory Centre (NordLab).

Greener residential environment is associated with increased bacterial diversity in outdoor ambient air

Contact with natural environments enriches the human microbiome, promotes immune balance and protects against allergies and inflammatory disorders. In Finland, the allergy & asthma epidemic became slowly visible in mid 1960s. After the World War II, Karelia was split into Finnish and Soviet Union (now Russia) territories. This led to more marked environmental and lifestyle changes in the Finnish compared with Russian Karelia. The Karelia Allergy Study 2002-2022 showed that allergic conditions were much more common on the Finnish side. The Russians had richer gene-microbe network and interaction than the Finns, which associated with better balanced immune regulatory circuits and lower allergy prevalence. In the Finnish adolescents, a biodiverse natural environment around the homes associated with lower occurrence of allergies. Overall, the plausible explanation of the allergy disparity was the prominent change in environment and lifestyle in the Finnish Karelia from 1940s to 1980s. The nationwide Finnish Allergy Programme 2008-2018 implemented the biodiversity hypothesis into practice by endorsing immune tolerance, nature contacts, and allergy health with favorable results. A regional health and environment programme, Nature Step to Health 2022-2032, has been initiated in the City of Lahti, EU Green Capital 2021. The programme integrates prevention of chronic diseases (asthma, diabetes, obesity, depression), nature loss, and climate crisis in the spirit of Planetary Health. Allergic diseases exemplify inappropriate immunological responses to natural environment. Successful management of the epidemics of allergy and other non-communicable diseases may pave the way to improve human and environmental health.

The biodiversity hypothesis postulates that the natural environment positively affects human physical and mental health. We evaluate the latest evidence and propose new tools to examine the halobiont environment. We chose to target our review at neuropsychiatric disorders, including depression, anxiety, autism, dementia, multiple sclerosis, etc. because a green prescription (exposure to green spaces) was shown to benefit patients with neuropsychiatric disorders. Specifically, our review consists of three mini reviews on the associations exploring: (1) ecological biodiversity and human microbiota; (2) human microbiota and neuropsychiatric disorders; (3) ecological biodiversity and neuropsychiatric disorders. We conclude that the environment could directly transfer microbes to humans and that human studies support the gut microbiota as part of the pathophysiology of neuropsychiatric disorders. Overall, the results from the three mini reviews consistently support the biodiversity hypothesis. These findings demonstrated the plausibility of biodiversity exerting mental health effects through biophysiological mechanisms instead of psychological mechanisms alone. The idea can be further tested with novel biodiversity measurements and research on the effects of a green prescription.

A Placebo-controlled double-blinded test of the biodiversity hypothesis of immune-mediated diseases: Environmental microbial diversity elicits changes in cytokines and increase in T regulatory cells in young children

What is needed for our children facing unprecedented challenges of modern time? Biodiversity, both for immunological and psychological well-being and resilience. That is also the keyword for the children with allergies and asthma. The cultural evolution with advanced technology and medicine along with major move to urban environment has profoundly changed our lifestyle and surroundings. We are increasingly disconnected from our evolutionary home, soil, natural waters, and air we used to breathe. The ecosystem of human body and mind has been tested, survived, and evolved closely in relation with other ecosystems. For balance and tolerance, immune regulatory circuits need training by microbes, biogenic chemicals, and close relation to natural environment throughout life. This is addressed by the biodiversity hypothesis of tolerance/resilience for health, supported by the pioneering real-world interventions and a few controlled studies. No need to go "back to nature," but we must take natural elements back to our everyday life to breathe, eat, drink, and touch. The change for better is plausible and cost-effective, as shown by the Finnish and other European initiatives, but needs contribution of the whole society.

Mountains are pivotal to maintaining habitat heterogeneity, global biodiversity, ecosystem functions and services to humans. They have provided classic model natural systems for plant and animal diversity gradient studies for over 250 years. In the recent decade, the exploration of microorganisms on mountainsides has also achieved substantial progress. Here, we review the literature on microbial diversity across taxonomic groups and ecosystem types on global mountains. Microbial community shows climatic zonation with orderly successions along elevational gradients, which are largely consistent with traditional climatic hypotheses. However, elevational patterns are complicated for species richness without general rules in terrestrial and aquatic environments and are driven mainly by deterministic processes caused by abiotic and biotic factors. We see a major shift from documenting patterns of biodiversity towards identifying the mechanisms that shape microbial biogeographical patterns and how these patterns vary under global change by the inclusion of novel ecological theories, frameworks and approaches. We thus propose key questions and cutting-edge perspectives to advance future research in mountain microbial biogeography by focusing on biodiversity hypotheses, incorporating meta-ecosystem framework and novel key drivers, adapting recently developed approaches in trait-based ecology and manipulative field experiments, disentangling biodiversity-ecosystem functioning relationships and finally modelling and predicting their global change responses.

In modern urban environments children have a high incidence of inflammatory disorders, including allergies, asthma, and type1 diabetes. The underlying cause of these disorders, according to the biodiversity hypothesis, is an imbalance in immune regulation caused by a weak interaction with environmental microbes. In this 2-year study, we analyzed bacterial community shifts in the soil surface in day-care centers and commensal bacteria inhabiting the mouth, skin, and gut of children. We compared two different day-care environments: standard urban day-care centers and intervention day-care centers. Yards in the latter were amended with biodiverse forest floor vegetation and sod at the beginning of the study. Intervention caused a long-standing increase in the relative abundance of nonpathogenic environmental mycobacteria in the surface soils. Treatment-specific shifts became evident in the community composition of Gammaproteobacteria, Negativicutes, and Bacilli, which jointly accounted for almost 40 and 50% of the taxa on the intervention day-care children's skin and in saliva, respectively. In the year-one skin swabs, richness of Alpha-, Beta-, and Gammaproteobacteria was higher, and the relative abundance of potentially pathogenic bacteria, including Haemophilus parainfluenzae, Streptococcus sp., and Veillonella sp., was lower among children in intervention day-care centers compared with children in standard day-care centers. In the gut, the relative abundance of Clostridium sensu stricto decreased, particularly among the intervention children. This study shows that a 2-year biodiversity intervention shapes human commensal microbiota, including taxa that have been associated with immune regulation. Results indicate that intervention enriched commensal microbiota and suppressed the potentially pathogenic bacteria on the skin. We recommend future studies that expand intervention strategies to immune response and eventually the incidence of immune-mediated diseases.

Native grasslands have been vastly transformed with the expansion of human activities. Applied fire regimes offer conservation‐based management an opportunity to enhance remaining grassland biodiversity and secure its persistence into the future. Fire regimes have complex interactions with abiotic and biotic ecosystem components that influence environmental heterogeneity and biodiversity. We examined the pyrodiversity–biodiversity hypothesis, which suggests that more species are supported where pyrodiversity, that is, the level of environmental heterogeneity associated with different fire regimes, is greater. A mesocosm‐type field experiment, maintained for 38 yr, was used to determine the response of plant diversity to 1‐, 2‐, 5‐ and 12‐yr fire‐return interval treatments, with early‐dormant, middormant and early–growing season burns. Our sampling regime was designed to assess the influence of fire treatments and combinations thereof, over spatial scale, on plant diversity. Pyrodiversity was maximized where fire regime diversity, simulated by varying the size of patches with different fire treatments, was greatest. Species richness was predicted to be reduced at short and long extremes of fire‐return interval, as suggested by the intermediate‐disturbance hypothesis. The influence of fire treatments on alpha and beta diversity, and plant functional groups, were tested using multivariate and Bayesian models. Multilevel models of plant height and growth form, with fire‐return interval, reflected the strong indirect influence of fire‐return interval on sward structure and the plant environment. The pyrodiversity–biodiversity and intermediate‐disturbance hypotheses were only partially supported and depended on the plant group and spatial scale of assessment. Although both frequent and infrequent burns made important contributions to overall species richness, richness peaked where 20–40% of the area was protected from frequent fires. The larger contribution of frequent burning to diversity was due to an interaction with scale and forb turnover over the trial area. Extremes in fire‐return intervals reduced forb richness, supporting the predictions of the intermediate‐disturbance hypothesis. Spring burns had a weak negative influence on forb alpha diversity, but only at small scales. For a meaningful contribution of management to plant diversity, traditional fixed biennial burns need to be supplemented with smaller patches burned with longer fire‐return intervals, and extremes in fire‐return intervals avoided.

Abstract AimPyrodiversity is the spatial or temporal variability in fire effects across a landscape. Multiple ecological hypotheses, when applied to the context of post‐fire systems, suggest that high pyrodiversity will lead to high biodiversity. This resultant “pyrodiversity–biodiversity” hypothesis has grown popular but has received mixed support by recent empirical research. In this paper, we sought to review the existing pyrodiversity literature, appraise support for the pyrodiversity–biodiversity hypothesis, examine potential mechanisms underlying the hypothesis and identify outstanding questions about pyrodiversity and future research needs.LocationGlobal terrestrial ecosystems.MethodsWe performed a systematic literature review of research related to pyrodiversity and the pyrodiversity–biodiversity hypothesis. We also examined how two individual species with distinct relationships with fire (spotted owl Strix occidentalis and black‐backed woodpecker Picoides arcticus) respond to pyrodiversity as case studies to illustrate underlying mechanisms.ResultsWe identified 41 tests of the pyrodiversity–biodiversity hypothesis reported from 33 studies; 18 (44%) presented evidence in support of the pyrodiversity–biodiversity hypothesis, while 23 (56%) did not. Our literature review suggested that support for the pyrodiversity–biodiversity hypothesis varies considerably with no consistent patterns across taxonomic groups and ecosystem types. Studies examining the pyrodiversity–biodiversity hypothesis often define pyrodiversity in different ways, examine effects at different scales and are conducted in ecosystems with different natural fire regimes, baseline levels of biodiversity, and evolutionary histories. We suggest these factors independently and jointly have led to widely varying support for the pyrodiversity–biodiversity hypothesis.Main ConclusionsClarifying the pyrodiversity–biodiversity hypothesis will be facilitated by stronger development of the different potential mechanisms underlying pyrodiversity–biodiversity relationships, which can be aided by examining how individual species respond to pyrodiversity. Future research would benefit from a closer examination of the role of scale (e.g. scale dependence) in pyrodiversity–biodiversity relationships, standardization of pyrodiversity metrics, broad‐scale mapping of pyrodiversity, and macroecological study of pyrodiversity–biodiversity relationships.

According to the hygiene and biodiversity hypotheses, increased hygiene levels and reduced contact with biodiversity can partially explain the high prevalence of immune-mediated diseases in developed countries. A disturbed commensal microbiota, especially in the gut, has been linked to multiple immune-mediated diseases. Previous studies imply that gut microbiota composition is associated with the everyday living environment and can be modified by increasing direct physical exposure to biodiverse materials. In this pilot study, the effects of rural-second-home tourism were investigated on the gut microbiota for the first time. Rural-second-home tourism, a popular form of outdoor recreation in Northern Europe, North America, and Russia, has the potential to alter the human microbiota by increasing exposure to nature and environmental microbes. The hypotheses were that the use of rural second homes is associated with differences in the gut microbiota and that the microbiota related to health benefits are more diverse or common among the rural-second-home users. Based on 16S rRNA Illumina MiSeq sequencing of stool samples from 10 urban elderly having access and 15 lacking access to a rural second home, the first hypothesis was supported: the use of rural second homes was found to be associated with lower gut microbiota diversity and RIG-I-like receptor signaling pathway levels. The second hypothesis was not supported: health-related microbiota were not more diverse or common among the second-home users. The current study encourages further research on the possible health outcomes or causes of the observed microbiological differences. Activities and diet during second-home visits, standard of equipment, surrounding environment, and length of the visits are all postulated to play a role in determining the effects of rural-second-home tourism on the gut microbiota.