The logic behind the biodiversity hypothesis is elegant and, on its face, compelling. Human immune systems evolved in intimate conversation with an enormous variety of environmental microbes—bacteria, fungi, and other microorganisms that thrived in diverse vegetation, soils, and water bodies. Regular contact with this microbial variety, the argument goes, helps train the immune system, keeping inflammatory responses in check. Deprive the immune system of that training, and it becomes prone to misfiring: overreacting to harmless allergens, inflaming airways, and producing the characteristic wheezing and breathlessness of asthma. Urban lifestyles, with their sealed buildings, sanitized surfaces, and simplified landscapes, dramatically reduce that microbial exposure. Climate change compounds the problem by degrading natural habitats and further depleting the diversity of plants and microbes in the environments where people live.

Testing this hypothesis rigorously, however, is far from straightforward. Biodiversity is not a single quantity but a family of related metrics, each capturing something different. Ecologists use indices such as the Shannon index, which combines the number of species present with how evenly they are distributed, to describe the diversity of vegetation in a landscape. Microbiologists, meanwhile, quantify bacterial richness—the sheer number of distinct bacterial types—and bacterial abundance, and they apply these measures both to environmental samples and to the microbial communities living in and on the human body. Because each metric reflects a different facet of biodiversity, and because different studies analyze their data with different statistical approaches, the resulting findings can point in seemingly contradictory directions. Reconciling them requires the kind of formal synthesis that only a meta-analysis can provide.

That is precisely what the Finnish-led team undertook. The researchers systematically assembled the empirical studies that had examined the association between exposure to environmental and human microbiota biodiversity and the development of asthma, and then combined their effect estimates using well-established statistical methods. For studies reporting biodiversity effects in comparable units, they calculated the standardized mean difference using the Hedges method, a technique that corrects for small-sample bias when pooling differences between groups. For studies reporting the likelihood of asthma outcomes, they summarized relative risks and odds ratios using the restricted maximum likelihood estimator, a random-effects approach that accounts for variability between studies. Confidence intervals of 95 percent were attached to each summary estimate, and the team formally assessed heterogeneity—the degree to which individual studies disagreed—and the potential for publication bias, the tendency of journals to favor studies that report statistically significant findings.

The results, published on 7 October 2026, are nuanced rather than triumphant. When environmental biodiversity was measured using the Shannon index of vegetation diversity, the summary effect estimate for the association with asthma was not statistically significant. In other words, across the pooled studies, living near more diverse vegetation did not translate into a clear, statistically detectable difference in asthma risk. This is a striking result, because the green-space narrative has become one of the most popular explanations for the rise of allergic disease in cities. The meta-analysis suggests that the simple story—more plant diversity, less asthma—does not hold up uniformly when all the evidence is weighed together.

Yet the picture changed when the researchers turned to a different biodiversity metric: bacterial richness in the environment. Here, the pooled evidence pointed to a significant protective association, meaning that greater environmental bacterial richness was linked to lower asthma risk. That finding aligns closely with the mechanistic core of the biodiversity hypothesis, which holds that exposure to a wide array of environmental microbes is what trains the immune system. But the authors are careful to temper the enthusiasm. The studies contributing to this estimate displayed considerable heterogeneity, meaning they varied substantially in their populations, settings, and methods, and the team also detected evidence of publication bias. Both issues caution against interpreting the protective signal as definitive; it is a promising lead, not a prescription.

The most surprising twist concerns the human microbiota itself. When the researchers summarized the standardized mean differences for human bacterial diversity and for bacterial richness and abundance, the estimates suggested that a high diversity and richness of bacteria in the human microbiota were associated with an increased risk of asthma—a direction opposite to what the biodiversity hypothesis might naively predict. However, when the same relationship was analyzed using odds-ratio-based estimates for bacterial richness and asthma, the association was not statistically significant. The discrepancy between the two analytical approaches underscores how sensitive conclusions in this field are to the choice of metric and statistical method, and it raises intriguing questions about whether the composition of the body’s microbial communities is a cause of asthma, a consequence of the disease or its treatment, or a correlate of other underlying factors.

Taken together, the meta-analysis delivers a clear methodological message: the relationship between biodiversity and asthma is real but variable, with both the direction and the magnitude of associations depending on the specific biodiversity metrics and analytical approaches used. Environmental bacterial richness emerges as the most consistent protective signal, while vegetation diversity indices and human microbiota measures yield mixed or even contradictory results. This variability is not a failure of the research program; it is a map of where the field needs to go. Future studies, the findings imply, should be designed with standardized biodiversity metrics, prospective cohorts that can distinguish cause from effect, and analyses that account for the confounding influence of urbanization, air quality, and socioeconomic factors.

The implications reach well beyond academic debate. If environmental bacterial richness genuinely protects against asthma, then urban planning could become a form of preventive medicine: designing cities with biodiverse green infrastructure—soils, vegetation, and water features that cultivate rich microbial communities—might reduce the burden of chronic respiratory disease. At the same time, the finding that human microbiota diversity can show a positive association with asthma risk warns against simplistic interventions such as indiscriminate probiotic supplementation or blanket exposure campaigns. The immune system’s relationship with microbes is a matter of balance and specificity, not sheer quantity.

What makes this study resonate beyond the asthma literature is its framing of biodiversity loss as a public health issue of the same magnitude as climate change itself, and one deeply intertwined with it. The same forces that are driving species to extinction and simplifying ecosystems are, by this evidence, reshaping the microbial landscapes that our immune systems depend on. The meta-analysis does not close the case on the biodiversity–asthma link, but it does something arguably more valuable: it clarifies which threads of evidence are strong, which are fragile, and which point in unexpected directions. In a century defined by environmental upheaval, understanding precisely how the living world’s diversity shapes human health may prove to be one of the most consequential questions in medicine—and this analysis marks a rigorous step toward answering it.

Subject of Research: The association between environmental and human microbiota biodiversity and the risk of asthma

Article Title: Meta-analysis of empirical evidence on biodiversity and the risk of asthma

Article References: Paciência, I., Sharma, N., Hugg, T. T., Rantala, A. K., Al-Delaimy, W. K., Jaakkola, M. S., & Jaakkola, J. J. K. (2026). Meta-analysis of empirical evidence on biodiversity and the risk of asthma. Nature Communications. https://doi.org/10.1038/s41467-026-77643-5

Image Credits: AI Generated

DOI: 10.1038/s41467-026-77643-5

Keywords: biodiversity, asthma, meta-analysis, microbiota, bacterial richness, Shannon index, immune regulation, urbanization, climate change, green space, chronic inflammatory disease, public health