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Restoration aims to return ecosystem services, including the human health benefits of exposure to green space. The loss of such exposure with urbanization and industrialization has arguably contributed to an increase in human immune dysregulation. The Biodiversity and Old Friends hypotheses have described the possible mechanisms of this relationship, and suggest that reduced exposure to diverse, beneficial microorganisms can result in negative health consequences. However, it is unclear whether restoration of biodiverse habitat can reverse this effect, and what role the environmental microbiome might have in such recovery. Here, we propose the Microbiome Rewilding Hypothesis, which specifically outlines that restoring biodiverse habitats in urban green spaces can rewild the environmental microbiome to a state that enhances primary prevention of human disease. We support our hypothesis with examples from allied fields, including a case study of active restoration that reversed the degradation of the soil bacterial microbiome of a former pasture. This case study used high‐throughput amplicon sequencing of environmental DNA to assess the quality of a restoration intervention in restoring the soil bacterial microbiome. The method is rapid, scalable, and standardizable, and has great potential as a monitoring tool to assess functional outcomes of green‐space restoration. Evidence for the Microbiome Rewilding Hypothesis will help motivate health professionals, urban planners, and restoration practitioners to collaborate and achieve co‐benefits. Co‐benefits include improved human health outcomes and investment opportunities for biodiversity conservation and restoration.

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Ecological restoration is a globally important and well-financed management intervention used to combat biodiversity declines and land degradation. Most restoration aims to increase biodiversity towards a reference state, but there are concerns that intended outcomes are not reached due to unsuccessful interventions and land-use legacy issues. Monitoring biodiversity recovery is essential to measure success; however, most projects remain insufficiently monitored. Current field-based methods are hard to standardize and are limited in their ability to assess important components of ecosystems, such as bacteria. High-throughput amplicon sequencing of environmental DNA (metabarcoding of eDNA) has been proposed as a cost-effective, scalable and uniform ecological monitoring solution, but its application in restoration remains largely untested. Here we show that metabarcoding of soil eDNA is effective at demonstrating the return of the native bacterial community in an old field following native plant revegetation. Bacterial composition shifted significantly after 8 years of revegetation, where younger sites were more similar to cleared sites and older sites were more similar to remnant stands. Revegetation of the native plant community strongly impacted on the belowground bacterial community, despite the revegetated sites having a long and dramatically altered land-use history (i.e. >100 years grazing). We demonstrate that metabarcoding of eDNA provides an effective way of monitoring changes in bacterial communities that would otherwise go unchecked with conventional monitoring of restoration projects. With further development, awareness of microbial diversity in restoration has significant scope for improving the efficacy of restoration interventions more broadly.

A relatively unaccounted ecosystem service from biodiversity is the benefit to human health via symbiotic microbiota from our environment. This benefit occurs because humans evolved alongside microbes and have been constantly exposed to diverse microbiota. Plants and animals, including humans, are organised as a host with symbiotic microbiota, whose collective genome and life history form a single holobiont. As such, there are interdependencies between biodiversity, holobionts, and public health which lead us to argue that human health outcomes could be improved by increasing contact with biodiversity in an urban context. We propose that humans, like all holobionts, likely require a diverse microbial habitat to appropriate resources for living healthy, long lives. We discuss how industrial urbanisation likely disrupts the symbiosis between microbiota and their hosts, leading to negative health outcomes. The industrialised urban habitat is low in macro and microbial biodiversity and discourages contact with beneficial environmental microbiota. These habitat factors, alongside diet, antibiotics, and others, are associated with the epidemic of non-communicable diseases in these societies. We suggest that restoration of urban microbial biodiversity and micro-ecological processes through microbiome rewilding can benefit holobiont health and aid in treating the urban non-communicable disease epidemic. Further, we identify research gaps and some solutions to economic and strategic hurdles in applying microbiome rewilding into daily urban life.

Principles of ecology apply at myriad scales, including within the human body and the intertwined macro and microscopic ecosystems that we depend upon for survival. The conceptual principles of dysbiosis (‘life in distress’) also apply to different realms of life—our microbiome, the macro environment and the socioeconomic domain. Viewing the human body as a holobiont—a host plus billions of microbial organisms working symbiotically to form a functioning ecological unit—has the potential to enhance personal and planetary health. We discuss this ecological perspective in our paper. We also discuss the proposals to rewild the microbiome, innovative microbiome-inspired green infrastructure (MIGI) and the basis of prescribing ‘doses of nature’. Particular emphasis is given to MIGI—a collective term for the design and management of innovative living urban features that could potentially enhance public health via health-inducing microbial interactions. This concept builds upon the microbiome rewilding hypothesis. Mounting evidence points to the importance of microbial diversity in maintaining favorable health. Moreover, connecting with nature—both physically and psychologically–has been shown to enhance our health and wellbeing. However, we still need to understand the underlying mechanisms, and optimal types and levels of exposure. This paper adds to other recent calls for the inclusion of the environment-microbiome-health axis in nature–human health research. Recognizing that all forms of life—both the seen and the unseen—are in some way connected (ecologically, socially, evolutionarily), paves the way to valuing reciprocity in the nature–human relationship. It is with a holistic and symbiotic perspective that we can begin to integrate strategies and address connected issues of human and environmental health. The prospective strategies discussed in our paper focus on enhancing our connections with the natural world, and ultimately aim to help address the global challenge of halting and reversing dysbiosis in all its manifestations.