Nature is our evolutionary home. A few theories on how biodiversity impacts our health.

Nature is our evolutionary home. Our species lived closely & dependably on it for millennia. The last 150 years changed our relationship a lot. Nature became more distant to the population due to radical urbanisation. The developments in hygiene reduced our exposure to all bacterias and viruses. At the same time, the loss of biodiversity and fragmentation of natural habitats in the environment took up speed. Socio-economic development increased the volume of pollutants in the environment. One could conclude that the immediate living environment in urban centres is quite sterile by now. In 2011, Leena von Hertzen, Ilkka Hanski & Tari Haahtela introduced the idea of 'natural immunity' that was developed later into Biodiversity Hypothesis of Health. The concept has a close connection to hygiene hypothesis, which was developed earlier, although, in my view, it does not reflect well the current evidence.  

The biodiversity hypothesis states that, reduced contact of people with natural environment and biodiversity may adversely affect the human commensal microbiota and its immunomodulatory capacity (von Hertzen, Hanski & Haahtela, 2011).

They argue that our immunity is closely connected to biodiversity of the natural world. Biodiversity loss, observed in recent decades, leads to an increase in immune dysfunction and disease. The conclusions were based on observational trends in asthma and allergic diseases, autoimmune diseases and many cancers (Figure below).

Figure: (von Hertzen, Hanski & Haahtela, 2011)Two global megatrends in biodiversity and public health. (A) Declining biodiversity since 1970 as measured by three indices. LPI, Living Planet Index; WBI, World Bird Index; WPSI, Waterbird Population Status Index. (B) Increasing trends in the prevalence of inflammatory diseases. Asthma and allergic rhinitis among military conscripts from 1966 to 2003.

So how is the health of our bodies connected to biodiversity in nature? The short answer is - microbes. Our body is constantly exposed to environmental and indigenous microbes through our respiratory tract, skin and guts. Their role is fundamental to our immunological tolerance and tissue integrity. According to the Biodiversity Hypothesis (Ruokolainen, Fyhrquist & Haahtela, 2016), it is essential to the healthy development of our immune system that we are sufficiently exposed to the diverse natural environments and microbes in them. They influence the development of the microbial community of our organism. The exposure 'trains' microbes in helping the immune system to identify actual threats from harmless allergens. While we already have the evidence for the relationship, the mechanism itself is still to be fully understood. Tari Haahtela has recently published (Haahtela, 2019, figure above) a comprehensive review of the current state of the research, referring also to longitudinal studies. The research focused primarily on allergies and inflammatory disorders as those highly dependent on microbial biodiversity. 

The evidence is building up from a developed world that our isolation from natural biodiversity is a double-edged sword. On the one hand, we are isolating from potentially harmful problems however, at the same time we are depriving our immunity of a change to build a tolerance to the majority of harmless elements.

Another perspective on biodiversity and health is the impact nature has on our mental health. One such point on influence would be the aforementioned exposure to biodiverse environment by gut-brain axis (Figure to right: The gut-brain axis. Carabotti et al.,2016). It is a relatively fresh discovery, but the one suspected for years: our guts are sensitive to emotions, but the connection goes both ways. The interaction is also dependent on the gut biota connecting the notion hypothetically to the Biodiversity hypothesis. 

Such case was made by Prescott et al. in 2016 paper connecting biodiversity to microbiota and mental well-being. In this perspective, since we are investigating also microorganisms present in humans, we are basically treated as multi-species organisms connected to local biodiversity which in turn affects health (with a focus on mental wellbeing). From this bio-psychological point of view, authors are calling the ongoing research an emergence of clinical ecology.

Looking at the bigger picture we can also think about the mental health effects of exposure to nature. Environmental psychology and neuroscience often base the conceptual framework of Biophilia Hypothesis. It assumes that humans have an innate tendency to favour and seek connections with nature and organic forms. In 2009 Mass et al., concluded that there is a relationship between how close to green spaces people live and the reports of anxiety and depression. In 2011, Julie Deana, Kate van Doorena and Philip Weinstein put forward the hypothesis that increased biodiversity in urban environments is associated with improved mental health and wellbeing. They have indicated that more research is desperately needed to prove the link.

Since then multiple studies analysed the problem from different angles: the mental restorativeness of the natural environment, developments in brain dysfunction & psychiatric disorders, the developmental mental resilience in children, impact of nature of sensory perception. The fundamental goal of the research in brain and mental health is to establish if and how the decline in biodiversity of the environment will impacts population. The special emphasis is put on urban clusters like those in a high chance of lack of access to a natural environment. 

Nature gave us a couple of 'yellow cards' in recent years as to our mutual relationship being abusive from our side. There is a common consensus that a lot needs to change in the nature of this relationship. The rapid spread of coronavirus and its still hypothesised possible source, made public opinion question if our (too) close relationship to nature is a factor in the spread of multiple contagion diseases in recent decades. However, is the radical suggestion to minimise contact with the natural environment in order to prevent a future pandemic, truly feasible? The explosion of human populations is profoundly changing environment and our lives. Are we too close to nature or too distant from it? It seems that the problem is multi-facet and complex and cannot be boiled down to simple solutions. Let us hope that ongoing research can inform the strategies and policies so the relationship could turn to be mutually rewarding.

IMS,

PS. All references are linked to the sources.

PPS. It is my personal, popular blog post intended to be informative with selection of review literature and peer-reviewed research papers. My current research project is closely related to this topic. I'm happy to answer all questions and provide more literature to those interested.

PPPS: Disclaimer: it represents my own, personal views.