Nature is good for you. It relieves stress and low mood, gets children moving and boosts the immune system. When the natural environments of our planet are feeling unwell, does our health suffer?
A connection to nature, preferably a strong connection to the abundant diversity of life, is good for you. The human gut and skin host an enormous quantity of microbes that affect our health, mind and development. Some of these microbes we inherit from previous generations, and some we acquire ourselves. Thanks to microbes, humans are part of their environment.
The lion's share of them are harmless or useful, making our mutual interaction worthwhile. A 'biodiversity hypothesis' developed in a research project headed by Professor Emeritus Tari Haahtela and the late Academician Ilkka Hanski claims that diverse exposure to natural microbes helps prevent, among other things, the onset of allergies. The hypothesis has been successfully tested among people living in North Karelia in Finland and across the border in the Russian Republic of Karelia. In the latter region, allergies are rare, with the microbial flora of people's skin being considerably richer.
Natural microbes in the skin keep the immune system fighting fit, whereas the absence of a connection to nature may cause the body to respond violently to otherwise harmless instances of exposure. Professor Harri Alenius, who heads the Human Microbiome research programme at the University of Helsinki, has described the phenomenon this way: "The immune system is like a pack of boys. The devil finds work for idle hands."
Research-based knowledge has brought about a turnaround in allergy recommendations. Instead of sterility and avoidance, appropriate sensitisation, normal living and a connection to nature are key - having soil under the fingernails is good for you.
The pandemic is a crisis of diversity
When references to the threat posed by the novel coronavirus began appearing in the media two years ago, the loss of biodiversity was fairly quickly highlighted as one of the causes of the outbreak. As is the case with many other causes of infectious diseases affecting humans, SARS-CoV-2 is a zoonotic virus, which means that it is transmitted between different animal species.
At the same time, pathogens too are an element of biodiversity. It would be easy to think that more diversity in nature would also increase the number of pathogens and other species that can infect humans. In principle, that is the case, but according to evolutionary biologist and researcher Tuomas Aivelo, not quite in practice.
"Actually, when habitats are destroyed to make room for human life, wildlife encounters people and domestic animals increasingly often. The buffers are missing," Aivelo notes.
Such buffers include space reserved for animals and other species. For instance, the Ebola epidemic of 2013 that originated in West Africa was traced back to a hollow tree on the edge of a village. The tree was inhabited by bats, which may have transmitted the virus to a boy playing in the tree. In turn, deforestation makes bats live closer and closer to human settlements.
"Some people have posited that greater biodiversity provides pathogens and parasites with a large number of potential carriers, reducing the risk of transmission to humans. It is as if transmission is diluted in a large group of potential carriers," Aivelo says.
This 'dilution effect' has been controversial, since it has been linked to ticks and borreliosis - namely, that reduced numbers of bank voles and other carriers would increase the number of infected humans. According to Aivelo, the dilution effect makes a difference in some cases, even if not in the case of ticks.
As animal and plant numbers dwindle, the microbes and parasites - Aivelo's primary research subjects - that depend on them also suffer. The endangerment of parasites too can be an unexpected health risk.
That is to say, the parasites that specialise in a specific species are the most vulnerable to the dwindling of host species populations. Generalists that thrive as parasites in a number of organisms fare better even if the population of an individual species collapses.
"These generalist parasite species are usually the ones that are also capable of jumping between species," says Aivelo.
In many cases, the presence of specialised parasites prevents generalist parasites from succeeding, which is why certain parasites are relevant with regard to risks associated with infectious diseases.
A diverse and healthy forest
Nearly a thousand forest species in Finland are endangered. To a nature hiker, this is first apparent through observations: there are fewer bullfinches and willow tits. In contrast, the profound effects of biodiversity remain unseen. Such as the buzz taking place in the root systems. The more mycorrhizal fungi a tree lives in symbiosis with, the healthier it is and the better it grows.
"Without mycorrhizal fungi, trees would not be able to extract much nutrition from the rocky Finnish soil," says Fred Asiegbu.
Asiegbu, a professor of forest pathology at Viikki Campus, studies root rot, a disease that spreads from pine and spruce stumps, causing losses of €50 million to Finnish forestry each year.
"The problem has been generated by forestry. If the diversity of tree species is reduced, diseases will spread.
In old diverse forests, root rot is unable to spread uncontrollably. In other words, with aspens and birches growing between spruces, the disease is not transmitted as effectively as in a single-species forest. Unfortunately, diverse cultivation is not optimal for forestry. Indeed, Asiegbu's research group strives to develop various methods for controlling the disease: chemical pesticides, forestry practices - and natural microbes.
"Our group found that a specific endophyte, that is, a microbe living in plants, found in the wild effectively inhibits the symptoms associated with root rot. Now, the group is investigating whether transmitting this microbe to saplings could be utilised as a means of preventing root rot," Asiegbu says.
Planetary health
Academy of Finland Research Fellow Riikka Puhakka is not a microbiologist, but she participated in the ADELE study investigating the exposure to nature of children in daycare. With more natural elements in the kindergarten yard, there were more beneficial microbes on the children's skin, but a connection to nature engenders more than just immunity.
"We also found that by bringing forest humus and transferable lawn to the yard, the children's games became more diverse and they got more physical exercise," Puhakka says.
In recent years, researchers in a range of fields have started talking about 'planetary health'. Human health is not a matter for the individual alone, but a process involving the environment.
"The status of the environment and human wellbeing are integrally linked," Puhakka sums up.
Puhakka is currently investigating this process in NATUREWELL, a project carried out at the Lahti University Campus that surveys young people's links to nature. The results obtained from interviews and questionnaires are interesting:
"They are not afraid of nature. And even though young people are said to be increasingly alienated from nature in public discussion, for most of them, nature and outdoor activities are important," Puhakka points out.
The accessibility of diverse natural environments and early experiences of nature emerge as important aspects of this link. Besides immunity, nature enhances mental wellbeing.
A connection to diverse nature is not an alternative miracle cure that can be used to replace Western medicine. Taking heed of research in agriculture and forestry can offer protection against pandemics and keep the environment vital for us and for future generations. Understanding the development of immunity could guide urban planning and public construction.
"At least daycare centres and schools in particular are places where a lot of good can be achieved through educational and environmental design," Puhakka says.
Diversity in general equals tolerance and toughness, or resilience, for both nature and humans. And there are differences between forests, too. Riikka Puhakka brings up a study conducted by Natural Resources Institute Finland, where old natural forests were found to promote recovery from stress and stimulate the mind even more effectively than small urban forests or young commercial forests.
That being said, the latter also make you feel good. And there is research evidence to back up that claim.
