The term “biodiversity” describes the diversity of living beings on Earth, the way they form communities, and their interactions with their ecosystems.104 Around the world, we are seeing a massive decrease in biodiversity, which is something that refers to loss of species, genetic losses, and the loss and degradation of natural habitats. The Intergovernmental Science Policy Platform on Biodiversity and Ecosystem Services (IPBES) estimates that 25% of animal and vegetable species are threatened with extinction. The current global extinction rate is already estimated to be tens, or even hundreds of times higher than the average over the past 10 million years.105 According to the latest Living Planet 2020 report published by the WWF and the London Zoological Society, global populations of mammals, fish, amphibians, and reptiles have diminished 68% the biomass of insects and the diversity of field species has diminished by more than 67 and 34%, respectively. In forests, the losses measured are of 41 and 36% respectively.107
Overall, the causes of these losses are complex and include changes in land use, the degradation of natural habitats, as well as the introduction of invasive species and diseases, the over-exploitation of natural resources, climate imbalance, and ecosystem pollution. The mass erosion of biodiversity compromises the ability of nature to provide the ecosystem services that life, including human life, depends on to live healthily. These services include, for instance, food and energy provision, as well as climate regulation. For instance, over 75% of world food crops rely on animal pollination. 105 The decline in insect populations observed risks having potentially disastrous consequences on food production. The loss of biodiversity is one of the planetary limit thresholds that is seen as having been crossed.5
Furthermore, the Covid-19 health crisis serves as a reminder of the links there are between disturbing natural ecosystems and increased risks of zoonosis. While these links remain complex and insufficiently understood, there is some proof that destroying natural habitats and changing the usage of ground soil have a destabilizing effect on the interactions between different species, including on those of host organisms and pathogenic vector species, and that therefore doing so risks contributing to the emergence of zoonotic diseases.108
Generally, if there is no wider awareness of the rhythm and consequences of the loss of biodiversity, and if drastic measures are not adopted, then the collapse of biodiversity will go on accelerating.105
Recognizing and better establishing the links between the integrity of ecosystems and human health could enable us to strengthen natural space and biodiversity preservation efforts.109-115 On the macro level, this is especially a matter of recognizing the total dependence of humans on ecosystem dynamics and the irreplaceability of these benefits.
On a more micro level, it begins by recognizing the benefits of contact with nature for individuals and populations. For instance, spreading awareness of the positive impact that green spaces and urban gardens have on the health of individuals and populations could encourage such spaces being incorporated into urban planning, thereby also offering sanctuaries for biodiversity.
Many studies, including qualitative ones, suggest that contact with natural environments is associated with both short- and long-term benefits on human health. For instance, a systematic review and meta-analysis conducted in 2018 suggests a relationship between green spaces and reducing blood pressure, heartrate, salivary cortisol, instances of type II diabetes and strokes, mortality all causes combined and cardiovascular mortality, and that they also have positive effects on pregnancy, cholesterol levels, and self-reported healthiness.116 Another systematic review published in 201817 that studies exposing children to nature shows doing so has a positive effect on mental health (stress, ADHD…). The authors note, however, that the heterogeneity of the different methods of evaluating contact with nature in the studies considered makes analyzing the results more complicated. A study by Engemann et al.118 cross-references data from national psychiatric registers and environmental remote sensing to show that growing up near non-built-up areas (farmland, green spaces, water), rather than in urban areas, when socioeconomic factors are taken into account, is associated with reduced rates of schizophrenia.
Various mechanisms contribute to the benefits green spaces can have on human health.109,111,119-123 By providing pleasant, calm spaces, as well as meeting points, natural spaces, parks, and urban gardens offer spaces that are ideal for physical activity and leisure activities, thereby contributing to reinforcing social capital.124,125 Natural spaces, or natural components, can also reduce perceived exposure to stressful environment. For instance, a forest can increase the physical or visual distance from a source of stress, like an industrial space or a highway, or diminish sound pollution perception.72,119 In urban areas, green spaces limit heat island effects, thus improving quality of life during heatwaves.
Natural spaces nonetheless seem also to have features that are intrinsically beneficial to health. Studies show, for instance, that physical activity conducted in a natural space is more beneficial than a similar activity that is carried out within a synthetic environment.123,126
Theories that take a psycho-evolutional approach offer various explanation for this phenomenon. The concept of “biophilia” describes a supposed inherent tendency in human beings to connect with parts of nature or natural processes. These links supposed-ly originate in evolutionary adaptations to natural environments in which humans have evolved. According to this theory, while this biological tendency is nowadays largely influenced by life experiences and socio-cultural input, it can explain the innate attention we pay to aspects of nature such as light, wind, smells, sounds, landscapes, and animals.127 Attention restoration theory suggests that natural landscapes are replete with things that encourage the recovery of concentration and focused attention-related cognitive abilities. This while offering fascinating features that require little focused attention, allowing us to recover our concentration abilities.128 Another theory, stress restoration theory, suggests that humans are predisposed to feeling positive emotions and having aesthetic experiences when presented with certain natural features, and that these positive feelings encourage stress reduction.129
Araujo et al.126 suggest that the particular benefit of physical activities conducted in natural environments has to do with the relationship that binds individuals to their environment. Individuals must continually adapt their movement to the inherent variability of the context of nature, choose what opportunities to seize, adapt to the wealth of information being received. This dynamic relationship requires individuals to engage on a sensory, emotional, bodily, and cognitive level with an environment that isn’t present in, say, gyms.
The latter provide uniform spaces, where social norms regulate the use of the space and of its tools, thereby offering users few opportunities for adaptation and creativity.
Just as a global decline in biodiversity is being documented, an emerging field of study is pointing to the role biodiversity plays in regulating the human immune system. Numerous studies draw a link between reduced exposure to microbial biodiversity, a reduction of the diversity in or an unbalance within human microbiota, and the growing prevalence of allergies and chronic inflammatory diseases in urban populations around the world.109,113,115,130-132 It is with this in mind that T. Haahtela has called upon the medical community to seriously consider the loss of biodiversity and the risks to immunoregulation that presents.115 He mentions an “adaptation crisis” for the immune system, which is struggling to adapt to the lightning-fast loss of biodiversity on a global scale and to lifestyle changes.
The human microbiome is closely tied to the microbial biodiversity that humans encounter in their environment, beginning with childhood and continuing throughout their lives. Exposure to certain micro-organisms in the environment allows the immune system to be stimulated and to modulate its tolerance.115,130,131 Regulating the immune system through biodiversity is thus described as an ecosystem service that is both crucial for health and insufficiently acknowledged.113
While the loss of biodiversity is a global phenomenon, urban environments in western countries are experiencing a particularly significant diminishing of microbial diversity. The “biodiversity” and “old friends” hypotheses particularly note the disappearance of certain specific micro-organisms that have evolved alongside humans.
In high-revenue countries, opportunities for close contact with animals, plants, and soil are significantly less than they used to be. Furthermore, natural construction materials, such as wood, wool, and clay, have been replaced by synthetic materials, which are often treated with biocides, and that are therefore not colonized by the same species that humans used to be regularly exposed to.113 These trends are reinforced by the loss of biodiversity in rural areas that is linked to the use of pesticides and the prevalence of agricultural monocultures.113,136
It is worth noting that the process that rules the relationship between human microbiota and environmental biodiversity remains poorly understood. A better understanding is needed in order to encourage lifestyles, living spaces, and urban spaces that offer healthy exposure to a certain amount of biodiversity, in order to reduce immune system pathology, all while avoiding allowing infectious diseases to develop.134,135
These various mechanisms are of course not mutually exclusive. The multiple benefits on health of contact with nature probably stem from a combination of effects, which can have short- or long-term consequences, which are difficult to measure and that it can be hard to untangle when researching the correlation between health and contact with nature119,123 (figure 8). This problem is rein-forced by the difficulty of defining “nature” and “contact with nature” and because of the fact that the experience of nature is partially subjective and replete steeped in cultural constructs, personal values, and aesthetic appreciations that can make results biased.
Humans depend on natural ecosystems to feed themselves. And yet, western societies have largely lost their connection to the soil and to food production, particularly with the advent of the food-processing industry, supermarkets, and ultra-processed foods. Community gardens and other forms of urban agriculture can contribute to reversing this trend, by making people more familiar with the earth, its dependence on the weather, the natural rhythm of plants, and how some ecosystems operate. All of these aspects can contribute to reinforcing contact with nature, developing a different relationship to food, reducing food waste, encourage consuming local, unprocessed produce, and thereby to reducing greenhouse gas emissions related to transportation, processing, and storage.
Community gardens have known benefits on individuals and populations.137,138 Beyond the health reasons, people’s primary motivation for getting into gardening is to have a pleasant, outdoor activity they can commit to, one that can occasionally provide food and that helps them evade stress. Having a concrete and tangible goal to focus on – producing food, for instance – as well as the aesthetic experience that gardens provide both contribute to creating meaning, satisfaction, a feeling of pride, and values for gardeners, and thus encourage feelings of well-being.124,137 Community gardens also contribute to strengthening social bonds and community involvement, by providing spaces to interact with close-ones and other gardeners in. 124,125,136,137 These social and emotional benefits, coupled with the benefits that come with physical activity and eating fresh, unprocessed food, are good for both physical and mental health. 137,138
Recent studies establish a link between immune system malfunction and less contact with biodiversity, due to lifestyles and a global reduction in biodiversity.113,115,130-133 Urban gardens can contribute to enriching urban biodiversity, providing an opportunity for people to come into contact with said biodiversity, and thus allowing the human microbiome to be enriched by it. This applies particularly to high-revenue countries where the impoverishment of the environmental and human microbiome is especially significant.
Numerous mental health interventions have shown that encouraging patient contact with nature had a beneficial effect on their health.139-141 One review Cochrane, showcases the overall benefit to health and well-being of conservation or environmental preservation activities out in the middle of nature (e.g.: vol-untarily picking up waste, planting trees, pathway maintenance).142 Qualitative studies have identified many mechanisms that contribute to improving the health and well-being of participants: an excuse to engage in a physical activity, better connection to the local environment and nature, taking some distance from stress factors, reconciling oneself with one’s spiritual values, better social relations, the feeling of belonging to a community, greater self-confidence, and better self-esteem by performing meaningful actions. More generally, many studies show the benefits to health and well-being of contact with nature, even though the data is essentially qualitative in nature and that the quantitative evidence surrounding these mechanisms often remains insufficient.
In an article entitled “The Evidence of Nature and the Nature of Evidence,” Frumkin highlights the huge opportunities in terms of preventing illness and promoting health that the many different experiences of contact with nature have to offer (figure 8). While calling on others to persevere with their research, he nonetheless questions the limitations of biomedical evidence in this area.120,122 We offer a synthesis of the overall co-benefits approach to contact with nature in figure 9.
• 25 % of plant and animal species are threatened with extinction.
• The global extinction rate is tens, if not hundreds of times higher than the average over the past 10 million years.
• Global mammal, fish, amphibian, and reptile populations have decreased by 68% since 1970.
• There are complex links between the decrease in biodiversity and the thinning out of the human microbiome (intestinal and skin flora, etc.)
• The level of exposure to biodiversity is in inverse correlation to the risk of developing allergies.
• Contact with nature has a measurable positive impact on the physical and mental health of humans.