Population Growth, Urban Intensification, Mice and Rat Studies, Human Mental Health and the Climate

Back in the 1960s, John Calhoun, a behavioural scientist, studied the effects of population density on mice and rats. When living in crowded circumstances, they exhibited antisocial behaviours that led to social disintegration and population collapse. Calhoun’s findings were published in a 1962 issue of Scientific American and inspired dystopian science fiction like Soylent Green.

Recently, reinterpretations of Calhoun’s data conclude that humans in similar circumstances might not experience what happened to the rat and mouse populations when crowded into confined spaces. That’s because some mice and rats didn’t change their behaviour. These particular subjects carved out sufficient space around them for family members and themselves. The ones that didn’t, however, displayed declining mental health, aggression, and antisocial behaviours.

As we battle climate change, urban planners are increasing population densities in city cores, building up rather than out. They are doing this to mitigate global warming through energy conservation, and minimizing the carbon footprint from transportation. Rather than use a polluting car, people can ride electrified mass transit or walk. But can urban crowding lead to overcrowding to produce results similar to Calhoun’s experiment in people?

A paper appearing in the December issue of the journal, Environmental Research, entitled “The evaluation of the 3-30-300 green space rule and mental health” describes an observed correlation between mental well-being and the presence of green space and green views.

An urban forester, Cecil Konijnendijk, has proposed a 3-30-300 rule which states that where people live they should:

• see at least three trees from a window in their home.
• reside in neighbourhoods with at least 30% tree canopies.
• have parks or green spaces no more than 300 metres (1,000 feet) away from their front door.

The study refers to the 3-30-300 rule in looking at a select population in the City of Barcelona, Spain. Involving 3,145 individuals between the ages of 15 and 97, the study compares mental well-being with the proximity of green views and green space. Why was Barcelona chosen? Because it has very little of either so applying the 3-30-300 rule was easy to differentiate the healthiness of the population. It turned out there was a significant correlation between citizens who lived in the few 3-30-300 areas and their use of fewer tranquillizers, sedatives or antidepressants than those living in areas with little in the way of green. These 3-30-300 people made fewer visits to psychologists and psychiatrists, lived healthier lives, experienced longer life expectancy, and had better cognitive function.

Is this a breakthrough study? It isn’t the first to look at the benefits of more green space. it isn’t the first to look at urban tree canopy coverage and overall health. But it is the first to look at both of those issues and combine that with green views from windows. It seems apt when you consider during the COVID-19 pandemic, how many people found themselves confined to their homes with only a window to look out of to see the world. Studying COVID-19 confinement and depression in the absence of green views or nearby green spaces will probably reinforce the findings of this Barcelona study.

Municipal Planners Take Note

What should this mean for municipal planners? Urban green spaces matter. Urban green spaces lead to healthier populations, reduce air pollution, lower greenhouse gas emissions, and are quieter. The problem, of course, is that urban land is expensive. As a result, green spaces get minimized when competing with other forms of land use. Central business districts in so many cities to this day are often treeless expanses of concrete, brick and glass. Trees, grass and flowers, take a back seat to buildings and other urban infrastructure.

Take the case study of my hometown, Toronto. It is Canada’s largest city with 2.7 million people and growing by 100,000 annually. The city has changed since my family moved here from Montreal in the 1950s when I was a little boy. Even back then Toronto had an extensive urban forest outside the downtown core because of the many ravines and small rivers that cut across the landscape. But the Central Business District (CBD) was far from green and remains so to this day. Back then people were moving uptown and away from the urban core. Today, the movement is reversed with tens of thousands of Torontonians living in condominium apartments in the heart of downtown.

The architects of these new homes submit plans usually with little interest in green views or nearby green space. For a time the less-than-green developers were largely left to proceed to create vertical glass and steel neighbourhoods separated by open courtyards of concrete with occasional planter boxes and small patches of lawn. But today, Toronto city planners are pushing back against the maximal land use view to shove as many people into as little space as possible without considering trees, grass and flowers. The ongoing tug-of-war between developers and urban green planners is cyclical. Right now, because of climate change, green is on the ascendant.

So what is Toronto’s current green status? A 2018 study states the forest canopy covers between 28.4 and 31% of the city. The estimated tree population is 11.5 million. That’s more than 4 per person, bettering the 3 in the 3-30-300 rule. And Toronto’s plan is to grow the canopy to cover 40% by 2050.

The plan, to date, makes no mention of mental health and well-being as the reason for planting more trees and creating more green spaces. Instead, the reasons given are:

• trees absorb water to stop floods.
• trees clean the air of pollution.
• trees provide shade to deal with the urban heat-island effect.
• trees reduce erosion for soil conservation
• trees reduce wind tunnel effects caused by buildings.
• trees decrease the energy needed for heating and cooling.

All of these are great rationales for increasing the tree canopy. But someone in urban planning, now should look at the 3-30-300 rule and apply it to strengthen the mental well-being of Torontonians. We don’t want to be the mice and rats of Calhoun’s study. We don’t want a dystopian future. Living in harmony with trees sounds like a plan.