Diversity as a key element of urban resilience

As climate change increases the frequency of storm events, flooding risk, prolonged drought, and other hazards, enhancing resilience becomes the goal of cities worldwide. How can societies prepare for and address a multitude of disturbances that can threaten the functioning of a city? A perhaps-unlikely answer is to increase diversity in all its forms. Economic, industrial, environmental, administrative, institutional, and sociocultural diversity are intertwined in cities, and diversity in all of these systems enhances resilience.

In the ecological sciences, increasing biodiversity is the best strategy for achieving resilience. When there are multiple species it is more likely that the extinction of some species will not completely change the state of an ecosystem. As one species becomes extinct, another similar one fills that niche, performing the same ecological functions of the extinct species and preserving the functioning of the entire ecosystem. In this sense, a system is resilient when the functioning of the system continues in spite of disturbances.

But ecological systems have been subject to disturbances throughout millennia – species go extinct all the time. Changes in state means that an ecosystem transforms into another ecosystem, but life continues after all, albeit in another state. For example, a forest burns and becomes a grassland, or a grassland is overgrazed and becomes shrub. However, in social systems, disturbances usually mean people dying or getting sick, economies collapsing, conflicts arising, migration, and often, human suffering and distress. In the current era of globalisation, disturbances in one place eventually are felt everywhere. Human systems normally cannot endure extreme changes of state. For this reason, as the world’s population becomes urbanised, resilience is becoming the goal of many cities around the word.

The concept of diversity as a means to increase resilience in social systems is not new. For example, increasing diversity to enhance resilience is very common in the financial world. The common saying “don’t put all of your eggs in one basket” applies to the distribution of funds in a portfolio of investment options. This way, if one investment does not grow as intended, the economic loss is compensated by earnings from other investments. The risk is distributed throughout a variety of investments. Distributing risk among different elements can be a successful strategy to achieve resilience. Increasing diversity distributes risk, which leads to more resilience and the whole portfolio of investments is more secure.

In cities, resilience also can be enhanced via a diversity of modes of transportation. A city that has infrastructure in place for rapid transit systems (e.g., trains or subway), buses, walking and biking paths, and automobiles is more resilient than a city that relies on just one (e.g., automobiles). In a resilient city, when a mode of transportation is disrupted, the population can find alternative modes of transportation and still make it to school or work. The system continues to function. However, even when diverse transportation options exist, a disaster can disrupt activities and a resilience plan is needed. For example, in 2015, Boston – a city with multiple modes of transportation – was seriously disrupted after several blizzards. Boston.com reported that Charlie Baker, the governor of Massachusetts, announced a “winter resiliency” plan that cost $83.7 million USD. The report says that the winter resiliency plan included an electrified third rail, snow plows attached to the front of some trains, snow fighter machines to clear railways, new communications, new heating equipment for the two train lines that were most affected, and a partnership with the Department of Corrections to have inmates help clear railways during storms. Because trains are the most reliable mode of transportation during snowstorms, Boston’s winter resiliency plan focuses on rapid recovery of the rail system. While the trains get back to work quickly after a snowstorm, the city works on clearing roads for buses and automobiles as usual. Getting people to their work places as fast as possible after a snow event helps support normal functioning of Boston’s economic sectors.

Diversity not only promotes stability in the face of change, but also increases overall vitality of a system on multiple subsystems, including businesses, industry, and social. In cities, a diversity of businesses and industries is very important because if one goes bankrupt, other businesses still can accommodate unemployed workers. An example of a city that did not have a diversity of industry is Detroit, Michigan. Even without climate change and natural hazards yet in the picture, the functioning of this city collapsed in the late 20^th century. Some decades before, in the early 20^th century, Detroit hosted the rise of the motor industry with Ford, Chrysler, and General Motors locating their automobile manufacturing businesses there. These businesses attracted many people, including immigrants and blacks from the South, and the city grew sixfold, becoming the fifth largest city in the United States (Gilder Lehrman Institute of American History).

However, Detroit’s neighborhoods and schools were racially segregated, and with racial segregation came income segregation. White populations enjoyed better salaries, better housing in better neighborhoods, and better schools. Racial tensions combined with union strikes to enhance the division of people. But most people in Detroit depended on the car industry, and when it suffered, most people also suffered. The city was not resilient to its own social and economic conditions, for without diversity in its major subsystems, a city may not be resilient to change. According to the Gilder Lehrman Institute of American History, Detroit experienced a decentralisation of the motor industry in the late 1960s, and the automation of labour, both of which significantly reduced assembly-line jobs. Unemployment and racial tensions drove white people (and their wealth) away from Detroit. Now Detroit has a population of 886,000 (compared to 1.8 million in the 1950 census) that inhabits a semi-deserted city dotted by empty old factory buildings. This is an example of a city where racial segregation and an emphasis on one type of economic activity led to a dramatic change of state, with detrimental effects for everyone.

City vitality is desirable for all interests. That quality attracts investment, businesses, tourism, art, and urban beautification. And diversity is a key element of city vitality.

The famous urbanist, Jane Jacobs, wrote about city vitality in her book The Death and Life of Great American Cities, and she emphasized the need to have a multiplicity of small businesses combined with an assortment of building types, and dwelling units. A range of types of small businesses means that many shops and restaurants are close to homes, so people can choose to walk or bike to these destinations. More people on the streets provide security from crime and customers for businesses, so in this instance high density can be helpful. Having a combination of businesses also leads to a mix of working hours. Jacobs recommends that bars stay open until late hours, because this draws people to the streets during nighttime. Likewise, bakeries that open early in the morning add people during this time of the day. Basically, this city design approach is the contrary of having one big box store, or an office district, because when these businesses close, an entire street can become isolated, and unsafe.

Research has shown that social cohesion – or sense of community – is an essential element for resilience. When a disturbance occurs in a city, people who possess a mixture of social connections are more likely to survive the disturbance and adapt more quickly to the new circumstances. In contrast, people who are isolated and have few social connections are more likely to suffer the consequences of the disturbance and even perish. For example, during the heat wave that hit Chicago in 1995, the victims were mostly poor people who did not have air conditioning systems and who would not open the windows for fear of crime. However, it was found that within this population, people who were more socially engaged were able to survive even if they were poor, without air conditioning, and living in crime-prone neighborhoods. They might have stayed with friends, or checked on each other. Another example of social cohesion as an element of resilience is the economic crisis of 2008 that hit the United States. That year, the housing market collapsed and many people lost their jobs. Again, those who were more socially engaged used their networks to find another job more rapidly than people who were isolated.

It’s possible to extrapolate this concept of diversity as a means to enhance resilience in most urban subsystems. For instance, increasing diversity of trees is more resilient. If we plant the same type of tree along the streets and there is a disturbance (e.g., pest, freeze) that negatively affects the selected tree species, the streets would be left treeless after that disturbance. However, if we plant a diversity of tree species, it is more likely that at least some tree species will survive the effects of the disturbance. This is also true for such other elements in a city as housing types in a neighborhood that include all types of income; land uses mixed in a neighborhood, types of greenspace that provide many kinds of opportunities for recreational activities; building materials; and routes.

With increasing instabilities in urban systems due to climate change, more options for water and energy sources are also essential in cities in order to increase resilience.

A palette of different energy sources is recommended by many power utility companies. This way, if one energy source is damaged, the utility company can supply power from another source and minimize the impact of disruptions. Even transitioning from the burning of fossil fuels and adopting renewable energy sources requires diversity. For example, solar energy alone cannot provide the power for a city all of the time because there is a lack of solar power at night, and also less power during cloudy days or in places with little daily sunshine. Even renewable-energy advocates who favour solar power recognise that that source needs to be combined with other energy sources (e.g., wind, thermal, hydro). In addition, solar power requires an array of solar generation locations. Having multiple locations is useful because as one location is experiencing cloudy weather conditions, another one might be experiencing sunny weather.

In a way, water security is also related to diversity. A city is more resilient in terms of water security when it has multiple water sources, particularly in cities in arid areas that frequently experience water scarcity. For example, until a decade ago the City of Tucson, Arizona, relied on groundwater as the main water source because surface water was seldom seen. However, the advent of the Central Arizona Project (CAP) canal, which brought surface water from the Colorado River, allowed water agencies to cease relying on the natural aquifer beneath the city. Now CAP water supplements the aquifer and is in fact used to recharge it. This process enhances supply and permits storage for future use. In addition to CAP water, Tucson is encouraging green infrastructure, where any pocket of greenspace can be transformed into stormwater-management infrastructure, that captures stormwater and allows its infiltration into the aquifer. Furthermore, Tucson has invested in wastewater-treatment plants to reuse reclaimed water for irrigating golf courses, and providing water for local ecosystems.

So, what would a resilient city look like?

As climate change and population growth threatens the functioning of cities across the globe, increasing diversity in all the subsystems of a city (economic, industrial, transportation, housing, society, water, energy, and land use) adds redundancy to the connections between subsystems, which increases the chances that as one connection fails under a disruption, others will still maintain the functioning of the whole system. A resilient city would feature multiple industries, businesses, modes of transportation, housing types, plant species, water sources, energy sources, institutions – and residents. In addition to diversity, it is critical to have in place resiliency plans that respond to emergency situations, and to continue to revise these plans in order to accommodate lessons learned from a city’s own experience and from that of others.

We can be certain that disturbances will continue to happen, and with greater frequency. Increasing diversity reduces the risk of all interests losing.