I’ve spent half my life in Tucson, Ariz., where javelinas graze on spiny cacti in people’s front yards, lizards climb speckled yellow trees, and rabbits seek cover in brittlebush, out of sight—but not out of smell—from coyotes. 

The Sonoran Desert, where Tucson was built, holds the most desert biodiversity on Earth. Tucson also lies within the Sky Island region, where mountain forests rise thousands of feet above a sea of arid lands, city, and ranchland, creating a huge assortment of ecosystems varying with elevation. 

Despite its pothole-ridden roads, unbearable urban heat, and noisy traffic, living here always felt different from most cities. It’s a unique energy living among high biodiversity. For myself, the mind feels clearer. I feel healthier. I feel more creative. 

From dry washes weaving through neighborhoods, to the smell of lush creosote bushes after rain, to pop-up galleries where Indigenous people sell their art pieces exemplifying a long, sacred connection to nature, Tucson exhibits biodiversity in every direction. And through initiatives like Mayor Regina Romero’s million trees plan and the Santa Cruz River Heritage Project, which refills an essential urban river with decontaminated wastewater, the city continues to nurture spaces that maintain its ecosystems.  

However, cities are fundamental biodiversity killers. Species richness, the number of different species in a given region, is 50 percent lower in cities than in “intact natural habitat.” 

More than half of the world’s population live in cities, benefitting from statistically higher salaries, rich nightlife, public transportation, and close proximity to healthcare options. But cities also prove to be conduits for pollution, depleted water resources, invasive species, deforestation, and desertification. Additionally, they produce more than 60 percent of global greenhouse gas emissions while making up less than 2 percent of the Earth’s surface, according to the United Nations.

Each of these issues contributes to habitat loss, the main driver of species extinction in today’s world. 

But it doesn’t have to be this way. 

Some cities, even with millions of human inhabitants and a billion tons of concrete, support thousands of documented species through calculated urban planning, policy, and determined nonprofit organizations. 

As we approach what some scientists call the sixth mass extinction, cities can be the change. Humanity must figure out how to build urban spaces that provide refuge for nonhuman life rather than forcing plants, animals, and the microbiome to fend for themselves. 

The following metropolises hold exceptional levels of biodiversity compared to the common concrete jungle. What can these cities teach us about the methods, logic, and benefits of maintaining urban biodiversity? 

Mexico City

• Indigenous knowledge goes a long way

Mexico City is the fifth largest city on Earth, supporting more than 22 million people. Somehow, despite rapid urbanization, it still manages to protect 59 percent of the land within its boundaries.

These government-designated “conservation lands” withstand multiple threats. Soaring housing prices plus “toothless” zoning laws force low-income residents to build illegal settlements in these areas. Urban water demand greatly surpasses the amount of water left in an aquifer beneath the conservation zones that supplies 60-70 percent of Mexico City. The story is far from perfect. 

But this amount of protected area gives life to the 4,000 documented species of plants and animals calling Mexico City home. 

Much of this biodiversity would not populate Mexico City if not for the ancient Aztecs.

Once a water-rich basin formed by five huge lakes, surrounded by Mexico’s Volcanic Axis, a diversity of climate, elevation, water availability, and volcanism created the incredible variety of life in the region that’s now Mexico City. The Aztec empire entered in the 14th century, where they formed a sustainable society, enhancing this variety rather than eradicating it. 

“All through these lakes, they created this system of floating islands that we call chinampas, which were used for agriculture in a very organic, very sustainable way,” said Tamara Blazquez Haik, a wildlife conservation photographer in Mexico City. 

Chinampas, also known as “floating gardens,” were large artificial islands made of stakes, mud, and interweaving reeds. The Aztecs piled mud and aquatic vegetation on top of the islands to create a rich layer of soil rising above the shallow lake waters. 

Covering more than 12 square kilometers, the chinampas system set the foundation for food production. They also added to the already notable landscape diversity that allowed many new species to find their ecological niches. 

Typically, the further you travel away from lake shores, the less biodiversity you will find. But the islands’ nutrient-rich soils, intercropping, tree cover, and freshwater canal systems, all harbored by the Aztecs, brought a remarkable 2 percent of the world’s unique species to the lush agro-archipelago. 

“These little islands, they created many many different kinds of shores, and that allowed for more species to thrive like the axolotl, and many bird species, and many fish species,” Blazquez Haik said.

Effective pollution management added to these ideal conditions for nonhuman life. Instead of releasing organic waste like food leftovers into the water, the Aztecs reused it as fertilizer. 

Some Aztec waste management practices do not translate well to today’s world. For example, they often reused human excrement for fertilization, and some people were put to death for wastefulness. However, 500 years after the Spaniards wiped out their civilization, drained the lakes, and destroyed the chinampas, Mexico City still applies some Aztec waste management logic, according to an article. 

For example, despite being the second biggest waste producer among the world’s megacities, Mexico City has reduced garbage in its landfills through heavy composting, recycling, and even turning waste into energy, which slows greenhouse gas emissions.

In most of the developed world, Indigenous peoples are left out of biodiversity decision-making, despite their long-practiced understandings and connections to the natural world. Indigenous peoples’ proven knowledge of nature should fortify them as jurisdictional authorities on biodiversity, but they’re constantly ignored.

Incorporating Indigenous knowledge into the way cities attract biodiversity could pave the way for healthy coexistence between humans and nonhumans. 

San Francisco

• Keep things native

In the U.S., there are only two regions considered biodiversity hotspots, which means they both have supreme amounts of native species but also face significant conservation threats. One is the Sky Island region near the U.S./Mexico border, and the other: California’s Floristic Province. 

Lining the entire Pacific coast of California, 61 percent of the hotspot’s plant species are found nowhere else on Earth. 

According to the Critical Ecosystem Partnership Fund (CEPF), the concentration of native plants, assorted landscapes, and Mediterranean-like climate sets the stage for more bird species to breed in the region than anywhere else in the U.S. Impressive numbers of mammals, reptiles, amphibians, and insects also inhabit the land. In fact, it houses 30 percent of all insect species in North America. 

In the upper half of the California Floristic Province lies San Francisco, the second most densely populated city in the U.S. This density leaves little room for biodiversity in the coastal city. 

“We haven’t really done well,” said Eddie Bartley, President of the Yerba Buena Chapter of the California Native Plant Society for the San Francisco Peninsula. “There were 769 known species of indigenous plant taxa here, and we’ve lost 34 percent of those.”

In San Francisco and beyond, native plant loss affects the entire food chain, from the fungi that colonize plant roots, to the apex predators that eat herbivores and omnivores. 

Today, the city and local nonprofits work diligently to protect native species. For example, the Green Connections Plan braids the streets with native greenery to create wildlife corridors between parks. 

But the legacy of introduced species in the 1800s and early-1900s still haunts urban biodiversity.

“There was all this rage to bring in exotic plants from other parts of the world, and that’s really reduced our native habitat,” Bartley said. 

Not all non-native species negatively affect landscapes. But when they do, the effects can be catastrophic. Non-native species, called invasive species when they cause harm, often outcompete native flora and fauna for resources, alter the landscape making it more prone to disaster, and spread disease. Invasives jeopardize 42 percent of the world’s endangered species according to the National Wildlife Federation. This will inevitably rise as climate change improves conditions for invaders. 

The yellow starthistle, a thorny Eurasian plant introduced to California around 1850, effectively “chokes out” San Francisco’s native plants. Smooth cordgrass, introduced to San Francisco shorelines in the 1900s by the U.S. Army Corps of Engineers, altered important habitats for the endangered California clapper rail and salt marsh harvest mouse, according to National Geographic. 

In 2008, the Bay Area had the world’s most non-native species. 

Let’s face it, plants are essential to the aesthetic beauty and functionality of cities, and sometimes exotic, non-native species fill that role. This is ultra-prevalent through the consistent rise of turf grass since the 17th century, a symbol of order, wealth, community values, and the white-picket-fence American dream. 

But San Francisco is now learning how important it is to prioritize species endemic to the region. 

In 2018, the San Francisco Board of Supervisors voted on a new policy, making local biodiversity a foremost concern. It passed unanimously. 

The policy encompasses 5 goals: Maintaining biologically rich ecosystems, equitable access to those ecosystems, community biodiversity stewardship, ecological planning and design, and resilience in the city. 

San Francisco looks to foster, not only conditions suitable for its native species, but a biophilic grace which people of all classes, races, and neighborhoods can enjoy, in a country where access to green space disproportionately caters to the wealthy. 

“That was a big step in the right direction,” Bartley said. 

 

Cape Town 

• The importance of citizen science

In 2019, the City Nature Challenge named Cape Town the world’s most biodiverse city. 

The Cape Floristic Region, a biodiversity hotspot, possesses the highest concentration of higher plant species in the world with the exception of tropical regions. Higher plant species, also known as vascular plants, are those with advanced characteristics such as seed-based reproduction and tissues that transport water, gasses, and nutrients. Some of Cape Town’s higher plant species include the Clanwilliam Cedar Tree and the King Protea, South Africa’s national flower. 

Lingering among this sky-high plant diversity are a number of magnificent vertebrates, from baboons to the South African penguin. 

Similar to many of the world’s biodiverse lands, urban and agricultural development threaten the integrity of Cape Town’s vulnerable, finite ecosystems. According to CEPF, agricultural expansion already envelopes 26 percent of the Cape Floristic Region, and even encroaches on high mountain ecosystems. 

Nearly 5 million people live in Cape Town, expanding 2 percent each year. Time is limited for the region’s biodiversity, which already falls victim to invasive species, rising sea levels, intensified fire risks, and drought, all fueled by climate change. 

In a region where apartheid-era structures denied many people access to natural resources and environmental education programs, CEPF believes that learning is key to solving this crisis. 

“The future of the Cape Floristic Region will only be assured if the public can be educated about the value of its biodiversity and if this knowledge can be translated into participation and political action,” CEPF said.

Ismail Ebrahim is the project manager for Custodians of Rare and Endangered Wildflowers (CREW), a Cape Town organization using citizen science to collect data on local plant species, threatened by some of the aforementioned issues. 

Citizen science, defined by National Geographic, “is the practice of public participation and collaboration in scientific research to increase scientific knowledge.” Unpaid volunteers often collect and submit data on scientific phenomena in their area, contributing to a large pile of information for researchers to interpret. It helps scientists but also helps the general public obtain the knowledge needed to fight environmental issues like climate change and biodiversity loss. 

“It’s just normal people going out there finding and collecting information,” Ebrahim said. 

Through the CREW program, citizen scientists affect governmental decision-making for the whole country of South Africa. 

“The data that comes from the citizen scientists contribute to our conservation mandates for the country,” Ebrahim said. 

It also builds public knowledge of the vast biodiversity of Cape Town. According to Ebrahim, many of their volunteers are at an “expert” level in understanding the ecology of the region. 

Cape Town holds a strong network of citizen science opportunities. For example, Cape Citizen Science uses iNaturalist, a social website where people share biodiversity observations, to report dying tree ferns. They look to identify landscape patterns that may be causing the region’s tree fern decline. Cape Research and Diver Development offers a citizen science program, allowing people to dive among the coral, kelp, and sea anemones of False Bay, to gather data for conservation. 

According to an article published in Frontiers in Climate, “data generated by citizen science groups have become an increasingly important source for scientists, applied users, and those pursuing the 2030 Agenda for Sustainable Development.” 

Citizen science accelerates action toward these 2030 goals set by the United Nations, some of which include sustainable cities, zero hunger, and clean water, by allowing massive amounts of research in a short period. It also infuses the public with knowledge, giving them the scientific background to make heavily informed decisions on issues that they care about. Somebody with vast knowledge of local biodiversity, for example, may be more likely to support public officials who protect it. 

 

Singapore

• Urban biodiversity lifts quality of life

Located just north of the equator, the small island city-state of Singapore hosts 5.7 million people. It’s one of the world’s most densely populated places. 

Some research shows that dense cities blemish quality of life. One study, which looked at U.S. counties, found that higher population density lowers happiness and life satisfaction. 

But Singapore consistently ranks the happiest place in Southeast Asia, per the World Happiness Report. 

The report evaluates happiness based on 6 factors: GDP, life expectancy, generosity, social support, freedom, and corruption. Access to nature directly improves some of these factors. 

As for life expectancy, one study documenting the lives of more than 100,000 women found that exposure to higher levels of green vegetation corresponded with less death. As for generosity, another study found that people are more charitable and willing to lend money when provided natural settings.

Once super polluted and spotted with only miniature pockets of nature, Singapore transformed itself after the 1960s into what many recognize as “The Garden City.” 

Singapore was a British colony for 144 years. The British arrived in 1819, and wiped out 90 percent of the island’s old-growth forest in the next 80 years, for timber, agriculture, and the creation of settlements, according to a Singapore National Parks Board report.  

Now the city, mostly made up of people with Chinese, Malaysian, and Indian ancestry, is at least 50 percent covered in vegetation. In the 1960s, when global environmental awareness was minimal, Prime Minister Lee Kuan Yew’s “Garden City” vision spread vegetation sideways and upwards, greening sidewalks, parks, and even high-rise buildings. 

Ever since, green space has deeply permeated Singaporean policy. Tree Planting Day, first held in 1971, is a national holiday of sorts, established to promote tree planting among students, grassroots leaders, and everyday residents. The 1975 Parks and Trees Act mandates government agencies and private developers to implement greenery into housing, roads, and parking lots, according to HistorySG. 

Singapore houses more than 300 densely forested nature parks and four established nature reserves, which serve “as core refugia for biodiversity,” according to the report. 

Refugia, plural for a refugium, are safe havens for species’ populations fragmented by harsh living conditions and disturbances. For example, a pond in the middle of a hot desert could provide refuge for a migrating dove, or a wandering mountain lion. Likewise, a dense tree canopy in the middle of Singapore provides refuge for lemurs and kingfishers. 

For leisure seekers, these spaces also provide access to nature bathing, a Japanese practice gaining momentum in Asian countries, defined by awakening your senses to the aromas, sights, and sounds of forests. 

For a long time, Singapore’s leaders have acknowledged the importance of green space for human well-being. 

“We have created a pleasant living and working environment for Singaporeans,” Yew said in a 1995 speech. “It is good for morale.”

He’s right. It is good for morale. Not only does urban green space aid biodiversity, combat climate change, and enhance air quality, but it works wonders for mental health. 

In Florence Williams’ “The Nature Fix: Why Nature Makes Us Happier, Healthier, and More Creative,” she meets up with experts across the globe, to better understand why the natural world uplifts quality of life so significantly. 

One point Williams references, provided by Yoshifumi Miyazaki, a physical anthropologist and vice director of the Center for Environment, Health, and Field Studies at Chiba University, is that humans are physiologically adapted to nature and its healing benefits. 

“Throughout our evolution, we’ve spent 99.9 percent of our time in nature,” he says in the book. “During everyday life, a feeling of comfort can be achieved if our rhythms are synchronized with those of the environment.” 

In Japan, where Williams spoke with Miyazaki, she also met Qing Li, an immunologist at the Department of Environmental Medicine at Nippon Medical School. Li found that tree smells can spur a 20 percent increase in NK cells, immune cells that protect us from disease. Nature exposure also lowers blood pressure and muscle tension.  

Of course, physical and mental health are intricately linked. When you keep your physical health in check, your mental health will likely follow, and vice versa. Just 20 minutes of nature causes a significant drop in cortisol, a marker of stress, according to a study. 

And the more biodiverse the nature experience is, the better. Research shows that improved mental health is associated with higher levels of plant and bird diversity. 

Global mental health is getting worse. According to the World Health Organization, 2007 to 2017 brought a 13 percent swell in mental health conditions and substance use disorders. It may be time for cities to better institute the well-studied mental health tool of biodiversity.