Unseen City: A tribute to urban nature

It was pure pleasure to read Unseen City (1).  Unlike most nature writing, Nathanael Johnson asks readers to notice and appreciate the urban nature that we tend to take for granted.  Ironically, the plants and animals that we see every day and in great numbers do not get the attention they deserve.  Most nature writing tends to focus on rare and remote species to which we have little access and often laments their absence where we live.  Conservationists often advocate for expensive programs to reintroduce rare species to urban centers where they haven’t lived for decades, if not centuries.

Johnson’s focus on the ordinary species around us is refreshing.  We were happy to take a break from the usual hand-wringing about loss of biodiversity and instead enjoy the richness and beauty of the nature we have.  It is our loss when we ignore the nature we have. Johnson’s intense focus on urban species reveals that they are every bit as interesting as the rare species we seldom see.  Johnson’s approach to nature is analogous to the optimist’s “glass-half-full” approach to life.

Another appealing aspect of Johnson’s approach is that his story is told from the perspective of a young father, introducing his toddler daughter to the mysteries of nature.  One of our primary concerns about the museumification of our parks by native plant advocates is that children are being deprived of the opportunity to interact with nature.  Being required to stay on trails or observe from behind fences is no way for children to appreciate the complexity and beauty of the natural world.  Johnson takes his daughter deep into the weeds to experience nature in a physical, tactile way. 

A few examples of the homely creatures in our cities

Johnson wrote his book while living in San Francisco and then in Berkeley.  So, the species he encounters and studies are those with which we are all familiar.  Here are some of the creatures he tells us about, with a few of the interesting things we learn about them.

  • Pigeons are reviled by many serious bird watchers. In fact, they are remarkable creatures in many ways.  They mate for life and they are extremely devoted parents.  They tend to nest in the same place and their ability to find and return to that nest from long distances is one of the reasons why humans have formed intense relationships with them.  There is a long tradition of keeping homing pigeons that are raced by their keepers in competitions that occur all over the world.  The pigeons are taken long distances from their nests and then timed on how long it takes them to return home.  Johnson tells remarkable stories about how pigeons overcome challenging attempts to prevent them from finding their way home.
  • Eastern grey squirrel. Creative Commons

    Squirrels are both extremely agile and very resourceful. Here is an example of how squirrels defeated an attempt to keep them out of a bird feeder: “…squirrels had to climb up through a vertical pipe, leap onto a blade of a spinning windmill, cling to it, and then sail off on the right trajectory to land on a platform.  Then they had to go paw over paw upside down along a suspended chain that passed through a series of spinning disks, negotiate a revolving door, run through a slack canvas tube, and keep their balance while crossing a pole covered with slick spinning rollers.  From there, it was a six-foot jump to another tunnel, through which they had to ride a sliding vehicle made to look like a rocket ship by pushing it along with their paws.  Finally, there was an eight-foot jump to the food.” (1)  I retell this to story to spare our readers the pointless effort of trying to prevent squirrels from raiding their bird feeders.

  • Turkey vulture in San Jose, California by Dan DeBold. Creative Commons

    The turkey vulture is another underappreciated bird. They eat primarily dead animals and many of those animals died of diseases or toxic chemicals and are rotten and maggot infested when they are finally found (by smell) and eaten by the vulture.  The digestive and immune system of the vulture is capable of detoxifying chemicals and killing bacteria and viruses in the dead animal.  In other words, the vulture is cleaning up the remains of dead animals.  India has learned the value of vultures the hard way.  They killed many of their vultures with an anti-inflammatory drug they were feeding to their livestock.  When their vulture population dwindled, they were buried in dead animals, many dangerously diseased and toxic.  We eradicate animals at our peril because we often don’t understand the roles they are playing in the ecosystem.

Defending novel ecosystems

In addition to asking his readers to appreciate the positive qualities of the creatures in our cities, he also asks us to reconsider the deep prejudice against them that has become the conventional wisdom.  Plants and animals that people believe were transplanted by humans into places where they did not exist in the distant past are considered “alien invaders” that dominate their predecessors, driving them out and reducing biodiversity.

This narrative, which originated in academic science as “invasion biology” in the 1960s, has become a popular story with the media, which is always attracted to scary stories.  The media is significantly less interested in the peaceful resolution of their horror stories.  With few exceptions, an introduced species that initially seems to be a problem eventually fades into the woodwork to become just another player in the ecosystem.  Johnson uses the Argentine ant as one of many examples of an introduced species that spread rapidly, but 20 years later has nearly disappeared.  In other cases, a species initially considered an unwelcome intruder becomes a valuable asset, such as zebra mussels which filter pollution from lakes and have become a source of food for diving birds.

Novel ecosystems are the future

Johnson concludes his book with this reminder that novel ecosystems have been created by human disturbance and that we should be grateful for the plants and animals that are capable of surviving our abusive treatment of the planet:

“The species that I’ve written about here are, at best, invisible, and at worst, reviled.  We honor least the nature that is closest to us.  As Courtney Humphries put it in Superdove, ‘We create and destroy habitat, we shape genomes, we aid the worldwide movement of other species.  And yet we seem disappointed and horrified when those plants and animals respond by adapting to our changes and thriving in them.’

“Because they are associated with human disruption, the organisms that spring up from our footprints look like corruptions of nature.  But I’ve come to see it the other way around:  These species represent nature at its most vital and creative.

“Nature never misses an opportunity to exploit a catastrophe.  When humans bulldoze and pave, nature sends in a vanguard of species that can tough it out in the new environment.  These invasive species are not nature’s destroyers, but rather its creators.  They begin setting up food webs, they evolve and diverge into new species.  Because humans purposefully import exotic plants—along with the insects, seeds, and microbes we accidentally bring in from around the world—cities are remarkable centers of biodiversity.  These creatures crossbreed, hybridize, eat one another, form cooperative relationships, and evolve.  And so, at a time when thousands of species are at risk of extinction because of our destruction of wilderness, new species are springing up in the new habitats we have created.” (1)

Worshipping the rare at the expense of the common

The ONLY known Raven’s manzanita plant is in the San Francisco Presidio. Its exact location is a secret to protect it.

Vast sums of money are being spent in often futile attempts to reintroduce rare plants and animals to urban environments where they have not lived for a long time.  The National Park Service and San Francisco’s Natural Resources Division are having little success with their efforts to reintroduce Mission Blue butterflies.  After over 30 years, the National Park Service has still not successfully germinated endangered Raven’s manzanita from seed.  These fruitless efforts are not just wasteful of resources, they also inflict damage on the environment by using pesticides and setting fires to eliminate competition and destroying trees to increase sunlight on rare plants and host plants of rare insects.

The veneration of rare plants and animals is often at the expense of the plants and animals that are adapted to present environmental conditions.  In Unseen City Nathanael Johnson invites us to place greater value on the ordinary creatures who are capable of living with us.  We can treat them with the respect they deserve by not destroying them in pursuit of a fantasy landscape populated by fantasy creatures that are not capable of surviving the changes we have made in the environment.


  1. Nathanael Johnson, Unseen City: The majesty of pigeons, the discreet charm of snails and other wonders of the urban wilderness, Rodale Wellness, 2016

“Restoration is just horticulture dressed up to look like ecology”

Peter Del Tredici was invited to speak at a conference sponsored by the Presidio Trust in San Francisco, “Bridging the Nature-Culture Divide Conference by the Cultural Landscape Foundation,” January 23, 2015.  Professor Del Tredici recently retired as senior research scientist at Harvard’s Arnold Arboretum after 35 years of service.  He is an Associate Professor in Landscape Architecture at the Harvard Graduate School of Design where he teaches courses on soils, plants and urban ecology.  He advocates for a pragmatic approach to urban landscapes, which values novel ecosystems for the functions they perform and their sustainability in stressful environments.

Professor Del Tredici’s presentation at the Presidio Conference was entitled, “Saving Nature in a Humanized World.”  The presentation is available on YouTube.

We attended his presentation, which was warmly received by an audience of about 180 people.  We paraphrase some of his key points.

Update:  Professor Del Tredici has requested that the following statement be added to this post:  “Professor Del Tredici has graciously allowed us to paraphrase–for educational purposes only–some of his key points in his lecture. In no way should his permission to reprint the lecture on this website be considered an endorsement of the political or ecological agenda of “Death of a Million Trees”


Professor Del Tredici began his presentation by complimenting the Presidio for what it has accomplished in the past 20 years and congratulating the Presidio for “…what it has become.  But I’m going to do something very different today.  I hope you’re ready for this.”

Professor Del Tredici spoke about spontaneous urban nature, some of which we control but a lot we do not control.  What does spontaneous nature look like?  The reason why this is important is because it’s about the future.  What is the world going to look like 20 years from now?  The answer to that question is in urbanized nature.

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Detroit is a depressing place from a sociological standpoint.  It is so economically depressed that the land has lost its value.  Forty percent of the land is no longer occupied or managed by public or private entities. From the standpoint of a botanist, it is a fascinating place because we can see how nature develops without human interaction.  Detroit is a case study for urban ecology.

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Globalized Ecology

The vegetation of most cities is as cosmopolitan as its human population.  Asa Gray’s “Manual of Botany” reports that 10.7% of plant species in Northeastern United States were non-native in 1856.  By the 1990s, 25-35% of plant species were non-native.  This number is not going down.  It’s a strongly upward trend over the past 150 years.  We can create little islands of native plants by eradicating non-native species, but the reality is that our ecology is becoming as globalized as our economy.  These changes mirrored the changes in the ethnic and racial composition of American cities.  The same forces that produce socio-economic changes in cities are also changing the biological environment.

Urbanized Environment

A significant portion of land area in the Northeast is fully urbanized.  Urbanization in the West is just as rampant as the Northeast.  Looking at an aerial view of Los Angeles, you can see that it is completely developed.  You can talk about what used to grow there, but the concept that there is a vegetation that is native to these current conditions, Professor Del Tredici said, “Personally I find that an absurdity.  I hate to be so harsh, but nothing is native to LA as it now exists.”

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  • Cities have distinctive environmental characteristics, such as the urban heat island effect. Cities are significantly warmer than rural adjacent areas, which means they are important predictors of the impact of climate change because they have already warmed as much as other places are projected to in the future.
  • Urbanized areas can also be defined by the amount of impervious surface they contain. When 25% or more of the land is covered with an impervious surface such as roads, parking lots, houses etc., the environment is urbanized from the standpoint of the vegetation because impervious surface fragments the environment, compacts the soil, and interrupts the hydrology.   Using the definition of 30% impervious surface, urbanization describes not only our cities, but also many of our suburbs.
  • Glaciation is analogous to the urbanized environment because the heavy equipment that is used to clear the land leaves in its wake compacted glacial till. What you find after the glaciers recede is barren land; the vegetation has to come back from nothing—a condition known as primary succession.

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  • One-sixth of the city of Boston is built on land fill. What is the native vegetation of filled soil?  There is no going back when you’re talking about filled urban landscapes.  Not quite as much of San Francisco is built on landfill, but most of the eastern and northern edges of the city are on landfill.

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  • There is a huge difference between native soils and fill soils. Fill soils support the development of novel ecosystems. Native ecosystems cannot be created without native soils.  There are some native species that are adapted to urban conditions, such as roadside areas.  Urbanized vegetation is a cosmopolitan mix of native and non-native.  Urbanization favors species that grow well in soils that are relatively fertile, dry, sunny, and alkaline.

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  • Where it snows, the roads are repeatedly salted to prevent dangerous, icy conditions. This creates alkaline conditions along roadsides to which many plant species are not adapted.

Urban Ecology

Professor Del Tredici studies modern urban ecology which was born in post-war Germany, where urban environments were reduced to rubble and ecologists began to study what was growing in that rubble.  That was the birth of modern urban ecology.  It’s important to study, not for what it used to be, but for what it is now and what it can become in the future.  Nature reclaiming the urban environment on its own terms is an interesting process, an evolutionary process that we should pay attention to.  Post industrial succession—the process of rebuilding ecology in an intensively urban environment– should be studied with the same level of academic intensity as we studied the post-agricultural succession in the Northeast.

Novel Ecosystems

When native forests are converted to urban ecosystems and then abandoned—as seen in Detroit– they don’t go back to their original state, rather they become novel ecosystems.  There is no going back.  Once we achieve the level of compaction and impervious surface of an urbanized environment we have limited what the landscape can become in the future.  Some of these changes are permanent.  There are long term disturbances caused by chronic stress factors that permanently alter ecological conditions.  Professor Del Tredici said, “These conditions are not reversible.  Invasive species aren’t going anywhere.  If you remove invasive species you are gardening.  When you garden you are deciding who lives and who dies.  You are just playing god.  This gives you the illusion of control, but it is a never-ending effort to control a process that can’t be controlled.”

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In 1996 the Arnold Arboretum was given a 24 acre parcel of derelict land, called Bussey Brook Meadow. In 2011, Del Tredici succeeded in preserving it as a site for research on urban ecology by leaving it alone.  The land had a 300 year history of use and abuse, all left more or less alone.  Plant species—both native and non-native–have sorted themselves out and restored a functional wetland in the middle of the site.  It doesn’t matter that it isn’t a native landscape if it is providing the necessary ecological functions.

The Bottom Line

Ecology is not about stasis, it’s about flux.  Stasis is achieved by maintenance, but the natural state is flux.  Evolution is based on competition, which species is the best adapted to current conditions.  Sustainability is about reducing maintenance in order to promote ecology.  Landscape architects look at the Bussey Meadow site and ask, “When are you going to fix it?”  Professor Del Tredici’s answer is, “I’m not sure this site needs to be fixed.  It has value just the way it is.”

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We have quoted Professor Del Tredici’s work in previous articles and we consider it important everywhere, but we bring this presentation to your attention today primarily because of where it was delivered.  The Presidio Trust has engaged in some of the most aggressive “restorations” in the San Francisco Bay Area and some of the most successful: Inspiration Point, El Polin Spring, Thompson Reach, etc.  All fish in Mountain Lake were recently poisoned in order to “restore” the lake to exclusively native species.  Pacific chorus frogs were recently reintroduced.  The intention is to reintroduce the Western Pond Turtle to Mountain Lake, a species that is notoriously easily disturbed and being considered for endangered status.  It is also a species that requires hundreds of meters of unshaded nesting habitat in proximity to its water source.

Tennessee Hollow
Tennessee Hollow “Restoration” is 270 acres, 20% of the Presidio. Presidio Trust photo.

These projects have required the destruction of thousands of trees because the native vegetation is grassland and scrub.  However, the Presidio has also made a commitment to the preservation of its historic, non-native forest which was planted by the military over 100 years ago.  Major investments have been made in reforestation of the aging forest with similar tree species.

The San Francisco Presidio, painting by Richard Beechey, 1826
The San Francisco Presidio, painting by Richard Beechey, 1826

In other words, the Presidio Trust seems to have assigned itself a schizophrenic mission to simultaneously destroy an existing landscape in order to re-create it and preserve that same landscape: the re-creation of an idealized landscape vs. preservation of the novel ecosystem within the historic forest.   We suppose that is one definition of “balance.”  However, we would like to believe that the invitation to Professor Del Tredici to speak of the sustainability of urbanized novel ecosystems is an indication that the Presidio Trust will assign more value to what exists and less effort to attempts to re-create an historic landscape that may no longer be adapted to the real world.