Urban Ecology

“One of the penalties of an ecological education is that one lives alone in a world of wounds. Much of the damage inflicted on the land is quite invisible to laymen. An ecologist must either harden his shell and make believe that the consequences of science are none of his business, or he must be the doctor who sees the marks of death in a community that believes itself well and does not want to be told otherwise.”

– Aldo Leopold, A Sand County Almanac

Definition of Urban Ecology

Urban nature at work in Alley Pond Park, Queens

Urban nature at work in Alley Pond Park, Queens

History. The study of urban ecology is a recent development. For decades, ecologists defined nature as the pristine green spaces far from people. True ecological observation could only occur away from human intervention, in the “wilderness”. It was universally held that cities were unnatural consortiums of weeds, pigeons and rats.

This perspective has changed within the last 25 years. Ecologists have begun to do their field studies closer to home. In the process, they have discovered that nature survives, and even thrives, in the city limits, with a surprising variety of native plants and animals. As formerly open spaces are devoured by development, understanding and protecting urban ecosystems is crucial to our environmental future.  More about the trend towards urban ecology, “Wild Cities, It’s a Jungle Out There” and “On the Taxonomy of the Naturalist

Additionally, studies done in “pristine” nature show that the concept of “wilderness” has been rendered an anachronism. All areas of the globe now bear the mark of humanity’s existence, even places as remote as the South Pole (ozone hole), or as untrammeled as Alaska (elevated levels of mercury in wildlife).

This warrants deeper consideration. Most people would consider dramatic disturbances, such as hurricanes, to be responsible for the destruction of natural areas. In fact, the opposite is true. Such storm events may have little long-term effect, whereas subtle, human-induced changes permanently alter nature. Ecology in urban natural areas is marred by disturbances from current and historical land uses. For example, in a recent study, forest that had been farmed by the Romans was compared with forest that had never been in cultivation. Thousands of years later, the former farms still have not recovered, sustaining more weedy plants and lower quality soils. (download the paper)

It behooves us to understand how this happens, for as a colleague recently put it, “every day the world becomes more and more like New York City, not the reverse.” For example, Staten Island in 1879 was a farming community with 30,000 people and 1,100 plant species. Staten Island today has 400,000 people and has lost 443 plant species.

Urban natural areas share some common attributes:

  • Higher percentage of paved/built land. Development of open space destroys 20,000 to 40,000 years of native, healthy soil development in one fell swoop.
  • Lack of plants underfoot due to use of mountainbikes and ATVs. Mineral soils are exposed, with the humus/seed bank layer gone. The only way to restore the area is through intensive measures such as plantings.

    Lack of plants underfoot due to use of mountainbikes and ATVs. Mineral soils are exposed, with the humus/seed bank layer gone. The only way to restore the area is through intensive measures such as plantings.

  • Increased use by humans. Given the dense population of people per park, our natural areas show signs of wear and tear – mountain bike tire ruts, charred stumps from arson, strewn litter, bare, unvegetated patches, exposed mineral soil. In Central Park, greater than 25% of the forested areas were bare ground, caused by pedestrians and cyclists. More than 2 1/2 miles of illegal bike trails were created in the North Woods section in one season.
  • More exotics, more of the time. Seeds transported by tires or carried by birds from landscape plantings mean proportionally many more alien species in urban that rural areas. And these non-native plants dominate these patches. In Chicago, one-third of all plants are exotic. About 150 species are so invasive, they are in 95% of the parks.
  • Replacing forests and fields with buildings and roads increases the amount of land impenetrable to precipitation. This “impervious cover” directs water offsite, unable to percolate into the ground as groundwater recharge. Brimming with pollutants, this overflow water has high volume and runoff rates, which create erosion gullies. This process, replayed over and over in urban areas, is especially deleterious to our wetlands, home to many of the city’s most imperiled plants.
  • Urban wetlands, such as this one, are under duress, due to increased runoff from paved areas and higher pollutant content in that surface water.

    Urban wetlands, such as this one, are under duress, due to increased runoff from paved areas and higher pollutant content in that surface water.

  • Urban wetlands are characterized by overall drier conditions, reduced nitrogen cycling and higher levels of pollutants than those in less developed watersheds. For more information, reference the work of Dr. Joan Ehrenfeld.
  • Poor air quality. High levels of air pollutants are responsible for more human deaths each year than car accidents or guns. Plants don’t like it either, because smog, acid rain, et al. increases infections, diseases and the likelihood of death. (Although stationery sites like factories shoulder all the bad pr, the source of these pollutants is mobile – our cars).
  • Urban Heat Island Effect. Trees and shrubs cool the air through shade and evaporation. Replacing them with roads and buildings, which release stored heat at night, makes for warmer evenings and increased A/C use. This extra heat (5-8 F) creates more rain and thunderstorms. Everywhere we go, we always take the weather with us. Learn more.

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Characteristics of an Urban Forest

Japanese knotweed (Polygonum cuspidatum) is one of the exotic invasives that is overrunning our urban natural areas.

Japanese knotweed (Polygonum cuspidatum) is one of the exotic invasives that is overrunning our urban natural areas.

  • Fewer native species and more exotic ones. Over a 50-year period, Pelham Bay Park, New York City’s largest, lost 2.8 native plant species every year, while it gained 4.9 new exotics annually. 1   These statistics are particularly frightening when compared with other such studies. For example, over a 100-year period, a large forest in metropolitan Boston lost one native plant species every year, while it gained a new exotic every 5 years. 2
  • Fewer plants in the herbaceous layer. Many of these wildflowers have ant-dispersed seeds that don’t travel far from the mother plant. For example, the seeds of Dutchman’s breeches (Dicentra cucullaria) advance only about 100 yards over a 1000-year period!
  • Trees and other vegetation are susceptible to air pollution.  Acid rain leaches minerals from the soil. Without their daily vitamins, plants develop brown spots and lose their leaves early. In this sickly state trees and shrubs are vulnerable to disease and infestation from insects. Leaves attempt to protect themselves from this toxic brew by secreting waxy layers in excess. When these leaves become part of the humus layer on the forest floor, they act to repel water.
  • Dutchman's breeches (Dicentra cucullaria) has ant-dispersed seeds that don't travel far from the mother plant.

    Dutchman's breeches (Dicentra cucullaria) has ant-dispersed seeds that don't travel far from the mother plant.

  • Dead wood is full of life. One-third of forest-dwelling species rely on rotting trees, logs, and branches for their survival. The removal of decayed timber leads to drastic declines in species such as insects, fungi, and lichens. Young “tidy” forests with little decayed wood are more susceptible to disease and erosion and the vagaries of climate change. Dead wood also functions as reservoirs, holding water over long periods of time. Such wet sites are important for plant and animal life during times of drought.
  • High squirrel and rodent populations are detrimental to plant survival. Herbaceous plants are damaged every time a nut is deposited into the forest floor, and nuts from which young oak and beech seedlings have sprouted become dinner. Squirrel densities are 10 times higher in urban than rural areas, mostly due to handouts from humans (because, gosh, aren’t they cute?) and lack of predators. On the grounds of the New York Botanical Garden in the Bronx, there are 25 squirrels per acre, with normal forest levels much lowers at 3-5 squirrels per acre.
  • Desert-like conditions. Between the heat island effect, reflected heat from pavement and buildings, glare, and wind exposure, urban plants are in dire need of a tall, cool glass of water. Consider that plants are already exist in soils made arid by pavement and compaction, their thirst may never be slaked.

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Urban vs. Rural Soils

Healthy urban soils support a wide variety of biological diversity, including mosses & lichens

Healthy urban soils support a wide variety of biological diversity, including mosses & lichens

Soils are the foundation of our indigenous flora. Native healthy soils beget native, healthy plants. But good soils are hard to find. They are not immune to the stresses of the urban environment. Urban soils differ from their rural counterparts in critical ways: they are warmer, more compacted, and hold less water and oxygen. This may result in lower seed germination and seedling survival. In metropolitan areas, soil processes (such as decomposition and nitrogen cycling), soil structure and soil ecology are constrained. Urban soils are higher in heavy metals and nitrogen than rural soils, which may account for their low populations of native soil invertebrates. They have more exotic earthworms and fewer mycorrhizal fungi, resulting in rapid leaf litter decay and nutrient cycling, and little accumulated organic matter (humus). In addition, studies of exotic vegetation demonstrate that some plants can change local soil chemistry to favor their own perpetuation and discourage the presence of natives.

Soil definition, texture & pH. It is important to recognize that soils themselves are ecosystems. They are a rich concoction of minerals, decayed plant material, living roots, microbes, fungi, animals and water. The presence and amount of each delimit the vegetative communities it may support. Texture (and site hydrology) determines the amount of air and water present. A soil’s texture is defined by the size and frequency of its particles, ranging from coarsely textured pure sand to medium-textured silts to very finely textured clay. Approximately equal parts of sand, silt and clay create loam, the celebrated soil texture of all garden gurus.

Ground cedar (Lycopodium digitatum) and other clubmosses are increasingly uncommon and can’t be restored. One reason for this is their critical reliance on soil fungi - mycorrhizae. Preservation of their habitat is the only means of insuring their future.

Ground cedar (Lycopodium digitatum) and other clubmosses are increasingly uncommon and can’t be restored. One reason for this is their critical reliance on soil fungi - mycorrhizae. Preservation of their habitat is the only means of insuring their future.

Most native plants prefer soil acidity from 4.5 to 6.5 pH (pH is “power of Hydrogen”, a scale measuring concentration of hydrogen ions (H+), with 0-6 being the acidic range, 7 neutral, 8-14 the basic range. A higher pH means there are fewer hydrogen ions. Since it is a logarithmic scale, a change of one pH unit reflects a tenfold change in the concentration of the hydrogen ion. Indigenous plants found in soils outside this pH range are usually poor competitors. In New York City’s forest soils, pH 5 to 5.5 is typical.

Mycorrhizae. Literally “fungus roots”, this group of fungi are critical to forest flora. They grow along plant roots, facilitating in the uptake of vital nutrients and water. Many plants, including our forest orchids, rely on them for their very survival. These organisms are so important and ubiquitous that a leaf layer 1 1/2 inches thick and a yard square could contain as much as 5,000 miles of these fungi. Mycorrhizae are very fragile. Their fungal filaments, which lie just below the soil surface, are easily destroyed.

Skunk cabbage (Symplocarpus foetidus) is a harbinger of spring. But is this common wildflower long for this world with all the ills we throw at its wetland haunts?

Skunk cabbage (Symplocarpus foetidus) is a harbinger of spring. But is this common wildflower long for this world with all the ills we throw at its wetland haunts?

Ecology of urban soil.

  • Soil organisms: The composition and abundance of soil organisms is vastly different. Overall the “good” organisms, the ones that play a desirable role in soil ecology, are less abundant in urban soils. These include mycorrhizal fungi and animals such as nematodes, mites and insects. The “bad and the ugly” would be certain bacteria and earthworms, which urban soils have in spades. We know, you are shocked. You always heard earthworms were good – but that’s for gardens, not forests.  See more here.
  • Heavy metals: We wish we meant Metallica, but in fact there is a glut of lead, nickel and copper at two to ten times the concentration of rural soils. High levels were found far from roads, so these pollutants were not from exhaust fumes, but had floated in as airborne particulates.
  • Moisture: Under certain conditions, urban soils repel water. This “hydrophobicity” is attributed to hydrocarbons (pollution again) and wax. Forest plants protect their leaves from air pollution by producing a waxy coating which acts as a barrier. These leaves die and become part of the soil in the form of decomposed organic material. Soil surfaces that eschew water do not bode well for seed germination or seedling survival.
  • Rare plants are not immune to the ills of human interactions. This sweetbay magnolia,  (Magnolia virginiana) a New York State-rare tree, has been not only tagged with grafitti, but the base has been hacked and burned as well.

    Rare plants are not immune to the ills of human interactions. This sweetbay magnolia (Magnolia virginiana), a New York State-rare tree, has been not only tagged with grafitti, but the base has been hacked and burned as well.

    Compaction: Heavy human usage is a hallmark of urban forests, and so is its concomitant ailment of soil compaction. Well-traveled areas lack herbaceous vegetation, since many of our low growing, non-woody plants are delicate. And while woody trees and shrubs appear unaffected, below ground they are being strangled. Naturally occurring pore spaces that give soil its structure are squeezed out by compaction. Air and water flow is stymied by tightly packed soil particulates, the result of bicycles, motorized vehicles and even inappropriate pedestrians. Thus there is little root respiration or growth.

  • Decomposition rates: The breakdown of organic material is faster in urban areas due to higher temperatures (heat island effect again) and earthworms. Organic matter in various states of decomposition is called “humus”. This layer is home to the soils’ living entities, including the seeds that will become the future forest. This decrease in decaying organics exposes more soil to erosion.
  • Sassafras (Sassafras albidum) saplings and wild geranium (Geranium maculatum) in Pelham Bay Park in the Bronx.

    Sassafras (Sassafras albidum) saplings and wild geranium (Geranium maculatum) in Pelham Bay Park in the Bronx.

    Nitrogen cycling: Nitrogen is critical to plant survival (which is why it is a component of fertilizers). So nitrification, the conversion of ammonium to nitrate by certain soil bacteria, is an important soil process. Urban soils are faster at this conversion, making them higher in nitrates than rural soils and less fertile ground for plant growth.

  • pH: Higher pH characterizes urban soils. Healthy, native soils are typically slightly acidic. Urban soils, with unwanted additions of concrete debris, road salts and ambient dust, are often greater than 6. Such alkalinity discourages most desirables, attracting exotics like mugwort and little else.
  • Exotic vegetation: As many as 80% of the seeds in forest soils (seed banks) may be non-native. Plants that are intolerant of shade, such as scraggly weeds of sunny roadsides, are replacing those plants that have adapted to deep canopy umbrage, like our forest lilies.

More about urban soils

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How Natural Areas and Gardens Differ

This garden in Avalon, NJ is working overtime to be offensive. Not only does is have large areas of woodchips, but it also features purple loosestrife (Lythrum salicaria) and Japanese barberry (Berberis thunbergii), two highly invasive exotic plant species.

This garden in Avalon, NJ is working overtime to be offensive. Not only does is have large areas of woodchips, but it also features purple loosestrife (Lythrum salicaria) and Japanese barberry (Berberis thunbergii), two highly invasive exotic plant species.

The outlook of the botanist and horticulturalist differ in their philosophies. For the botanist, natural areas are dynamic, ecologically functioning units made up of biological organisms that are suited to the specific locality. Thus forests, wetland meadows, grasslands and dunes are managed with the goal of achieving and retaining ecological integrity, e.g. invasives are removed, human-induced threats are limited and ameliorated. For the latter, greenery is infrastructure. Gardens, lawns, and street trees are aesthetic elements that add interest to the visual landscape. A naturally occurring diverse suite of plants is replaced with the conformity of a limited, repetitive plant palette. Sites are managed to maximize ornamentation and human uses.

Healthy natural areas are characterized by:

  • Abundance of native plants & animals – A complex, living ecosystem adapted to its environment, having evolved over thousands of years. In the wild, individual flora and fauna emerge, grow, reproduce and die. These life cycles contribute to the long-term survival of the species.
  • Numerous vegetative layers – Tall canopy trees tower over saplings and shrubs, which cast shade on the wildflowers, grasses and ferns along the forest floor. Vines intermingle amongst the layers. This layering, found in all systems but most profoundly in our forests, confers structural complexity.
  • Rich, healthy soils – Soils are fecund, diverse, living ecosystems. Natural areas retain their native soils, which are infused with decaying plant matter and teeming with life. Unlike inert turf and mulch, the ground layer is a low nitrogen system. Soil improvements occur naturally through the addition of organic material and loosening of compact layers by plant roots.
  • Natural disturbances – Changes in species composition over time (succession) is the norm. Large- and small-scale natural disturbances create a patchwork of habitats and diversity of species over the landscape. Repetition of functional units allows for system-wide endurance of shifting environmental factors.

Horticultural settings are characterized by:

  • Abundance of exotic plants & animals – A simple system filled with flora chosen for aesthetic characteristics. Promotion of plants that tolerate extremes (road salts, cement-like soils) and endure artificial conditions (sidewalk pits, fill soils). Individual plants are impermanent, installed for short-term, and discouraged from reproduction (mowing, deadheading). Favorite haunt of earthworms.
  • Simplistic vegetative layers – Ecosystem processes of decay and renewal are overridden by the compulsion to make the area tidy (raking leaves, pulling “weeds”). Such cosmetic manipulation does not acknowledge the complexity of our natural systems.
  • Overworked soils – Overly fertilized, watered, and treated with pesticides. These soils are unable to support vegetation without continual amendments and improvements. High nitrogen levels are unnatural and encourage invasives. The average suburban lawn may use as much as 10,000 gallons of drinking water during one summer, and uses 10 times the pesticides as an acre of farmland.
  • Human disturbances – The methods for preparing a garden bed (turning the soil, grubbing) are a familiar ritual. Yet such continual maintenance decreases the complexity of the system. Many of these human disturbances cause more damage than most natural systems can endure. Perpetual strain on conditions increases vulnerability to natural stresses.

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Tell me more about nature in New York City.

Dominated by soft rush (Juncus effusus) and fragrant water lily (Nymphaea odorata) is Lily Pond, an example of an intact urban ecosystem, located in Blue Heron Park, Staten Island.

Dominated by soft rush (Juncus effusus) and fragrant water lily (Nymphaea odorata) is Lily Pond, an example of an intact urban ecosystem, located in Blue Heron Park, Staten Island.

“Skyscraper national park.” Kurt Vonnegut’s description supports the widely held view that New York City is a paean to the built environment. This collective image includes towering edifices, taxied roadways and neon billboards. The last thing one would expect in this milieu is nature. Yet sprinkled throughout the five boroughs are approximately 28,000 acres of city parkland. Discounting ball fields and swing sets, nearly half of these have significant areas of flora and fauna. They harbor the city’s true treasures: freshwater wetlands, salt marshes, beaches, and forests. Ensconced within these ecosystems are more than 40% of New York State’s rare and endangered plant species.

Still, it isn’t easy being green in the Big Apple. Over the past century, 75% of the city’s woodlands, wetlands and meadows have been destroyed. The persistent pressure of urbanization and its concomitant ailments has driven many of the City’s native plants to the brink of extirpation. We have already lost 43% of our flora including such treasures as the yellow fringed orchid (Platanthera ciliaris) and swamp pink (Helonias bullata).

The most recently published flora of the five boroughs relates a disturbing tale. Of 1,357 native plants ever recorded, only 778 species remain. Somewhat resilient were trees and shrubs. This may be attributed to the protection afforded by their tough outer bark. Overtime, as suitable habitat shrinks and these trees and shrubs are unable to recolonize, they too will pass.

Pink ladyslipper (Cypripedium acaule), is one of the few orchid species that has managed to survive in NYC. But for how much longer?

Pink ladyslipper (Cypripedium acaule), is one of the few orchid species that has managed to survive in NYC. But for how much longer?

Plants that disappeared more quickly were less resilient non-woody ground layer vegetation, like ferns, wildflowers, and grasses. Annuals and biennials, with their short life cycles, were also in decline. Flora of wetlands and open meadows were also hit hard. 3 These herbaceous plants, the foundation of plant species diversity, appear to be acutely vulnerable to anthropogenic disturbances. The latter is especially germane on an archipelago teeming with 8 million humans. In Manhattan and Brooklyn, exotic plants already outnumber the surviving native species.

Retention of most of our native plants is not reason enough to pull out the party hats. The bad news is that the majority occur infrequently, with many known from only one location. Some plant groups have been utterly decimated. Sadly, the orchid family illustrates this point well. Once upon a time, there were 30 species of orchids in our fair city. Today only six species remain, consisting of only eleven populations. 4 There is one other orchid; the exotic broad-leaved helleborine (Epipactis helleborine). Visiting from Europe, this species is found in more and more parks every year.

Trailing arbutus (Epigaea repens), once common on Staten Island, now extinct in New York City. This picture was taken in the Delaware Water Gap.

Trailing arbutus (Epigaea repens), once common on Staten Island, now extinct in New York City. This picture was taken in the Delaware Water Gap.

Staten Island is New York City’s Xanadu, a place where many plant species have taken refuge. With bucolic vistas and 10% of its land preserved, the island bolsters the city’s native plant numbers. Yet its greenery still suffers the slings and arrows of human existence. Since 1990, Staten Island has lost more than 30% of its indigenous flora, including nodding trillium (Trillium cernuum) and wood lily (Lilium philadelphicum var. philadelphicum). Scarier still, no plant or habitat type was invulnerable to the onslaught of urbanization. And ecologically Richmond County is a microcosm for the rest of the country – a store of biological diversity struggling against high human population densities. Most of these species cannot be grown or transplanted successfully. Habitat protection is the only hope for their future.

New York has a higher percentage of open space than any major city in United States.  At 25.7  percent of its acreage, 53,000 acres of city (which has 28,000 acres), state, and federal, is parkland; more than Los Angeles and Philadelphia combined.  But Gotham’s spaces serve 8 million people, more than any other urban park system.  New York City spends a paltry sum on its green acres, only $41 per resident per year, with next to none of it going toward management of natural areas (monitoring population dynamics, maintaining habitat for rare plants, etc.).  By contrast, Seattle and Chicago annually spend much more per capita – $164 and $108 respectively.

Profile of NYC Department of Parks & Recreation’s Natural Resources Group and the work they do from September 2010, New York Magazine: “How Did New York City Become a Diverse Ecological Hotspot?

For information on finding natural areas in New York City, NYC Parks Department “Forever Wild” program.  More about the Forever Wild sites in this New York Times article, “Next Subway Stop, the Wilderness”

“God might have made a more beautiful place than Staten Island, but He didn’t.”
- George W. Curtis, 19th c. author and SI resident

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  1. Decandido, R. 2004. Recent changes in plant species diversity in urban Pelham Bay Park, 1947-1998. Biological Conservation 120:129-136.
  2. Drayton, B. and R.B. Primack. 1996. Plant species lost in an isolated conservation area in Metropolitan Boston from 1894 to 1993. Conservation Biology 11(1): 30-39.
  3. DeCandido, R., A.A. Muir, and M.B. Gargiullo. 2004. A first approximation of the historical and extant vascular flora of New York City: Implications for native plant species conservation. Journal of the Torrey Botanical Society 131(3): 243-251.
  4. The six remaining orchid species are: large coralroot {Corallorhiza macullata} (known from one population, a handful of individuals), pink lady’s slipper {Cypripedium acaule} (known from three locations), downy rattlesnake plantain {Goodyera pubescens} (known from one population, three individuals), whorled pagonia {Isotria verticillata} (known from two locations), bog twayblade {Liparis loeselii} (known from one location, few individuals), nodding ladies tresses {Spiranthes cernua} (known from four populations, a handful of individuals each).