SINCE the industrial revolution, cities have been the subject of a peculiar
love-hate relationship in Western culture. On the one hand, we tip our hats to
their importance in the rise of civilisation. On the other, we pinch our noses
at their billowing smokestacks, mountainous garbage dumps and over-ripe sewers,
castigating them as the very antithesis of wilderness. Nature, wrote the
American philosopher and naturalist Henry David Thoreau, 鈥渇lourishes most alone,
far from the towns where [humans] reside鈥�.
But if an alien from another planet were to visit, would it disagree? Would
it not see cities as it saw rainforests and coral reefs: as just another of
Earth鈥檚 living systems? And would it not note that everything in the city is
ultimately governed by the same biological and physical processes that shape
environments everywhere? According to Steward Pickett, the answer is yes. No,
he鈥檚 not a Martian. He is an ecologist with the Institute of Ecosystem Studies
in Millbrook, New York, and he contends that our biases have blinded us to the
fact that even in a modern metropolis Mother Nature still calls the shots.
Now it is his job to identify how. As project director for the Baltimore
Ecosystem Study he has been given the task of leading some two dozen scientists
on one of the US government鈥檚 most radical large-scale expeditions into the
field of ecology. The project is currently being funded to the tune of $1
million a year and is expected to last several decades. This is the first time
that an entire city has been looked at as an ecosystem. The researchers鈥� goal is
to understand Baltimore鈥檚 hydrology, its microclimate, its nutrient cycles and
its flow of energy and to tease out predator-prey relationships and the
competition among species in this habitat. 鈥淚鈥檓 excited to find out how [nature]
goes on in cities because before now, we just haven鈥檛 looked. This is ecology鈥檚
last frontier,鈥� says Pickett.
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What makes this study novel is the researchers鈥� acceptance that a certain
naked ape is intricately linked to the system, just like any other plant or
animal. 鈥淥ne of the biggest challenges we have in ecology is understanding the
role of people and human institutions in ecological processes,鈥� says Pickett.
The researchers are modifying standard ecological models to incorporate
buildings, bridges, roads and all aspects of human behaviour鈥攅verything
from consumer habits to the decision-making processes of our legal, political
and educational institutions. They believe that these human components interact
with biology and biogeochemistry in ways that are as organised as the
relationships found in an old growth forest. And making these connections will
bring us one step closer to a new age of environmentally aware city planning,
one in which civic planners balance nitrogen and carbon along with the
budget.
Throughout much of this century, the dominant view of how ecosystems work has
been influenced by the ideas of Frederic Clements, the American plant ecologist
who was the first to describe habitats as a series of closed loops that together
form a self-regulating system, much like a living organism. It was also Clements
who described how plant communities develop through successive stages,
eventually settling into a relatively stable stage. From these concepts grew the
idea鈥攕till entrenched in popular culture鈥攖hat nature tends towards
balance and harmony.
But many ecologists no longer see it this way. For a start, they now realise
that the relationships within an ecosystem are much more open. So factors
originating beyond the borders of a given habitat can influence the make-up of
the soil, air, water and biodiversity inside.
What鈥檚 more, individual habitats themselves are not as harmonious as Clements
suggested. Nature rarely settles into equilibrium because disruptions
continually disturb that balance. A grassland fire, an infestation of rodents or
a bad storm, for example, can all break what were once thought to be homogeneous
landscapes into a series of distinct patches.
Concrete desert
At first glance, it鈥檚 hard to see how cities fit this or any other ecological
model. For one thing they involve massive imbalances of inputs and outputs, with
huge quantities of resources coming in and equally huge quantities of pollution
and garbage going out. Then there鈥檚 the sheer volume of pavement鈥攁 true
desert if there ever was one. 鈥淧eople argue with me and say urban areas are
abnormal, that there is no ecosystem there the way we think of ecosystems,鈥� says
Rich Pouyat, a soil scientist with the Baltimore project. 鈥淲e argue that鈥檚
hogwash. I go so far as to say that if we cannot adapt our current ecological
models to urban ecosystems, then we need to go back and re-evaluate those
尘辞诲别濒蝉.鈥�
Urban ecologists admit they have never studied an animal quite like humans.
What they do say is that cities, despite all appearances, remain networks of
biological relationships chained to the laws of the physical world. And while
the city may have disrupted many of the relationships that once existed in its
place, there is still air chemistry, a water system, flow of energy and a mix of
species, including humans, that continue to act on and influence each other,
just as they do in the 鈥渨ild鈥�.
The aim of the Baltimore project鈥攁nd one other like it in Phoenix,
Arizona鈥攊s to build a coherent picture of the many strands of
relationships that form the urban ecosystem. To do this, the Baltimore
researchers are mapping and cataloguing the entire city from the abandoned lots
of the inner core to the sprawling suburbs. 鈥淲e鈥檙e not ever going to understand
every connection,鈥� admits Pickett. 鈥淵ou try to understand certain key things and
then trace those interactions out.鈥�
It鈥檚 early days, but isolated studies in the past suggest that there is some
interesting biology to uncover. In the mid-1980s, ecologist Mark McDonnell was
working on a project for the New York city parks department when he stumbled
across a survey that had been done 50 years earlier on a 45-acre patch of old
growth forest in the New York Botanical Gardens. The survey, part of a job
creation project during the Depression, gave the location and species of every
tree over three inches in diameter. McDonnell, curious to see how the forest had
changed, resurveyed the same patch of trees. Something dramatic had happened.
What had been a hemlock forest鈥攖he habitat that predated the
city鈥攚as now a mixed bag of species that included Norway maple, black
cherry and ornamental Asian cork trees. Without any direct human intervention,
the forest had been completely transformed.
Obviously the new vegetation had emerged from seeds broadcast by wind or
wildlife from the surrounding city, where cork trees and other species are
planted in streets and gardens. But how did they manage to gain a foothold in
territory as foreign as a well-established hemlock stand? What caused all the
ecological components buttressing the old habitat to allow the invasion of such
a ragtag collection of newcomers? Was it as simple as saying that hemlocks just
can鈥檛 survive in the modern city?
These questions intrigued Pouyat, who quit his job with the parks department
so that he could devote his energies to research. Pouyat began studying the soil
in oak stands growing under urban, suburban and rural conditions. He found that
as you move from country to city there is a gradual change in a wide variety of
parameters: levels of copper, lead and nickel all rise; soil temperature
increases by between 2 and 3 掳C; and there are fewer microinvertebrates,
such as mites and springtails, fewer species of fungi and fewer fungus-feeding
nematode worms.
But soil in the city forest does have something it lacks in rural forests:
thriving populations of several species of earthworms. What did this all add up
to? When Pouyat put leaves from the same tree on the different plots, he found
that the city forest actually had the highest decomposition rates鈥攁 fact
borne out by the observation that city forests have a thinner layer of leaf
litter.
Looking at other studies carried out since 1996 by Margaret Carreiro, a
microbial ecologist at Fordham University in New York, Pouyat now suspects that
organisms that live in urban forest create their own type of soil. Organic
nitrogen in the leaf litter and other forest debris tends to be converted into
inorganic nitrate, whereas in rural forests the by-product of this same process
is more often ammonium. All of which leads back to the trees. 鈥淲e think this
fundamental change in the chemistry of the soil affects the competitive
interactions of various trees,鈥� says Pouyat. 鈥淎nd part of the success of the
non-native trees may be because they are more efficient users of the
苍颈迟谤补迟别.鈥�
The urban forest has become its own unique world in other ways, too. In 1997,
plant ecologist Gary Lovett from the Institute of Ecosystem Studies looked at
particles that enter the system from the atmosphere by sticking to leaves and
then being washed into the soil by rain water. Lovett found plenty of nitrogen,
which is not surprising since nitric acid is one of the by-products of car
exhaust, but he also discovered that trees endure a constant shower of calcium.
This probably comes from wind eroding the calcium carbonate in concrete, and
seems to play an important role in neutralising nitric acid in the soil.
Now Pouyat and the Baltimore team are trying to make more connections between
urban trees and the concrete jungle. They are looking at how different
combinations of vegetation, buildings and open space affect wind speed,
temperature, humidity and other microclimatic factors. A three-year study in
Chicago has already shown that trees increase or decrease the energy efficiency
of buildings depending on where they鈥檙e planted and what materials and designs
are in the building. By incorporating such knowledge into landscaping decisions,
building managers can save on energy costs, which will have an impact on
emissions from power plants. Even decisions such as which type of tree to plant
might make a difference. 鈥淐ertain species, such as oak, emit a lot of volatile
organic compounds which are known to lead to ozone formation,鈥� says David Nowak,
an urban forest ecologist with the US Forest Service in Syracuse. 鈥淚f you鈥檙e
trying to reduce that ozone, maybe you want to shift away from certain species
that make it.鈥�
Many connections between humans and other urban species are not so simple.
When local officials or a neighbourhood group decide to plant street trees, for
example, it may affect property values. This can raise the socioeconomic status
of an area, with a subsequent decline in the number of rodent species in
adjacent forest patches. Pets also have an impact on the city environment. They
are often rapacious carnivores, creating mayhem in local animal and bird
populations, which in turn will affect vegetation. Even the shape of our cities
influences the wildlife they contain.
Structure is now known to be a very influential factor in ecosystems. No one
really knows how the shape of a city determines what species live where, but the
Baltimore researchers are hoping to find out. The wildlife ecologists are
starting with birds鈥攃ounting species and measuring populations at several
plots, in a range of land-use categories to see how distribution is linked to
factors ranging from the shape of buildings to the structure of vegetation.
The hydrologists are also interested in structure. They are looking at how
the flow of nutrients and water differs in areas dominated by buildings, roads,
parks or lawns. They have set up several stations to monitor nitrogen, carbon
and phosphorus entering and leaving the ecosystem via the atmosphere, through
soil, in groundwater and streams and finally in nearby Chesapeake Bay. Their
hope is that city planners will one day be able to improve local water quality
by knowing how and where to incorporate novel infrastructure designs or
materials into their daily decision-making process.
As for the magnitude of the disturbance large-scale human habitation
represents, sediment core samples from Baltimore show how vegetation in the area
has altered since the end of the last ice age, 10 000 years ago. These indicate
some dramatic changes since the arrival of Europeans, including the virtual
disappearance from the local bay, over the past 30 years, of microscopic plant
forms known as macrophytes鈥攁 decline unmatched in the 2000 years that have
been studied so far.
Murky waters
Researchers blame murkier waters caused by several factors including
increased sedimentation from deforestation and phytoplankton blooms fuelled by
agricultural fertilisation. Macrophytes are important food for shellfish and
juvenile fish, so the impact of their decline reaches even seafood-loving
humans.
The team is also taking a more overt look at the impact of human behaviour on
the urban ecosystem, using questionnaires, census data and that bedrock of the
consumer culture, market surveys. 鈥淲e can look at your education, income, age
and so on and have a pretty good idea of your recreational behaviour, the amount
and types of food you consume, the trash you produce,鈥� says social ecologist
Morgan Grove. 鈥淧utting it together we鈥檒l start to come up with some idea of how
you as an ecological agent affect consumption of energy, use of water, use of
materials and so on.鈥� This will help identify which behaviours (anything from
dumping oil down the drain to how home owners discard lawn clippings) make the
most impact on air, water and soil quality.
No city is an island. Knowing the impact each has at the regional and global
level should open the door to understanding how to manage the system.
鈥淪ustainable cities and quality of life are very much on our minds,鈥� says
Pickett. 鈥淲e want to be able to put interfaces on our models that will allow
planners and even regular citizens to sit down at a computer screen and say,
`Let鈥檚 see what happens if we tear down these abandoned buildings and put this
kind of park or that kind of vegetation on it or that kind of built structure.
What鈥檚 that going to do for our local microclimate, for our runoff, for our
water management?'鈥�
At this point it鈥檚 still hard to imagine a modern-day Thoreau pouring his or
her heart into passionate prose about the nature of cities. But who knows? 鈥淚f
we鈥檙e ever going to achieve sustainability,鈥� says Pickett, 鈥渨e need to
understand that it鈥檚 not people versus nature, it鈥檚 people and nature. If we can
help people understand that the city, too, is a complex natural system, then we
might have a way to better connect them with the natural world.鈥�