Life in the fast lanes

Urban areas are "living organisms" in themselves and create their own ecosystems

Published: Wednesday 30 June 1999

-- nature flourishes most alone, far from the towns where (humans) reside, wrote the American philosopher Henry David Thoreau. According to him, cities do not have an ecosystem of their own. Frederic Clements, the American plant ecologist went a step further and describes habitats as a series of closed loops that together form a self-regulating system. Clements also describes how plant communities develop through successive stages, eventually settling into a relatively stable stage. The belief that nature tends towards balance and harmony finds its roots in Clement's ideas ( New Scientist , No 2178).

Till very recently, environmental scientists took this theory for granted. But many ecologists no longer see it this way. They believe that relationships within an ecosystem are influenced by factors that are outside a given habitat. Besides, individual habitats themselves are not as harmonious as Clements suggested. A grassland fire, an infestation of rodents or a bad storm, for instance, can break what were once thought to be homogenous landscapes into a series of distinct patches.

Steward Pickett, an ecologist with the Institute of Ecosystem Studies in Millbrook, argues that a modern metropolis is like a living organism. He says if a Martian visited a metropolis on Earth, it would find cities no different from rainforests and coral reefs. It would look at cities being governed by biological and physical processes smilar to that of living organisms like human beings. That is to say, a city, to sustain itself has to consume certain things (fossil fuel) and then excrete (pollution). Picket's analogy is based on a project that he has undertaken as director for the Baltimore Ecosystem Study. It is the first project of its kind where a city is being looked at as an ecosystem. The researchers' goal is to understand Baltimore's hydrology, its microclimate, its nutrient cycles and its flow of energy and to figure out predator-prey relationships and the competition among species in the habitat.

"One of the biggest challenges we have in ecology is understanding the role of human institutions in ecological processes," says Pickett. To do that, researchers are attempting innovations with standard ecological models to incorporate social aspects of human behaviour like legal, political and educational interests, and material actions like constructing roads, buildings and bridges. They argue that these human actions interact with biology, geography and chemistry around them in ways similar to those found in old growth forests. If these connections are made, the researchers say, it will go a long way allowing civic planners to accommodate nitrogen and carbon in their budget.

But still, it is hard at first glance to see how cities fit into any ecological model. For one thing, they involve massive inputs and outputs -- huge quantities of resources coming in and equally huge quantities of pollution and garbage going out. "People argue with me and say urban areas are abnormal, that there is no ecosystem the way we think of ecosystems," says Rich Pouyat, a soil scientist with Baltimore project. "That's hogwash. If we cannot adapt our current ecological models to urban ecosystems, then we need to go back and re-evaluate those models."

Urban ecologists say cities have networks of biological relationships chained to the laws of the physical world. This is not to say that the relationships are static. On the contrary, they are dynamic. The networks that once existed get disrupted and new ones emerge, but the air chemistry, water system, flow of energy and mix of species, including humans, continue to act on and influence each other, just as they do in the "wild". The aim of the Baltimore project -- and another one in Phoenix, Arizona -- is to build a coherent picture of the many strands of relationships that form the urban ecosystem. To do this, Baltimore researchers are mapping and cataloguing the entire city from the abandoned lots of the inner core to the sprawling suburbs.

It is too early to come to any conclusion, but 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 came across a survey that had been done 50 years earlier on a 18-hectare patch of old growth forest in the New York Botanical Garden. The survey gave the location and species of every tree that was over three inches in the forest. McDonnell decided to again survey the forest. He found that the nature of habitat had undergone a dramatic change. What had been a hemlock forest -- the habitat that predated the city -- was now a mixed bag of species that included Norway maple, black cherry and ornamental Asian cork trees. The new vegetation had emerged from the seeds bought by wind or wildlife from the surrounding city. What is significant is that the transformation took place without any human intervention. But how did they manage to gain a foothold in foreign territory? 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 cannot survive in the modern city?

These questions intrigued Pouyat, and he 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 C; and there are fewer microinvertebrates, such as mites and springtails, fewer species of fungi and fewer fungus-feeding nematode worms.

The soil in the city is different from that of the rural forests, the latter lacks thriving populations of several species of earthworms. As a result, when Pouyat put leaves from the same tree on the different plots, he found that the city forest actually had the highest decomposition rates -- a fact borne out by the observation that city forests have a thinner layer of leaf litter.

After closely looking at the results of other studies carried out since 1996 by Margaret Carreiro, a microbial ecologist at Fordham University in New York, Pouyat suspects that organisms that live in urban forests 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 byproduct of this same process is more often ammonium. All of which leads back to the trees. "We think this fundamental change in the chemistry of the soil affects the competitive interactions of various trees," says Pouyuat. "And part of the success of the non-native trees may be because they are more efficient users of nitrate."

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. Another study has shown that trees increase or decrease the energy efficiency of buildings depending on where they are planted and what materials and designs are in the building. By incorporating such knowledge into landscape decisions, building managers can not only save on energy costs, but also reduce emission levels as fewer power plants would be required to produce the same amount of energy. Even decisions such as which type of tree to plant might make a difference. "Certain species, such as oak, emit a lot of volatile compounds which are known to lead to ozone formation," says David Nowak, an urban forest ecologist with the US Forest Service in Syracuse. "If you're trying to reduce ozone, may be you want to shift away from certain species that make it."

Connections between humans and other urban species are not so simple. For example, if people decide to plant street trees, it may affect the value of property in the area. 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 affects vegetation. Even the shape of our cities influences the wildlife they contain. Structure is 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 -- counting 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 the 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 their ecosystem via the atmosphere, through the 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 of microscopic plant forms known as macrophytes from the local bay over the past 30 years -- a decline unmatched in the 2000 years that have been studied so far.

Since macrophytes are important food for shellfish and juvenile fish, the impact of their decline reaches even seafood loving humans. Several factors, including increased sedimentation from deforestation and phytoplankton blooms fuelled by agricultural fertilisation, are blamed for murkier waters. 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. "We 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 and the trash you produce," says social ecologist Morgan Grove. "Putting it together, we'll 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." This will help identify which behaviours 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, the study should open the door to understanding how to manage the system.

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