Years ago, one of us—Ludwig—had business in the seaside city of Aqaba on the Gulf coast of southern Jordan. It took most of a day for my colleague and me to be driven down the ancient King’s Highway from Amman. We arrived late in the afternoon and gave our drivers the evening off. After checking in to our hotel, we wandered the town, keeping to the seaside road.
We came upon what seemed to be a vacant lot tucked into a shoreline corner between the beach and a canal that ran back into the city. On closer inspection, it turned out to be an archaeological site. Under excavation was a street from an ancient town, with the foundations of small, rectangular buildings built wall to wall. Informative signs identified the town’s souk (market), and explained the architecture of the buildings. Most were combinations of commercial space—shops or storehouses—with residences. With the quiet waters of the Gulf of Aqaba meters away, and the sun setting behind some date palms, it was easy to imagine life in the trading town of Ayla around 600 AD.
Older ruins—dating to Roman times and beyond, possibly to the middle of the first millennium BC—were reputed to be nearby on the shore. Within Ayla itself, the narrow street with open building fronts didn’t seem that different from a modern souk in the crowded alleys of any Middle Eastern city. You could feel the heat of the day slipping away as the markets opened, smell the dates and lemons for sale, hear the calls of the cloth-sellers and metal workers. Ayla was one of the first conquests of the Islamic Caliphate rising from Arabia. Its part on the world stage didn’t change much, only the place taxes were sent. Ayla remained a busy trading crossroads where land and water transportation met. Small industry prospered. The population supported itself on the combination of local produce and import/export business.
It immediately occurred to us to seek the sources of fresh water and agricultural production that kept Ayla going. Alas, these hinterlands—except for the waters of the Gulf itself, which undoubtedly provided fish and shellfish--are now buried beneath infrastructure of the modern city. But it occurred to us that, at least in pre-Roman times when trade was likely a smaller proportion of the city’s life support system, it must have been a tough place to support a sizeable population.
And what was that population size? Well, it fluctuated, depending on whether Ayla was in the trading route mainstream or sidelined by alternatives. It may have changed with climate, as well, which varied over the life of the town from at least 1500 BC to near 1500 AD. Absolute population figures are not available. The infrastructure as I saw it could probably support no more than a few thousand individuals, including local providers of food, water, raw materials, and products. But the fact remains: this was a city on a body of salt water backed against a desert. Not the most propitious place for a city.
Consider the problems such a location entails. Fresh water is hard to come by. Pasture land is sparse. Trash doesn’t decompose in the salty soils. The air may be humid, at least spot-on the coast, but it carries the constant winds and grinding sands of the desert. And the location is exposed, from both land and sea, to invaders.
We were struck by the oddity of the location, and shocked at how much like a modern city Ayla looked. Many of our questions—why this particular place? How did they provision? What did it cost someone—in time, money, effort, risk—to live here? What benefits made the investments worthwhile?—were similar to the questions systems ecologist ask all the time.
Until that warm sunset on the Aqaba shoreline, it hadn’t occurred to us to think of cities as ecosystems. After that, we never thought of them any other way.
 a briefing-level history of Ayla is available at http://www.unesco.org/csi/act/jordan/preproject.htm
 a more detailed account built from a report of sequential archeological excavations is at http://oi.uchicago.edu/research/pubs/ar/91-92/aqaba.html
 a detailed discussion of population characteristics, but not size, is “Scholars and Society at Early Islamic Ayla.” Journal of the Economic and Social History of the Orient 38 (1995): 417-428. Presently available at http://upenn.academia.edu/PaulMCobb/Papers/445308/Scholars_and_Society_at_Early_Islamic_Ayla
Mesoamerica took a complicated and lengthy path to settlement . The process was slowed by a comparative lack of easily-domesticated livestock. The only sheep and goat species occupied high alpine regions beyond the reach of domestic horticulture. The only cattle were the Arctic musk ox and the bison, neither of which possessed the characteristics needed for domestication . In South America, high altitude camelids (llama, vicuna, alpaca) proved to be domesticable, but their restriction to alpine habitats limited their value.
The other problem was lack of diversity in wild grains. Western Hemisphere hunter-gatherers had a large variety of plants to utilize. Along the way, semi-domesticated and domesticated varieties of peppers, tomatoes, squash, gourds, beans, avocados, papayas, and guava contributed to the human diet. Eventually, quinoa was domesticated, and soon thereafter, maize. At this point, several thousand years after agricultural settlements had appeared in Eurasia, urbanization came to the Americas. Stone infrastructure is preserved at high, dry altitudes in Central and South America. These mountain peoples also domesticated the potato, a process that required quite a bit of time as the wild plant was laced with toxic glycoalkaloids. Selection for size apparently helped tame the toxins and provided a path to safe, palatable potato tubers .
Maize/corn was a particularly problematic domesticant. First planted about 5000 BCE, it remained a small grain and the cob was chewed entire (instead of harvesting kernels) for several thousand years after that. Productivity in planted vs. wild varieties increased only slightly, due to more favorable growing conditions. It wasn’t until about 2000 YBCE that selected or perhaps hybrid maize increased productivity substantially. Until that time, foraging and hunting and gathering were more productive life styles than agriculture-based settlement.
After maize was successfully domesticated and productivity increased, “urban” style villages appeared throughout the Americas. Today, we know best the peoples who created large-scale stone infrastructure in relatively arid climates. This includes peoples of the southwestern U.S., the Maya in modern-day Mexico, and alpine dwellers including the Inca and the Aztecs. However, agricultural villages, including large settlements with extensive infrastructure, occurred throughout North and South America. In many cases, the buildings—dwellings and ceremonial centers—were constructed of wood and earth, and have not survived to the present day as well as the stone structures of some ancient American peoples.
Climate change was an important factor in the development of urban centers in the Western Hemisphere. Postglacial dry periods are believed to be largely responsible for the decline of urban centers in the southwestern US, including the Anasazi and Hohokam people who depended on intricate irrigation systems for survival . Great urban centers of the Maya failed after thousands of years, at least partially due to climate change (the other important factor may have been that the urban population simply exceeded the carrying capacity of the surrounding landscape). Humans lived even in the Atacama desert in the high Andes, which for thousands of years has gotten little or no precipitation.
Climate change had the effect of “cycling” some American urban cultures back to hunter-gatherer status after long periods of prosperity. Ponting [1, page 51] says that the time lag in agriculture-based urbanization meant that “…when the first Europeans arrived in the Americas in the 16th century they found a society that was in many ways comparable to those of Mesopotamia in about 2000 BC.” That is only partially true. Aztec and Inca peoples were in a stage of intense urbanization. Others, like the Maya and Anasazi, had already declined by the time Europeans “discovered” the “New World”.
PeopleSystems and Sustainability: This Week in the Global Environment.
Locking and Loading the Landscape Weapon
Throughout history, manipulation of land and water have been part of the arsenal that human beings apply to each other in war and other acrimonious relationships. Classic cases include flooding via dam breaching (as in the World War Two bombing of upstream dams on the Ruhr River), wildfire (burned crops and croplands in Russia and the Ukraine during World War Two), landform reconfiguration (the enormous anti-tank trenches around Kuwait in the early 1990s) and others.
One matter not usually viewed as an issue of war and weaponry is the Irish potato famine of the 1840s. The ecological facts are straightforward. Over time, millions of Irish farmers switched from diverse crops and crop cycles characteristic of smallholding farms to potatoes. And not just any potatoes. Cheap, vulnerable potato varieties, grown mostly for export. It was the English, at the time nominal overlords of Ireland, who shunted Irish agriculture into potatoes. According the Washington Post (book review dated 13 January 2013), the English saw the potato famine as a necessary offset to an inherent “laziness” of the Irish, and also as a way to solve some of their own regional overcrowding by moving people from densely populated areas to parts of Ireland recently rendered under-populated. Two excellent books on the topic came out early this year. American John Kelly published The Graves Are Walking: The Great Famine and the Saga of the Irish People (published by Henry Holt); and Tim Pat Coogan published The Famine Plot: England’s Role in Ireland’s Greatest Tragedy (Palgrave Macmillan).
These books take different analytical approaches. Kelly is driven by cold facts and documents the “nuts and bolts” of the potato famine. Coogan is more polemic, perhaps befitting an Irish historian.
The facts are that Irish farms by the 1840s eschewed crop diversity and grew potatoes. And only potatoes. And only one variety of potatoes. The result, when a fungus Phytophthora infestans hit and spread, was an ongoing sequence of crop failures. By 1851, Ireland’s population was reduced from more than eight million people to about six and a half million. And the country has yet to recover. The population remains below the level of 1845.
Basically, the English instituted a system that virtually assured mass starvation and emigration. Basic ecological principles tell us that intensive and large scale monoculture of an inherently physiologically weak crop will result in crop failure. In general, a year of crop failure would trigger a response in the form of more diverse plantings going forward. In mid-19th century Ireland, the farms continually planted potatoes. Year after year. And crop failures occurred year after year. The English accomplished their bizarre objective of depopulating Ireland simply by assuring that a single, vulnerable crop was monocultured for decades. Big problem, easily predictable and easily solved, if the English had any interest in “solving” it.
Landscape manipulation as a large-scale weapon continues deep into the twenty-first century. In Vietnam in the 1970s, the U.S. applied infamous herbicide Agent Orange to denude forested lands. In Africa in the Sudan and Somalia, factional warfare includes crop destruction, land ruination, and usurpation of arable areas.
In most wars, ecosystem degradation is an outcome. In Ireland in the 1840s, it was a weapon. A weapon that continues in use. This illustrates a fundamental problem of ecosystems science. Our work is filtered through political and social systems. It doesn’t take an ecologist to apply the landscape weapon. But repairing, restoring, or eliminating landscape weaponry definitely requires ecological expertise. As far as I know, there is no uniformed service of ecologists to play these issues out during war. Perhaps it is time to rethink this. The National Oceanic and Atmospheric Administration, a component of the U.S. Department of Commerce, has a uniformed service of vessel operators. Maybe the U.S. Department of Defense needs an ecological services branch to monitor and respond to weapons oriented application of the biosphere. An odd thought, I know. But one that seems to be of growing utility in a world where wars are now mostly local or regional and susceptible to such weaponry. With a little forethought, USDOD might innovate to obviate or neutralize landscape weapons.
What do you think? Is there a role for ecology in wartime military? This presumably means continual support of ecologists in peacetime as well. But the investment might well be worthwhile. The case of England and Ireland illustrates why!
The roots of urbanization are oldest in the Middle East. But they are not alone. On a time scale not quite synchronous with the villages and towns of western Asia and Turkey, urban settlements appeared and began to grow in Middle America and central China.
On 11th Street, we saw people escaping local population traps by taking their culture and their skills on the road, expanding the urbanizing area regionally. It turns out that important and large-scale impacts of early villages and towns lie in the spread of the life style into landscapes peopled by hunter-gatherers. Each region—Middle East, Middle America, and Central China—contributed idiosyncratically to the First Great Transition. Because the crops, animals, tools, and intellectual artifacts were so different, it is worth our while to explore the spread of urbanization within each region. Let’s begin with the Middle East, which wedged its settlement culture onto a vast European landscape peopled with hunter-gatherers.
Before we turned the corner, we talked about the tools and skills that allowed sedentary peoples to spread beyond the borders of their settled territories. For people emerging from the fertile crescent and moving into the hunter-gatherer-dominated landscapes of Eastern and Central Europe, this “toolkit” has been given a name. It is called the “Neolithic Package”, and it provided the easy way out of the local or regional population trap built into the earliest villages and towns .
The Neolithic Package consisted of ground and flaked stone tools, pottery, rectangular timber buildings, domesticated sheep, goats, cattle and pigs, and cultivated cereals. Depending on where and when, the cereals might have been wheats of various origin (emmer, einkorn, etc.), barley, oats, millet and/or others. With this toolkit, settled people under population pressure in place could shift location relatively easily and safely. Perhaps the clearest way to view it is to see it as familiarity. Moving to a new location where ecosystem specifics might differ greatly from those at home, the emigrants could fall back on comfortable and familiar techniques and tools to overcome local exigencies. They didn’t have to start over every time they entered a new habitat. The stone, pottery, and building technologies were valid over an enormous range of environmental conditions. Once you’d mastered the Neolithic Package, you could not only survive, but prosper over a broad swathe of landscape with varied vegetation, climate, geology, and hydrology.
Climate shifted the balance on a year-to-year basis between newcomers settling in via the Neolithic Package and the local hunter-gatherer societies whose stability in place and time varied. Cunliffe (page 109) points out that active trade between groups was likely a given. Stationary farming communities could trade foodstuffs, stone tools, and pottery for game, furs, and timber. Even people might leave and join particular groups, based on individual predilection for stability or adventure.
The landscape, of course, was not infinite. Eventually, all useable land would be occupied or in play as part of hunter-gatherer territory. The carrying capacity (number of individuals an area of land could support) varied with environmental quality. Good growing conditions, abundant game, dense timber meant a landscape could support many more people than a drier, poorer, barer area. There is evidence (Cunliffe p 111) of violence among groups when times got tough. Soon thereafter, different styles of pottery and tool cultures showed that the European landscape was saturated—as many people were packed into the ecosystem as could be accommodated.
A similar process, albeit differing in specifics of the mobile “package” villagers took with them on the road, occurred in the Far East (China) and Meso America. We will compare those cultures when we get to 13th Street.
Meanwhile, the people-saturated landscape provided a feedback system that increased the size, area and power of settlements. The less land available for successful hunting and gathering, the greater the pressure to join up with sedentary villages where division of labor helped assure quality of life and sufficient nutrition. It was this time, around 5000 YPB, that humankind stepped inevitably and forever on the path to urbanization.
 Cunliffe page 88 ff
Word origins can be clues to conceptual relationships. The word “ecology” comes to us from the Greek oikos, meaning farm in the sense of house, home, or hearth. Logos means to study or investigate. Ecology is the study of where and how living things—in our case, people—live. The word “economics” is etymologically closely related. The basis is the same oikos, with nomos meaning management, counting, or accounting. Economics is the management of biological systems.
Key to the First Great Transition is linked changes—feedback loops—in the ecology and economics of human settlements. One suite of changes reflects human impacts on the ecosystem in place. The effects of permanent settlements are more intense, longer-lasting, and profound than those of seasonal or temporary hearths. Indeed, researchers  now identify long-lost cities in the Fertile Crescent by mapping “anthrosols”—soil systems created and maintained by human beings. In the arid Middle East, soils were drastically altered by the presence of ancient towns and cities. Human-impacted “anthrosols” are higher in organic content, have a finer texture, are lighter in color, and have different hydrogeological responses than native soils. Where anthrosols occur, people once lived in towns and cities.
Anthrosols result directly from human activities. Breakdown of mud bricks, build-up of trash and organic waste, surface impacts from movement of people, animals, and carts all contributed to the creation of the uniquely human soils that remain in place to this day.
Changes came to more than soils in settled areas, of course. Permanent water or adequate precipitation are necessary for agriculture, which alters the landscape drastically from its primordial, non-farmed status. Transport—inter-city exchange, primarily for trade, extends the impact on the landscape to areas well beyond the immediate boundaries of the city.
The full spectrum of landscape impacts resulted from the combined effects of ecology and economics. In the Levant, villages like Jericho, Ain Ghazal, and Tel abu Herera flourished between 8,000 and 6,000 YBP. Each town had hundreds of residents, and evidence of varied and differentiated labor. There is evidence that these earliest urban centers traded as far away as modern-day Turkey, as well as within and among the regional towns .
Timber is, of course, and indeed was, a severely limited resource in the arid Middle East. When settled communities improved the construction of buildings, one of the things they did was cover mud bricks with lime plaster. Manufacture of lime plaster requires massive amounts of wood. What forests there were in the area of each city were hit hard by the use of lime plaster in building.
Recovery of woody vegetation was impaired by foraging goats and sheep. The more people present, the more food was needed, the larger the affiliated population of sheep and goats. Larger cities thus degraded larger landscape area—the “urban footprint” expands with regional human population.
In addition, farming was necessarily associated with permanent water flowing in deeply incised wadis. Under these circumstances, soil erosion as a result of agriculture and animal-raising was inevitable.
In sum, the environmental impacts associated with villages, towns and cities led to ecosystem deterioration. As the local economy depended directly (via plant and animal food production) or indirectly (via trade in extractable resources such as obsidian and turquoise) on the health of the ecosystem, economic deterioration inevitably accompanied the landscape decline . Many important villages, towns and cities were forced to abandonment by the human population, leaving us with the large number of Tel mounds that show us where people lived in ancient times.
Was there a way out of this urban ecological trap? Well, the Levant and fertile crescent remain populous to this day, so it is clear that these problems were solved in antiquity. And they were solved primarily via what we now recognize as economic activities. It is clear that ecosystems are not solely driven by biology and geology. Human intellect and actions were important ecosystem control parameters 7,000 YBP, as they are today .
Two persistent towns were the walled village of Jericho and the larger Anatolian settlement of Catal Huyuk. Each depended on a very specific aspect of the ecosystem. There was a permanent fresh-water well at Jericho supporting the population in place. Catal Huyuk sat astride a deposit of high-quality obsidian.
At Catal Huyuk, where division of labor (and therefore apportioning of food resources to support non-producers, that is, the obsidian quarriers and tool makers) was already occurring, reacting to local overpopulation was relatively seamless. Along the trade routes for the obsidian were many other smaller villages and settlements. All it took to reduce the population pressure was to release the valve for emigration. Artisans took their stone and their tools and their skills and moved on to the next game. Where their products were sufficiently valuable, local economies shifted to serve and preserve their art and industry. Economy comes to the rescue of the ecology!
Now, to Jericho. A walled village, so space is automatically and artificially limited. The single well complex provided all the fresh water for the entire community. What happened when the number of people needing water exceeded what the wells could provide? Well, the populace had developed for themselves a successfully diverse ecosystem management program. They domesticated local grains and animals, farmed on an annual cycle, and had the technical “team” abilities to make such a program work elsewhere. Things too crowded, water insufficient? Take your skill sets—and your team—on the road. Move to the next location with resources to support annual-cycle farming and permanent fresh water.
What’s happened in both cases, and likely thousands of other settlements, is that economic complexities made it possible to overcome ecological contretemps. For the first time in history, our management of the natural system—the nomos—enabled and supported its inherent operations—the logos.
Looking back, this is clearly logical and in a sense the “no brainer” way out of the local population growth trap. From the perspective of millennia, it was the trigger for economics and ecology to be inexorably and forever intertwined components of what made human beings successful. This is a theme that carries our entire inquiry forward from here. At around 7,000 YBP, we became economic as well as ecologic creatures. Much drama, and much good and bad, were to flow from this linkage.
[Redmon page 110]
 PNAS paper
 Redmon page 106
 Ponting page 45 ish
One summer, our band of gatherers and their aurochs came upon a small valley with idyllic conditions. The slopes were grown over in rich pasture grass. Unusually dense thickets of both wild barley and oats, a permanent stream, a copse of crabapple, and scattered thickets of nuts, acorns, plums and berries made it the perfect place to spend some time. In fact, as the seasons transitioned, we found we had harvested enough grain and fruit that we could store it and make it through the winter. Easy enough to fence the aurochs in (truncating their instinct to move as a herd) and craft more permanent dwellings than we were used to.
The first spring was a little difficult as the group had never before tried to make one season’s harvest last into the next. But the aurochs were in milk, and green edibles appeared early enough to prevent hardship. In fact, the storage shelters each held residual seeds of rye and barley, and the group expanded the grain fields by scratch-plowing the sod and planting the winter’s leftovers. To their surprise, they found by midsummer that they were gathering enough to feed the entire community, and the entire community did not have to forage actively to do so.
And here is the key to the “First Great Transition” . People had sufficient calories, varied foods for good nutrition, and time on their hands. With the extra time (that, in prior years, had been devoted to migration, hunting and foraging), shelters could be strengthened and redesigned for year round use. Rather than prepare movable shelters of skins and frames, solid buildings of logs and/or sod could be built with the intention of occupation for multiple years. The building process absorbed some of the total of spare time the “easy pickings” gave the group. But there was still time to spare.
Since every member of the band no longer had to do the “jack of all trades” trick, people started to specialize. A couple of older children became adept at flaking high quality blades and spear and arrow points. And they could do so prolifically, since the tribe as a whole contributed to their sustenance and they no longer had to supply their own food while making points for the whole group. Similarly, individuals or small subgroups began manufactories for baskets and wicker fish traps, for pottery made from the clay of the stream bank, maybe even, some time later, manipulation of fiber for rope and woven cloth.
Remember back to that delicious pastry and espresso we had on Avenue A? Here we have, in place, a complex social structure with sufficient material wealth (food, water, raw materials) to support differential and specialized labor. This yielded a feedback loop that increased the quality of life with every passing season. The specialists in blades and points made better blades and points with years of practice. Pottery became more sophisticated and useful. And the processes of planting and harvesting became easier as crops adapted to domestication. Life got better and easier. And the band grew. More individuals could be fed, and prove useful in the different arts being practiced, so more children were born. Early childhood deaths declined. More buildings were erected to support more families. Here, in simplistic terms, is the first actual “town” according to the conditions specified by our caffeine-fueled discussions back on Avenue A. We have people engaged in differential activities, supported by other components of the community. Which other components became more efficient as, for example, quality arrowheads fed back on hunting and made hunting easier and less costly of time and effort. Despite its tiny size, our little valley is humming with social and economic activity, with an expanding population.
And that is how the next level of difficulty (and sophistication) arise.
First, the difficulty. Carrying capacity is the population level that a particular portion of landscape can support. Clearly, our idyllic valley can support far more people than a less diverse and verdant area of similar size somewhere on our traditional herding route. But it is not infinite. Now that we’ve got labor specialization, our population has to find a balance between those producing non-comestible products and the total available nutrition. It is easy to overshoot that mark (see discussion of flour beetles and the early science of population biology discussed coming up in Avenue B). And when we do, people either leave or starve, often both.
However. Remember the “quality” aspect of our production. High quality basketry, pottery and weaponry give us two ways out of the demographic trap. First, such goods make it easier and safer for subgroups to break off from the main group and move off to occupy new areas, if such are available. Rather than having to repeat the food production/manufactury/domesticated farming cycle, these groups can pretty much move right in, assuming land is available.
Second, we have goods to trade with other groups. Trade provides two additional adaptive responses to our overpopulation problem. For one thing, trade can bring us goods and services allowing us to increase our population in place by providing necessary items we haven’t developed for ourselves. For another, it provides a way out for individuals, families, or small subgroups. Via marriage or simple compatibility, some of our people can move off with the other groups who cross the paths of our valley home. In this way, we have a “pressure release valve” that allows us to bleed away some of the overpopulation without disrupting our comfortable and successful life.
Outmigration and trade. Ways to take our village to the next step on the way to becoming a fully functioning urban ecosystem. And bringing economics into partnership with our ecology, of necessity complicating our seemingly simple lifeways.
At this point, we need to take a divergence from walking the cross-town streets and stroll up Avenue A. The Avenues in this context are expositions of general principles or important sideline items that are not directly part of the story of development and maintenance of urban ecosystems.
In this case, since 9th Street is closing in on sedentary communities of villages or towns, it is important for us to define “urban” or “city” as the primary focus of this book. Defining “urban” proves to be something of a challenge. All of the easy-to-obtain definitions are circular—“urban” refers to cities, “cities” refers to urban areas! Not particularly helpful for present purposes. Let’s survey some definitions and come up with one that works in the context of historical ecosystems analysis.
The American Heritage Dictionary Third Edition (1992 Houghton Mifflin) defines “urban” as “of, relating to, or located in a city” and “characteristics of the city or city life”. AHD defines “city” as “a center of population, commerce, and culture; a town of significant size and importance”. A “town” then, is “a population center, often incorporated, larger than a village and usually smaller than a city”. Most useful of these definitions is that for a city as a focus for population, commerce and culture. But this does not help us differentiate an emergent city from any other gathering of human beings, any of which involves commerce and culture.
Let’s turn to the growing urban ecology literature in search of a working definition. Nilon et al.  ask “What is Urban?” Their answers involve human population density (1,600 people per square kilometer, and 50,000 people in the “central city” of a “metropolitan area”. From the environmental analytic perspective, these hard-number definitions don’t help us evaluate the ecosystems parameters involved in the shift to and ongoing functions of, cities.
McDonnell  defines “urban ecology”: “integrates both basic (i.e. fundamental) and applied (i.e. problem oriented) natural and social science research to explore and elucidate the multiple dimensions of urban ecosystems”. This definition is at least process-oriented. But neither McDonnell or any of the other contributors to this volume wrestle with definitions of “urban” or “city”. As seems to be the case in general for publications in the field, a common understanding of what constitutes “urban” and “city” is assumed. Given that (as we’ve seen on our walk from 1st to 9th streets), human aggregation is a continuum. There is no ecologically credible threshold for specifying where, in that continuum, a city becomes.
N.E. McIntyre  in a paper titled “Urban Ecology: Definitions and Goals” wrestles specifically with the need for and aspects of, effective definitions of “urban” and “city”. His fundamental premise is that “urban” is a relative phenomenon—what is “urban” in inland Australia might be considered “countryside” in heavily urbanized Japan. McIntyre doesn’t leave us hanging, though. He concludes that urban areas in general are characterized by high human population density, ubiquity of built structures, altered climate (urban heat island effect, most pronounced at night), landscape dominated by anthropogenic impervious surfaces, high concentrations of man-made chemicals, altered ecosystem productivity, and a large ecological footprint. This multiple parameter approach is useful as far as it goes. In particular, it provides a consistent basis for comparative analysis of extant landscapes, identifying and characterizing present-day urban ecosystems vs. the environmental matrix in which they are embedded.
For our purposes here, wandering up Avenue A seeking good coffee and maybe a pastry, we need a simpler definition. A definition that will let us understand the ecosystems thresholds of human aggregations of all kinds, and when sufficient thresholds are crossed to call an aggregation “urban”.
Complicating our task is the need for a definition of “urban” and/or “city” that contrasts with human-dominated landscapes in general. In fact, there are no longer any “pristine” places on earth. The hand of man reaches the entire biosphere, and works constantly to transform ecosystems from those touched by man to those dominated. For example, a large pasture is certainly an artifact of human change, and would not exist without continuing human inputs of money, energy, chemicals, and animals. But a pasture is not “urban”, it is not a component of “city”. Of course, pastureland may be within the ecological footprint of one or more urban areas. In the present day, with efficient transportation and highly efficient ways to transfer money, one pasture may be part of the ecological footprint of many cities. Years ago I worked in the Hashemite Kingdom of Jordan. One evening, a ship pulled into the port of Aqaba, and began to unload sheep. By the thousands. Those sheep were pastured in Australia, but the ecological footprints of Aqaba and Amman reached those antipodal pastures.
For present purposes, I propose an empirical definition of “urban” and “city”. A city is a) a human aggregation with complex social structure such that differential labor is integral (that is, diverse production occurs in place); b) inhabited by dense human populations directly linked by transportation and communication; c) dependent on resource production at locations remote from the city itself. Note that this definition explicitly does NOT specify a human population size threshold. This is because the historical continuum of human aggregation yielded relatively small physical entities that encompass these traits and must thus be considered “urban”. Early Jericho, for example, was on the verge of shifting from “town in a large landscape” to city, dominating that landscape.
In cities, humans directly affect key ecosystem parameters. Impervious surfaces (indeed the built environment as a whole) dominate the landscape, net flow of useful (high quality) energy is into the city, low quality (heat) energy is dissipated, net flow of necessary goods (water, food) is inward.
In summary, while a pasture is indeed a landscape component created by humans, the day-to-day processes are not governed by people. Contrast that with a parking lot, which is under the direct control and maintenance of people on a daily basis.
It is important to separate the city itself—the urban “core”—from the ecological footprint. As we resume our progress on the city streets, we will have this in mind. We will document ways in which management of the city per se and its remote footprint are different, yet essential to the holistic urban ecosystem.
 Nilson, C.H., A.R. Berkowitz, and K.S. Hollweg 2003. Introduction: Ecosystem understanding is a key to understanding cities. Pages 1 – 13 in Understanding Urban Ecosystems, edited by A.R. Berkowitz, C.H. Nilon, and K.S. Hollweg. Springer, NY.
 McDonnell, M.J. 2011. The history of urban ecology, pages 5 – 13 in Urban Ecology, Editor-in-Chief Jari Niemela, Oxford University Press, Oxford.
 McIntyre, N.E. 2011. Urban Ecology: Definitions and Goals. Pages 7 – 16 in I. Douglas, D. Goode, M.C. Houck, and R. Wang, editors, The Routledge Handbook of Urban Ecology, Routledge/Taylor & Francis, NY.
An important question arises with the realization that hunting-gathering lifestyles are not dystrophic. The question is—why agriculture ? The answer may be population pressure. One thing a hunter-gatherer society needs is land. Remember over on 8th Street that we were following our little herd of aurochs over hundreds of square miles of pasture and migration passage, gathering along the way. If our family or tribal group became too large for our aurochs and foraging land to support, the easy ecological solution was for part of the group to break off and operate as a separate entity. The problem with this solution is that it is finite. Only a few iterations, and the landscape is too packed with hunter-gatherer groups to support any additional. At this point, the solution becomes the converse—rather than spreading out to exploit more land, find a way to make the land support more people.
And that happens by agriculture. In simple terms, harvest concentrates on a subset of the food plants available, and particularly productive locations become the focus of that harvest. In southwest Asia, the “fertile crescent” from the Tigris and Euphrates valleys through Turkey and in the Levant, wild ancestors of wheat, peas, chickpeas, and lentils grew (and still grow) over their natural ranges. As people focused their harvest on individual patches or subpopulations of these species, a forced-selection process occurred. Humans harvested all available seeds, and replanted a subset to assure the following year’s growth. This selected against individual plants with high levels of self-dispersal, as humans favored those that were easy subjects for hand planting.
In addition, the plant species chosen by humans were weedy species, present in disturbed ground and able to survive drought and flood in turn. “Disturbed ground” is just what humans provide in abundance anywhere they settle, tightening the domestic connections.
Simultaneously, as people settled in place in small groups, their semi-domesticated animals settled with them. The animals—cattle, sheep, goats, camels—favored or at least could thrive on plants other than those selected by humans for cultivation. This helped shrink the trophic footprint of each group, as a parcel of land could provide provender for both people and animals.
This process of domestication and in-place specialization of people, plants, and animals occurred roughly simultaneously in three regions—southwest Asia, China, and Central and South America. The plant and animal species differed in each region, of course. But these areas provided the ecosystems focus for the transition from itinerant to settled human life.
Between 10,000 and 7,000 BP, sedentary human groups began to develop urban infrastructure. Around 6500 BP, Jericho was a walled town of 10 acres. Catal Huyuk was a larger town in present-day Turkey. Three or four thousand years later, towns appeared in China, with the population dependent on millet and rice. In Mesoamerica, towns developed on a foundation of maize, palm, squashes, and potatoes.
Crossing 9th Street was a major step on the human path to the urban ecosystem. At this point, towns and villages are appearing across the landscape around the world. Now, social differentiation and size expansion will become important parameters in the development of the city system.
 The transition to agriculture, its regional specifics, and general ecological drivers are discussed concisely and effectively in Clive Ponting’s A Green History of the World, Penguin Books 1991. The topic merits a full chapter in Ponting’s book, titled The First Great Transition.
Let’s pop the lid on our environmental tool kit and explore some of what’s available at this point in human history. Most conspicuously, we’ve got our Swiss Army Knife of gadgets—fire. To this point, we’ve used fire for everything from providing social framework as the hearth to a method by which a few individual people can transform entire herds from dangerous cohabitants to high quality food and shelter. More sophisticated tools include stone blades, used for everything from killing to cleaning to butchering a kill. We’ve got our fellow-traveling pack members—dogs. At this stage in our relationship, dogs help us hunt and we help dogs eat. And dogs contribute to that hearth—the warm and secure feeling we have in smart, skilled, diverse friends and family. Most importantly, we’ve got our brain. And at this moment in human history, it is a particular aspect of human brain power—the ability to hypothesize, test, recall and act on positive findings—that is critically important.
The animals we follow and domesticate further—in our case, let’s assume we’re in the forests and savannah landscape of eastern Europe, working with a small herd of aurochs—give us enormous potential. We shape our animals into food, shelter, and weapons and end up following them as leaders. We have to get our animals to proper forage for each season. They need to get to certain places in the land at certain times of year. Of necessity, we go with them. Along the way, we bring that brain power into play, at first rather informally. We notice that some places our herds pass have plants that provide important and nutritious plants. In fact, we find that places that support our herds every year have higher volumes and higher quality food plants of the same species that are scrawny and less nutritious where they are not fertilized annually.
This is a critical point in the history of the biosphere. Henceforth, human beings are not just fellow travelers in the ecosystem, working within the constraints provided by the cycles of water, nutrients, gases and biota. This is the point at which human beings begin to exert brain-mediated control of those constraints. We begin to change from passenger to pilot. A transition that has continued for more than 10,000 years to get us where we are today.
The sophisticated hunter-gatherer lifestyle is not, as sometimes popularly perceived, brutal, difficult and short. In fact, with the complete technological toolkit, groups of sustainable size, and effective relationships with domesticated animals, life was rather safe, sane and comfortable . With one major, consistent difficulty remaining in the everyday life of human beings. The difficulty is the incessant, unavoidable, absolute need to travel regularly in a familiar landscape.
Travel, even with domesticate animals from aurochs for food to dogs for protection is dangerous, logistically difficult, and a hindrance to further development of that social pull of the hearth bringing more and more people together peacefully and comfortably.
In addition to fertilizer, people noticed that ground that is “worked”—dug up, disturbed, turned over on an annual basis—maybe where the herd was traditionally bivouacked—yielded improved quality and quantity of plant crops . It was just a short step from passive ground working to harnessing the animals and actively employing them to plow and disk the soil. Here is an important feedback loop in the trophic network of humans and their food. By staying in one place and taking care of the same plot of land year after year, the landscape can provide both animal products (meat, blood, milk, hide, bone) and vegetable. The better the care taken of the parcel of land, the better the yield of plants for food and fiber.
However. We remain at the mercy of the ecosystem for selecting and providing the plants that prosper in our now carefully nurtured homestead. We’ve involved ourselves in an ecological poker game—if we’re lucky, we draw the best plant species cards from the deck which contains multiple species. The pool of plant material available in any area is greater than what ends up being utilized, and the selection is a matter of chance, depending on what seeds reach what land when and under what conditions.
Notice that we now have in place all the social and physical parameters needed for a group of human beings to transform into a sustainable landholding society. Except for one. An important one. The plant material we are relying on are selected for us, more or less at random, by nature, with little or no input from us. Fields and fields of, for example, purslane, are certainly edible and nutritious, and can feed both man and beast. But there are many drawbacks to purslane as a staple or primary food source, ranging from its mucousy texture to bitter flavor and tough, herbivore-fighting texture. If we could identify the plant species we wanted every year in every place, we would have the last brick needed to build the wall of human civilization.
As with animals, it turns out only a few species of plants of the hundreds of thousands possible are effective and ideal for interaction with human beings. When we stop for an espresso and some pastry on 9th Street, we’ll learn the intricate story of the process by which humans and plants became intimates, completing a protosociety that provided the basis for the massive transformations that made human beings into an urban animal and transformed the biosphere into a human construct.