Making the Case for Landscape EcologyAn Effective Approach to Urban Sustainability

Jianguo (Jingle) Wu

S Oc 01

ABSTRACT Urban sustainability is one of the most pressingand chaiienging tasks facing humanity today because cities arethe primary sources of major environmental problems, the cen-ters of economic and sociai developments, and home to morethan half of the worid popuiation. While the ecological, economic,and sociai dimensions of sustainabiiity are equaliy importantin principie, the ecology of cities is arguably ieast studied. Butthis situation has been changing rapidly in recent years, in thispaper, the author compares and contrasts different perspectivesin urban ecology and examines their relevance to urban sustain-ability While aii perspectives are usefui in some ways, the au-tbor argues, a landscape ecology perspective that integrates ele-ments of sustainabiiity science seems most comprehensive andeffective. This integrative perspective views humans as powerfulecosystem engineers" or agents that are criticaiiy important for

developing urban sustainabiiity. It focuses on the human land-scape scaie that is iarge enough to include key ecoiogicai andsocioeconomic processes and smail enough to aliow for detailedmechanistic studies. The landscape ecology approach also em-phasizes the interrelationship between urban landscape patternsand ecoiogicai/socioeconomic processes on different scales, andencourages place-based research that integrates ecology withplanning, design, and other social sciences.

KEYWORDS Urban ecoiogy, landscape ecology, urban sustain-abiiity. landscape planning and design

We are at a historic turning point in human civi-lization as Homo sapiens is transforming from

a predominantly agrarian to a mostly urban species(Figure 1). The global human population has grown ex-ponentially since the Industrial Revolution in the late1700s, rising from 1 billion around 1800 to 2 billion in1927,4 billion in 1974, and 8 billion by 2025 (United Na-tions 2004a). The world urban population bas increasedmuch faster than the rural population, rising from 14percent in 1900 to 29.1 percent in 1950, 47 percent in2005, and about 61 percent by 2030. The rural popu-lation has essentially stopped growing, and all futurepopulation growth is expected to occur in urban areas,most of which will take place in developing countries.'While the global population is likely to stabilize around9.1 billion by 2100 (United Nations 2004b), urbaniza-tion will continue even after 2100 because of continu-ing rural-to-urban migrations. Therefore, urban areaswill increasingly become the primary habitat for bu-

mans although their consumption of resources reachesinto diverse ecosystems near and far.

This increasingly urban nature of humanity hasa number of profound environmental, economic, andsocial implications for the world's future. Although ur-banized areas cover about 2 percent of the earth's landsurface, they account for 78 percent of carbon emis-sions, 60 percent of residential water use, and 76 per-cent of the wood used for industrial purposes (Brown2001). The environmental impacts of urbanization areexceptionally intense locally and go far beyond the citylimits to have regional and global consequences. The"ecological footprints" of a city for different resourcescan be hundreds of times as large as its physical size.̂Here I briefly discuss several environmental effects ofurbanization to lay the groundwork for later explora-tion on urban sustainability.

First, urbanization can significantly influence localand regional climate by altering land cover patterns andconsequently surface radiation regimes and energy bal-ance. The best-documented example of anthropogenicclimate modifications is the urban heat island (UHI)phenomenon, tbat is, cities tend to have higher air andsurface temperatures than their rural surroundings(Voogt 2002; Arnfield 2003}.̂ For example, in metropoli-tan Phoenix, Arizona, urbanization increased the meandaily temperature by 3A°C (or 5.58^) and nigbttimeminimum temperature by 5°C (or 9.0"F) over a periodof 50 years, resulting in the doubling of "misery hoursper day" for humans and a significant increase in energyconsumption for cooling, and heat stress on biologicalorganisms (Baker et al. 2002). Second, urbanization sig-nificantly affects water resources due to increased percapita use of fresh water and contamination of watersources by sewage and wastes in cities. Also, alteredland cover patterns and increased impervious surfacescan significantly change the flow paths of surface waterand hydrologic cycling of the urhan landscape. Third,cities are the major producers of greenhouse gases andair pollutants that cause health problems for humansand the environment.' Fourth, urbanization is the most

= 3.0

1.0

0.01950 1960 1970 1980 1990 2000 2010 2020 2030

Year

Figure 1. The dynamics of world urban and rural populationsbetween 1950 and 2030 (United Nations 2004a). The projectionof future population growth shown here was based on the mediumvariant scenario (i.e., assuming that fertility will decline from 2.6children per woman today to slightly over 2 children per womanin 2050).

drastic form of land-use change that affects biodiver-sity and ecosystem services intensively and extensively.Urbanization results in the loss and fragmentation ofnatural habitats as well as the introduction of exoticspecies (Hope et al 2003; Martin, Peterson, and Stabler2003). Numerous studies have shown that urbanizationhas profound effects on biodiversity, net primary pro-ductivity, watersbed discharge characteristics, and bio-geochemical cycles (Pickett et al. 2001; Whitford, Ennos.and Handler 2001; Gregg, Jones, and Dawson 2003).

The myriad environmental problems caused byurbanization indicate that most if not all our cities arenot sustainable (McGranahan and Sattertbwaite 2003).-'If global urbanization continues the way it has. theseproblems will likely accelerate explosively as humansbecome progressively more urban. To gain a glimpse ofthis possible future, just tbink of the differences in lifesupporting needs, aspirations, and ecological footprintsbetween a farmer in India and an urbanite in New York,and then magnify them by several billion times. Un-doubtedly, the performance of cities will increasinglydetermine human and environmental health as well asregional and global sustainability. As Kofi Annan (2002),the former secretary-general of the United Nations,stated: "The future of humanity lies in cities."

How can we develop more sustainable cities? Giventhe various environmental problems associated witbcities, ecology ought to have much to offer. However,has this been the case? How can we make ecological ap-proaches more relevant and effective for urban sustain-ability? In this paper, I address these questions by firstreviewing different perspectives in urban ecology, andthen proposing a transdisciplinary framework for study-ing and developing sustainable landscapes.

Different Perspectives in Urban EcologyA major goal of urban ecology is to understand the re-lationship between the spatio-temporal patterns of ur-banization and ecological processes. Cities may differdrastically in their architectural appearance and envi-ronmental settings, but one commonality is that thediversity and spatial arrangement of their landscapeelements undoubtedly affect and are affected by physi-cal, ecological, and socioeconomic processes withinand beyond their boundaries. Ecologists have longstudied the effects of the spatiaj pattern of urbanizationon ecological processes (Stearns and Montag 1974; Su-kopp 1990, 1998; Loticks 1994; Breuste, Feldmann, andUhlmann 1998). Recognizing that any classification isbiased by the classifier, be it an educated human or aprogrammed computer, I distinguish five urban eco-logical perspectives based on their research focus andmethodology (Figure 2).

These perspectives seem to have stemmed fromthree traditions: ecology in cities (EIG), ecology of citiesas socioeconomic structures (EOC-S), and ecology ofcities as ecosystems (EOC-E). From the EIC tradition, abioecology approach has evolved with a focus on howurbanization affects tbe distribution and dynamics ofplants and animals in cities, and much of this researchis related to urban biogeography. For example, bota-nists have documented the diversity and distributionof plants in cities for several decades (see Sukopp 1990;Sukopp 1998). To some extent, this EIG perspective hasinfluenced ecology-based concepts in design and plan-ning of cities. Second, the socioecology perspective hasfollowed the EOG-S tradition that views cities as socio-economic structures and applies ecological concepts

42 Landscape Journal 27:1-08

Ecoiogy in Citieswithout Socioeconomics

Figure 2. An illustration of theevolution of urban ecologicalperspectives.

(1) Bio-ecologyperspective

Perspectives in Urban Ecology

Ecology of Citiesas Ecosystems

(4) Integrattveurban ecosystern

perspective

(5) Urban landscapeecoiogy perspective

Ecology ot Urban Landscapes

Ecology of Citiesas Socioecortomic Structures

(Z) Socio-ecoiogyperspective

" Londscape ecology" Landscape orchiteclure• Landscape aesthetics• Urban planning and design• Urban sociology• 6iodiversity conservatian• Decision making science• Sustainability science and

*^ technology

Legend Biological components Q Socioeconomiccomponents

and principles such as competition, succession, inva-sion, and niche theory to understand them. This per-spective is epitomized by the Chicago school of urbanecology that defined the field as the study of the rela-tionship between people and their urban environment(Thio 1989). This is essentially "tbe" urban ecology typi-cally seen in sociology textbooks, whose influences canbe found in socially based concepts about cities in de-sign and planning.

Tbe other three perspectives have evolved out of theEOC-E tradition that considers the city as an ecosystemwitb both socioeconomic and biological components(Figure 2).The urban systems perspective is exemplifiedby studies that focus mostly on socioeconomic processeswitb bioecological components considered, but only asa factor (for example, Forrester 1969), and tbose that ex-amine the functioning and dynamics of urban areas asdisturbed ecosystems with a predominant emphasis onbioecological components. A more balanced empha-sis on both bioecology and socioeconomics is foundin the integrative urban ecosy.stem approach, whichis exemplified by the "Total Human Ecosystem" model

of Naveb and Lieberman (1984), or "bolistic landscapeecology" (Naveh 2000). This approach emphasizes theprocess interactions, feedbacks, control mechanisms,and totality of coupled human-environment systems.Since tbe 1990s, a landscape ecological perspective tourban studies bas emerged as ideas of beterogeneity,scale, and patch dynamics have pervasive influences inecology and environmental sciences (Wu and Loucks1995; Pickett et al. 1997; Zipperer et al. 2000; Luck andWu 2002; Blascbke 2006). This urban landscape ecol-ogy perspective views cities as spatially heterogeneouslandscapes that are composed of multiple interactingpatches within and beyond the city limits (Pickett et al.1997; Zipperer et al. 2000).

So, are these urban ecological perspectives ad-equate for urban sustainability research and practice?The simple answer is no. While all of them are useful incertain ways, some are more relevant than others. Thefive perspectives seem to have distinctive characteris-tics in their research focus and methodology (Figure 3).Tbe bioecology perspective views cities as severely dis-turbed ecosystems and humans as disturbance agents.

Wu 43

Bio-ecology perspective

Urban systems perspective

Integrative urban ecosystem perspective Socio-ecology perspective

Urban landscape ecology perspective

Emphasis on Bio-Ecology Emphasis on Socloeconomlcs

Emphasis onInterdisciplinarity and Transdisciplinarity

Urban areas as disturbed ecosystemsHumans as disturbance agentsBasic ecology in urban environmentSpaliotemporal patterns oforganisms and human influencesLacking cross-disciplinaryintegration with social sciences

* Cilies as ecosystems/landscapes* Humans as an integral component* Interactions between biophysical and

socioeconomic processes* Interdisciplinary and transdisciplinary

across natural and social sciences* Highly relevant to urban sustainabihty

* Cittes as socioeconomic systems* Humans as the primary or the

only system components" Ecological principles and

methods used only as metaphors* Lacking cross-discipiinary

integration with naturai sciences

Rgure 3. Different perspectives in ur-ban ecology and their major characteris-tics in terms of assumptions, researchemphases, and methodology.

adopts a biology-centered, basic science approach, andoffers little interdiscipliiiarity between natural and so-cial sciences. The socioecology approach, on the otherhand, views cities as socioeconomic systems designedfor human welfare, and tends to deemphasize the im-portance of biodiversity and ecosystem services, thusagain discouraging cross-disciplinary interactions be-tween natural and social sciences. The urban systemsperspective and the integrative urban ecosystem per-spective are centered on the principles and method-ology of the systems approach, consider humans asintegral components of the urban systems, and encour-age interdisciplinary and problem-solving research.Although the systems approach has proven to be quitepowerful in studying feedbacks and process interac-tions, its ability to deal witb spatial heterogeneity of eco-logical and socioeconomic patterns, which is essentialin urban studies, is limited. The urban landscape ecol-ogy perspective is tbe most inclusive approach in whichall previous approaches can be integrated together ascomplementary elements. In tbe next section, I attemptto explore why this is so and how this perspective mayhelp us to study and develop urban sustainability.

A Landscape Ecology Approachto Urban SustaJnabilityLandscape ecology is the science and art of studyingand influencing the relationsbip between spatial pat-tern and ecological processes on multiple scales (Wuand Hobbs 2007). The "science" of landscape ecologyfocuses on the theoretical basis for understanding theformation, dynamics, and effects of spatial heteroge-neity, whereas the "art" of landscape ecology reflectsthe humanistic and holistic perspectives necessary forintegrating ecology, design and planning, socioeco-nomics, and management practices. Landscape ecol-ogy promotes interdisciplinary and transdisciplinary

approaches,^ and landscape optimization and sustain-ability are among tbe top research priorities in the field{Wu and Hobbs 2002, 2007; Ahem 2005). The functionand sustainability of cities influence and are influencedby urban morphology and spatial processes of socio-economic activities. These networks of activities as-sociated witb cities are geographically distributed inspace and operate on multiple scales ranging from lo-cal neighborhoods to the region and beyond. Thus, thespatially explicit, pattern-process approach developedin landscape ecology seems quite appropriate for urbansystems.

Landscape ecology can significantly contribute toand gain from sustainability science, a rapidly emerg-ing transdisciplinary field that provides the scientificbasis for sustainable development (National ResearchCouncil 1999: Kates et al. 2001; Parris and Kates 2003).Sustainability science focuses on tbe dynamic interac-tions between nature and society, and addresses issuesof self-organizing complexity, vulnerability and resil-ience, inertia, thresholds, complex responses to mul-tiple interacting stresses, adaptive management, andsocial learning (National Research Council 1999; Kateset al. 2001; Clark and Dickson 2003). Thus, it is commit-ted to place-based and solution-driven research thatintegrates environmental, economic, and social dimen-sions encompassing local, regional, and global scales.In ligbt of sustainability science, urban sustainabilitymay be considered as the dynamic capacity of an urbanarea for adequately meeting the needs of its present andfuture populations through ecologically, economically,and socially sound planning, design, and managementactivities.

Landscape ecology is essential for sustainabil-ity research and practice for several reasons. First, thehuman landscape {for example, a watershed or metro-politan area) represents the most operational scale for

44 Landscape Journal 27:1-08

Figure 4. The scope of landscapeecology and its relationship tosustainability science. L«na»cBpB Ecology Contributm to

SiMtalnablllty Sclenca (SS|:• Tne human landscape {or region J

as a bask: spatial unrf for studyingenii mainialnrg sustaJnabfHiy

' PrQvtHirtg a hiiBraicnical and Inift-graiivB scokiglcal basis

' Provldlns ho^sttc approaches IDboooBcoBgical syslems

• ProvWing ifwoiy and mellwds lotsludying Bpattal neterogeneity andlataiing panem to crocess

• PrauMmg rneinods ard meWca kirquantifying ButiamaCility=icpv>dimi theoiy and melhods torscaling and uncerTaiirty analysis

Studying and maintaining sustainability because it rep-resents the smallest scale below which nature-societyinteractions cannot be adequately addressed (Forman1990). For example, to tackle the problem of urban sus-tainability, focusing on individual cities without ade-quate consideration of their surrounding landscapes isnot sufficient because important neighborhood inter-actions and top-down constraints would be missed. Onthe other hand, studying the sustainability of a particu-lar city by focusing on the global urban system is un-likely to produce practical outcome because of the lackof details. Also, Gobster et al. (2007] has recently arguedthat, although environmental problems and human ex-periences may take place on a range of scales, the scaleof landscape surroundings represents the human "per-ceptible realm." Wliile perception is not always reality, itcertainly helps actions in reality. Thus, the metropolitanlandscape arguably is the most effective focal level ofstudy for urban sustainability, which provides a com-mon ground that facilitates interactions and integra-tion among biophysical, socioeconomic, and planningand design disciplines (Figure 4).

Second, landscape ecology provides a hierarchicaland integrative ecological basis for dealing with issuesof biodiversity and ecosystem functioning at multiplescales. For example, the principles and insights of eco-logical hierarchy theory (O'Neill et ai. 1986; Wu 1999)can clarify our understanding of urban hierarchies andecological footprints. Third, landscape ecology has al-ready developed a number of interdisciplinary andtransdisciplinary approaches to studying nature-societyinteractions (e.g., Opdam, Steingrover, and van Rooij

2006; Pedroli and Pinto-Correia 2006; Potschin andHaines-Young 2006). Fourth, landscape ecology offerstheories and methods for studying the relationships be-tween spatial patterning and biophysical and socioeco-nomic processes. These relationships will not only helpus better understand the interactions between urbanform and function, but also can guide design and plan-ning practices (e.g., in terms of alleviating urban heat is-lands or increasing urhan biodiversity}. Fifth, landscapeecology provides a suite of methods and metrics that arehelpfi.ll for developing sustainability indicators (e.g., Bl-aschke 2006). Finally, landscape ecology provides boththeoretical and methodological tools for dealing withscaling and uncertainty issues that are fundamental tonature-society interactions (Wu et al. 2006), particularlyin urban landscapes where human-induced contingen-cies abound.

Urban sustainability certainly has much to do withlandscape planning and design. Recent developmentsin urban planning and design also seem to support anurban landscape ecological perspective. Platt (1994)provided a lucid discussion of how the concepts of openspace in North American cities have evolved in relationto urban design and planning. The "Picturesque Rural-ity" favored "the establishment of large, lavishly plantedurban parks," hut "put less emphasis on functional util-ity than on aesthetic effect through landscape designand horticulture." The "City Beautiful" monumental-ism "emphasized large, geometric plazas embellisheduith fountains, statuary, and forma! landscaping."while the "Garden City" notion advocated open spacesof different fonns (for example, practical community

Wu 45

parks and individual garden plots) as major elementsof the city and throughout the core of the city (Platt1994). Although the City Beautiful and Garden City wereamong the most influential paradigms in urban designand planning, modern urban design and planningprinciples have moved beyond an initial focus on cityform and human interests (Steiner 2000; Ahern 2005).Efforts by urban planners, designers, and architects tocombine urban morphology with ecological function-ing and efforts by ecologists to integrate the "ecologyin cities" with socioeconomic patterns and processeshave brought both sides much closer to a commonperspective^a landscape ecological perspective of cit-ies (Musacchio and Wu 2004). Yet, explicitly integrat-ing landscape ecology principles witb architecture andplanning for achieving urban sustainability is still in theearly stages, and many challenges and opportunitiesare expected (Ahern 2005).

Urban sustainability needs a spatially explicit,multi-scaled, and cross-disciplinary framework so thatenvironmental, economic, and social components areintegrated. Landscape ecology provides such a frame-work with theories and methodologies that facilitateinterfaces with natural and social sciences, particu-larly, on the scale of metropolitan landscapes. Urbansustainability is intrinsically related to landscape sus-tainabilit)'. Indeed, it is hard to imagine how anyonecan develop a sustainable city without consideringthe bigger landscape and global contexts as indicatedby its ecological footprint. A sustainable world is mostlikely composed of sustainable regions and landscapes,which in turn consist of sustainable cities designed notonly with sustainahle materials but also spatial patternspromoting environmental, economic, and social func-tioning. These spatial patterns of urbanization from thelocal city to the entire globe are all important for achiev-ing sustainability because of the hierarchical linkagesand influences between the levels of organization.However, as cities are engines of economic growth andcenters of diversity and change (Hinrichsen, Salem, andBlackburn 2002), regional and global sustainability mayincreasingly hinge on urban sustainability. The perfor-

mance of major cities is thus a barometer of the regionor nation in which they reside. Therefore, our challengeis not to stop urbanization, but ratber to speed up tbeprocess of designing and planning better cities.

DiscussionCities epitomize the creativity, imagination, and mightypower of humanity. Cities are magnificent because ofspectacular architecture; cities are inspirational be-cause fascinating stories of human history abound.Cities are attractive because resources, opportunities,and comfort coincide, and yet cities can be ugly anddreadful because poverty, pollution, and destructionof nature often reach the extreme. However, influencedby the natural history tradition, ecologists have longignored cities as legitimate study areas until recently(Collins et al. 2000). As a result, ecologists know moreabout the habitat of penguins in the Antarctic than theecology of cities in which they reside; they know moreabout the ozone hole in the stratosphere than the urbanbeat island in their neighhorhoods. Most existing eco-logical studies in urban areas have focused primarily onthe negative impacts of human activities on biodiver-sity and ecosystem processes. There even seems to havebeen a perception that ecologists are professionally atodds with developers, architects, engineers, and thelike hecause almost everything that humans do to na-ture was traditionally viewed as ecologically negative.Thus, ecology has been viewed historically as a "subver-sive science" by many (Kingsland 2005). The reality isthat scientists, engineers, and artists of all kinds mustwork together in order to achieve urban sustainability.This need necessarily begets a sbift in research empha-sis from "ecology in cities" to "ecology of cities" and to"sustainability of cities" through interdisciplinary andtransdisciplinary collaborations. It is encouraging thatthe current research activities in urban ecology seemto move toward this direction (Breuste, Feldmann, andUhlmann 1998; Zipperer et al. 2000; Pickett et al. 2001;Musacchio and Wu 2004).

Urhanization should not he viewed merely as acause for environmental problems, but also as an in-

46 Landscape Journal 27:1-08

evitable path to regional and global sustainability. Git-ies have lower per capita costs of providing clean water,sanitation, electricity, waste collection, and telecom-munications, while offering better access to education,jobs, health care, and social services. At tbe present, ur-ban areas are home to more than 50 percent of the worldpopulation, while accounting for less than 2 percent ofthe earth's land surface. To achieve regional and globalsustainability, we need to restrain the sprawl of humanhabitations and minimize the ecological footprints ofcities and individuals within them. This encourages ur-banization upwards (instead of outwards). In this regard,the idea of contact cities, or "arcology" (architecture +ecology), proposed by the renowned architect PaoloSoleri, may indeed be relevant to urban sustainability,and needs to be practiced beyond Arcosanti, a smallexperimental tovm 70 miles north of Phoenix, Arizona(see http://wvvw.arcology.com/). Although intensifiedurbanization does not necessarily reduce human im-pacts on the environment, this actually is happening inmany developed countries when more land is set asidefor natural recovery (Bisborrow 2006).

Urban sustainability requires the active partici-pation of humans. In this regard, it may he helpful toview humans as the most mighty ecosystem engineersfor creating sustainable landscapes rather than just dis-turbance agents as in traditional ecology. In tbe naturalworld, most species adapt to their environment, butsome, known as "ecosystem engineers," actively createand maintain tbeir own babitats (Jones, Lawton, andShachak 1994). Beavers are a prototypical ecosystemengineering species who build dams with tree brancbesto create a suitable habitat for themselves. As a conse-quence, the hydrology, vegetation, and function of thewhole landscape are altered significantly. However, evenif beavers cbange a natural landscape from one state toanother, their influences are usually confined withinthe local landscape. In the process of building cities,on tbe other hand, humans directly eradicate parts orthe whole of ecosystems, replace organisms with con-crete and bricks, and substitute biological complexitywith artificial regularity. Tbe scale of human influences

on tbe environment through urbanization can be ofglobal reach. Beavers cannot possibly destroy tbe eartbecosystem no matter bow diligently they work; but hu-mans can if their greed for consumption is not tamed.The rapid urbanization around the globe will not onlycbange the face of the earth ecosystem, but also pro-foundly influence tbe future of bumanity.

Urban systems are spatially extended complex sys-tems that often exhibit critical tbresbolds and surprises(Wu and David 2002; Alberti and Marzluff). Alan Kay,a pioneer of object-oriented programming and per-sonal computing, once said; "The best way to predicttbe future is to invent it." Developing sustainable cities,bowever, may be more difficult than inventing anythinghumans have ever created. This requires a paradigmshift in the study of cities and an unprecedented level oftransdisciplinarity. Tbe integration between landscapeecology and sustainability science seems to provide aneffective approacb to acbieving tbe goal of urban sus-tainability.

ACKNOWLEDGMENTS

This paper was based on an invited presentation given at theH. W. S. Cleveland Symposium in Landscape Architecture: "Mythsand Realities of Ecology, Design, and Ecosystem Health in theMetropolitan Landscape." held at Ihe University of Minnesota,April 21-22, 2006. 1 would like to thank Laura Musacchio for hercomments on the paper. My research in urban ecology and sus-tainabiiity science has been supporied in part by National ScienceFoundation under Grant No. BCS-0508002 [bioconiplexity/cnh)and DEB 9714833 (CAP-LTER).

NOTES

1. ln 2005, the percentage of urban population was 37 percentfor China and 28 percent for India, as compared to 79 per-cent for Canada and the United States, 89 percent for theUnited Kingdom, and 91 percent for Australia (PopulationReference Bureau 2005). However, this urbanization gap be-tween developed and developing regions has been narrow-ing steadily since 1975. By 2030, the urban population willreach 57.1 percent In developing regions and 81.7 percentfor developed regions (United Nations 2004a).

2. Ecological footprint is measured as the land area necessaryfor sustaining the current levels of resource consumption

Wu 47

and waste discharge by a population {Wackernagel and Rees1996). Cities often have huge ecological footprints becauseof their colossal demands for water, food, and energy as wellas their enormous output of wastes and pollutants. For ex-ample. Luck et al. (2001) calculated that among the 20 ma-jor metropolitan regions of the continental US. Phoenix—adesert city in the southwest—had the largest ecological foot-print for water (4.3 times the size of the metro area), food(116.1 times the metro area), and carhon assimilation (244.6times the metro area).

3. UHI is attributable primarily to the replacement of vegeta-tion by impervious surfaces that reduce or eliminate evapo-transpiration by plants and soil. In this case, much of thesolar radiation energy used for evapotranspiration in natu-ral landscapes becomes available for heating the air and sur-faces in cities. Also, thermal radiation from the heated urbansurfaces, often reflected many times by artificial walls beforeescaping into space, further increases the air temperature incities, especially during nighttime.

4. Air pollutants from urban centers are emitted primarily fromtransportation and industrial sources, including sulfur ox-ides (SOx). nitrogen oxides (NOx), carbon monoxide (CO),suspended particuiate matter (SPM), and greenhouse gases.Most serious air pollution problems (for example, photo-chemical smog, acid rain, and tropospheric ozone) occur inurhan areas.

5. Sustainability is commonly defined as the ability to meetfundamental human needs today while conserving the life-.support systems of the earth for future generations (NationalResearch Council 1999, Kates et aJ. 2001). The three pillars ofsustainability are environmental, economic, and social sus-tainability.

6. The terms interdiscipUnarity and transdisciplinarity havedifferent connotations in the literature. Here I adopt the us-age by Tress. Tress, and Fry (2005): interdisciplinary researchinvolves multiple disciplines that interact closely to achievea common goal based on a concerted framework, whereastransdisciplinary research has both close cross-disciplinaryinteractions and participation from nonacademic stakehold-ers and governmental agencies guided by a common goal.

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AUTHOR JIANGUO (JINGLE) WU is Professor of Ecology andSustaJnability Science in the School of Life Sciences and GlobalInstitute of Sustainability, Arizona State University, Tempe. Hiscurrent research interests include landscape ecology, urban ecol-ogy, and sustainability science. He is the author of 5 books andmore than 130 journal papers and book chapters, and currentlyserves as the editor-in-chief of Landscape Ecology.

-. 2007. Key Topics in Landscape Ecology. Cambridge: Cam-bridge University Press.

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