time for a change: dynamic urban ecology

Time for a change: dynamic urbanecologyCristina E. Ramalho and Richard J. Hobbs

School of Plant Biology (M090), The University of Western Australia, 35 Stirling Highway, Crawley, Perth, WA 6009, Australia

Review

Contemporary cities are expanding rapidly in a spatiallycomplex, non-linear manner. However, this form of ex-pansion is rarely taken into account in the way thaturbanization is classically assessed in ecological studies.An explicit consideration of the temporal dynamics,although frequently missing, is crucial in order to under-stand the effects of urbanization on biodiversity andecosystem functioning in rapidly urbanizing landscapes.In particular, a temporal perspective highlights the im-portance of land-use legacies and transient dynamics inthe response of biodiversity to environmental change.Here, we outline the essential elements of an emergingframework for urban ecology that incorporates the char-acteristics of contemporary urbanization and thusempowers ecologists to understand and intervene inthe planning and management of cities.

Challenges for urban ecology in a rapidly urbanizingworldNot only is the world experiencing an unprecedented urbantransition [1,2], but contemporary urbanization also differsmarkedly from historical patterns of urban growth [3] (Box1), thus imprinting a unique signature on contemporarycities (Figure 1). Indeed, such cities are largely youngurban landscapes that have expanded rapidly over thecourse of the major urban transition that started in1950 and that has accelerated steeply over the past10–20 years [3]. Importantly, contemporary cities are in-creasingly expansive and dispersed landscapes [3,4], whichgrow and age in a spatially complex, non-linear manner [5].Consequently, they display multifaceted patterns of vari-able density across space and time, in which high densitybuilt-up areas can be finely interspersed with lower densi-ty, rural and natural areas [3,6]. By contrast, historicallydeveloped cities are contained areas that grew slowly, overseveral centuries or decades, in a relatively linear manner,through concentric and compact rings of development [3].Contemporary urbanization has major implications for theecology of cities, requiring ecologists to acknowledge thephenomenon actively in terms of the ways that they inter-vene in, and study, cities.

As cities expand, protected areas that are currentlyoutside city boundaries will soon become embedded in urbanlandscapes [4,7–9]. Furthermore, other natural areas andpreviously managed land with conservation value (e.g. oldfields) will be largely reduced to small and scatteredurban remnants. Whereas cities were previously relatively

Corresponding author: Ramalho, C.E. ([email protected])

0169-5347/$ – see front matter � 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.tree.2011.

confined spaces and therefore conservation of remnant eco-systems within their boundaries was not a priority, this is nolonger the reality. In fact, the conservation of urban remnantecosystems will become increasingly important for severalreasons. First, especially in areas with high beta-diversity,remnants provide the only remaining habitat for manyspecies [10]. Second, they provide ecosystem services (e.g.water infiltration, microclimatic amelioration, sequestra-tion of air pollutants, recreation and esthetics) that improvethe urban environment and enhance the wellbeing andquality of life of urban dwellers [11–13]. Third, urban rem-nants are the primary connection that many humans have tothe natural world [14]. Preventing the extinction of thisexperience [15] is important for conservation far beyond cityboundaries [16].

Urban ecological research is largely framed by a con-ceptual approach that assumes that urbanization and itsinduced environmental changes decrease in a linear gra-dient from the core to the city fringes [17]. This assump-tion, as well as oversimplifying urban environments [6,18],does not fit with the non-linear and complex growth ofcontemporary cities. Equally important, a static approachneglecting the young and rapidly evolving nature of thoselandscapes (and consequent ecological implications) ispredominant across current urban ecology frameworks.This might result from a slow recognition of the unprece-dented spatial and temporal scale of contemporary urban-ization [19]. Regardless of its cause, this mismatch hasmajor consequences for the scope of urban ecological re-search and calls for an urgent revision of the way in whichurbanization is assessed in ecological studies.

Here, we review how urbanization is evaluated in eco-logical studies. We identify key drawbacks of currentconceptual frameworks, emphasizing the misleadingassumptions of linear variation in urbanization intensityand age, the simplification of the set of intervening driversand the lack of a temporal approach. We then propose anemergent framework for urban ecology: the DynamicUrban Framework. This incorporates an explicit temporalperspective that considers land-use legacies and time-lagged ecological responses to ongoing environmentalchange. Furthermore, it includes a conceptual and analyt-ical structure in which relationships between interveningdrivers can be analyzed in a mechanistic manner. Here, itfocuses on remnant ecosystems, but is extendable to othercomponents of the urban environment. Finally, the frame-work can be incorporated or used in conjunction with otherconceptual frameworks with a stronger multidisciplinaryfocus [20,21]. It is time for a change in the way in which

10.008 Trends in Ecology and Evolution, March 2012, Vol. 27, No. 3 179

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Box 2. Lost in translation

Several terms and urbanization measures originating from social

sciences, geography and urban planning are widespread in the

ecological literature and have been adopted in core research

aspects, such as study design. However, these are often used

without a critical assessment of their ecological meaning [92] or of

whether they reflect the actual range of disturbance to which

ecosystems are exposed in the study context. Urban ecological

research is hence wedded to broad, vague terms and measures of

urbanization, which impedes ecologists from gaining a more

mechanistic understanding of the ecology of cities.

The problem above might result from two reasons. First, urban

ecology has had its major methodological and conceptual develop-

ment in social sciences, geography and urban planning, and has

only recently emerged in mainstream ecological research [44,93].

Consequently, relevant bodies of knowledge from those fields might

have been poorly transferred into ecological language and focus.

This has the pernicious consequence that recent advances in the

other fields are little recognized in urban ecological research. For

instance, whereas the spatial and temporal complexity of cities is

comprehensively acknowledged in urban planning (e.g. [94]),

ecological studies still approach those characteristics in a rudimen-

tary way. There is therefore a gap between fields, and important

considerations get lost in translation.

Second, the use of broad urbanization terms and measures has

been promoted as a common platform for data collection and

integration across different fields and in comparative studies

[18,95]. However, it is almost impossible to determine the definition

of single terms or a set of urbanization metrics that are universally

applicable [96,97]. The quest for integration in such a multi-

disciplinary field is important, but must happen in parallel with

the development of specific and ecologically driven vocabulary,

concepts and theories.

Box 1. A rapidly urbanizing world

Since 2008, for the first time more than half of the human population

of the world (3.3 billion) lives in urban areas and this number is

expected to reach 5 billion by 2030 [1]. This figure reflects an

unprecedented urban transition, with characteristics that are

different from any other moment in history. In a thorough recent

review [3], it was shown that contemporary urbanization differs

markedly from historical patterns of urban growth in terms of scale,

rate, location and form. First, the scale and rate of urban expansion,

both in terms of population growth and land-cover change, are

extraordinary. For instance, between 2000 and 2030 middle-sized

cities with populations of 500 000 to 1 million are expected to triple

their area [2]. Second, the location of urbanization is shifting.

Indeed, whereas the first urban transition (1750–1950) took place in

Europe and North America, increasing their urban population from

15 million to 423 million, the second urban transition (1950–2030) is

happening largely in Africa and Asia and will increase their urban

population from 309 million to 3.9 billion in only 80 years [2,91]. By

2030, these countries will contain 80% of the world urban

population. Third, the shape of the cities has changed. Whereas

historical cities were contained and well-defined areas that grew

through concentric rings surrounding a dense urban core, con-

temporary cities are no longer sharply defined and are increasingly

dispersed and expansive [3]. Furthermore, the patterns of urban

sprawl differ between countries. Indeed, in places such as the USA

and Australia, suburbanization is predominant and consists of

single-family residential development. In developing countries and

some European cities, sprawl occurs predominantly through peri-

urbanization, a more disordered development that expands along

urban corridors spreading out from metropolitan regions and

incorporating small towns and rural areas [3].

Review Trends in Ecology and Evolution March 2012, Vol. 27, No. 3

ecologists view, study and intervene in cities, so that theycan have a more active and positive role in the planningand management of the places in which most humans nowlive.

Conceptual frameworks in urban ecologyThe urban-to-rural gradient framework

The urban-to-rural gradient approach [17] has framedmost ecological studies analyzing the effects of urbaniza-tion on biodiversity and ecosystem functioning [18,22,23].For example, it aided in the understanding of how speciesrichness varies across urban–rural gradients [22,24] andin response to important urbanization drivers, such aspopulation density [25]. This framework views cities pre-dominantly as monocentric or sometimes polycentricagglomerations that grow through concentric rings sur-rounding a dense urban core [26]. Most importantly, theframework assumes that urbanization and its inducedenvironmental changes vary along linear gradients be-tween the urban core and the peripheral rural matrix [6].These include changes in land cover, species assemblages,the chemical and physical environment, and disturbanceregimes [26].

Framed by the urban-to-rural gradient, urbanization isdepicted and assessed in ecological studies in two mainways [18,26]. A first group of studies simply uses broadzoning categories that are defined subjectively based on thegeneral landscape context [27] or along a geographicaltransect [28] (Box 2). Such studies compare responsesbetween sites located in, for instance, urban, suburbanand rural areas [29]; urban, rural and natural areas[27], or city centre, city edge and peri-urban areas [30].Alternatively, linear distance to the city centre has been

180

used as a precise measure of the gradient [31]. A secondgroup of studies combines gradient analysis with land-scape metrics [32,33] and/or land-use types [34]. In thefirst case, census, cartography and remote-sensing dataare used to quantify socio-economic, land cover, land useand built infrastructure variables in or around the studysites. These variables are used individually or aggregatedas proxies to characterize the degree of urbanization.Commonly used metrics measure population density[32], income [35,36], percentage of impervious surface[37], housing [38] and road density [31]. This approachhas recently featured in studies aiming to define standard-ized measures of urbanization to be used in comparativestudies [26,39].

Other frameworks

Other conceptual frameworks have been proposed in urbanecology, whose use has been restricted to a few specific casestudies [20,21,40]. These frameworks are strongly based onthe integration of social and environmental sciences. Im-portantly, they have an ecosystem focus, exploring thelinks between human and biophysical drivers, patternsand processes, to understand the relationships betweenurbanization and ecosystem functioning. The Human Eco-system Model [40,41] is strongly rooted in social sciencesand, together with the hierarchical patch dynamics frame-work [42] and watershed models [43], provides the concep-tual and analytical core for the urban Long-termEcosystem Research (LTER) projects in Baltimore andCentral Arizona-Phoenix [44,45]. Other frameworks in-clude the Human Modification Framework [18], the

(a)

(c) (d)

(b)

TRENDS in Ecology & Evolution

Figure 1. Contemporary cities. Aerial perspectives of Chicago (a,b) and Houston, USA (c,d) illustrating how contemporary urbanization imprints a unique signature on

cities. Contemporary cities are largely young and rapidly evolving urban landscapes that have expanded dramatically over the past few decades, during the second major

world urban transition. These cities no longer have a compact development, but instead are highly expansive and dispersed, sprawling in fractal or spider-like

configurations [5], and embedding functioning or decaying fragments of other land uses (e.g. agriculture, forestry or remnant vegetation) in the rapidly changing matrix.

The complex spatial patterns of urban growth reflect not only past landscape configurations, but also current socioeconomic and political processes, such as planning,

transportation costs, agglomeration economies and market prices [3]. Reproduced with permission from R. Hobbs (a,b) and C.E. Ramalho (c,d).


Multidimensional Biocomplexity Framework [46] and theemerging framework based on the LTER Baltimore Eco-system Study [47].

Limits to current approachesLinear gradients do not fit with the characteristics of

contemporary cities

Urban-to-rural gradient studies oversimplify cities [6,18].Initially, this simplification was important in developingan understanding of these highly complex human-modifiedecosystems. However, the underlying assumption of alinear gradient in the urbanization-induced environmentalchanges does not fit with the spatial-temporal character-istics of contemporary urbanization. Indeed, the fact thatcontemporary cities grow in a rapid, complex, non-linear,dispersed and expansive manner, means that urbanizationintensifies and ages in patchy and complex spatial patternsacross the landscape, rather than in a linear gradient(Figure 1). Consequently, the environmental or ecologicalconditions in one focal remnant patch depend not on itsposition along the linear gradient, but on the character-istics of the neighboring patches. In a similar way, rem-nants closer to the city have not necessarily been isolatedfor longer than remnants in rural areas, and remnants

close to each other might have been isolated for differentlengths of time (Figure 2). This means that the use ofcategorical or quantitative measures of geographical lineardistance in urban ecological studies can be ambiguous andmisleading.

Simplification of the set of intervening drivers

Urban-to-rural gradient studies using landscape metricsand/or urban land-use types can partially capture some ofthe non-linear heterogeneity and complexity of cities. Nev-ertheless, these studies still oversimplify urban environ-ments, as they often ‘flatten’ several human andenvironmental drivers into a reduced number of aggregat-ed variables used in study design and data analysis[6,18,21], although there are a few exceptions [35,36,48].The aggregated representation of drivers does not fullyencapsulate the complex dynamics in urban ecosystems,because it neglects the role of a broader set of drivers andtheir interactions affecting remnant biodiversity and eco-system functioning. These drivers include, for instance,landscape fragmentation, disturbance regimes, local envi-ronmental conditions and the features of the local environ-ment that are not affected by urbanization. Bioticresponses to environmental changes associated with

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Remna nt vegetati on Urban matrixKey:

Sa mpling siteAgricult ural matrix

(a)

(b)

(c)

(d)

Curren t10 years ago20 years ago30 years ago


Figure 2. Urban growth is a dynamic process in space and time. Traditional approaches measuring the degree of urbanization in study sites often neglect the temporal

dynamics of landscape change, with only the most recent spatial configuration and surrounding land uses taken into account. This approach provides only a snapshot in

which all the dynamics that led to that captured spatial moment are neglected, severely limiting understanding into the past and future. In this figure, the landscape context

and spatial configuration of four different remnants (a–d) are represented along a period of 30 years. A ‘snapshot’ approach taken at the current time would classify these

remnants in the same category. However, their trajectories of landscape change show that the intensity of exposure through edge effects to the disturbance processes

originating in the surrounding urbanized areas is different in the four cases. While remnant (a) has been isolated for 30 years, with the same spatial configuration, remnant

(b) has only recently had its area reduced to the same size and, in the near past, was part of a much larger and continuous remnant. This means that, in remnant (a), the

sampling site has been highly exposed through edge effects to the urban disturbance processes from the immediate vicinity, and is probably highly degraded, unless it has

been targeted by management and restoration efforts. In remnant (b), the exposure to an edge is relatively recent, and communities in the sampling site might or might not

already exhibit an altered composition and structure owing to the current spatial configuration. Whereas in (a) and (b) the major driver of landscape fragmentation was

urbanization, in remnants (c) and (d), the major driver was agriculture. This means, first, that the isolation history of the remnant could be much older and, second, that both

(c) and (d) have been exposed for a long time to an agriculture matrix and its disturbance processes, only recently being exposed to urbanization. Solely considering the

current landscape context means that land-use legacies from the surrounding agricultural matrix prior to the onset of urbanization are missed.


urbanization might be masked if such factors are ignored[49]. In fact, the action and interaction of multiple drivers,including those that are unique to cities, is responsible fordifferent processes and dynamics of disturbance [19,50]that can even decouple fundamental ecological mecha-nisms [51]. Predator–prey relationships can break downbecause synanthropic predators become strongly subsi-dized by anthropogenic resources [52]. Urban speciesassemblages can also be determined mainly by stochasticprocesses rather than by mechanisms such as interspecificcompetition [53,54]. For these reasons, single or simplecombinations of aggregated urbanization measures mustbe used with caution, and an explicit quantification of the

182

intervening drivers and their interactions is required. Theabsence of such an approach limits the capacity to under-stand and forecast the effects of urbanization on remnantbiodiversity and ecosystem functioning, as well as theircombined effects with other global change drivers, such asclimate change [55,56]. Moreover, it diminishes the abilityto provide ecologically derived guidelines for managementand restoration [10,57].

Other frameworks [20,21,40] are generally based on acomprehensive set of human and biophysical drivers. How-ever, the focus on social sciences, and ecosystem processesand functions rather than on biodiversity dynamics hasreduced the ecological utility of those frameworks and


might explain their lack of application in urban ecologicalstudies. This simplification is not problematic in the studyof human-created and highly managed habitats (e.g. parks,lawns, or green roofs). However, it becomes relevant ifapplied to remnant ecosystems, as aspects such as frag-mentation and associated direct and indirect effects onbiodiversity are not properly accounted for.

Lack of a temporal perspective

Another major limitation of current conceptual frame-works is the absence of an explicit temporal perspective.Although the importance of temporal dynamics is wellrecognized in ecology [58–61], this has been incorporatedless well into the study of cities. However, urbanization age[35,36,62,63], time-lagged social factors [47,64], the devel-opment history of the cities [65] and their agrarian legacies[66] have all been shown to be important factors in deter-mining current urban biodiversity and ecosystem patterns.

The importance of a temporal perspectiveAn explicit temporal perspective in the study of contempo-rary cities is crucial for several reasons. First, cities arehighly dynamic landscapes and, therefore, a dynamic ap-proach is required to study them. Indeed, the configura-tion, composition and function of patches in the urbanmosaic are dynamic. For example, as urban growth occursthrough infilling, scattered remnant vegetation is clearedin stages, sometimes over several decades, as differentsuburbs are developed. Moreover, vegetation conditionin parks and reserves changes as a consequence of naturaland human-driven disturbance regimes and restorationefforts, which vary across time, influenced by climaticand socio-economic drivers [67]. Backyard species compo-sition and structure also change, influenced by gardeningpractices and fashions, and local socio-economic drivers[64,67,68]. Furthermore, as urban populations and de-mand for land increase, block subdivision and demolitionfor higher density construction also increase. Finally, citiescan ‘shrink’ because of population loss, employment declineand/or economic downturns, which can result in the pas-sive or forced abandonment of entire neighborhoods, andcommercial and industrial areas, a phenomenon called ‘de-urbanization’ [69].

Second, contemporary cities are young and rapidlyevolving landscapes [70] that have been through recentlarge-scale habitat destruction and land-use changes. Insuch emergent landscapes, remnant ecosystems are likelyto be strongly shaped by past land uses [71], and time-lagsmight mask remnant biodiversity response to ongoingfragmentation and environmental change [60,65,72,73].Considering these two aspects is of major importance tothe understanding of biodiversity patterns and processes(e.g., invasion and extinction) in rapidly urbanizing land-scapes.

Land-use legacies

Past land use can affect ecological systems with lastinglegacies that persist over time, sometimes for hundreds tothousands of years [74,75]. These effects can remain evenafter land-use change and after other more recent distur-bance processes begin operating [66,71,76]. Depending on

the land use within and surrounding remnant ecosystemsprior to urbanization (e.g. agriculture, livestock grazing, orindustrial activities), there might be legacies that influencecurrent biotic and/or abiotic ecosystem components. Forinstance, in expanding European cities, the biodiversity innewly formed urban remnants might have been reducedlong before urbanization owing to historical agriculturalland uses [65]. Agrarian legacies can also affect urbanremnants soils. In Arizona, for example, residential yardsconverted from farms had double the organic matter,nitrogen and phosphorus than yards converted from nativedesert [66].

Time-lagged responses to fragmentation

Biodiversity responses to urbanization-induced fragmen-tation might show a temporal delay [72]. This depends onseveral factors, including the species turnover rate, rem-nant area, landscape connectivity and disturbance inten-sity. Temporal delays are shorter for species with higherturnover rates (e.g. annual vs perennial plants), for smallerand more isolated remnants, and following small pertur-bations [72]. In old cities, remnant biodiversity might belargely shaped by the disturbance processes originatingfrom the surrounding urban matrix. However, in youngand rapidly urbanizing landscapes, communities are likelyto be gradually adjusting to the novel environment. Duringthis transient period, biological communities might bebetter explained by previous rather than current remnantand landscape spatial configurations [72,77,78]. Further-more, these communities contain a transient species poolthat might include species that will go extinct once thetransient period is over (i.e. extinction debt) [72]. In asimilar way, they might not yet be affected by the invasionof exotic species, which is more likely to occur once rem-nants re-equilibrate (i.e. invasion credit) [60,73,79].

Failure to consider land-use legacies effects and tran-sient dynamics in the response of biodiversity to fragmen-tation can have major consequences for the scope of urbanecological research. Indeed, it might lead ecologists toclassify remnants with different fragmentation trajectoriesand legacies in the same class of urbanization. Essentially,communities at different stages along the course of adjust-ment to the surrounding environment are mixed more orless indiscriminately and independently of their past. Thiscan lead to incorrect and misleading study design(Figure 2) and, ultimately, to contradictory and unexpectedresults. Research on the species richness–area relationshipis an example in which misleading interpretations arelikely to arise if time is not considered, because if datawere collected in remnants with different ages, any areaeffect might be masked by the differences resulting fromdifferent trajectories of fragmentation [80].

Towards an emerging framework in urban ecologyA new approach to studying the ecology of cities is neededthat incorporates: (i) awareness that urbanization intensityand age needs to be assessed based on the analysis of thefocal remnant patch and neighboring landscape, rather thenon its position along a linear geographic transect; (ii) amechanistic perspective, considering the role of multipledrivers and their direct and indirect effects on remnant

183


ecosystems; and (iii) an explicit temporal perspective, ac-knowledging land-use legacies and time-lagged responses toenvironmental change. A more effective emerging frame-work would incorporate three essential elements (Figure 3).

A first element is a comprehensive set of interveningfactors selected using an ecologically oriented perspective.These factors can quantify drivers, patterns, or processes,and include: (i) human factors (e.g. socio-economic, demo-graphic and built infrastructure); (ii) environmental fac-tors affected by urbanization, including landscape-scale(e.g. fragmentation and land use) and local-scale factors(e.g. disturbance regimes and local environmental condi-tions); and (iv) environmental factors unaffected by urban-ization (e.g. geological or geographical). An ecologically

Urban-to-rural gradient studies

(a)

(c)

Urban

Suburban

Rural

(

(b)

Socio-economicsand urban land use

Ecologicalresponse

Present area andconnectivity

Presarea an

Figure 3. Dynamic Urban Framework: an emerging framework for the study of cities. Th

ecosystems using either categorical classes or quantitative measures of linear distance

combination of those with socio-economic, land cover, land use, or built infrastructu

comparison between ecological responses across different urban classes or on the sin

Dynamic Urban Framework uses a temporal perspective that places the focal urban remn

urban landscape and their past spatial configurations and land uses (d). It also uses an

variation in the community or ecological process of interest (d). A hierarchical perspectiv

drivers and their direct and indirect effects on the ecological community or ecological

184

oriented approach is important to guide in the identifica-tion of the factors relevant to the ecological questionaddressed. On the one hand, this requires focus on thespecies or community of study, because the response to theenvironment is species and/or trait specific [81,82]. There-fore, environmental attributes and scales meaningful toone species or community might not be relevant to another.On the other hand, it demands a careful analysis of thestudy area. For instance, if a study is undertaken in asuburban-type landscape, then income or education levelmight be more appropriate drivers of ecological variation inthe area than is human population density.

A second element is an explicit temporal perspective.Given that urban and landscape ecology have focused

Dynamic urban framework

Land-use legacies

Urbanization age

Past remnantconfigurations

Local environment

Socio-economics,urban land use

e)

Land-uselegacies

Urbanizationage

Socio-economicsand

urban land use

(d)

Disturbanceregimes

Localenvironmental

conditions

Ecologicalresponse

ent and pastd connectivity


e urban-to-rural gradient approach classifies the degree of urbanization of remnant

between the city centre and the rural matrix (a) (remnant vegetation in black); or a

re metrics (b) (road density depicted here). Data analysis usually focuses on the

gle effects of a simplified set of explanatory variables (c). A more comprehensive

ants in their trajectory of change, analyzing the length of time they have been in the

ecological perspective that identifies the variables that best describe the range of

e is used to understand the causal and interacting relationships between multiscale

process of interest (e).

Box 3. Measuring the temporal dynamics of landscape change

Landscapes are complex systems with two main vectors of dynamism

and change: space and time [83]. Landscape and urban ecology have

developed their main body of knowledge from research on spatial

patterns. However, temporal dynamics have often been ignored and

there are very few consistent examples of case studies, nomenclature,

or conceptual frameworks supporting research along the temporal

axis of ecological variation. Nonetheless, historical geographic data,

such as aerial photographs and cartographic maps, are available and

can be used to assess how urban landscapes changed through time.

Here, we suggest three types of variable that can be used to quantify

temporal dynamics of landscape change in urban ecological research.

Urbanization or remnant age

Urbanization or remnant age reflects the time since the urban patch was

developed or the remnant patch was isolated and surrounded by

urbanization, respectively [36,62,63]. This variable can be used to

quantify the length of exposure to the urban environment and the time

lag in ecological responses to urbanization-induced fragmentation;

Past remnant and landscape attributes

Past remnant and landscape attributes refer to patch and landscape

spatial configurations (e.g. remnant area and landscape connectivity)

[77,98], land cover (e.g. urban cover) and socio-economic attributes

(e.g. population density and build-up density) [64] that can be

quantified in a time series. Future studies could develop time-

weighted variables, measuring the age of attributes of interest;

Landscape fragmentation drivers

Landscape fragmentation drivers are variables identifying the main

drivers of landscape fragmentation and isolation of the focal remnant

patch (often agriculture, urban or industrial development). Such

variables can be used to track the presence of land-use legacies.


mostly on the analysis of spatial patterns, a deliberate shiftis needed from that purely spatially oriented approachtowards a perspective that recognizes landscapes withtwo main vectors of change: space and time [83]. From atheoretical perspective, this involves the incorporation ofimportant conceptual constructs, such as extinction debts[72,77], invasion credits [60,73] and land-use legacies [71].From a methodological perspective, it demands consider-ation of the intervening factors mentioned above from aspatial and temporal perspective. Here, we underline theimportance of characterizing the temporal dynamics oflandscape change, including urbanization or remnantage, past remnant and landscape attributes, and fragmen-tation drivers (Box 3). Temporal dynamics of landscapechange are likely to be particularly important when: (i)urbanization is relatively recent; (ii) there is a range oftime since remnant isolation and/or urbanization; and (iii)there is a range of previous land uses.

A third element is a conceptual and analytical structurewhere the relationships between and among driving andresponse factors are analyzed in a mechanistic manner.

Box 4. A temporal perspective in the planning and management

Urban planning

Identification of remnant sizes

Extinction debt research aids in the understanding of how biodiver-

sity varies in time in response to remnant size and connectivity,

variables that are often the scope of urban planning decisions.

Therefore, it can provide guidance on the selection of remnant sizes

and landscape configurations that will allow reasonable conservation

outcomes in the future.

Identification and prioritization of remnants to set aside for

conservation

A temporal perspective considering the remnant age and past land

uses can provide insight into the biodiversity value of particular

remnants and, therefore, can be used in prioritization for conserva-

tion. Priorities could be, for instance, those remnants without

significant land-use legacies and those that were recently fragmented.

Management and restoration

Managing and restoring remnants that have land-use legacies

A temporal perspective considering land-use legacies adds realism to

the formulation of goals and understanding of outcomes in restora-

tion. The presence of land-use legacies might mean that ecosystems

This can be achieved using a hierarchical approach. Urbanecosystems are more likely to be described as heterarchicalrather than hierarchical systems, in the sense that differ-ent factors might or might not be related by causal relation-ships, depending on the conditions and scale of analysis[84,85]. Nevertheless, a hierarchical approach provides amiddle ground where the complexity of these coupledhuman–nature systems can be accommodated, and theirmultidimensional nature partitioned into smaller, moremanageable subsystems [42,86,87]. The hierarchicalpatch-dynamic framework [42] provides an integrativeapproach to spatial analysis, whereby the nested structureof spatial and temporal patterns and processes in urbanlandscapes can be depicted [88]. This framework providescore structure to urban LTER projects in the USA[20,44,47] and its use should be further encouraged. Fur-thermore, structural equation [89] and Bayesian hierar-chical modelling [87,90] are promising statistical tools toinvestigate the complex networks of causal and interactingrelationships between multiple factors, and their directand indirect effects on remnant biodiversity and ecosystem

of urban remnants

have passed biotic and/or abiotic thresholds that might impede

restoration [71]. Furthermore, if thresholds were crossed, ecosystems

are likely to require specific interventions that are not required in

remnants not subject to those legacies. For example, whereas

prescribed burning and mechanical overstorey thinning were im-

portant drivers of the plant community in post-agricultural Pinus

palustris woodlands in the south-eastern USA, these actions had

barely any effect on historically forested sites [99].

Improving the habitat quality of remnants in transient periods ofadaptation

In rapidly urbanizing landscapes where natural areas were cleared for

urban development, the transient period in which remnant biodiver-

sity gradually adjusts to the novel urban scenario provides a unique

opportunity for action [65]. Interventions should improve the habitat

quality of these remnants and target: (i) the core patch with

restoration efforts and the design of margins and tracks that minimize

influence from humans and external processes; and (ii) the buffer

areas [100], by improving connectivity and enhancing the urban

matrix at various scales, from the individual garden to the neighbor-

hood or suburb [68]. These interventions should target priority

remnants and also those where keystone species are present and

whose extinction are predicted to have cascade effects on the survival

of other species [77].

185


functioning [87,90]. Finally, keeping in mind the heter-archical nature of urban ecosystems is essential becausethe importance of different ecological–social drivers andtheir temporal and spatial boundaries is fluid [85]. Thisflexibility is fundamental to a dynamic approach.

Application to planning, management and restorationin contemporary citiesA critical approach to the assessment of urbanization inecological studies will expand the ability and scope ofurban ecological research to better intervene in the plan-ning, management and restoration of remnant ecosystemsin contemporary cities. First, a proper identification of thedrivers controlling remnant ecosystems elucidates wheremanagement and restoration efforts should focus, helpingto formulate meaningful management guidelines and tai-lor strategies of action. This is important not only tomaximize conservation outcomes, but also to minimizecosts. Second, a temporal perspective considering land-use legacies and time-lagged ecological responses to frag-mentation places current condition of an ecosystem in thecontext of its trajectory of change [71], enhancing theunderstanding not only of observed patterns, but alsothe processes and dynamics that generate and maintainthem (Box 4).

Concluding remarksIn the context of a rapidly urbanizing world, it is importantto consider the complex growth, relative youth and dynam-ic nature of contemporary cities if ecologists want to moveforward in the study and conservation of the places wheremost humans live and work [10]. Failure to consider thesecharacteristics compromises the scope of urban ecologicalresearch, potentially leading to ill-designed studies andpartial or misleading research outcomes. Furthermore, itlimits the ability of urban ecology to provide meaningfulguidance to planning, conservation and restoration incities. Here, we have suggested the essential elements ofan emerging Dynamic Urban Framework. From a concep-tual perspective, this framework is based on ecologicaltheory that urgently needs to be incorporated into main-stream urban ecological research. In particular, the tran-sient dynamics in biodiversity response to environmentalchange, including extinction debts [72,77] and invasioncredits [60,73], implications of land-use legacies for con-servation [71], hierarchical patch dynamics [42,87] andhierarchical modelling [87,90], all need to be incorporated.From a practical perspective, the Dynamic Urban Frame-work: (i) is grounded in the area and community of study;(ii) places the process of urbanization and its effects onbiodiversity and ecosystem functioning in a temporal con-text; and (iii) depicts the observed ecological responses asthe result of multiple measurable factors that relate andinteract at different spatial and temporal scales. As awhole, the conceptual and practical elements of the frame-work can be a first step towards the foundation of a newapproach to the study of cities.

AcknowledgementsC.E.R. was funded by a Portuguese National Science Foundation(Fundacao para a Ciencia e a Tecnologia) doctoral scholarship. The

186

authors thank Lauren M. Hallett for her encouragement and input, andMichael Perring, Kris Hulvey and two anonymous reviewers for usefulcomments on earlier versions of the manuscript.

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