Urbanization Biodiversity and Conservation

8/10/2019 Urbanization Biodiversity and Conservation

1/9

Urbanization, Biodiversity, and ConservationAuthor(s): Michael L. McKinneySource: BioScience, Vol. 52, No. 10 (Oct., 2002), pp. 883-890Published by: University of California Press on behalf of the American Institute of BiologicalSciencesStable URL: http://www.jstor.org/stable/1314309

Accessed: 19/07/2010 15:41

Your use of the JSTOR archive indicates your acceptance of JSTOR's Terms and Conditions of Use, available athttp://www.jstor.org/page/info/about/policies/terms.jsp . JSTOR's Terms and Conditions of Use provides, in part, that unlessyou have obtained prior permission, you may not download an entire issue of a journal or multiple copies of articles, and youmay use content in the JSTOR archive only for your personal, non-commercial use.

Please contact the publisher regarding any further use of this work. Publisher contact information may be obtained athttp://www.jstor.org/action/showPublisher?publisherCode=aibs .

Each copy of any part of a JSTOR transmission must contain the same copyright notice that appears on the screen or printedpage of such transmission.

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new formsof scholarship. For more information about JSTOR, please contact [email protected].

American Institute of Biological Sciences and University of California Press are collaborating with JSTOR todigitize, preserve and extend access to BioScience.
http://www.jstor.org/stable/1314309?origin=JSTOR-pdfhttp://www.jstor.org/page/info/about/policies/terms.jsphttp://www.jstor.org/action/showPublisher?publisherCode=aibshttp://www.jstor.org/action/showPublisher?publisherCode=aibshttp://www.jstor.org/page/info/about/policies/terms.jsphttp://www.jstor.org/stable/1314309?origin=JSTOR-pdf


2/9

Articles

Uranzti on odivers i n d Conservat i on

MICHAEL . McKINNEY

mong the many human activities that causeabitat loss (Czech et al. 2000), urban development

produces ome of the greatest ocal extinction rates and fre-quently eliminates he large majority of native species (Valeand Vale 1976, Luniak 1994, Kowarik 1995, Marzluff 2001).Also, urbanization s often more lasting than other types ofhabitat oss. Throughout much of New England, or exam-ple, ecological uccession s restoring orest habitat ost fromfarming and logging, whereas most urbanized areas n thatregion not only persist but continue to expand and threatenother ocal ecosystems Stein et al. 2000).

Another great conservation challenge of urban growth sthat it replaces he native species that are lost with wide-spread weedy onnative species. This replacement onsti-tutes the

processof biotic

homogenizationhat threatens o

reduce the biological uniqueness of local ecosystems Blair2001). Urban-gradient tudies show that, for many taxa, orexample, plants (Kowarik 1995) and birds and butterflies(Blair nd Launer 1997), he number of nonnative pecies n-creases oward centers of urbanization, while the number ofnative species decreases.

The final conservation hallenge f sprawl s its current nd

growing geographical xtent (Benfield et al. 1999). A review

by Czech and colleagues 2000) finds that urbanization n-

dangers more species and is more geographically biquitousin the mainland United States han any other human activ-

ity. Species hreatened y urbanization lso tend to be threat-ened by agriculture, ecreation, oads, nd many other humanimpacts, emphasizing the uniquely far-reaching ransfor-mations that accompany urban sprawl.

About 50% of the US population lives in the suburbs,with another 30% iving in cities (USCB 2001). Over 5% ofthe total surface area of the United States s covered by urbanand other built-up areas USCB 2001). This s more and hanis covered by the combined total of national and state parksand areas preserved by the Nature Conservancy. More omi-

nously, the growth rate of urban land use is acceleratingfaster han land preserved s parks or conservation areas bythe Conservancy figure 1). Much of this growth s from the

THE IMPACTS F URBANIZATION N

NATIVE PECIES RE POORLY TUDIED,

BUT EDUCATING HIGHLY URBANIZED

HUMAN POPULATION BOUT THESE

IMPACTS AN GREATLY MPROVE PECIES

CONSERVATION N ALL ECOSYSTEMS

spread of suburban housing. It is estimated, for example,that residential

yards occupy 135,000acres in the state of

Missouri MDC 2002). This residential andscape epresentsnearly 1% of the total area f Missouri nd s nearly hree imesthe area occupied by Missouri state parks.

Here I review he growing iterature hat documents howurban (and suburban) expansion harms native ecosystems.This knowledge can aid conservation efforts n two majorways. One is through he use of ecological principles-suchas preserving emnant natural habitat nd restoring modifiedhabitats o promote native species conservation-to reducethe impacts of urbanization n native ecosystems. Rare and

endangered pecies sometimes occur in urbanized habitats(Kendle nd Forbes 1997, Godefroid 001) and thus could beconserved here. Managing he arge mount of residential eg-etation 1% of the state area, snoted above) n ways hat pro-mote native plants and animals could also make a significantcontribution o conservation.

Michael L.McKinney e-mail: [email protected]) sa professor fgeologicalsciences nd director f the Environmental tudies rogram t the Departmentof Geological Sciences, University of Tennessee-Knoxville, noxville, TN37996. His current esearch nterests ocus on the mpacts f urbanization n

biodiversity. n addition o his professorial esponsibilities t the university, estrives o educate hegeneral ublic about conservation n all ecosystems.2002 American nstitute of Biological Sciences.

October 002 / Vol. 52 No. 10 * BioScience 883


3/9

Articles 4

120

developed land0

100

0

80- national parks

60

0 040 O

0

20state parks

A A A AA A A

a

o

m Conservancy

1965 1970 1975 1980 1985 1990 1995 2000

Year

Figure 1. Amount of land covered n the lower 48 states, by category. Source: All data arefrom Statistical Abstract of the United States or the years shown, except or Nature Con-servancy data, which is from Stein and colleagues 2000).

A second way n which he study of urban cology an serveconservation s by helping to develop a more ecologicallyinformed public. Providing a well-informed public could bethe most important application f urban ecology, as a meansof promoting effective onservation f native pecies Kendleand Forbes 1997). Because 80% of the American public ivesin or near urban areas, here are many opportunities or cre-

ating an informed public hat can wield enormous economicand political pressure o promote conservation olicies. Peo-

ple who live in urban environments often have a great ap-preciation of many urban species, uch as birds (Clergeau tal. 2001). Indeed, esidents f suburban nd urban areas endto place a much higher value on species conservation hanthose living in rural areas Kellert 1996). This is reflected n

voting behavior: Legislators rom highly urbanized statesand districts end to be more supportive f strengthening he

Endangered pecies Act (Mehmood and Zhang 2001).Unfortunately, hese conservation opportunities are hin-

dered by the very poor ecological nowledge f typical Amer-

ican urbanites. A survey of Texas high school students, or ex-ample, showed that 60% of the students misidentified he

opossum as a rodent and that ecological nderstanding f hu-man effects on biota was even poorer; nly 2% of the studentsknew that raccoons tend to benefit from many human ac-tivities (Adams et al. 1987).

The urban-rural gradient:General patternsUrban-to-rural radient tudies xamine hanges n plants andanimals along a transect rom the inner city to surrounding,less-altered cosystems; hey also show what happens o sur-

rounding native ecosystems sur-ban sprawl xpands. General pat-terns hat emerge rom these stud-ies are described below.

Physical gradients. Physicalchanges along the gradientstrongly nfluence available abi-tat for native pecies.A number ofreviews Sukopp nd Werner 982,Medley et al. 1995, Pickett et al.2001) show increases in thesephysical hanges, s one moves o-ward he urban core, n such met-rics as human population density,road density, air and soil pollu-tion, average ambient tempera-ture ( heat island effect), aver-age annual rainfall, soil

compaction, soil alkalinity, andother ndicators f anthropogenicdisturbance. The percentage ofarea that is impervious surface(pavement, asphalt, buildings)ranges rom well over 50% at the

urban core to less than 20% at the fringe of urban expansion(figure 2). In addition, he amount of subsidized nergy andmatter mported for use by humans and available o other

species ncreases oward he urban center Collins et al. 2000,Pickett et al. 2001).

Habitat-loss gradient. Thesephysical hanges roducea gradient of natural habitat oss that steepens rom rural ar-eas toward he urban center. As habitat s lost, it becomes n-

creasingly fragmented into more numerous but smallerremnant patches Medley et al. 1995, Collins et al. 2000). Thelost natural habitat s then replaced by four types of alteredhabitat hat become progressively more common toward heurban core. The four types of replacement habitat are istedbelow, in order of increasing habitability to most native

species and decreasing proportion of coverage oward theurban core. The latter three types are based on Whitney(1985).

1. Built habitat: uildings nd sealed urfaces, uch asroads

2. Managed egetation: esidential, ommercial, nd other

regularly maintained reen spaces

3. Ruderal egetation: mpty ots, abandoned armland,and other green pace hat s cleared ut not managed

4. Natural emnant egetation: emaining slands of origi-nal vegetation usually ubject o substantial onnative

plant nvasion)

884 BioScience a October 002 / Vol. 52 No. 10


4/9

oArticles

Diversity changesalong theurban-ruralgradientIt is probably ntuitive o eventhe most casual observer hatthe increasing ragmentation

of natural habitat by humandisturbances n the directiontoward urban centers willtend to reduce species rich-ness (number of species) inthat direction. here are, how-ever, many variables hat canaffect he rate and consistencyof species oss along the gra-dient, so empirical tudies arecrucial in measuring urban

impacts.

Urban core, low diver-sity. Many tudies documentthat the lowest species diver-sities along the urban-rural

gradient occur in the inten-

sively built nvironments fthe urban core. This has been shown for many taxa, nclud-

ing plants Kowarik 995), birds and butterflies Blair 2001),many insects (Denys and Schmidt 1998, McIntyre 2000),and mammals (Mackin-Rogalska t al. 1988). In all thesetaxa, he number of species at the urban core s reduced o lessthan half of that found in the rural, more natural areas at the

opposite end of the gradient figure 2).Blair 2001), or example, ound ust 7 summer esident ird

species n the central business district of Palo Alto, Califor-nia, compared with 21 species that inhabited a natural area

(preserve) outside the city limits. Similar reductions werefound for birds and butterflies n other cities, as shown byBlair's 2001), and especially by Marzluff's 2001), compre-hensive compilation of studies on urbanization mpacts onbirds.

Much of the reduction n richness s obviously caused bythe loss of vegetation. The number of species of animal axa,such as birds (Shugart t al. 1975) and insects (Majer 1997),

tends to correlate with the number of plants n an area. Also,area covered by vegetation is a good predictor of speciesnumbers for birds (Goldstein et al. 1986); mammals, am-

phibians, nd reptiles Dickman 1987); and nsects McIntyre2000).

As over 80% of most central rban areas s covered ypave-ment and buildings (Sukopp and Werner 1982, Blair andLauner 1997), ess than 20%, herefore, emains as vegetatedarea. Furthermore, he remaining vegetated habitat oftencontains ow plant diversity sa result of erosion, rampling,pollution, nvasion or cultivation of a few nonnative pecies,and many other human disturbances. Also, mowing, prun-

ing, and other common landscaping practices further re-duce the volume of the remaining vegetation Gilbert 1989,Adams 1994).

Suburban diversity: Peak or plunge? ome tudies n-dicate that species richness ends to be higher n areas withlow to moderate evels of human development such as out-lying suburban developments) han n more natural ural ar-eas such as preserves. his suburban eak n species numbersis evident n many axa, such as mammals (Racey and Euler

1982), birds and butterflies Blair 2001), bumblebees Pawli-kowski and Pokorniecka 990),ants (Nuhn and Wright 1979),lizards Germaine nd Wakeling 000), and plants (Kowarik1995).

An explanation often suggested for this suburban peak(e.g., Blair and Launer 1997, Germaine and Wakeling 000,Blair 2001) is the intermediate disturbance hypothesis. Theinitial human impacts of suburban sprawl are sometimes

relatively mild, with only a few housing subdivisions n amatrix of largely atural r agricultural abitat. This promotesenvironmental heterogeneity, ecause different habitats oc-cur alongside one another. Such habitat diversity s enhanced

by the fact that individual homeowners often make ndivid-ualistic choices n the plants that they cultivate Hendersonet al. 1998).

In addition to providing spatial heterogeneity, hese an-

thropogenic abitats re ypically ery productive Falk 1976),being highly subsidized n scarce resources, ranging fromwater o nutrients e.g., fertilizers). Cultivated lants nclude

many ornamentals hat often bear ruits and seeds hat are uti-

Conservation trategiesAcquire emnant abitats Restore managed nd ruderal habitats

Speciesrichness

Rural Urban ringe Suburbia Urban ore

Surface rea < 20% mpervious 20%-50% mpervious > 50% mpervious

Urban biotas Avoiders Adapters ExploitersExamples Forest nterior pecies Edge species Commensals

Figure . Urban-rural radient. This sa verygeneralized nd simplified epiction f changesin surface rea, pecies ichness, nd composition, s compiled rom a number fsources is-cussed n the text. Two basic onservation trategies with respect o urban prawl re shown atthe top.



5/9

Articles

lized by animals, specially birds and bats (Munyenyembe tal. 1989,Adams 1994). Some animals have adapted o the di-rect consumption of human resources Adams 1994) hat areprovided accidentally garbage) or intentionally bird ood).

In contrast o the above, other studies how that suburbanareas have reduced pecies diversity ompared o less-alteredrural habitats (figure 2). For example, Marzluff's 2001) com-

pilation of 51 bird studies ound that 31 of the studies 61%)showed ower species richness in suburban and other areasof human settlement, ompared with more natural ural ar-eas. The remaining 20 studies reported either an increase orno change in diversity with increasing human settlement.The 51 studies covered a wide range of geographic and nat-ural settings, so it is difficult o identify which variables de-termine whether a rise or fall of species richness occurs withincreasing ettlement and suburban development.

Teasing part hese variables, uch as the role of the naturalsetting, s clearly a priority or further work on urban-ruralgradients. Bell (1986), for example, has suggested hat ur-

banization n a tropical rain forest may have different ffectson local species richness han urbanization n other naturalsettings, because rain forest birds have exceptional difficultyadapting o human settlements.

Local extinctions during housing development.Areas of active development end to have ow biodiversity e-cause of the devastating mpact on native pecies of most res-idential and commercial development methods. Before on-struction of most residential nd commercial buildings, t iscommon for developers o remove most vegetation and eventopsoil (Sharpe t al. 1986). This reduces construction costs

by allowing equipment ready access o the construction ite.A study of the fate of natural vegetation during urban de-

velopment n Wisconsin ound that only about one-third ofthe original egetation was not destroyed Sharpe t al. 1986).The loss of native vegetation and total vegetated area) has a

negative mpact on native animal diversity. Bird pecies rich-ness declined dramatically n the early tages of housing con-struction compared o preconstruction iversity) n California(Vale and Vale 1976) and Poland (Luniak 1994).

Once construction s finished, some of the area s paved,which removes it as habitat for nearly all species. In PaloAlto, California, or example, 25% of the area of residential

communities s covered y pavement Blair nd Launer 997);another 20% of the area s covered with housing. Of the re-maining nonpaved portions, much is replanted with (usuallynonnative) grasses, shrubs, and trees (Wasowski and Wa-sowski 2000).

Conservation strategies. Habitat conservation an uti-lize preservation nd restoration figure 2). The most effec-tive (and cheapest n the long term) strategy s to preserve smuch remnant natural habitat as possible. Many studies de-scribe how native species richness n a remnant habitat n-creases with the area of that habitat. This s true or many axa,

including birds (Tilghman 1987), mammals Dickman 1987),and plants (Dawe 1995).

One way o preserve emnants n housing developments sto retain predevelopment vegetation. A number of recentbooks, uch as TheLandscaping evolution Wasowski nd Wa-sowski 2000), have pointed out the benefits of retaining pre-existing vegetation when building new homes. Unfortunatelyfor conservation goals, his type of construction s rarely un-dertaken by most residential eal estate developers. Althoughostensibly related o cheaper costs of mass construction, e-taining more predevelopment egetation s less expensive nthe long term (Dorney et al. 1986) and is preferred y manyhomeowners (Wasowski nd Wasowski 000).

A major nfluence n natural emnants s the matrix, r thetype of habitat, hat surrounds hem. Remnants re often em-bedded n a highly disturbed matrix hat also serves as a con-tinuous source of nonnative species. A major challenge sthat remnant habitats are open to colonization by nonnativespecies of invasive lants Luken 1997) and predatory nimalssuch as housecats and

dogs (Marzluff 2001).These nonna-

tive invaders and predators an greatly reduce he ability ofthe remnant habitat to support native species, especiallybirds. In the language of population biology, hese remnantsbecome population sinks hat are unable to support self-sustaining populations of the native species.

Restoration trategies: uccession nd cultivation.Conservation strategy can also focus on restoring nativespecies n managed and ruderal habitats. n natural ecosys-tems, biotic succession ncreases he number of plant and an-imal species after a disturbance Gibson et al. 2000). This isalso true of ruderal nd managed habitats hat remain undis-turbed ong enough for succession o occur. Various tudieshave documented how succession ncreases pecies diversityin ruderal nd managed ommunities, or example, ncreasedplant diversity n urban ots (Crowe 1979), ncreased rthro-pod diversity n restored ommunities Majer 1997), and in-creased ird species ichness n residential ommunities Valeand Vale 1976, Munyenyembe t al. 1989, Luniak 1994). As a

consequence, lder residential reas usually nearer he urbancore) tend to have higher species richness han younger ones

(e.g., Munyenyembe t al. 1989).The studies cited above how that the accumulation ate of

new species during uccession s initially ery rapid and s sub-

stantially lower after he first few years and especially afterthe first decades. Aside from increasing otal diversity, co-logical succession also often reduces the diversity of non-native species n an area Gibson et al. 2000), many of whichrely on disturbance o sustain heir populations Luken 997).

Another restoration trategy o increase native biodiversityin managed habitats s to cultivate a variety of plant species.Cultivation with native plant species may benefit not only na-tive plant populations ut also native animal populations. orexample, native bird species richness in Australia (Mun-yenyembe t al. 1989) and North America Sears nd Anderson1991) ends o positively orrelate with the volume and species

886 BioScience * October 002 / Vol. 52 No. 10


6/9

Articles

diversity of native vegetation. Similarly, he percentage ofnative nsect pecies n a fauna has been found o correlate iththe percentage of native plant species (Crisp et al. 1998).Landscaping olf courses with native plants can benefit manylocal native bird species (Terman 1997).

Compositional hanges alongthe urban-rural radientSpecies ary n their ability o adapt o the often drastic phys-ical changes along the urban-rural gradient (Gilbert 1989,Adams 1994). Although there are probably many ways tocategorize hese changes n species composition, many bird(e.g., Goldstein t al. 1986,Maeda and Maruyama 991,Blair2001) and mammal (e.g., Nilon and VanDruff 1987) studieshave concluded hat species along the gradient an be classi-fied, for convenience, nto three distinct categories eflectingtheir reaction o human activities. Using Blair's 2001) terms,these categories are urban avoiders, urban dapters, nd

urban exploiters (figure 2). While birds are the best-

studiedtaxa for work on urban-rural

gradients,hese three

categories have also been used for work on butterflies Blairand Launer 1997) and izards Germaine nd Wakeling 000).

These categories how that, even in highly modified envi-ronments, species are nonrandomly assembled n ways that

approximate ommunity assembly processes n nature. Eachof these assemblages as a distinctive et of ecological char-acteristics hat reflect he impacts of urban sprawl on native

species. One of the most important raits hat separates hethree categories s the extent to which species depend onhuman-subsidized esources o exist in an area (Johnston2001). As subsidized resources ncrease toward the urbancore, there is a concurrent increase in species that utilizethem. Urban exploiters re generally ommensals hat are al-most entirely ependent n human ubsidies i.e.,obligate ar-asites). Urban adapters re able o utilize ubsidies ut are ac-ultative n that they also widely use natural wild-growing)resources. Urban avoiders end to rely only on natural re-sources Johnston 2001).

Characteristics f urban avoiders, dapters, ndexploiters. Because birds, mammals, and, to a lesser extent,plants are he best-studied axa along urban-rural radients,they will be the major ocus here. Urban voiders re speciesthat are very sensitive o human persecution and habitat dis-

turbances. The first species to disappear n the proximity ofhumans are usually arge mammals, specially redators, e-cause hey are actively ersecuted, elatively are, nd have owreproductive ates. Thus, cougars, bison, and elk were amongthe first to disappear after European settlement began(Matthiae and Stearns 1981). Avian urban avoiders ncludespecies dapted o the interior f large, ld forests, uch as tree-foraging nsectivores, eotropical migrants, nd many ground-nesting birds hat are very sensitive o the presence f humansand pets (Whitcomb et al. 1981, Beissinger and Osborne1982, Sears and Anderson 1991, Adams 1994). Plant speciesthat are very sensitive o human activities would nclude ate-

successional (old-growth) and wetland plants (Stein et al.2000), the loss of which s attributable o our tendency o clearforests and drain wetlands for agricultural nd settlementgoals.

Urban dapters reoften ound n the matrix of human anduses that occur n suburban andscapes. or plants, arly uc-cessional pecies are common in managed uburban abitats,

such as residential ards and commercial as well as unman-aged ruderal abitats (e.g., undeveloped ots). These early uc-cessional plants nclude both cultivated pecies avored yhu-mans (e.g., turfgrass, ast-growing ornamental shrubs, andtrees), as well as weedy species that are common in both

managed and unmanaged uburban abitats. The most com-mon weedy species are wind-dispersed lawn weeds (e.g.,dandelions, crabgrass) and bird-dispersed nvasive shrubs(e.g., privet, pokeweed) hat commonly grow on cleared, n-tended andscapes Crowe 1979). Botanically, uburban and-

scapes are often characterized s structurally pproximatingsparsely orested avanna r grassland ommunities Dorneyet al.

1984).This s

apparentlyn

aesthetically referredand-

scape or most suburbanites Henderson et al. 1998).Among animals, urban adapters ypically nclude many

species often referred o as edge pecies, which are adaptedto forest edges and surrounding pen areas Whitcomb t al.1981, Adams 1994). These animals exploit many foods, in-

cluding human-subsidized oods, such as cultivated lants and

garbage. The great abundance f such subsidized oods is onereason why these animal urban adapters ften attain an abun-dance and biomass that is much greater than in naturalareas Adams 1994,Marzluff 001).Another eason s that nat-ural predators f these animals are usually eliminated by hu-man activities Gering and Blair 1999).

For birds, urban adapters nclude a high proportion ofcertain eeding guilds. These nclude omnivores and groundforagers, such as the American robin and many corvids(crows, ays); eedeaters uch as finches; and aerial weeperssuch as swifts (Whitcomb et al. 1981, Beissinger and Os-borne 1982,Sears and Anderson 1991,Adams 1994,Johnston2001). Each of these three guilds seems to be responding odifferent spects f human mpacts. The highly productive i.e.,fertilized) awn and ornamental plant ecosystem provides arich source of invertebrate nd plant foods (Falk 1976) for

ground gleaners, while seedeaters avor bird feeding stationsand many ornamental plants that produce seeds (Adams

1994). Aerial weepers ake advantage f the many open ar-eas, including pavement, over suburban habitats and thehigh abundance of many flying nsects, especially hose thatare attracted o artificial ights. Tree, hrub, and cavity nestersare also common among urban adapters Johnston 2001).

As most mammals ack he high mobility of flight possessedby birds, ife in suburban nvironments poses different hal-lenges. Nevertheless, mammalian urban adapters re able tofind shelter rom intensive human activity as well as exploitrich sources of food provided by humans (Matthiae andStearns 1981, VanDruff and Rowse 1986, Nilon and Van-Druff 1987). One group of mammalian dapters inds refuge

October 002 Vol. 2 No. 10 * BioScience 87


7/9

Articles

through heir burrowing habits. Groundhogs, ottontail ab-bits, moles, and skunks are examples of successful daptationto human proximity n suburbia. Trophically, hese animalsderive much food from the rich subsidies of suburban awns,including apidly rowing grasses, rnamental plants, and n-vertebrates Falk 1976).

Another group of mammal adapters ncludes species hatrequire adjacent orest fragments (e.g., in cemeteries andparks) or shelter Dickman 1987). These pecies ypically or-age for human-subsidized food supplies in surroundingareas. Some are medium-sized omnivores (especially rac-coons and opossums) that forage n garbage, egetable gar-dens, and other resources provided by humans. Others aremedium-sized carnivores, uch as foxes and coyotes, thatconsume a wide variety of prey. As with birds, elimination oflarge predators in addition o subsidized esources) eads overy high population densities of urban adapter mammalspecies (Crooks and Soule 1999).

Urban exploiters, ften called synanthropes e.g., Sukoppand Werner 1982, Mackin-Rogalska t al. 1988, Johnston2001), are very (often otally) dependent n human resources.The abundance of urban exploiters s usually not dependentupon the amount or types of vegetation Lancaster nd Rees1979,Nilon and VanDruff 987,Mackin-Rogalska t al. 1988,Johnston 001). The combination f predator elease preda-tor removal, uch as the extermination f wolves and cougars)with abundant ood subsidies llows hem to attain normouspopulation densities Lancaster nd Rees 1979,Adams 1994).

Urban xploiters robably epresent he most homogenizedof the world's biotas (Blair 2001). Unlike urban adapters,which are largely composed of early successional species

from nearby cosystems, urban exploiters re composed of avery small subset of the world's species; hese exploiters arewell adapted o intensely modified urban nvironments her-ever humans construct hem across he planet (Adams 1994,.Johnston 2001, Marzluff 2001).

Urban environments ypically ave more n common withother cities than with adjacent natural ecosystems Sukoppand Werner 1982), so urban exploiters re often not native oa region (Adams 1994, Kowarik 1995, Blair 2001), but tendto leapfrog rom city to city. Thus, rock doves, tarlings, ouse

sparrows, Norway rats, and the house mouse are ound in allcities in Europe (Mackin-Rogalska t al. 1988) and North

America (Adams 1994). This is also true for urban plants(Whitney 1985).

Among plants, urban exploiters end to be ruderal peciesthat can tolerate high levels of disturbance, specially rassesand annuals see reviews n Sukopp and Werner 1982,Whit-ney 1985, Kowarik 995). Examples nclude wind-dispersedweeds that colonize abandoned ndustrial and commercial

properties and plants that can grow in and around pave-ment. Adaptive traits that are typical of urban-exploitingplants nclude tolerance o high levels of air pollution (espe-cially smog and acidic fog); trampling; and alkaline, com-pacted, and nitrogenous oils.

Avian urban exploiters are often species evolutionarilyadapted o cliff-like ocky areas and therefore re preadaptedto the devegetated oncrete edifices of very urbanized areas(Lancaster nd Rees 1979,Adams 1994). Common examplesinclude the rock dove and peregrine alcon. Another groupof avian exploiters onsists of cavity-nesting pecies hat areable o inhabit human dwellings. Examples nclude he house

sparrow, house finch, and European starling. Trophically,avian urban exploiters end to be ground-foraging eedeatersor omnivores Lancaster nd Rees 1979, Adams 1994).

Mammalian urban exploiters find shelter in humandwellings and exploit the rich food sources n or near them.Trophically, hey are usually omnivorous (Adams 1994) andinclude such familiar pecies as the house mouse, black orbrown rat, and insects, including a variety of cockroachspecies.

Increasing nonnative species oward he city. Manystudies have found that the number (and proportion) of

nonnative pecies ends o increase long he urban-rural ra-dient, moving toward he urban center. n general, he pro-portion of species hat s nonnative goes from ess than a fewpercent n rural areas o over 50% at the urban core. Thesechanging proportions apply to plants in the United States(Whitney 1985) and Europe Kowarik 995) and birds n theUnited States (Blair 2001). The population density of non-native pecies--both mammals Mackin-Rogalska t al. 1988)and birds (Marzluff 001)-also tends to increase he nearerthey are to the urban core.

The increase n nonnative species toward he urban corereflects number of human causes. One is that higher human

population densities nearer he urban core produce ncreas-

ing importation propagule ressure ) f nonnative pecies,for example, the cultivation of nonnative plants (Mackin-Rogalska t al. 1988, Kowarik 995). Another cause s the in-

creasing amount of disturbed abitat toward the urbancore, which provides opportunities or nonnative species of

plants (Kowarik 1995, Luken 1997) and animals (Adams1994, Marzluff 2001) that can utilize the new resources.

Conservation implications of compositionalchanges. In their book Urban Nature Conservation, endleand Forbes 1997) note that, as highly urbanized reas re gen-erally occupied by species that thrive n the presence of hu-

mans, there will be relatively ew rare native species of con-servation oncern n areas f high human population density.They review some examples, lowever, f rare species of in-sects and plants found in highly urbanized areas; habitatconservation and restoration ould be planned or sites thatharbor uch species. Not surprisingly, ost rare pecies n ur-banized areas are ound sites hat have escaped high-intensitydevelopment Godefroid 001). Sites where rare pecies mostcommonly occur include city parks, cemeteries, railroadtrackways, vegetated areas under transmission lines, andother public rights-of-way hat are protected rom develop-ment (Gilbert 1989, Kendle and Forbes 1997).

888 BioScience October 002 Vol. 2 No. 10


8/9

o rticles

Aside rom the conservation f rare native species, knowl-edge of the species composition of urban biodiversity an bevery useful as an educational ool to better understand he nat-ural world. An enhanced appreciation f nature by the 80%of the American public that lives in this environment ouldpromote more effective political and economic action. Ex-amples f such knowledge nclude better ducation f the pub-lic in the natural history of local species and problems withnonnative species (Kendle and Forbes 1997).

ConclusionsUrbanization s a rapidly growing cause of many environ-mental problems (Benfield et al. 1999). The impact of ur-banization s documented n the growing iterature on theurban-rural gradient. These studies how consistent changesin species richness and species composition along the gradi-ent.

Species richness of many taxa often declines along thegradient, with the lowest richness o be found in the urban

core. Urban planners hould find ways o preserve biodiver-sity as cities expand outward and subsequently modify nat-ural habitat. uch efforts would most ikely ocus on preservingas much remnant natural habitat as possible, as opposed tomost current and development echniques, which removemost natural vegetation during construction.

Where ntensive and development has already occurred,native animal biodiversity an be increased by revegetationwith a diversity f native plant species. Protecting his reveg-etated habitat rom disturbance o allow ecological uccessionwill not only enhance plant and animal diversity but alsotend to reduce the diversity of nonnative species. Unfortu-

nately, most current landscaping ends to revegetate withnonnative plant species in unnatural spatial distributions(Henderson et al. 1998, Wasowski nd Wasowski 000) andarrests uccession hrough he management of those ecosys-tems (at great inancial cost; Kendle and Forbes 1997).

Species omposition lso shows pronounced hanges longthe urban-rural gradient. Most notable is that nonnativespecies become proportionately ore common toward he ur-ban core. Urban avoiders nclude native species such as largepredators nd forest-interior especially nsectivorous) birdsthat disappear quickly n the initial stages of suburban en-croachment, nless special ffort s made to retain arge ractsof native habitat and reduce human persecution of species.

Urban adapters, mammals and birds that are mainlyadapted o forest edges and open areas, lourish n suburbanhabitats, specially older subdivisions where ecological suc-cession has advanced nd produced xtensive evegetation. r-ban adapters are very mportant or biodiversity ducation,because half of the American public ives n a suburban en-vironment USCB 2001). Public biodiversity ducation wouldbe most effective f we draw on these familiar uburban om-munity assemblages nd species o promote an understand-ing of concepts such as ecological uccession and the role ofnative plants n promoting native animal diversity. Becauseof its enormous size, wealth, and political nfluence, a more

ecologically informed suburban population could greatlyimprove he social support or conservation f native speciesin all ecosystems.

References itedAdams E, homas K, in ,Weiser .1987. rban igh chool tudents'

knowledge fwildlife. ages 3-86 nAdams W, eedy L, ds. nte-

grating an ndNature n theMetropolitan nvironment. olumbia(MD): ationalnstituteorUrbanWildlife.

Adams W. 994. rbanWildlife abitats. inneapolis:niversityfMin-nesota ress.

BeissingerR,Osborne R. 982. ffectsfurbanizationnavian ommunityorganization. ondor 4: 5-83.

BellHL. 986. ccupationfUrban abitats yBirdsnPapua ewGuinea.LosAngeles: estern oundation fVertebrate oology.

Benfield K,RaimiMD,ChenDD. 1999.OnceThereWereGreenfields.NewYork: ational esource efense ouncil.

Blair B. 001. irds nd utterflieslong rban radientsn wo coregionsof heU.S. ages 3-56 nLockwoodL,McKinney L, ds.Biotic o-mogenization. orwellMA): luwer.

Blair B, auner E. 997. utterfly iversitynd uman and se: pecies

assemblages longn urban

radient. iologicalonservation 0:

113-125.Clergeau ,Mennechez ,Sauvage ,Lemoine .2001.Human erception

and ppreciationfbirds: motivationorwildlife onservationnur-ban nvironments fFrance. ages 9-86 n Marzluff M, owman ,Donnelly , ds.Avian cologynanUrbanizing orld. orwellMA):Kluwer.

Collins JP, Kinzig A, Grimm NB, Fagan WF, Hope D,Wu J, Borer WT. 2000.A new urban ecology. American Scientist 88: 416-425.

Crisp PN, Dickinson KJM, Gibbs GW. 1998.Does native nvertebrate iversityreflect native plant diversity? A case study from New Zealand and im-

plications or conservation. Biological Conservation 3: 209-220.Crooks KR, Soule ME. 1999. Mesopredator elease nd avifaunal xtinctions

in a fragmented ystem. Letters o Nature: 563-566.Crowe TM. 1979. Lots of weeds: nsular phytogeography f vacant urban ots.

Journal f Biogeography : 169-181.Czech B, Krausman PR, Devers PK. 2000. Economic associations among

causes of species endangerment n the United States. BioScience 50:593-601.

Dawe GFM. 1995.Species density n relation o urban open space. Land Con-tamination and Reclamation : 114-116.

Denys C, Schmidt H. 1998. Insect communities on experimental mugwortplots along an urban gradient. Oecologia 113: 269-277.

Dickman CR. 1987. Habitat ragmentation nd vertebrate pecies richnessin an urban environment. Journal f Applied Ecology 24: 337-351.

Dorney JR, Guntenspergen R,Keough R,Steams E 1984.Composition ndstructure f an urban woody plant community. Urban Ecology :69-90.

Falk H.1976. Energetics f a suburban awn ecosystem. Ecology 7: 141-150.

Gering JC, Blair RB. 1999. Predation on artificial ird nests along an urban

gradient:Predation isk or relaxation n urban environments.

Ecogra-phy 22: 532-541.Germaine SS, Wakeling BE 2001. Lizard species distributors and habitat

occupation along an urban gradient n Tucson, Arizona, USA. Biologi-cal Conservation 7: 229-237.

Gibson DJ,Adams ED, Ely JS,Gustafson DJ,McEwen D. 2000. Eighteen earsof herbaceous ayer ecovery f a recreation rea n a mesic forest. Jour-nal of the Torrey Botanical Society 127: 230-239.

Gilbert OL. 1989. The Ecology of Urban Habitats. London: Chapman andHall.

Godefroid S. 2001. Temporal analysis of the Brussels lora as indicator or

changing environmental quality. Landscape and Urban Planning 52:203-224.

Goldstein EL, Gross M, DeGraaf RM. 1986. Breeding birds and vegetation:A quantitative ssessment. Urban Ecology 9: 377-385.



9/9

Articles

Henderson PB,Perkins H, NelischerM. 1998. Residential awn alterna-tives:A study f their distribution, orm and tructure. andscape ndUrban lanning 2:135-145.

Johnston E2001.Synanthropic irds f North America. ages 9-67 nMarzluff M,Bowman ,DonnellyR,eds.Avian Ecology n an Urban-izingWorld. orwell MA):Kluwer.

Kellert R. 1996.TheValue f Life. Washington DC): sland ress.Kendle ,Forbes . 1997. Urban Nature Conservation. ondon: hapman

and Hall.Kowarik . 1995.On the role of alien pecies n urban lora nd vegetation.Pages 5-103 n Pysek ,PrachK,RejminekM,Wade M, ds.Plant n-vasions-General spects ndSpecial roblems. msterdam Nether-lands): PBAcademic.

Lancaster K,ReesWE. 1979.Bird ommunities nd he tructure f urbanhabitats. anadian ournal f Zoology 7:2358-2368.

Luken O. 997.Conservation n the context f non-indigenous pecies. ages107-116 nSchwarz W, d.Conservation nHighly ragmented and-scapes. ondon: hapman ndHall.

LuniakM. 1994.The development fbird ommunities n new housing s-tates n Warsaw. emorabilia oologica 9:257-267.

Mackin-Rogalska ,Pinowski ,SolonJ,Wojcik . 1988.Changes n vege-tation, vifauna, nd mallmammals n a suburban abitat. olish co-

logical tudies 4: 293-330.Maeda ,Maruyama . 1991.Early allurban ird ommunities fHobart,

Tasmania. amashina nstitute f Ornithology 2:56-69.Majer D. 997. nvertebrates ssist herestoration rocess: nAustralian er-

spective. ages 12-237 n Urbanska M,Webb NR, Edwards J, ds.Restoration cology ndSustainable evelopment. ambridge UnitedKingdom): ambridge niversity ress.

Marzluff M. 001.Worldwide rbanization nd ts effects n birds. Pages19-47 n Marzluff M,Bowman ,Donnelly ,eds.Avian cologyn anUrbanizing orld. orwell MA):Kluwer.

Matthiae E, teams 1981.Mammals n forest slands n southeastern is-consin.Pages 5-66 n Burgess L,Sharpe M,eds.Forest slandDy-namics n Man-Dominated andscapes. ewYork: pringer-Verlag.

McIntyre E.2000.Ecology furban rthropods: review nd a call oac-

tion. Annals f the Entomological ociety f America 3: 825-835.MedleyKE,McDonnellMJ,Pickett TA. 995.Forest-landscape tructure

along nurban-to-rural radient. rofessional eographer 7:159-168.Mehmood R,Zhang DW. 2001.A roll call analysis f the Endangered

Species ctamendments. merican ournal fAgricultural conomics83:501-512.

[MDC]Missouri epartment f Conservation. 002.Landscaping orback-

yard wildlife. 3 September 002;www.conservation.state.mo.us/nathis/

backyard)Munyenyembe J,Harris ,Hone J,NixH. 1989.Determinants fbird pop-

ulations n an urban rea. Australian ournal f Ecology 4: 49-557.

Nilon CH, VanDruff LW. 987.Analysis f small mammal community dataand applications o management of urban greenscapes. roceedings fthe National Symposium on Urban Wildlife 2: 53-59.

Nuhn TP, Wright CG. 1979.An ecological urvey of ants n a landscaped ub-urban habitat. American Midland Naturalist 102: 353-362.

Pawlikowski ,Pokorniecka .1990. Observations n the structure f bum-blebee communities of the town-forest areas n Torun Basin, NorthPoland. Acta Universitatis Nicolai Copernici Biologia 37: 3-22.

Pickett STA,Cadenasso ML, Grove JM, Nilon CH,Pouyat

RV,Zipperer

WC,Costanza R. 2001. Urban ecological ystems: Linking errestrial, colog-ical, physical, nd socioeconomic omponents f metropolitan reas.An-nual Review of Ecology and Systematics 2: 127-157.

Racey GD, Euler DL. 1982. Small mammal and habitat esponse o shoreline

cottage development n central Ontario, Canada. Canadian ournal f Zo-

ology 60: 865-880.Sears AR, Anderson SH. 1991. Correlation etween birds and vegetation n

Cheyenne, Wyoming. Pages 75-80 in Adam LW, Leedy KL, ds. WildlifeConservation n Metropolitan Environments. Columbia (MD): Na-tional Institute or Urban Wildlife.

Sharpe DM, Stearns F, Leitner LA, Dorney JR. 1986. Fate of natural vegeta-tion during urban development of rural landscapes n southeasternWisconsin. Urban Ecology 9: 267-287.

Shugart HH, Anderson SH, Strand RH. 1975. Dominant patterns n bird pop-

ulations of the eastern deciduous orest birds. Proceedings f the Sym-posium on Management f Forest nd Range Habitats or Nongame Birds2: 90-95.

Stein BA, Kutner L,Adams J.2000. Precious Heritage. Oxford United King-dom): Oxford University Press.

Sukopp H, Werner P. 1982. Nature n Cities. Strasbourg France): Councilof Europe.

Terman MR. 1997. Natural inks: Naturalistic olf courses as wildlife habi-tat. Landscape nd Urban Planning 38: 183-197.

Tilghman NG. 1987. Characteristics f urban woodlands affecting reedingbird diversity and abundance. Landscape and Urban Planning 14:481-495.

[USCB] US Census Bureau. 2001. Statistical Abstract of the United States.

Washington DC): Government Printing Office.Vale TR, Vale GR. 1976. Suburban ird populations n west-central Califor-

nia. Journal f Biogeography : 157-165.VanDruff LW, Rowse RN. 1986. Habitat ssociation f mammals n Syracuse,

New York. Urban Ecology 9: 413-434.Wasowski A,Wasowski . 2000. The Landscaping evolution. Chicago: Con-

temporary Books.Whitcomb RF,Robbins CS, Lynch F,Whitcomb BL,Klimiewica MK, Bystrak

D. 1981. Pages 125-206 n Burgess RL,Sharpe DM, eds. Forest sland Dy-namics n Man-Dominated Landscapes. New York: Springer-Verlag.

Whitney GG. 1985. A quantitative nalysis of the flora and plant commu-nities of a representative midwestern U.S. town. Urban Ecology 9:143-160.

890 BioScience - October 002 / Vol. 52 No. 10

Urbanization Biodiversity and Conservation

Documents

Transcript of Urbanization Biodiversity and Conservation