6
avian communities, and landscape
Miller', JenniferM. Fraterrigol,N. ThompsonHobbs23,
David M.
. WiensI
Colorado State University,
317 WestProspect.Fort Collins. CO 80523 USA
ColoradoState University,Fort Collins. CO 80523 USA
Birds, Colorado, gradient, landscape ecology, policy, study design,
urbanization
Humansettlementis a prevalentsourceof land-usechangeworldwide, but our
understandingof the effects of settlementon avian communities is limited.
Settlement has been characterizedas a gradient that extends from urban
developmentto exurban and rural areas. An important advantageof the
gradientapproachis the potentialto identify thresholdsin the responseof birds
to settlement. Although gradientshave been used in studying the effects of
urbanizationon birds, relatively little attention has beenpaid to the exurban
end of the spectrum,despitethe potential for this type of developmentto affect
large expansesof habitat. Studies at relatively fine scales are useful for
investigatingthe influence of proximate factors, such as vegetationstructure
and composition,on birds in human-dominatedareas. However,such studies
must be coupledwith investigationsat broaderscalesto gain a more complete
understandingof the ways that human settlementaffects bird communities.
Urban-wildlandgradientscan be quantified using remotely-sensedimagery in
combination with data on the intensity and pattern of settlement. However,
metricsusedto describepatternsof settlementareonly usefulto the extentthat
they representsomethingmeaningful to birds. The extent of the landscape
mosaic surroundinga study site that needsto be quantified dependson the
18
Chapter6
goals for a particular study. We proposea researchprotocol for studyingthe
effectsof settlementon birds that may be usefulat multiple scales. Study sites
are distributedamong land-coverand settlementtypes, and replicatedsurveys
are progressivelyaggregatedfrom small to large scales. Thesedata serveas
the basis for interpolation, using traditional statistical tools and geostatistical
methods,to areasnot sampled. To be effective,conservationmust be focused
on landscapemosaics,not habitat patches. Areaswherepeople live and work
are importantcomponentsof thesemosaics.
1.
INTRODUCTION
Development associated with human settlement has emerged as a
prevailing source of land-usechange throughout the world (Berry 1990;
United Nations Centrefor Human Settlements1996;Cohen 1997;Marzluff
2001). In the U.S.A. alone,nearly 6 million ha of forest and rangelandwere
convertedto housing and various settlement-relatedinfrastructurebetween
1982and 1992(NRCS 1995). Such developmentis not evenly distributed
geographically, nor is it limited to urban areas. Extensive landscape
transformationin the Rocky Mountain region, for example,is the result of a
populationgrowth ratethat is threetimes the nationalaverage.Coloradois a
bellwether of change for this region, with 6 of the 10 fastest-growing
countiesin the nation (including the top three)- all exceeding6% population
increasesper year between 1990 and 1998 (U.S. Bureau of the Census
1998). Although much of this growth is manifest in typical urban and
suburbansprawl, the conversionof agricultural and forested land to lowdensity residentialdevelopmentis occurring at unprecedentedrates(Knight
et al. 1995,Riebsameet al. 1996).
Urbanization clearly harms some native bird species(Marzluff 2001).
Our understandingof the mechanismsunderlying such effects has been
explored by Bolger (2001) and Marzluff (2001), but remainsrudimentary,
largely becausemany ecologistshaveavoidedhuman-dominatedsystemsas
locationsfor study. In the relatively few studiesthat havefocusedon settled
areas, researchers have usually concentrated on urban or suburban
environmentswhile areasof lower residentialdensity have generally been
ignored. Furthermore,most studieshave been conductedat relatively fine
scales(but also seeBlair 1996,Nilon et al. 1995,Clergeauet al. 1998). For
thesereasons,the scientific basisfor avian conservationin developedareas
remains weak, particularly in areas of lower settlementdensities and at
landscapeand regionalscales.
Our goal is to contribute to the dialogue on the direction of future
researchon bird communities in human-dominatedareas. We begin by
examining gradientsof human settlementwith specialattentionto portions
of such gradients that have thus far received scant attention by avian
119
Next, we consider issues of spatial scale and stress the
of a multi-scale approach to examining the effects of
Finally, we discussresearchneedsas well as several
in designing and implementingeffective policies for
GRADIENTS OF HUMAN SETTLEMENT
have treated "urban" as a separatetype of land-cover,
,
or "grassland." This is similar to the way that
.
. in land-usedatabasessuchas
..~_.-" (Anderson et al. 1976), where the only
is betweenurban and non-urbanareas.However,the true extent
. may be vastly underestimatedby such conceptualizations
"urban" is an endpointon a continuumof settlementintensity- a
that extends from sparsely populated rural areas to large cities
2001). When we comparedenselypopulatedurban areasto
,
-- - ---
-- - --
."
~,
.
shrubs. Suburban areas often give way to exurban residential
-
-
~
Ultimately, exurbandevelopmentgradesto rural areas,
Settlement
is not a matter
paradigm"
has
long
been
used
in
1967)
at a variety
1981).
and
---~
function
of
of
spatial
The
ecological
ecology
and
scales
underlying
systems
as a basis
has been
(T erborgh
assumption
vary
in
for
applied
to
1 971 ,
is that
space,
and
this variationcan enhanceour knowledgeof system-level
McDonnell
and
Pickett
. gradient"
a "powerful
organizing
(1990)
and
tool"
have
described
suggested
for
that
ecological
urbanization
the
gradient
research
as a
paradigm
in human-
.-.
. gradients are indeed complex. In major metropolitan areas,
extending from the urban core reveal an interspersionof dense
development, parks, lower-density suburbs, and smaller city
. A designbasedon linear transectsis usefulfor
120
Chapter6
A.
¥
~
~
. .
&--+
~
Residential
urban
L--1-'i - ;8
B,
Figure 6.J. Humansettlementalong the northernFront Rangeof Colorado(both panels:Fort
Collins-top, Loveland-bottom,and Greeley-right). (A) Areasclassifiedasresidentialurban
land-coverfrom the USGS 1:250,000LU/LC map series. (B) The full gradientof housing
densitiesfrom urbanto rural using U.S. CensusBureaublock-level data
121
-
'
of development
(seeSavardandFalls2001),but as
(2001) note, urban-ruralgradientsare not purely geographical.
-. .'to examiningthe effectsof settlementon birds doesnot
study sites to be aligned in this fashion and other ways of
. . may be more informative. One might comparesites
of different types or intensitiesof development(Blair
2001; Ctergeau et at. 1998), or examine the effect of varying
. habitattype (Friesenet at. 1995,
-.
2001).
importantadvantageof the gradient approach,as opposedto binary
.
,(e.g., urban vs. rural or suburbanvs. wildland), lies in its
--- for identifying thresholds or breakpointswhere human impacts
markedchangesin biotic responses(Wiens 1989). Such thresholds
be found, for example, in measuresof human density, patterns of
or distance from developed areas (Bolger et al. 1997).
are important simply becausethey offer a clearer basis for
such as the determinationof buffer distancesfor sensitive species
.
.
.
recentlysurveyed41 peer-reviewedstudiesof avian communitiesin
-- humansettlementthat were publishedbetween1970and 1999. Of
investigations,12 utilized a gradient-basedresearchdesign. In most
the gradientswere restricted in the levels of developmentthat were
only five studiescoveredthe spectrumfrom urban or suburbanto
In fact, the low-density portion of the gradient has not
much attention generally (only four of the 41 studies included
sites) despite the increasing pervasivenessof this type of
2.1
Exurban
Development
In Colorado,exurbandevelopmentexpandedat a paceof 8.3% annually
between1960 and 1990 - nearly three times the rate of population growth
(Theobald2000). This type of low-density developmentis expectedto
continuein Colorado and elsewhere,fueled by the growth of recreation-,
service-,and information-basedindustries; dissatisfactionstemming from
various problems associatedwith urban and suburban living; aesthetic
considerations;
and recreationalopportunities. Locationsnear federal lands
suchas National Parks and National Forests - places often viewed as
conservation
areas- appearto be particularly attractive(Nelson 1992,Howe
etal. 1997,Marzluff et al. 1998).
Although individual buildings are more widely spaced in exurban
developments,
this does not necessarilymeanthat the effects of settlement
122
Chapter6
are restricted to a small portion of the landscape. As rural areas are
converted to exurban residential development,road density is likely to
increase. Existing roads will experiencegreater traffic becausewidely
disperseddevelopmentmeansmore vehicles per householdand more trips
per day per vehicle (Romme 1997).
Roadsare associatedwith an effect zone - an areaof ecological impact
on either side of the road itself - that extendsoutward from tensto hundreds
of meters (Forman and Alexander 1998). Numerousspeciesof grassland
birds and forest birds have beenfound to avoid roads(Ferris 1979,van der
Zande et al. 1980). Researchin the Netherlandsshowed that the effectdistances(the distance from a road at which population decreaseswere
detected)varied with traffic density and habitat; 60% of the bird species
presentoccurredat lower densitiesnear roads,and songbirddeclineswere
coincidentwith thresholdsin traffic noise(Reijnen et al. 1995,Reijnenet al.
1996). Moreover, evidencesuggeststhat mammalianpredatorstravel along
low-traffic roads (Bennett 1991, Forman 1995) and increasedrates of nest
predation have been found near low-traffic road edges(Small and Hunter
1988). Roads may turn source habitats into sinks by increasingmortality
rates (Mumme et al. 2000) and roads may also alter avian community
composition by attracting edge species,particularly in forested habitats
(HanowskiandNiemi 1995).
One can imagine that buildings also are associatedwith an effect zone,
although this phenomenon has not received as much attention from
ecologists. A building-effect zone with reference to birds might be a
function of humanactivity, noise, or free-rangingpets. Although the effect
of anyone building on the surrounding matrix is probably small, the
cumulative effect of many buildings may be substantial(Theobald et al.
1997). Building effects might cause avoidancein some species(O'Dell
2000), whereasspeciesthat are tolerant of human presencemay benefit.
Given the rapid increaseand dispersednatureof exurbandevelopment,this
little-studiedportion of the gradientof humansettlementhasthe potentialto
alter the structureof bird communitiesover largeareasin someregions.
3.
SPATIAL SCALE
3.1
Local Scales
Traditionally, ecologistshave focused on within-habitat diversity when
conductingresearchon bird communitiesand have ascribeddifferencesin
avian diversity to variation in habitat features such as foliage-height
diversity or horizontal habitat heterogeneity(MacArthur and MacArthur
6. Urban bird ecology
123
1961,Karr and Roth 1971, Willson 1974). In an effort to reduce spatial
variationand thereby allow "fair" comparisons,sampleshave usually been
collectedover small plots or quadrats,usually < 0.5 km2,locatedin a single
habitattype.
Studydesignsbasedon within-habitat diversity also have beencommon
in researchon urban birds. Severalworkers have examinedrelationships
betweenavian community composition and attributes of urban woodlots
(Tilghman 1987) or parks (Gavareski 1976, Cicero 1989) including
vegetation,patch area, or the presence of human artifacts such as
recreationaltrails. Similarly, DeGraaf and Wentworth (1986) described
suburbanbird assemblagesin Amherst, Massachussetts,and attributed
differencesin avian community structure to various aspectsof the built
environment,particularly those relating to vegetation(see also Savardand
Falls2001).
Nest loss in developedareasalso has been examinedrelative to local
habitatfeatures (Haskell et al. 2001). For example, Major et al. (1996)
found that nest predation increasedwith nest height and tree density in
residentialbackyardslocated in Australian urban areas. Miller and Hobbs
(2000) examined nest predation in lowland riparian areas adjacent to
suburbandevelopmentalong the Front Rangeof Coloradoas a function of
distancefrom recreationaltrails. Mammaliannestpredatorstendedto avoid
trails, whereas avian predatorstended to prey more on nests near trails
(Miller and Hobbs 2000), similar to the effects of buildings and human
activityon nestingbirds reportedby OsborneandOsborne(1980).
Studiesat local scaleshave advancedour understandingof proximate
factorsthat affect bird distributions in areasof humansettlement,but many
questionsremain to be investigated. Vegetationstructureand floristics, for
example,have been frequently cited as important determinantsof habitat
suitability for some species(Beissinger and Osborne 1982, Green et al.
1989,Jokimaki and Suhonen 1993, Rolando et al. 1997, Germaineet al.
1998). However, an exclusive focus on the fine-scaleattributesof humandominatedareasreflects the belief that local featuresare synonymouswith
"habitat." Evidence suggeststhat additional factors operating at broader
scalesalso influenceavian communitystructure.
3.2
LandscapeContext
Landscapecontext may influenceecologicalprocessessuchas thosethat
detenninelocal avian diversity becausebirds are highly mobile organisms
and likely respond to habitat features across a range of spatial scales
(Hostetler 200I). Hilden (1965) suggestedthat habitat selection by
migratorybirds could best be viewed as a hierarchicaldecisionprocessthat
124
Chapter6
starts at regional and landscape scales and proceeds to fine-scale habitat
characteristics. If this is true, then we might sometimes expect similar
habitats to support different bird assemblages, depending on the structure
and composition of the surrounding landscape matrix.
Pearson (1993) found that much of the variation in the abundance and
diversity of wintering birds could be explained solely by landscape variables.
Several workers have reported a positive relationship between the width of a
riparian area and avian diversity (Stauffer and Best 1980, Darveau et al.
1995, Hodges and Krementz 1996), but this relationship does not always
hold when wider riparian areas are surrounded by urban or suburban
development (Miller et al. 2001).
Many mechanisms may mediate the effects of landscape context on avian
communities in a given habitat type. Birds nesting in forest fragments may
be subjected to different nest predators and experience different rates of nest
predation depending on the nature of the areas surrounding these patches
(Andren 1992, Marzluff and Restani 1999). Szaro and JakIe (1985) found
that birds associated with riparian habitats comprised nearly a third of avian
assemblages found in desert scrub but that this percentage decreased with
distance from the riparian zone. Clearly, conclusions drawn on the sole
basis of local habitat features without consideration of landscape context
may be misleading.
When assessing the effects of landscape context, ecologists often have
focused on landscapeswhere the primary anthropogenic impacts involve the
conversion of land-cover types such as forest and woodland to clearcuts and
cropland, commonly thought of as habitat fragmentation (Harris 1988,
Saunders et al. 1993, Schwartz 1997). Indeed, contemporary thinking about
anthropogenic impacts on landscapes is dominated by the fragmentation
paradigm, by the idea that the integrity of many ecosystems is diminished by
human activities that isolate well-defined habitat patches in an ecologically
compromised matrix. Although this paradigm may apply well to landscapes
in which the primary effects of human land-use involve the wholesale
conversion of land-cover types, including native habitat remnants embedded
in high-density urban areas (e.g., Central Park in New York City), it is not
especially faithful to the effects of settlement at lower densities.
In their study of the Golden-cheeked Warbler (Dendroica chrysoparia),
Engels and Sexton (1994) noted that it is often difficult to delineate patches
in urban and suburban areas in a meaningful way because the effects on
birds of the defining features of built environments, such as streets and
buildings, are poorly understood. Engels and Sexton (1994: p. 287)
observed that the woodlands in the vicinity of Austin, Texas ".. .are more
frequently speckled by urban structures than isolated by them into discrete
islands." Generalizing this idea, we suggest that in many instances the
~
125
human
settlement
ml!Y
be
better
represented
as
.
used
by
ecologists
when
images
--
-~--~
canopy
or
:..
cover,
but
aerial
forested
quite
studying
photos,
areas.
"perforations"
fragmentation
may
fail
Two
different
sites
with
~
at
to
capture
may
respect
the
be
similar
to
of
Furthermore,
the
data
broad
scales,
true
extent
in
intensity
terms
of
of
of
,
frequentlyusedby urbanplannersandgeographers,
are
measuring
boththeintensityandpatternof settlement
(Alberti
,
--'
2001). Parcel maps, for example,may be acquiredfrom county tax
offices and provide spatially-explicit information on building
building age, and zoning restrictions for a given parcel. Such
in combinationwith remotely-senseddata, provide the meansfor
gradientsof human settlement. This being said, it should be
: ability to computean ever-expandingarray of landscape
Intact
Fragmentation
Perforation
Figure6.2. Contrastinglandscapepatternsassociatedwith habitatfragmentationandhabitat
perforation.Black areasin the lower panelsrepresenthabitatremoval. Fragmentationresults
in the large-scaleconversionof land-covertypes(by timber harvestor agriculture,for
example)and discretehabitatpatchessurroundedby a highly-alteredmatrix, whereashabitat
perforationresultsin a landscapespeckledwith disturbancepoints (suchas individual houses
or clustereddevelopments).
126
Chapter6
metrics for the purpose of describing settlementpatterns far exceedsour
ability to relatethesemetricsto ecologicalprocessesin a meaningfulfashion
(Gustafson1998,Turner et al. 2000). Once the metrics are computed,the
more-difficult task of determiningthe extentto which they representpatterns
that are importantto birds still remainsto be done.
How much of the surroundinglandscapematrix should be quantified?
This is a vexing questionthat only recently has begun to be addressedby
ecologists. The answer likely depends on a suite of factors, but two
important considerationsare the ecological traits of the species under
investigation and the portion of the urban-rural gradient that is being
examined. Different species may respond to the landscapeat different
scales,dependingon traits such as body size and trophic status(Hostetler
2001). Thus, a study of the effects of settlementon habitat use by raptors
may require measurements
over a much broaderareathan a similar study on
songbirds.
The type of developmentthat occurs in the surroundinglandscapealso
may dictate the scale of study. The pattern or intensity of development
within, say, 100 m of an urban study site may be quite different than that
occurring within 500 or 1000 m. Becauseexurban developmentis, by
definition, more diffuse, spatial patterns usually change more gradually
comparedto urban areas. One way in which investigatorshave tried to
assessthe scale at which birds are respondingto their environment is by
determining the amount of variation in avian community structure (e.g.,
richness, density, composition, species presence/absence)
explained by
landscapemeasuresbeyond that accountedfor by local habitat features
(Pearson1993,Knick and Rotenberry1996,Bolger et al. 1997,Saab1999).
As a final point, we note that most researchon human settlementhas
been conducted in forested landscapes(Marzluff et al. 2001). In this
context, it is not surprisingthat ecologistshave assertedthat urbanizationis
synonymous with the simplification of habitat structure compared to
undevelopedareasand leads to overall reductionsin avian diversity (Geis
1974,Aldrich and Coffin 1980,Beissingerand Osborne1982,DeGraafand
Wentworth 1986). However,developmentalsocan result in a more complex
habitat structure compared to the presettlement landscape. Human
settlement in the arid and semi-arid regions of North America often is
accompaniedby the planting of trees,shrubs,and expansivelawns as people
try to recreatethe familiar landscapesfrom which they came (Limerick
1987). As a result, habitat structure becomes more complex and the
diversity of birds associatedwith the built environmentin theseareasis, in
some cases,greater than that of the predevelopmentlandscape(Guthrie
1974, Vale and Vale 1976, Sodhi 1992). The same may be said of
development in areas formerly dedicated to intensive agriculture, as
6. Urban bird ecology
127
monoculturesare replaced with more diverse habitats. Again, landscape
contextmatters.
3.3
Regional Perspectives
Most investigationsof the effects of urbanizationon bird communities
havebeensomewhatrestrictedin terms of spatial extent. In our review of
aviancommunity studies in areasof human settlement,the majority were
conductedin a single city or town (n = 32). Five studiescomparedbird
communitiesacrossmore than one city, but only a handful were extendedto
include the matrix in which the cities were embedded. Three studies
examinedeffects at a county-wide scale and only two consideredeffects
acrossmore than one county. As a result, the cumulative effects of
settlementat regional scalesare virtually unknown.
A regional perspectiveis important, both in assessingthe cumulative
effects of settlement and in managing human population growth. For
example,Marzluff(2001) notedthat growth control in Portland,Oregon,has
simply transferred development pressure to outlying communities.
Furthermore,just as landscapesare perforatedby individual buildings and
smallresidentialclusters,regionsare perforatedby cities, towns, and larger
suburbanenclaves surrounded by exurban development and linked by
transportationnetworks. Thus, a regional perspective is a necessary
componentof comprehensiveavian conservationstrategiesif they are to be
effective(Knopf and Samson1994).
4.
A MULTI-SCALE RESEARCH PROTOCOL
Becausewe will never be able to measureprocessesand statesfor more
thana tiny fraction of a region, we need samplingdesignsthat allow us to
extrapolateto unmeasuredareas. We therefore propose a multi-scale
researchprotocol for assessingthe effects of development on avian
communities(Fig. 6.3). This protocol could be usedto comparethe effect of
differentpatternsor densitiesof settlementon birds within a city, between
sitesat different points on the urban-ruralgradient,or acrossa region. Data
obtainedwith this method also could be integratedwith spatially-explicit
modelsof land-usechange,basedon demographicand economic drivers
(Theobald and Hobbs 1998), to identify important habitats that are
particularlyvulnerableto future residentialdevelopment.
Traditionalavian censustechniques(point or line transects,circular plots)
in combinationwith fine-scale habitat measurements(vegetationstructure
andfloristics) arethe foundationof this method. Setsof local surveysare
128
Chapter6
~
Selectpatchesof eachland type in the study
areausing a stratified-randomprocedure
~
Randomlyselectsurveypoints within
replicatepatchesof eachland type
~
Randomlyselectsmall plots in
the vicinity of eachsurveypoint
Field data
Bird surveys
Small-plot
measurements
Large-plot
measurements
~
Test model predictions
with new field data
~
Developspatialmodel integratingfield
data,remotesensingimagery,and
measuresof humansettlement
~
Interpolateto areasnot sampled
Land-useplanning
Figure 6.J. Multi-scale researchprotocol for assessing
the effectsof developmenton bird
communitie~
6. Urban bird ecology
129
distributed across the spectrum of land types in the study area using a
geographic information system (GIS) and a stratified random design (Fig.
6.3; step 1). For our purposes, land types are defined by measures of human
settlement intensity (road density, building density, etc.) superimposed on
habitat types (forest, grassland, etc.) to represent the full gradient of
settlement occurring in the focal area. Undeveloped sites also could be
included. Bird survey points are placed within patches of each land type
(step2) and smaller plots are placed in the vicinity of each point (step 3); the
exact locations of points and plots are determined using a stratified-random
procedure. Appropriate habitat measurements (canopy cover, shrub cover,
etc.) are taken over the larger plots, or the area over which birds are counted
at each point, and fine-scale habitat measurements (leaf litter, coarse woody
debris, etc.) are taken in the smaller plots. The sampling design is
hierarchically nested so that replicated surveys and habitat measures can be
progressivelyaggregated at broader scales.
A spatial model is then developed that can be used to correlate broadscaledata derived from satellite imagery or aerial photography and measures
of settlement pattern or intensity with fine-scale data on bird communities
and habitat structure (step 4). To examine spatial dependence among
variables, one might use traditional statistical methods such as stepwise
regression in combination with geostatistical tools such as trend-surface
analysis(Gittens 1968). It then becomes possible to interpolate to areas not
sampled (Robertson 1987; step 5). Land-use planning often involves the
application of data-based models to unsampled areas and this is preferable to
planning in the absence of field data. Optimally though, a model's
performancewould first be assessedby testing its predictions in a new set of
studysites (step 6).
In addition to their use in applied research, these methods can be used to
addressbasic ecological questions. For example, beta diversity is generally
thought to increase with an increasing number of habitat types. A recent
study conducted in the Colorado Front Range of the Rocky Mountains
explored the extent to which this relationship is altered by the effect of
human settlement (Fraterrigo 2000). The overall study area encompassed
two counties and was dominated by ponderosa pine (Pinus ponderosa),
Douglas fir (Pseudotsuga menziesii), lodgepole pine (Pinus contorta), aspen
(Populus tremuloides), and alpine meadows. The research focused on
severalsmall towns that lie along a north-south axis. The basic premise was
that if areas of human settlement tend to be more similar to one another in
terms of habitat structure than they are to the pre-development landscape
(Schneider 1996), their respective bird communities should exhibit greater
similarity to one another than to avian assemblages in the surrounding
matrix. There is evidence to support this assertion in larger developments at
130
Chapter6
lower elevationsin other regions (Aldrich and Coffin 1980,Beissingerand
Osborne1982,DeGraafand Wentworth 1986),but herethe emphasiswas on
smallersettlementsor exurbanareasin westernconiferousforests.
5.
RESEARCHNEEDS
There is a pressingneedfor more researchon virtually all aspectsof the
relationshipbetweenurbanizationand bird communities. This appliesto all
points along the urban-wildland gradient and across scalesfrom local to
regional. Of course,no single study can addressmore than a few aspectsof
settlement's effects on birds. Particularly at broader scales, logistical
constraintsmake it difficult to include a sufficient number of study sites to
teaseapartmore than a few "treatment"effects,evenif we relax assumptions
about strict replication (Hargrove and Pickering 1992). Therefore,
researchersmust consider the objectives of a project carefully and
realistically. This being said, we offer somethoughtson areasof research
wherethe needfor additional study is particularlyacute.
Our emphasisin this paper on researchconductedover relatively broad
scalesshouldnot be construedto meanthat investigationsat local scalesare
unnecessary. An understandingof the interaction betweendevelopment,
local habitat features, and bird communities is fundamental to accurate
interpretation of patterns discerned over landscapesand regions. Data
collectedat smallerscalesalso are useful in providing guidelinesfor habitat
enhancementto individual property owners, neighborhoodorganizations,
and municipal governmentagencies.For example,an urgentneedexists for
information on the proximateeffects of streetsand vehiculartraffic, various
sorts of humanactivity, and different patternsof tree and shrubplanting on
birds. The relative value of native versus exotic vegetationalso requires
further examination, particularly the insect loads that different species
support (Beissingerand Osborne 1982) and their effect on nest predation
rates(Schmidtand Whelan 1999).
We believe that investigationsaimed at determiningthe effect zone or
"footprint" of individual housesor clustersof buildings shouldbe affordeda
high priority by ecologists. Such data are useful in an urban or suburban
context, but they are critical in assessing the impacts of exurban
development.As we have noted,very little researchhas beenconductedon
this part of the urban-wildlandgradient,althoughthis type of development
potentially affects vast expansesof habitat. When examiningthe effects of
featuresof the built environmentat local scales,it is important that some
considerationbe given to the natureof the surroundinglandscape.
6. Urban bird ecology
131
At larger scales, we urge workers to addressthe entire gradient of
development,from urban to wildland, in their study designs. There are
logisticalchallengesinherentin suchan approachand it is almost inevitable
that a tensionwill exist betweencollecting an adequateamount of data at
localscalesand visiting a sufficient numberof sitesat different points along
thegradient. Nevertheless,suchstudiesare necessaryto placesmaller-scale
effortsin a broadercontext and to make fair comparisonsamong different
intensitiesof settlementon the gradient. Until there is a greaterbalancein
the spatial scales that researchersaddresswhen conducting research,a
comprehensiveunderstanding of the effects of development on bird
communitiesis unlikely to emerge.
6.
POLICY AND MANAGEMENT IMPLICATIONS
Over the last few decades, scientists have become increasingly interested
in the role that their research might play in the environmental policy process.
Here,our goal is not to provide an exhaustive treatment of the intricacies of
policy development regarding natural resource management - this has been
done elsewhere (e.g., Ostrom 1991, Lee 1993). Rather, we wish to share a
few observations that stem from our attempts (particularly by authors NTH
and DMT) to provide scientific support for land-use decisions that affect
humansettlement (Theobald et al. 2000).
Historically, ecologists and conservation biologists have exerted their
greatestimpact on environmental protection by influencing decisions at the
national level. The Clean Air Act, the Clean Water Act and the Endangered
SpeciesAct all are examples of policies that extend "top-down" from the
federal government to influence local actions. However, most decisions
regarding residential and commercial development are made at the base of
the government hierarchy by county governments, municipalities, and
landowners(Heatley 1994).
This creates several challenges for implementing conservation policy.
The first challenge is created by the diffuse nature of land-use decisions.
Many seemingly small decisions - choices made at many different times and
locations - accumulate to cause large-scale impacts. The fact that these
choicesare diffuse in time and space means that it is difficult for experts to
inform them. It follows that many local decisions are made without the
benefit of scientific input regarding their ecological impacts. Second, the
fact that many local jurisdictions, including individual landowners, are
responsible for land-use decisions implies that conservation policy at the
regional scale must include input from numerous sources. Achieving
consensus or even agreement among these many jurisdictions may be
132
Chapter6
impossible, and when consensus is reached it might be forged from
compromises that fail to deal with fundamental conservation issues. Finally,
the large degree of discretion granted to landowners by the U.S. Constitution
(Smith 1993, Cullingworth 1997) means that conservation policy developed
by government may have very little impact on what happens on the
landscape. Unless we are content with merely cleaning up the infrastructure,
it will be imperative to find ways to compensate private landowners for not
developing their land.
Rising to these challenges will require the development of strategies for
conserving and managing biodiversity at multiple scales and the integration
of actions by state, county, and municipal governments with those of
landowners. This is a daunting task, but there are encouraging examples to
guide our efforts (DeGrove 1992, Lee 1993, Beatley 1994, Duerksen et al.
1997). These multi-scale strategies must achieve reasonable compromises
between maintaining local and regional diversity of species and habitats.
Achieving conservation objectives in the face of rapid land-use change
requires a new set of responses by scientists. We must assure that data and
analyses are both readily understood and easily accessible to policy-makers
and to the citizens who wish to influence decisions in established review
processes. We must be willing to work locally to contribute to the solution
of regional problems. Finally, we must emphasize that conservation should
be focused on landscape mosaics, not habitat patches, and that developed
areas are important components of these mosaics. Rather than writing off
settled areas as having no conservation value, the challenge is to enhance the
habitat value of these lands through better design.
ACKNOWLEDGEMENTS
We thank M. Hostetler, M. Alberti, and J. Marzluff for helpful
suggestionson an earlier draft of this manuscript.
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