Upload
others
View
0
Download
0
Embed Size (px)
Citation preview
Short-Term Response of Forest Birds to Experimental Clearcut Edges (Respuestas a Corto Plazode las Aves de Bosque a Bordes Creados Experimentalmente por Tala Rasa)Author(s): Marc-André Villard, Fiona K. A. Schmiegelow and M. Kurtis TrzcinskiReviewed work(s):Source: The Auk, Vol. 124, No. 3 (Jul., 2007), pp. 828-840Published by: University of California Press on behalf of the American Ornithologists' UnionStable URL: http://www.jstor.org/stable/25150340 .
Accessed: 28/01/2013 08:25
Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at .http://www.jstor.org/page/info/about/policies/terms.jsp
.JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range ofcontent 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].
.
University of California Press and American Ornithologists' Union are collaborating with JSTOR to digitize,preserve and extend access to The Auk.
http://www.jstor.org
This content downloaded on Mon, 28 Jan 2013 08:25:02 AMAll use subject to JSTOR Terms and Conditions
^^ The Auk 124(3):828-840, 2007
[ jJk ? The American Ornithologists' Union, 2007.
^TL Printed in USA.
SHORT-TERM RESPONSE OF FOREST BIRDS TO EXPERIMENTAL CLEARCUT EDGES
Marc-Andre Villard,1'4 Fiona K. A. Schmiegelow,2 and M. Kurtis Trzcinski3
^Canada Research Chair in Landscape Conservation, Departement de biologie, Universite de Moncton, Moncton,
New Brunswick E1A 3E9, Canada;
2Department of Renewable Resources, University of Alberta, Edmonton, Alberta T6G 2H1, Canada; and
department of Biology, Dalhousie University, Halifax, Nova Scotia B3H 4J1, Canada
Abstract. ?Numerous studies have addressed the potential consequences of
increasing the density of edges through human activities, but most have docu
mented responses to existing edges. Here, we monitored the response of seven
forest bird species to experimentally created edges around five plots (10 ha, n = 3;
25 ha, n = 2) in the boreal mixed-wood forest of Alberta, Canada. We also mapped
bird detections in six control plots (10 ha, n = 5; 25 ha, n =
1). The focal species were Least Flycatcher (Empidonax minimus), Red-eyed Vireo (Vireo olivaceus),
Yellow-rumped Warbler (Dendroica coronata), Black-throated Green Warbler (D.
virens), Ovenbird (Seiurus aurocapilla), Mourning Warbler (Oporornis Philadelphia), and White-throated Sparrow (Zonotrichia albicollis). In the two breeding
seasons
following experimental clearcutting, we
quantified birds' responses to edges in
the absence of substantial edge-induced changes in vegetation by comparing the
distribution of detections between treatment and control plots. We predicted that
forest-edge specialists would be attracted to edges, forest-interior specialists would
avoid them, and interior-edge generalists would show a neutral response. None of
these predictions was consistently supported among plots and years, except in the
Mourning Warbler. However, none of the significant responses was the opposite of predictions. We also predicted that postharvest colonization of treated plots
would mainly involve forest-edge specialists, whereas most local extinctions
would involve forest-interior specialists. Only the colonization of 10-ha fragments followed the predicted pattern. The relatively neutral response to forest edges we
observed suggests that, in general, boreal forest birds do not respond to the edge
itself or to proximate cues of edge proximity. Rather, significant responses may be
delayed until edge-to-interior gradients in vegetation are established. Received 30
March 2005, accepted 14 July 2006.
Key words: edge effects, field experiment, forest landscape, forest management,
habitat fragmentation, spot mapping, vegetation gradients.
Respuestas a Corto Plazo de las Aves de Bosque a Bordes Creados Experimentalmente por Tala Rasa
Resumen.?Muchos estudios han abordado las potenciales consecuencias del
aumento de la densidad de bordes generados por las actividades humanas.
Sin embargo, la mayoria de estos han documentado las respuestas a bordes
previamente existentes. En este estudio, monitoreamos la respuesta de siete
especies de bosque a bordes creados experimentalmente alrededor de cinco
parcelas (10 ha, n = 3; 25 ha, n =
2) en el bosque boreal mixto de Alberta, Canada.
4E-mail: [email protected]
828
This content downloaded on Mon, 28 Jan 2013 08:25:02 AMAll use subject to JSTOR Terms and Conditions
July 2007] Response to Experimental Edges 829
Tambien mapeamos las detecciones de las aves en seis parcelas control (10 ha,
n = 5; 25 ha, n =
1). Las especies focales fueron Empidonax minimus, Vireo olivaceus,
Dendroica coronata, D. virens, Seiurus aurocapilla, Oporornis Philadelphia y Zonotrichia
albicollis. En dos epocas reproductivas subsecuentes a los cortes experimentales, cuantificamos las respuestas de las aves a los bordes en ausencia de cambios
substanciales en la vegetacion inducidos por el borde, comparando la distribucion
de detecciones entre las parcelas control y de tratamiento. Predijimos que las
especies especialistas de borde serian atraidas a los bordes, que las especies de
interior de bosque los evitarian y que las especies generalistas de interior y de
borde tendrian una respuesta neutra. Ninguna de estas predicciones se
cumplio sistematicamente entre parcelas y anos, excepto para O. Philadelphia. Sin embargo,
ninguna de las respuestas significativas fue opuesta a las predicciones. Tambien
predijimos que la colonizacion post-cosecha de las parcelas de tratamiento
involucraria principalmente a
especies especialistas de borde, mientras que la
mayoria de las extinciones locales involucraria a especies especialistas de interior
de bosque. Solo la colonizacion de fragmentos de 10 ha siguio el patron predicho. La respuesta relativamente neutra a los bordes de bosque que observamos sugiere
que, en general, las aves de los bosques boreales no responden al borde en si ni a
sehales directas de la proximidad del borde. Las respuestas significativas podrian ocurrir mas tarde, una vez que los gradientes en la vegetacion desde el interior del
bosque hacia los bordes se hayan establecido.
Habitat edges influence the distribution of
many bird species: some cluster their territories
along edges, whereas others tend to avoid them
(Kroodsma 1984, Lanyon and Thompson 1986, Noss 1991, McCollin 1998, Renfrew et al. 2005).
Compared with the number of forest-edge "spe
cialists," relatively few species have been shown
to actively avoid forest edges (Baker et al. 2002), and even fewer exhibit consistent edge avoid
ance across studies (McCollin 1998, Villard
1998). It is critical to determine whether this lack of consistency is real, or whether it reflects
the lack of a general conceptual framework to
properly analyze, interpret, and compare pat terns in edge response (Ries et al. 2004).
Among the variety of mechanisms that
may explain significant positive or negative
responses to forest edges by woodland birds,
we distinguish three types: "access," "proxi
mate cues," and "ultimate cues." First, spe cies may be attracted toward edges because
they facilitate access to resources in adjacent
patches (Lanyon and Thompson 1986, McCollin
1998, Ries and Sisk 2004). Second, species may respond to proximate environmental cues
found near edges, such as
particular vegetation structures (McCollin 1998, Imbeau et al. 2003),
microclimatic conditions (Chen et al. 1999),
edge-related variations in food abundance
(Burke and Nol 1998, Ibarzabal and Desrochers
2004), or perhaps the presence of interspecific
competitors (Bollinger and Gavin 2004) or cer tain nest predators (Winter et al. 2000, Chalfoun
et al. 2002, Ibarzabal and Desrochers 2004,
Morton 2005). Third, ultimate effects of edges on
reproductive success may promote dispersal either away from or toward edges (e.g., Foppen and Reijnen 1994, Bollinger and Gavin 2004).
Thus, birds may respond either to proximate cues (the forest edge itself or conditions associ
ated with edge habitat) or to ultimate cues (e.g.,
edge effects on vital rates). When proximity to
an edge has consequences on individual fit
ness, an ability to use proximate cues would
represent a major advantage for individuals
possessing it.
To date, most studies investigating the spa tial response of forest birds to edges have been conducted in sites with pre-existing edges (e.g.,
Hansson 1983, Kroodsma 1984, Noss 1991,
Germaine et al. 1997, Flaspohler et al. 2001,
Manolis et al. 2002). In such cases, responses to proximate cues from the vegetation may be
more difficult to distinguish from responses to other proximate cues (microclimate, abun
dance of food, or nest predators), because the
This content downloaded on Mon, 28 Jan 2013 08:25:02 AMAll use subject to JSTOR Terms and Conditions
830 VlLLARD, SCHMIEGELOW, AND TrZCINSKI [Auk, Vol. 124
influence of the vegetation is difficult to control
(but see Kristan et al. 2003). However, edge-to interior gradients in forest vegetation
are not as
strongly developed in the first years following clearcut harvesting (Harper and Macdonald
2002), providing opportunities to reduce con
founding effects.
Here, we measured the short-term response of forest bird species to experimentally cre
ated clearcut edges. We mapped bird territories
during the breeding season preceding clearcut
harvesting of the forest adjacent to our study
plots, and during the two subsequent breeding seasons. Seven species
were common enough
among plots and years to be included in the
analyses: Least Flycatcher, Red-eyed Vireo,
Yellow-rumped Warbler, Black-throated Green
Warbler, Ovenbird, Mourning Warbler, and
White-throated Sparrow (see Table 1 for scien
tific names). Collectively, these species represent a wide range of nesting and foraging strategies
(ground, shrub, and canopy nesters; ground for
agers, foliage gleaners, and aerial insectivores). We also examined population turnovers (local
extinctions and colonizations) for all other song bird species present in the study plots.
We tested the following predictions: after
clearcutting, (1) species classified as forest-inte
rior specialists would establish territories farther
from actual forest edges than from control-plot
boundaries, (2) species considered to be forest
edge specialists would defend territories located
closer to clearcut edges, whereas (3) territory location in relation to clearcut edges
or control
plot boundaries would not differ significantly for
interior-edge generalists. Finally, we
predicted that (4) in treatment plots, most postharvest col
onization events would involve forest-edge spe
cialists, and that most local extinctions would be
detected among forest-interior specialists. Ries
and Sisk (2004) pointed out that such a priori pre dictions should be made only when information
on habitat quality and resource distribution is
available for both sides of the edge. In the study area, only one of the seven species considered,
the White-throated Sparrow, is known to nest
and forage in recent clearcuts (Hannah 2001).
Experimental edges were sharp immediately
postharvest, with scattered shrubby patches and woody debris left in clearcuts. Therefore,
it is safe to assume that clearcuts represented nonhabitat for six of the seven species examined
during the present study. Preference or avoidance of forest edges by
certain species has important conservation
implications (Ries et al. 2004). For example, these phenomena would strongly influence the
amount and quality of habitat available for a par ticular species in a given landscape. Although
edge avoidance is routinely assumed by authors
Table 1. Comparison of predicted and observed responses of seven passerine bird species to
experimental clearcut edges in the first two years postharvest (see text for details).
Predicted Observed Observed
Species response (10-ha plots, P) (25-ha plots: R, P / F, P)
Least Flycatcher Neutral No data Yr 1: At (0.002) / N (0.90)
(Empidonax minimus) Yr 2: At (0.001) / N (0.22)
Red-eyed Vireo Avoidance Yr 1: N (0.94) Yr 1: N (0.27) / Av (0.076)
(Vireo olivaceus) Yr 2: N (0.33) Yr 2: N (0.42) / N (0.36)
Yellow-rumped Warbler Avoidance Yr 1: N (0.47) Yr 1: N (0.25) / N (0.51)
(Dendroica coronata) Yr 2: N (0.18) Yr 2: N (0.67) / N (0.78) Black-throated Green Warbler Avoidance No data Yr 1: Av (0.033) / N (0.63)
(D. virens) Yr 2: Av (0.055) / N (0.13) Ovenbird Avoidance Yr 1: N (0.17) Yr 1: N (0.11) / N (0.86)
(Seiurus aurocapilla) Yr 2: N (0.19) Yr 2: Insufficient data
Mourning Warbler Neutral Yr 1: N (0.29) Yr 1: N (0.44) / N (0.98)
(Oporornis Philadelphia) Yr 2: N (0.62) Yr 2: N (0.22) / N (0.24) White-throated Sparrow Attraction Yr 1: N (0.61) Yr 1: N (0.59) / N (0.37)
(Zonotrichia albicollis)_Yr 2: N (0.79)_Yr 2: N (0.68) / N (0.86) Note that there are two pairwise comparisons per year among 25-ha plots. Responses consistent with predictions are in bold.
See text for details on study design. Abbreviations: R = riparian fragment, F =
upland fragment, N = neutral, At = attraction,
Av = avoidance.
This content downloaded on Mon, 28 Jan 2013 08:25:02 AMAll use subject to JSTOR Terms and Conditions
July 2007] Response to Experimental Edges 831
who refer to the concept of forest-interior spe
cies, this phenomenon remains largely untested
(Villard 1998). In spite of the fact that our study area is located in an
extensively forested land
scape with a recent history of anthropogenic
landscape change (Schneider 2002), forest types are still patchily distributed as a result of natu
ral disturbance regimes (mainly fire) and varia
tions in soils or drainage. Hence, a response to
forest edges could have evolved in bird species
occupying this landscape.
Methods
Study area. ? The present study
was conducted
from 1993 to 1995 near Calling Lake, Alberta
(55?N, 113?W), as part of a broader study on
the effects of experimental forest fragmentation on boreal birds (Schmiegelow et al. 1997). We
selected 11 study plots of two different sizes
(eight 10 ha, three 25 ha) in mature-to-old boreal
mixed-wood forest. In each plot, the dominant
tree species were trembling aspen (Populus
tremuloides) and balsam poplar (P. balsamifera), with scattered white spruce (Picea glauca) and
paper birch (Betula papyrifera). All plots were characterized by
a dense shrub layer dominated
by alders (Alnus tenuifolia, A. crispa), wild roses
(Rosa spp.), and lowbush cranberry (Viburnum
edule). Stand age varied from 90 to 130 years, and density and height of white spruce and alders increased with stand age (for details, see
Schmiegelow et al. 1997). The experimental design is illustrated in
Figure 1. Plots were located within continuous
forest (C: controls), or forest-fragment sites of
10 ha (three plots: F2-F4) or 40 ha (Rl and Fl) surrounded by clearcuts >100 m wide. We used
study plots of two sizes: (1) 10-ha plots within continuous forest (controls; n =
5) or surrounded
IKSp^BBaMHFw ^^MP^"- JgWiMiWlMllW 8hk I9BW8BHB BMPa. ffiB
1 0.5 0 1 2 3 ^^i=^^^^H^=i^=i^^^^B Kilometers
Fig. 1. Map of study area, 200 km north of Edmonton, Alberta. Calling Lake occupies the north
east corner. Plot codes refer to controls (C), upland fragments (F), and riparian (lakeshore) frag ments (R). Smaller plots are 10 ha in size and larger ones 25 ha.
This content downloaded on Mon, 28 Jan 2013 08:25:02 AMAll use subject to JSTOR Terms and Conditions
832 Villard, Schmiegelow, and Trzcinski [Auk, Vol. 124
by clearcuts on all sides (fragments; n = 3), and
(2) a 25-ha control plot (n =
1) and others within
larger (40-ha) fragments (n =
2). Plot Rl was con nected to a
riparian buffer strip, whereas all other
treatment plots (F1-F4) were completely isolated from nearby mature forest. It should be noted
that forest structure and composition within plot Rl was very similar to that of other plots because
it was located in upland forest, 100-300 m from
the riparian edge (Fig. 1). Experimental clearcut
ting took place during the winter of 1993-1994,
following the flagged site boundaries. Seven of the 11 plots
were surveyed
one year before har
vesting and two years after, whereas four were
only surveyed one year postharvest.
Because distance to the nearest edge varies as
a function of the number of edges and plot size
(10 ha and 25 ha), we performed separate analy
ses for each plot size and conducted pairwise
comparisons between 25-ha plot CI and plots Rl and Fl, respectively (see below). We mod
eled each year separately to allow within-year
spatial comparison with our controls. Pre- and
postharvest data were also used to examine pat terns in local extinctions and colonizations.
Bird surveys. ?
Every year, observers visited
each plot eight times. We mapped the loca
tions of all birds seen or heard and recorded all occurrences of countersinging among conspe
cific males following the procedure described in Bibby et al. (1992). For each species,
a mini
mum of two detections separated by >10 days was
required to register a territory (Bibby et al.
1992). In most cases, the number of detections
within a given territory exceeded three (Fig. 2).
For a given plot-year, we conducted analyses
only on species occupying at least two territo
ries. In two cases (Ovenbird; Fig. 3E, F), too few
detections fell within the control plot to permit statistical analysis.
Statistical analyses.?To quantify responses to
forest edges bordering experimental clearcuts,
we measured the distance from the location of
each bird detection within a defined territory to
the nearest forest edge in treatment plots, and
to the nearest plot boundary in controls. In the
case of 25-ha plots, we matched actual clearcut
edges with corresponding boundaries imposed on the control plot (Fig. 1). For example, for sta
tistical comparison between plots Fl and CI, we
measured the distance from each bird detection
to the nearest clearcut edge in Fl, and to the
nearest plot boundary in CI to the west, north,
or east. Because territories are nested within
study plots and individual bird detections are
nested within territories, we modeled responses to edge using linear mixed models (Pinhero and Bates 2000). In 25-ha plots, each species'
response to forest edges was estimated using the following model:
Vijk =
Pi + bj(i)
+ Zijk
i = l,...3,j
= l/...j,k
= l,...k
where y^k
= distance of an observation from the
nearest clearcut edge, i is site, j is territory within
site, and k is a detection within a territory. This
model specifies territory within site [bj(i)]
as a
random effect, and site as a fixed effect. The bj(i)
and etjk
values are assumed to be independent random variables with N(0, o2b), N(e, o2). The |3, values estimate the mean response of a
species to the forest edge at a site. Because the distribu
tion of possible distances from the nearest clear
cut edge differed between the two 25-ha plots, we ran separate models to compare each plot with the 25-ha control plot.
Smaller (10-ha) plots were replicated within treatments and controls. Our replicate plots
may have differed for many reasons; thus, this
nested structure allowed us to estimate the
between-site variability as a random effect and
to test for a treatment effect (T = forest fragment
or control) on the distance of bird detections
from the edge:
yTijk=PT +
bi(T) +
bj<i) +
EW
However, the fact that 10-ha plots contained
fewer territories than larger ones made it more
difficult to estimate the variance in the response
of territory holders at a site, b:(i).
Using every spot-mapped bird detection in
the analysis provides a more detailed assess
ment of response to edge than using average
distance-to-edge per territory, or average dis
tance from territory centroids to nearest edge. Detections within a territory are modeled as
independent measures of an individual bird's
activity in relation to the nearest edge. We
explicitly modeled the correlation in observa
tions collected within territories by estimating a random effect of territory distance from the
nearest edge (bj(i)). In other words, we assumed
randomness within a territory and modeled a
territory effect. Considering distance to nearest
This content downloaded on Mon, 28 Jan 2013 08:25:02 AMAll use subject to JSTOR Terms and Conditions
July 2007] Response to Experimental Edges 833
Clearcut ....-"" . "*"
/^^ / \ - '""" a-1 ~\~?^-.(.
, *y One year i T \i_^
^^r>> preharvest R1 f*?500m?*
Clearcut _(. <y
;
Clearcut .'" ... \ \\ \ .^^^
( *\[^/[-' \ \ v postharvest
I- / /^"^^x Two years K| ^>=-^^ / .' 'V ".'J- postharvest
w rrVtiln Fig. 2. Changes in the number and distribution of Black-throated Green Warbler territories in
plot Rl before and after experimental harvesting. The location of the spot-mapping grid in rela
tion to experimental clearcuts is shown (top left). Each dot (right) represents a bird detection on
spot maps.
edge as a random effect could be problematic for abundant species because of the saturation
of available habitat but, typically, <50% of a
study plot was occupied by a given species. We
could also have compared the observed distri
bution of detections to randomly generated ones, but a
comparison to actual control data
seemed more biologically relevant.
Predictions. ?
Each species' response to experi mental forest edges was
predicted using existing classifications (Freemark and Collins 1992, Miller et al. 2004). Because species' assignment to cat
egories differed among authors in some cases,
we reclassified them as follows: (1) generalists: Least Flycatcher and Mourning Warbler; (2) forest-edge specialist: White-throated Sparrow;
This content downloaded on Mon, 28 Jan 2013 08:25:02 AMAll use subject to JSTOR Terms and Conditions
834 Villard, Schmiegelow, and Trzcinski [Auk, Vol. 124
25-ha plots: one year preharvest
A B 300 -i 300 -i
225 -L 225 J
:
p^ihjipiii:
U n ij ij \-i
,i 0 J 19 6 12 17 5 3 9 11 2 3 14 7 14 10 Q J 19 15 12 15 5 4 9 8 2 2 14 15 14 18
C1 R1 C1 R1 C1 R1 C1 R1 C1 R1 C1 R1 C1 R1 C1 F1 C1 F1 C1 F1 C1 F1 C1 F1 C1 F1 C1 F1
LEFL REVI YRWA BGNW OVEN MOWA WTSP LEFL REVI YRWA BGNW OVEN MOWA WTSP
^ 25-ha plots: one year postharvest
>? C D CD 300 -i 300 -.
O) P = 0.002 P = 0.03 P = 0.08
"? G) 225 -J J T 225
-j
fV. 0 -I 18 10 11 12 12 11 9 10 4 3 13 9 16 13 Q J 18 14 11 14 12 9 9 8 4 2 13 16 16 16
co "?^?r~~'?"?"?^?r? ?|?r~n? " ?r~n?|~~'?"?r?1 .? C1 R1 C1 Rl C1 R1 C1 R1 C1 R1 C1 R1 C1 R1 C1 F1 C1 F1 C1 F1 C1 F1 C1 F1 C1 F1 C1 F1
Q LEFL REVI YRWA BGNW OVEN MOWA WTSP LEFL REVI YRWA BGNW OVEN MOWA WTSP
25-ha plots: two years postharvest
E F 300
-| 300 n
P = 0.001 P = 0.06
225 J J T 225 -J
0 J 14 12 10 11 5 14 9 7 2 3 5 10 9 14 q J 14 20 10 14 5 9 9 5 2 2 5 15 9 13
C1 R1 C1 R1 C1 R1 C1 R1 C1 R1 C1 R1 C1 R1 C1 F1 C1 F1 C1 F1 C1 F1 C1 F1 C1 F1 C1 F1
LEFL REVI YRWA BGNW OVEN MOWA WTSP LEFL REVI YRWA BGNW OVEN MOWA WTSP
Fig. 3. Mean distance (? SE) from bird detections to nearest clearcut edges or
corresponding control
plot boundaries in three 25-ha plots (Fl =
upland fragment, Rl = riparian fragment; CI =
control; see
Fig. 1), one year before harvest (A and B) and 1-2 years postharvest (C and D; E and F). Number of
territories within each study plot is indicated below mean distances, and P values are indicated above
when significant. The number of Ovenbird detections per territory in plot CI (two years postharvest)
was insufficient to allow statistical analyses. See Appendix for bird species codes.
This content downloaded on Mon, 28 Jan 2013 08:25:02 AMAll use subject to JSTOR Terms and Conditions
July 2007] Response to Experimental Edges 835
and (3) forest-interior specialists: Red-eyed
Vireo, Yellow-rumped Warbler, Black-throated
Green Warbler, and Ovenbird. We reclassified
Red-eyed Vireo as a forest-interior specialist because it consistently avoided edges in other
studies (reviewed in Villard 1998). Accordingly, we
predicted that (1) territories of forest-interior
specialists would be located farther from the forest edge, when accounting for random varia
tions observed in the control plot, and that (2)
territories of forest-edge specialists would be
located closer to edges, whereas (3) territory locations of interior-edge generalists would be
similar in control and fragments. Finally, we
predicted that (4) most postharvest colonization
events would involve forest-edge specialists and most local extinctions would be detected
among forest-interior specialists. We compared the proportions of local extinctions and coloni
zations between forest-edge and forest-interior
specialists, pooled separately for 10-ha frag
ments, 40-ha fragments, and controls. All sites
were combined, whether they were monitored
one or two years postharvest. Species present in
a given plot before harvest and absent in the first or second years postharvest
were considered
locally extinct. Similarly, species absent before
harvest and present in either year postharvest were considered to have colonized.
Results
The classification of species according to
their perceived response to forest edges was
not a good predictor of their observed response
to experimental clearcut edges. On the basis of
statistical significance, 11 of the 36 responses observed were consistent with predictions
(Table 1). Six of these 11 responses were asso
ciated with the predicted neutral response of
the Mourning Warbler. Even when responses matched predictions, this was not the case for all
plots and years considered (Table 1). Significant edge avoidance was observed only in three
cases involving two species (Red-eyed Vireo
and Black-throated Green Warbler). Ovenbird
responses also suggested edge avoidance, but
the differences were not significant (Table 1;
Figs. 3 and 4). Our predictions assumed that published clas
sifications of species' responses to edge are based
on solid empirical evidence, which may not be
the case (Villard 1998). If we disregard these
A 100
| One year postharvest
80 - I
::h '! lni q, 43 3 3 4? 43 43 O) ? "
,_r_^_.T_^. 7__....-,_,_, g? CF CF CF CF CF
REVI YRWA OVEN MOWA WTSP
2
o B 2 10? i Two years postharvest
Q 80 - ~T~
"I* _T ,T tt 20 -
3 3 53 5 2 33 53
CF CF CF CF CF REVI YRWA OVEN MOWA WTSP
Fig. 4. Mean distance (? SE) from bird detec tions to nearest clearcut edge
or corresponding
control-plot boundary in 10-ha plots, (A) one
year and (B) two years after harvesting. The
number of territories within the study plot is
indicated below mean distances, and P val
ues are indicated above when significant. See
Appendix for bird species codes.
predictions and consider instead that the seven
species examined form a representative sample
of avian life histories in the boreal mixed-wood
forest, our results suggest that most boreal
forest bird species respond weakly to clearcut
edges per se and to environmental conditions
associated with recently created forest edges
(access to resources in adjacent patches, sharp
gradients in microclimate, and possibly changes in biotic interactions). However, because more
territories could straddle control-plot boundar
ies than actual edges, bird detections should
have been closer to control-plot boundaries by chance alone. Thus, our
analyses provide a very conservative assessment of attraction to edges and a liberal test of edge avoidance.
Our last prediction pertained to local extinc
tion and colonization events. Proportions of
local extinctions and colonizations differed
This content downloaded on Mon, 28 Jan 2013 08:25:02 AMAll use subject to JSTOR Terms and Conditions
836 Villard, Schmiegelow, and Trzcinski [Auk, Vol. 124
significantly when comparing 10-ha plots and
controls for both forest-interior specialists and
forest-edge specialists (G test, Gad]
= 9.77, P =
0.021), whereas the difference between 40-ha
plots and controls was not significant (P =
0.20). As predicted, most colonization events in frag ments involved forest-edge specialists (Fig. 5), but this was evident only in 10-ha fragments, where species turnover was greatest. There
were relatively few local extinctions, either in
fragments or controls, and the proportions of
forest-interior and forest-edge specialists going
locally extinct were similar. Interestingly, the
most striking pattern was the large proportion of colonization events in control plots involv
ing forest-interior specialists. These paralleled a
sharp increase in overall abundance in these
sites in the second year following harvest
(Schmiegelow and Hannon 1999).
Discussion
In spite of the diverse life histories of the seven
species considered, the dominant pat tern was one of neutral short-term response to
clearcut edges, especially in the smaller frag ments (10 ha). Neutral responses were predicted
in only two of those seven species. At the level
of species assemblages, the predicted influx
of forest-edge specialists was observed in 10
ha fragments, but numerous colonizations by
20 j-1
18 10-ha fragments H
16 El 40-ha fragments I 14 Controls r?i
8 12 I I 10 I
Is ^ I
1 1 LE(I) LE(E) C(l) C(E)
Fig. 5. Number of local extinctions (LE) and
colonizations (C) recorded in the study plots over the first two years following experimen tal clearcut harvesting. Data are
grouped by habitat-use category (I
= forest-interior special
ists, E = forest-edge specialists).
forest-interior specialists were also observed in
control plots. This latter result mirrors patterns in overall abundance, where total numbers of
individuals detected by point counts in the control plots increased significantly in the sec
ond year following harvest (see Schmiegelow et al. 1997, Schmiegelow and Harmon 1999). By contrast, overall number of individuals detected
in the treatment areas remained stable, despite
apparent colonization by forest-edge special ists (F. K. A. Schmiegelow unpubl. data). Thus,
experimental harvesting resulted in shifts in bird distribution across the study area in the first two
years postharvest (see Schmiegelow et al. 1997),
but these shifts were not reflected in predicted short-term responses by individual species on
the basis of existing edge-response classifica
tions. However, none of the patterns observed
was the opposite of predictions (i.e., avoidance
instead of attraction or vice versa). Therefore,
one could attribute the low predictive success
of the edge-response classification we tested to
alternative factors, such as low statistical power
(Murcia 1995). Nevertheless, to our knowl
edge, the present study is among the few field
experiments measuring forest birds7 response to
edges that feature spatial and temporal controls
and, at the 10-ha scale, replication. How can we
interpret these results? Four
types of explanation come to mind: (1) environ
mental conditions in our study region
or study sites are
atypical, (2) our results reflect a lack of
sensitivity in our approach to the measurement
of species' responses or low statistical power,
(3) existing classifications of bird species with
regards to their response to edges are invalid,
or (4) many boreal bird species do not or cannot
use proximate cues to detect edge habitat.
Disturbance regime and response to forest
edge.?As noted above, the study area is
characterized by an active fire regime, and
large-scale anthropogenic disturbance (for
estry, exploration for oil and gas) is recent.
Hence, one could hypothesize that forest
bird populations of north-central Alberta
are relatively unresponsive to the treatment
because they have evolved in a fine-grained,
highly dynamic landscape (Mutch 1970, Perera et al. 2004) where habitat edges dominate "natural" landscape mosaics. Longer-term
monitoring of responses at a regional scale is
necessary to determine whether newly created
anthropogenic edges elicit stronger responses
This content downloaded on Mon, 28 Jan 2013 08:25:02 AMAll use subject to JSTOR Terms and Conditions
July 2007] Response to Experimental Edges 837
as exposure increases in conjunction with
expansion of development activities.
Sensitivity of our approach. ?
Although we are
confident in our quantitative approach to the
detection of edge response, we recognize that
our results are conservative with respect to the
detection of edge attraction. In addition to the
fact that territories could straddle the boundary of control plots, bias might arise from the spa
tial location of territorial males. Because males
spend large amounts of time patrolling terri
tory boundaries, differences in bird locations
between control and treatment plots could be
reduced if territory boundaries in the latter tend to correspond to clearcut edges. Only
one of the
seven species, the White-throated Sparrow,
was
predicted to exhibit significant attraction toward
edges, but we may have overlooked such
responses in other species, owing to the conser
vative approach we used. While spot mapping,
only once did we detect an individual of one of the other species considered here in an adja cent clearcut (male Mourning Warbler <10 m
from forest edge; M.-A. Villard pers. obs.).
Thus, White-throated Sparrow was the only
species likely to frequently use supplementary resources (sensu Dunning et al. 1992) in experi
mental clearcuts during the time-frame of this
study (see also Hannah 2001). Another complicating factor is the variabil
ity in abundance among species examined.
Although we
registered few territories for some
species (e.g., Ovenbird), contributing to low
power with which to detect effects, other spe cies were much more abundant, which may also
have reduced the probability of detecting shifts in response to experimental edges because of
territory packing. Mourning Warbler would fit
this pattern. On the other hand, Least Flycatcher still showed a
significant attraction to edges in
Rl, even though its density reached 10-18 ter
ritories in 25 ha (Fig. 3). The present study also points to an
impor tant tradeoff in sampling design when inves
tigating edge effects. It is necessary to sample several sites to quantify among-site variability.
However, maximizing the number of sites sam
pled may incur a cost in estimating a site-level
response, because the number of individuals
sampled at each site decreases when plot size
is reduced. Further, in the present study, vari
ability in distance from forest edge was greater
among territories than among 10-ha sites
(M.-A. Villard et al. unpubl. data), which indi
cates that larger plots containing more territories
provide a more accurate assessment of a spe
cies' response to edges. Future studies should
include replicated plots large enough to analyze
complex, additive edge effects (Fernandez et
al. 2002). For example, Black-throated Green
Warblers appeared to avoid the northeastern
corner of plot Rl to a greater degree than the
southeastern corner (Fig. 2), probably because
the northern and eastern plot boundaries were
directly adjacent to clearcuts, in contrast to the
southern plot boundary. Prediction of species response to forest edges.?
Villard (1998) questioned the validity of classifi cations of bird habitat-use with respect to forest
edges. His argument was based on the lack
of empirical evidence provided in support of those classifications, and on their poor perfor
mance when confronted with such data. Since
then, a few studies have provided empirical evidence to classify species (e.g., Porneluzi and
Faaborg 1999, Brand and George 2001, Bollinger and Gavin 2004). Here, we used existing clas
sifications strictly to formulate predictions. Our
results confirm that such classifications may not
be consistent across the range of a given species
and in variable landscape contexts, especially in
the case of forest-interior specialists. Use of direct or indirect cues. ?The general lack
of response of forest bird species to the abrupt forest edges that were created in this experi
mental study suggests that direct cues of prox
imity to edge play a minor role in breeding-site
selection. Unfortunately, we did not document
possible ed^e effects on reproductive success
and, therefore, cannot investigate potential costs of the observed lack of response to edges.
However, other investigations of edge effects
conducted in the same study area confirm
that edge effects on reproductive performance
were either weak or inconsistent (Cotterill and
Hannon 1999, Song and Hannon 1999, Lambert
and Hannon 2000). In summary, our results confirm that clas
sifications of species' habitat-use with respect to forest edges require further empirical test
ing. More replication at the larger plot scale
would also be desirable. The relative neutral
ity we observed in forest birds' responses to
sharp, recent forest edges is consistent with the
suggestion that birds may respond to edge-to interior gradients in vegetation (McCollin 1998,
This content downloaded on Mon, 28 Jan 2013 08:25:02 AMAll use subject to JSTOR Terms and Conditions
838 Villard, Schmiegelow, and Trzcinski [Auk, Vol. 124
Imbeau et al. 2003) rather than to edges per se.
Thus, a species7 response to edge will vary both
with landscape context and with time since
edge creation.
Acknowledgments
This research was supported by discov
ery grants from the Natural Sciences and
Engineering Research Council of Canada
to M.A.V and F.K.A.S., the Alberta/Canada
Partnership Agreements in Forestry (F.K.A.S.), and the Alberta Wildlife Enhancement Fund
(F.K.A.S.). We thank S. J. Hannon for her col
laboration in designing the broader experimental
study and for her assistance with field logistics. This research was made possible by the excellent
collaboration of the staff at Alberta-Pacific Forest
Industries. We are grateful
to our field assistants
R. Brown, S. Diggon, M. Eggen, A. Jansen, K.
Hannah, P. Heaven, K. Lisgo, M. Lomow, H.
Lefebvre, M. Ruebel, I. Schmelzer, J. Wojnowski, and to N. McVarish, T. Morcos, I. Robichaud, and
K. St. Laurent for geographic information system
(GIS) assistance. E. K. Bollinger and three anony mous reviewers made insightful comments on a
draft of the manuscript.
Literature Cited
Baker J., K. French, and R. J. Whelan. 2002. The
edge effect and ecotonal species: Bird com
munities across a natural edge in southeast
ern Australia. Ecology 83:3048-3059.
Bibby, C. J., N. D. Burgess, and D. A. Hill. 1992.
Bird Census Techniques. Academic Press,
San Diego, California.
Bollinger, E. K., and T. A. Gavin. 2004.
Responses of nesting Bobolinks (Dolichonyx oryzivorus) to habitat edges. Auk 121:
767-776.
Brand, L. A., and T. L. George. 2001. Response
of passerine birds to forest edge in coast red
wood forest fragments. Auk 118:678-686.
Burke, D. M., and E. Nol. 1998. Influence of
food abundance, nest-site habitat, and forest
fragmentation on breeding Ovenbirds. Auk
115:96-104.
Chalfoun, A. D., M. J. Ratnaswamy, and F. R.
Thompson III. 2002. Songbird nest predators in forest-pasture edge and forest interior
in a fragmented landscape. Ecological
Applications 12:858-867.
Chen, J., S. C. Saunders, T. R. Crow, R. J.
Naiman, K. D. Brosofske, G. D. Mroz, B. L.
Brookshire, and J. F. Franklin. 1999.
Microclimate in forest ecosystem and land
scape ecology. BioScience 49:288-297.
Cotterill, S. E., and S. J. Hannon. 1999. No evi
dence of short-term effects of clear-cutting on artificial nest predation in boreal mixed
wood forests. Canadian Journal of Forest
Research 29:1900-1910.
Dunning, J. B., B. J. Danielson, and H. R.
Pulliam. 1992. Ecological processes that
affect populations in complex landscapes. Oikos 65:169-175.
Fernandez, C, F. J. Acosta, G. Abella, F. Lopez,
and M. Dias. 2002. Complex edge effects fields
as additive processes in patches of ecological
systems. Ecological Modelling 149:273-283.
Flaspohler, D. J., S. A. Temple, and R. N.
Rosenfield. 2001. Species-specific edge effects on nest success and breeding bird
density in a forested landscape. Ecological
Applications 11:32-46.
Foppen, R., and R. Reijnen. 1994. The effects of
car traffic on breeding bird populations in woodland. II. Breeding dispersal of male
Willow Warblers (Phylloscopus trochilus) in relation to the proximity of a highway.
Journal of Applied Ecology 31:95-101.
Freemark, K. E., and B. Collins. 1992. Landscape
ecology of birds breeding in temperate for
est fragments. Pages 443-454 in Ecology and Conservation of Neotropical Migrant Landbirds (J. M. Hagan III and D. W.
Johnston, Eds.). Smithsonian Institution
Press, Washington, D.C.
Germaine, S. S., S. H. Vessey, and D. E. Capen.
1997. Effects of small forest openings on
the breeding bird community in a Vermont
hardwood forest. Condor 99:708-718.
Hannah, K. C. 2001. Patterns in habitat quality for
the White-throated Sparrow (Zonotrichia albi
collis) in a recently logged landscape. M.Sc.
thesis, University of Alberta, Edmonton.
Hansson, L. 1983. Bird numbers across edges
between mature conifer forest and clearcuts
in Central Sweden. Ornis Scandinavica 14:
97-103.
Harper, K. A., and S. E. Macdonald. 2002.
Structure and composition of edges next
to regenerating clear-cuts in a mixed-wood
boreal forest. Journal of Vegetation Science
13:535-546.
This content downloaded on Mon, 28 Jan 2013 08:25:02 AMAll use subject to JSTOR Terms and Conditions
July 2007] Response to Experimental Edges 839
Ibarzabal, J., and A. Desrochers. 2004. A nest
predator's view of a managed forest: Gray Jay
(Perisoreus canadensis) movement patterns in
response to forest edges. Auk 121:162-169.
Imbeau, L., P. Drapeau, and M. Monkkonen.
2003. Are forest birds categorized as "edge
species" strictly associated with edges?
Ecography 26:514-520.
Kristan, W. B., Ill, A. J. Lynam, M.V. Price, and J. T.
Rotenberry. 2003. Alternative causes of
edge-abundance relationships in birds and
small mammals of California coastal sage scrub. Ecography 26:29-44.
Kroodsma, R. L. 1984. Effect of edge on breed
ing forest bird species. Wilson Bulletin 96: 426-436.
Lambert, J. D., and S. J. Hannon. 2000. Short
term effects of timber harvest on abundance,
territory characteristics, and pairing success
of Ovenbirds in riparian buffer strips. Auk 117:687-698.
Lanyon, S. M., and C. F. Thompson. 1986. Site
fidelity and habitat quality as determi
nants of settlement pattern in male Painted
Buntings. Condor 88:206-210.
Manolis, J. C, D. E. Andersen, and F. J.
Cuthbert. 2002. Edge effect on nesting success of ground nesting birds near regen
erating clearcuts in a forest-dominated land
scape. Auk 119:955-970.
McCollin, D. 1998. Forest edges and habitat
selection in birds: A functional approach.
Ecography 21:247-260.
Miller, J. R., M. D. Dixon, and M. G. Turner.
2004. Response of avian communities in
large-river floodplains to environmental
variation at multiple scales. Ecological
Applications 14:1394-1410.
Morton, E. S. 2005. Predation and variation in
breeding habitat use in the Ovenbird, with
special reference to breeding habitat selec
tion in northwestern Pennsylvania. Wilson
Bulletin 117:327-335.
Murcia, C. 1995. Edge effects in fragmented for
ests: Implications for conservation. Trends
in Ecology and Evolution 10:58-62.
Mutch, R. W. 1970. Wildland fires and ecosys tems: A hypothesis. Ecology 51:1046-1051.
Noss, R. F. 1991. Effects of edge and internal
patchiness on avian habitat use in an old
growth Florida hammock. Natural Areas
Journal 11:34-47.
Perera, A. H., L. J. Buse, and M. G. Weber.
2004. Emulating Natural Forest Landscape Disturbances: Concepts and Applications. Columbia University Press, New York.
Pinhero, J. C, and D. M. Bates. 2000. Mixed
effects Models in S and S-PLUS. Springer Verlag, New York.
Porneluzi, P. A., and J. Faaborg. 1999. Season-long
fecundity, survival, and viability of Ovenbirds
in fragmented and unfragmented landscapes. Conservation Biology 13:1151-1161.
Renfrew, R. B., C. A. Ribic, and J. L. Nack. 2005.
Edge avoidance by nesting grassland birds:
A futile strategy in a fragmented landscape.
Auk 122:618-636.
Ries, L., R. J. Fletcher, J. Battin, and T. D.
Sisk. 2004. Ecological responses to habitat
edges: Mechanisms, models, and variability
explained. Annual Reviews of Ecology and
Systematics 35:491-522.
Ries, L., and T. D. Sisk. 2004. A predictive model
of edge effects. Ecology 85:2917-2926.
Schmiegelow, F. K. A., and S. J. Hannon. 1999.
Forest-level effects of fragmentation on boreal
songbirds: The Calling Lake fragmentation studies. Pages 201-221 in Forest
Fragmentation: Wildlife and Management
Implications (J. A. Rochelle, L. A. Lehmann,
and J. Wisniewski, Eds.). Brill, Leiden, The
Netherlands.
Schmiegelow, F. K. A., C. S. Machtans, and
S. J. Hannon. 1997. Are boreal birds resilient
to forest fragmentation? An experimental
study of short-term community responses.
Ecology 78:1914-1932.
Schneider, R. R. 2002. Alternative futures:
Alberta's boreal forest at the crossroads.
Federation of Alberta Naturalists and Alberta
Centre for Boreal Research, Edmonton.
Song, S. J., and S. J. Hannon. 1999. Predation in
heterogeneous forests: A comparison at nat
ural and anthropogenic edges. Ecoscience 6:
521-530.
Villard, M.-A. 1998. On forest-interior species,
edge avoidance, area sensitivity, and dogmas
in avian conservation. Auk 115:801-805.
Winter, M., D. H. Johnson, and J. Faaborg.
2000. Evidence for edge effects on mul
tiple levels in tallgrass prairie. Condor 102:
256-266.
Associate Editor: E. K. Bollinger
This content downloaded on Mon, 28 Jan 2013 08:25:02 AMAll use subject to JSTOR Terms and Conditions
840 Villard, Schmiegelow, and Trzcinski [Auk, Vol. 124
Appendix. Bird species investigated in the present study.
Code Species Classificationa
LEFL Least Flycatcher I/E REVI Red-eyed Vireo I YRWA Yellow-rumped Warbler I BGNW Black-throated Green Warbler I
OVEN Ovenbird I MOWA Mourning Warbler I/E WTSP White-throated Sparrow E
a Modified from Freemark and Collins (1992) and Miller et al. (2004); see text.
Classification key: I = forest-interior specialist, I/E = interior-edge generalise E =
forest-edge specialist.
This content downloaded on Mon, 28 Jan 2013 08:25:02 AMAll use subject to JSTOR Terms and Conditions