Effects of Introduced Canada Geese (Branta canadensis) on Native Plant Communities of the Southern Gulf Islands, British Columbia

Abstract

BioOne sees sustainable scholarly publishing as an inherently collaborative enterprise connecting authors, nonprofit publishers, academic institutions, research libraries, and research funders in the common goal of maximizing access to critical research. BioOne (www.bioone.org) is an electronic aggregator of bioscience research content, and the online home to over 160 journals and books published by not-for-profit societies, associations, museums, institutions, and presses. Exotic invasive species are recognized worldwide as a threat to native ecosystems (Lake & Leishman, 2004) and a leading cause of native species decline (Levine & D'Antonio, 2003). Recent work suggests that these threats may be amplified further when co-invaders facilitate each other's success (Simberloff & Von Holle, 1999; Mitchell et al., 2006), but relatively few examples of such interac-tions have been studied in detail. In the Southern Gulf Islands of British Columbia (BC), recent experiments show that exotic grasses increased at the expense of native species in the presence of non-migratory Canada geese (Branta canadensis) in coastal meadows (Best & Arcese, 2009). Because non-migratory Canada geese in this area are themselves introduced, and the "maritime meadows" in which they breed have been the subject of a federal "multi-species at risk" recovery strategy (Parks Canada Agency, 2006), we examined potential links between the introduction of non-migratory Canada geese to southwest-ern BC and the dominance of non-native grasses in mari-time meadows on 39 islands distributed throughout the Southern Gulf Islands.

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BioOne sees sustainable scholarly publishing as an inherently collaborative enterprise connecting authors, nonprofit publishers, academic institutions, research
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Effects of Introduced Canada Geese (Branta canadensis) on Native Plant
Communities of the Southern Gulf Islands, British Columbia
Author(s): Miriam Isaac-Renton, Joseph R. Bennett, Rebecca J. Best and Peter Arcese
Source: Ecoscience, 17(4):394-399. 2010.
Published By: Centre d'etudes nordique, Universite Laval
DOI: 10.2980/17-4-3332
URL: http://www.bioone.org/doi/full/10.2980/17-4-3332
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17 (4): 394-399 (2010)
Exotic invasive species are recognized worldwide as
a threat to native ecosystems (Lake & Leishman, 2004)
and a leading cause of native species decline (Levine &
D’Antonio, 2003). Recent work suggests that these threats
may be amplified further when co-invaders facilitate each
other’s success (Simberloff & Von Holle, 1999; Mitchell
et al., 2006), but relatively few examples of such interac-
tions have been studied in detail. In the Southern Gulf
Islands of British Columbia (BC), recent experiments
show that exotic grasses increased at the expense of native
species in the presence of non-migratory Canada geese
(Branta canadensis) in coastal meadows (Best & Arcese,
2009). Because non-migratory Canada geese in this area
are themselves introduced, and the “maritime meadows”
in which they breed have been the subject of a federal
“multi-species at risk” recovery strategy (Parks Canada
Agency, 2006), we examined potential links between the
introduction of non-migratory Canada geese to southwest-
ern BC and the dominance of non-native grasses in mari-
time meadows on 39 islands distributed throughout the
Southern Gulf Islands.
Effects of introduced Canada geese (Branta canadensis)
on native plant communities of the Southern Gulf
Islands, British Columbia1
Miriam ISAAC-RENTON & Joseph R. BENNETT, Department of Forest Sciences and Centre for Applied Conservation Research,
University of British Columbia, 2424 Main Mall, Vancouver, British Columbia V6T 1Z4, Canada.
Rebecca J. BEST, Department of Evolution and Ecology, University of California-Davis, Storer Hall, Davis, California 95616, USA.
Peter ARCESE2, Department of Forest Sciences and Centre for Applied Conservation Research, University of British Columbia,
2424 Main Mall, Vancouver, British Columbia V6T 1Z4, Canada, peter.arcese@ubc.ca
Abstract: Recent experiments suggest that introduced, non-migratory Canada geese (Branta canadensis) may be facilitating
the spread of exotic grasses and decline of native plant species abundance on small islets in the Georgia Basin, British
Columbia, which otherwise harbour outstanding examples of threatened maritime meadow ecosystems. We examined this idea
by testing if the presence of geese predicted the abundance of exotic grasses and native competitors at 2 spatial scales on
39 islands distributed throughout the Southern Gulf and San Juan Islands of Canada and the United States, respectively. At
the plot level, we found significant positive relationships between the percent cover of goose feces and exotic annual grasses.
However, this trend was absent at the scale of whole islands. Because rapid population expansion of introduced geese in the
region only began in the 1980s, our results are consistent with the hypothesis that the deleterious effects of geese on the cover
of exotic annual grasses have yet to proceed beyond the local scale, and that a window of opportunity now exists in which to
implement management strategies to curtail this emerging threat to native ecosystems. Research is now needed to test if the
removal of geese results in the decline of exotic annual grasses.
Keywords:Branta canadensis, Canada geese, exotic species, Garry oak ecosystem, islands, maritime meadow, native
plant communities.
Résumé : Des expériences récentes suggèrent que les bernaches du Canada (Branta canadensis) non migratrices introduites
pourraient faciliter la dispersion d’herbes exotiques et le déclin de l’abondance d’espèces de plantes indigènes sur les petits
îlots du bassin de Georgia, en Colombie-Britannique, qui par ailleurs possèdent de remarquables exemples d’écosystèmes
menacés de prairie maritime. Nous avons examiné cette idée en testant si la présence d’oies pouvait prédire l’abondance
d’herbes exotiques et de compétiteurs indigènes à 2 échelles spatiales dans 39 îles distribuées dans toute la partie sud des îles
Gulf au Canada et dans les îles San Juan aux États-Unis. À l’échelle de la parcelle, nous avons trouvé des relations positives
significatives entre le pourcentage de couverture de fèces d’oies et d’herbes annuelles exotiques. Cependant, cette tendance
était absente à l’échelle d’îles entières. L’expansion rapide des populations d’oies introduites dans la région ayant débutée
seulement dans les années 1980, nos résultats sont compatibles avec l’hypothèse que les effets délétères des oies sur la couverture
d’herbes annuelles exotiques ne se font encore sentir qu’à l’échelle locale et qu’ainsi une fenêtre d’opportunité existe en ce
moment pour mettre en oeuvre des stratégies de gestion visant à réduire les risques associés à cette menace émergente pour
les écosystèmes indigènes. Des recherches sont donc nécessaires afin d’évaluer si le retrait des oies pourrait causer un déclin
des herbes annuelles exotiques.
Mots-clés : Bernaches du Canada, Branta canadensis, communautés de plantes indigènes, écosystème du chêne de Garry,
espèces exotiques, îles, prairie maritime.
Nomenclature: Douglas et al., 1998–2002; Mowbray et al., 2002.
Introduction
1Rec. 2009-11-24; acc. 2010-09-20.
Associate Editor: Stephen Vander Wall.
2Author for correspondence.
DOI 10.2980/17-4-3332

ÉCOSCIENCE, VOL. 17 (4), 2010
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Contrary to prevailing assumptions, Canada geese
were infrequent winter visitors to southwestern BC prior
to the 1960s, when goslings from Minnesota, Ontario,
Saskatchewan, Alberta, and south-central British Columbia
were inter-bred and then introduced to establish “a breeding
population that would allow a harvestable excess” (Butler
& Campbell, 1987, reviewed in Smith, 2000). Initial intro-
ductions of geese to the BC Lower Mainland (Butler &
Campbell, 1987; Smith, 2000) led to the establishment of
non-migratory breeding populations (Vermeer & Davies,
1978), facilitated their translocation to Vancouver Island,
and were accompanied by hunting closures to encour-
age population growth (Smith, 2000). By 1978, however,
translocations were also undertaken to reduce goose num-
bers in urban and agricultural areas, where they were
increasingly perceived as a nuisance and potential health
hazard (Smith, 2000), including translocations to Sidney
Island in the Southern Gulf Islands (Figure 1; Smith, 2000;
G. Kaiser, M. Chutter, pers. comm.). At Kerr Island (3 km
north of Sidney Island, Figure 1), where records date back
to 1968, Canada geese were first noted in August 1974
(A. J. Brumbaum, pers. comm.). On Mandarte Island, where
ornithologists have worked annually since 1960 (Drent
et al., 1964; Smith et al., 2006), Canada geese first nested
in 1983 (P. Arcese, pers. observ.), about the same time they
colonized the Isabella islets (J. Thornburn, pers. comm.).
Similarly, the North American Breeding Bird Survey first
recorded non-migratory Canada geese on its long-term
Seabird Island, Victoria, Sunshine Coast, and Chemainus
routes in 1976, 1989, 1991, and 1993, respectively, where
populations continue to increase (Sauer, Hines & Fallon,
2008). As a consequence of these introductions, non-native
Canada geese are now abundant on many small islands in
the region (Best & Arcese, 2009) and well-established in
southwestern BC (Campbell et al., 1990; Smith, 2000).
Much literature also indicates that geese influence plant
species abundance and community composition directly
by removing biomass and altering competitive interactions
among species (Mulder, Ruess & Sedinger, 1996; Zacheis,
Hupp & Ruess, 2001) and indirectly by increasing nitrogen
cycling or deposition (Cargill & Jefferies, 1984; Smith,
Craven & Curtis, 1999), soil salinity (Mulder, Ruess &
Sedinger, 1996), and soil erosion (Smith, Craven & Curtis,
1999). In BC, Best (2008) and Best and Arcese (2009)
used exclosures and greenhouse experiments to show that
geese reduced the persistence of native plant species and
facilitated the development of “grazing lawns” dominated by
exotic grasses on small islands colonized by non-migratory
Canada geese. Best and Arcese (2009) further suggested
that geese disperse exotic grasses between feeding and
breeding sites, given that 92% of 25 germinants in goose
feces collected on nesting islets in the Southern Gulf Islands
were exotic species, and 80% were exotic annual grasses
(Poa annua, 72%; Aira praecox, 8%). These authors also
reported a positive correlation between the intensity of
goose grazing and abundance of exotic annual grasses in
experimental plots and attributed this correlation to the fact
that, under herbivory, annual grasses form short, highly
branched mats that can cover large areas (Best & Arcese,
2009; see also Cargill & Jefferies, 1984; Person et al., 2003).
The colonization of small islands by non-migratory
Canada geese is a consequence of their preference for nest
sites with high visibility, adjacent to open water (Williams,
1967; Van Wormer, 1968; Breen, 1990; Smith, Craven &
Curtis, 1999). Unfortunately, many of the islands colonized
to date harbour the last examples of Garry oak and
associated ecosystems (Fuchs, 2001; Garry Oak Ecosystem
Recovery Team, 2002; Gedalof et al., 2006; MacDougall
et al., 2006) with high native species cover (Gonzales,
2008; Best & Arcese, 2009; M. Isaac-Renton, J. R. Bennett,
R. J. Best & P. Arcese, unpubl. results). Garry oak and
associated ecosystems include maritime meadow and
coastal bluff ecosystems, which together make up among
the most biologically diverse and threatened ecosystems in
Canada (Garry Oak Recovery Team, 2002; Lea, 2006; Parks
Canada Agency, 2006). Exotic plants are identified as a key
threat to the persistence of these ecosystems (Fuchs, 2001;
Garry Oak Recovery Team, 2002, Parks Canada Agency,
2006), and they continue to increase in prevalence (Roemer,
1995; Lilley & Vellend, 2009).
Given that non-migratory Canada geese are introduced
to the region and appear to be facilitating the invasion
of threatened ecosystems by exotic plants, we wished to
test further the results of Best and Arcese (2009) and to
determine if the effects they described at the plot level are
detectable at the scale of whole islands. Specifically, we
expected to find positive relationships between the percent
cover of goose feces and exotic annual grasses at the plot
level, particularly of P. annua and A. praecox, given their
observed occurrence in goose feces (Best & Arcese, 2009).
We also expected to observe a negative relationship between
goose feces and native forb cover. Third, to test if goose
presence influenced plant community composition at the
scale of whole islands, we used the presence and average
number of nests per island over 4 y as proxies for goose
activity and compared these indexes with the cover of exotic
annual grass and native forbs averaged across plots.
Methods
STUDY SITES
We visited 39 islands in the Southern Gulf Islands, BC,
and San Juan Islands, Washington (WA), distributed from
the Winchelsea islands near Nanaimo, BC, to Lopez Island,
WA (Figure 1). Our surveys focused on smaller islands
(0.1–13.3 ha) because these are favoured as breeding sites
by geese (see Introduction).
DATA COLLECTION
We used 1- × 1-m quadrats to estimate percent cover
of all vascular plants < 1 m in height and the percent
cover of goose feces. We used percent cover to estimate
goose impacts on plant communities because Best (2007)
notes that this metric gave results similar to those based on
more laborious counts of individual plant stems. Quadrat
locations were chosen in stratified-random fashion,
according to the following protocol: 25- × 25-m grids
were superimposed on islands using aerial photographs
and Geographic Information System software (ArcGIS
9.2; ESRI, Redlands, California, USA), and then at the

ISAAC-RENTON ET AL.: EFFECTS OF INTRODUCED GEESE ON NATIVE PLANTS
396
intersection of grid lines, a minimum of 5 sample locations
(more on larger islands) were chosen randomly. Overall, we
surveyed 278 quadrats on 39 islands.
Potential goose nesting areas were thoroughly searched
from April to late May, 2005–2008 to record all nests with
evidence of use in the current season, including nests with
eggs, shell fragments, down, or goslings. In total, 98 goose
nests on the 39 islands were tallied over 4 y.
DATA ANALYSIS
We summarized data on percent cover of plants by
species into the functional groups of Best and Arcese
(2009): exotic annual grasses, exotic annual forbs, native
annual forbs, and native perennial forbs. At the plot level,
percent cover of goose feces was used as a proxy for goose
disturbance. We used generalized linear mixed models,
with a Gaussian error distribution and island area as a
random covariate, to test for relationships between percent
cover of goose feces and percent cover of exotic annual
grasses, exotic annual forbs, native annual forbs, native
perennial forbs, P. annua, and A. praecox. This approach
was superior to using standard GLM because we sampled
multiple quadrats on multiple islands of different size.
At the island level, we used the same dependent variables
but instead used the average number of nests on an island
over the duration of the study period as predictor variables
and proxies for goose impact. At both the plot and island
level, the following transformations were used to improve
normality: plant percent cover data were arcsine square-root
transformed, and other continuous data (cover goose feces,
island size, mean number of nests) were log transformed.
Results
As predicted, the cover of exotic annual grasses
increased significantly as percent cover of goose feces
increased in plots (Table I). The percent cover of exotic
annual forbs also increased with goose fecal cover, but
was only marginally significant. In contrast, we found no
relationship between cover of native annual forbs and the
FIGURE 1. Focal survey locations in relation to Vancouver Island, BC (A). Surveyed islands with (grey circle) or without (black triangle) nesting geese in
the Winchelsea (B) and Southern Gulf and San Juan Islands (C, black star indicates Sidney Island, BC).
TABLE I. Estimated intercepts (G0) and slopes (G1) from generalized linear mixed models to predict the percent cover of 6 indicators of vege-
tative cover by the log-transformed percent cover of goose feces at the plot level (standard errors in parentheses; DF indicates the degrees of
freedom, T the t-statistic, and P the significance level for the test of G1= 0).
Model Y G0G1DF(G1)TP
Arcsine Sqrt. Exotic Annual Grasses 0.152 (0.021) 0.460 (0.175) 238 2.630 0.009
Arcsine Sqrt. Exotic Annual Forbs 0.074 (0.011) 0.204 (0.108) 238 1.885 0.061
Arcsine Sqrt. Native Annual Forbs 0.153 (0.019) 0.093 (0.185) 238 0.502 0.615
Arcsine Sqrt. Native Perennial Forbs 0.559 (0.029) –0.490 (0.259) 238 –1.891 0.060
Arcsine Sqrt. P. annua 0.001 (0.0009) 0.030 (0.015) 238 2.028 0.044
Arcsine Sqrt. A. praecox 0.033 (0.007) 0.417 (0.078) 238 5.337 < 0.0001

ÉCOSCIENCE, VOL. 17 (4), 2010
397
percent cover of goose feces, but we did find a marginally
significant decline in the cover of native perennial forbs as
goose fecal cover increased. We also found a strong positive
relationship between the cover of A. praecox and cover of
goose feces (Table I), as well as a weak positive relationship
between the cover of P. annua and cover of goose feces.
However, contrary to our predictions, we found a
negative relationship between the cover of exotic annual
grasses and average number of goose nests on islands and
no relationship between the cover of exotic annual forbs and
average number of nests on islands (Table II). Interestingly,
we also found significant positive relationships between the
cover of native annual and perennial forbs and the average
number of goose nests on islands (Table II).
Discussion
As predicted, we observed that the cover of exotic
annual grasses increased with the cover of goose feces
at the plot level, consistent with the experimental results
of Best (2008) and Best and Arcese (2009). Geese may
facilitate the introduction of exotic annual grasses by
importing exotic seeds to nesting islands in their guts (Best
& Arcese, 2009) or on their bodies (Clausen et al., 2002).
Once the exotic grasses are introduced, geese may facilitate
their establishment further by inhibiting competitors and
increasing available soil nitrogen via defecation (Cargill
& Jefferies, 1984; Best, 2008). Geese may also stimulate
vegetative growth in exotic grasses directly by grazing, a
process that leads to the creation of grazing lawns in other
ecosystems (Cargill & Jefferies, 1984; Person et al., 2003).
Best and Arcese (2009) proposed that some or all of
these mechanisms led to a positive feedback loop between
non-migratory Canada geese and the exotic annual grasses
they graze in rural and urban landscapes of southwestern
BC. We found only a weak positive relationship between
the cover of P. annua, a common food and the predominant
germinant in Canada goose feces (Best & Arcese, 2009),
and percent cover of goose feces at the plot level (Table I).
Instead, the positive link between exotic annual grasses and
geese that we observed was due mainly to the increased
occurrence of A. praecox, a widespread species of shallow,
disturbed soils, that germinated from goose feces much less
often than P. annua (Best & Arcese, 2009; Table I). Overall,
therefore, our results are consistent with the hypothesis
that geese facilitate the spread of exotic annual grasses on
recently colonized islands but are inconclusive regarding the
specific mechanism of facilitation.
An alternate explanation for the positive relationship
between goose disturbance and exotic annual grasses at
the plot-level could be that trampling by geese impedes
natives by enhancing the competitive advantage of
disturbance-tolerant exotic annual grasses. Disturbance,
such as trampling, facilitates exotic species invasion in
other ecosystems (Hobbs, 2001; MacDougall et al., 2006).
The positive relationship we observed between the cover of
A. praecox and goose feces is consistent with the trampling
hypothesis because A. praecox is common in disturbed
soils throughout the Southern Gulf and San Juan Islands
(M. Isaac-Renton, J. R. Bennett, R. J. Best & P. Arcese,
unpubl. results). Last, across the full geographic extent of
our study, it is also possible that geese simply prefer habitats
also favoured by exotic annual grasses (Conover, 1991; Best
& Arcese, 2009). For example, A. praecox is a drought-
tolerant species often found at the perimeter of islands,
the same areas sought out by nesting geese (Williams,
1967; Van Wormer, 1968; Smith, Craven & Curtis, 1999).
Working near an epicentre of goose introductions, Best
and Arcese (2009) used exclosures to manipulate goose
presence and showed a direct effect of geese on these
grasses rather than a simple correlation between them.
However, because we collected only correlative evidence of
the association of goose presence and exotic species cover,
we cannot rule out that indirect effects also played a role in
affecting their co-distribution over the entire study area.
Contrary to our predictions we found no relationship
between the cover of goose feces and native annual forbs
in plots (Table II) and only a weak negative relationship
between the cover of goose feces and native perennial forbs.
Because these 2 plant groups are strongly influenced by
soil depth and competition with exotic perennial grasses
(MacDougall et al., 2006), it is possible that the influence
of geese on the islands we studied remains small relative to
other drivers of plant community composition. For example,
the islands we surveyed occur over a much wider area than
those studied experimentally by Best and Arcese (2009;
Figure 1), which were colonized soon after the introduction
of geese to southwestern BC. Similar to Best and Arcese
(2009), we did observe a marginally significant positive
relationship between the cover of goose feces and exotic
annual forbs (Table I). Further work is therefore needed
to test if the results of Best and Arcese (2009) are general,
specific to a region or community type, or simply represent
the initial stages of plant community change associated with
a relatively recent co-invasion of exotic geese and plants.
Interestingly, the positive relationship between goose
presence and exotic annual grasses at the plot level was
not observed at the scale of whole islands when using the
number of goose nests as a proxy for disturbance. In fact,
TABLE II. Estimated intercepts (G0) and slopes (G1) from generalized linear mixed models to predict the percent cover of 4 indicators of vege-
tative cover by the average number of goose nests on islands over 4 y (standard errors in parentheses; for conventions see Table I).
Model Y G0G1DF(G1)T P
Arcsine Sqrt. Exotic Annual Grasses 0.219 (0.028) –0.170 (0.065) 37 –2.616 0.013
Arcsine Sqrt. Exotic Annual Forbs 0.092 (0.016) –0.042 (0.036) 37 –1.182 0.245
Arcsine Sqrt. Native Annual Forbs 0.108 (0.027) 0.146 (0.062) 37 2.369 0.023
Arcsine Sqrt. Native Perennial Forbs 0.464 (0.039) 0.247 (0.091) 37 2.721 0.010

ISAAC-RENTON ET AL.: EFFECTS OF INTRODUCED GEESE ON NATIVE PLANTS
398
our results were generally opposite to our predictions,
showing that the cover of exotic annual grasses declined,
while the cover of native annual and perennial forbs
increased, on islands with more goose nests. We suspect
that these trends reflect a preference of nesting Canada
geese for islands that are undisturbed by humans and thus
also support relatively intact native flora on an island-wide
basis. Because goose nests tend to occur at the periphery
of islands, this may also limit their initial impact at the
interior of islands, where many of our randomly located
plots occurred. In contrast, Best and Arcese (2009) located
their experimental plots systematically around areas with
evidence of goose nesting. Thus, it is reasonable to expect
that the effects of geese described by Best and Arcese
(2009), and also observed here at the plot level, will become
detectable at the level of whole islands as goose populations
grow and time since their colonization accumulates, an
hypothesis that could be tested by conducting systematic
surveys of islands close to and far from original points
of goose introduction. If true, our results and those of
Best and Arcese (2009) suggest that managers still have
time to reduce the rate of exotic species introduction and
biodiversity loss by reducing the size of introduced Canada
goose populations in the Georgia Basin and preventing
nesting by introduced geese on islands that still support
native maritime meadow plant communities.
Acknowledgements
We are indebted to M. Chutter, G. Kaiser, R. Butler, J. Evans,
D. Gurd, and A. Breault of Environment Canada and the BC
Ministry of Environment for generously providing information
on goose introductions and to M. Flint for data on the distribution
of goose nests in 2004-05. Our work was funded by an Natural
Sciences and Engineering Research Council (NSERC) Discovery
grant to P. Arcese, USRA to M. Isaac-Renton, NSERC CGS
to J. R. Bennett, and generous donations by H. and W. Hesse.
J. Thornburn kindly confirmed the arrival date of geese at the
Isabella islets, and A. J. Brumbaum kindly allowed us to use Kerr
island and report her observations.
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