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

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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.
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.
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
395
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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... First, invasive annual grasses can rapidly convert perennial grasslands into annual grasslands and increase fire risk (Abatzoglou & Kolden, 2011;Davies, 2011;Balch et al., 2013;Garbowski et al., 2021). Second, a population of non-native resident Canada geese introduced in the 1980s has rapidly expanded across the San Juan and Gulf Islands, with strong evidence that they are degrading meadow habitats at alarming rates (Best & Arcese, 2009;Isaac-Renton et al., 2010;Bennett et al., 2011). Third, due to the changing social perceptions of hunting, as well as the loss of primary predators in the islands, deer are rapidly exploding in numbers across the islands, which are dramatically altering the structure and richness of island plant communities (Martin, Arcese & Scheerder, 2011;Arcese et al., 2014). ...
... Moreover, invasive species are impacting protected and imperiled ecological communities well beyond the frontier of human settlement and development (Seabloom et al., 2006). In particular, this study supports the detailed evidence demonstrating the negative consequences of introduced Canada geese (Best & Arcese, 2009;Isaac-Renton et al., 2010;Bennett et al., 2011Bennett et al., , 2013 as well as the negative consequences of deer herbivory on the native flora of small meadow islands (Martin, Arcese & Scheerder, 2011;Arcese et al., 2014Arcese et al., , 2018. Without the rapid and concerted effort to control both deer and Canada goose populations, the long-term viability of these small island meadow communities is in significant doubt. ...
... These have spread widely throughout the San Juan and Gulf Islands and now nest on many smaller islands where they are not disturbed by human visitors and many predators. Unlike the native Canada geese, that were largely migratory and relatively uncommon, the year-round presence of these resident birds is rapidly changing the flora of islands where they nest in abundance through herbivory, nutrient and alien species introductions, and nest building(Bennett et al., 2011;Best & Arcese, 2009; Dawe & Stewart, 2010;Isaac-Renton et al., 2010).The threat of invasive species amplifies the risks of island extirpations due to small island size(Wilcove et al., 1998), prone to extinction and colonization events(MacArthur & Wilson, 1967). Global and regional rarity compounds the risks of small populations of plants living on small islands. ...
Thesis
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The core premise of the Anthropocene is that we have unintentionally altered the earth so much that we have entered a new geological period. One of the most concerning of these unintentional consequences is the widespread movement of species across continents. This movement is causing natural communities to become simpler and more self-similar, a process called biotic homogenization. This thesis explores how much biotic homogenization is occurring and could occur in the future within the flora of the San Juan Island archipelago of Washington State, which is a hotspot of floristic diversity. This thesis addresses five main questions 1) what proportion of the flora are alien species, 2) are rare species disproportionately impacted by alien species, 3) what factors influence the number and distribution of alien species, 4) how much biotic homogenization could occur in the future, and 5) is biotic homogenization occurring now? Currently, alien species comprise between 38 and 47% of the San Juan Island flora, and most alien species present are invasive in other parts of the United States. Invasive species are most common in meadow habitats which also have the greatest number of rare and imperiled species. The most important factors determining the frequency of alien species are residence time, invasiveness, island size, and how impacted the island is by human development. In addition, because most of the alien flora has recently arrived, the future flora could become up to 20% more similar by 2079. Finally, current evidence suggests the most diverse small meadow islands are rapidly losing native species and being mostly colonized by alien species. The synergistic impacts of invasive annual grass, introduced Canada geese, and over-abundant black-tailed deer are hastening this change. However, each island is changing uniquely, currently causing no directional change towards homogenization or differentiation.
... However, recent studies also indicate that overabundant ungulates and geese can simplify native plant and animal communities dramatically in the Gulf and San Juan archipelagos (Gonzales and Arcese 2008;Best and Arcese 2009;Isaac-Renton et al. 2010;Martin et al. 2011;Arcese et al. 2014;Skaien and Arcese 2018), raising the possibility that these species interact with non-native earthworms to facilitate plant species invasion (Fisichelli et al. 2013;Davalos et al. 2014Davalos et al. , 2015a. Although native to the region, black-tailed deer (Odocoileus hemionus columbianus; rarely exotic fallow deer, Dama dama) populations burgeoned in these islands following the extirpation of large predators and more recent bans on human hunting (Gonzales and Arcese 2008;Martin et al. 2011;Arcese et al. 2014). ...
... Although native to the region, black-tailed deer (Odocoileus hemionus columbianus; rarely exotic fallow deer, Dama dama) populations burgeoned in these islands following the extirpation of large predators and more recent bans on human hunting (Gonzales and Arcese 2008;Martin et al. 2011;Arcese et al. 2014). Intensive efforts to introduce Canada geese (Branta canadensis) to the region similarly led to exponential increases in population size and the establishment of resident populations on islets with few terrestrial predators (Best and Arcese 2009;Isaac-Renton et al. 2010). As a consequence, selective herbivory by deer (Gonzales and Arcese 2008;Arcese et al. 2014) and the fertilization and transportation of exotic seeds by geese (Best 2008;Best and Arcese 2009) has resulted in most intact examples of Garry oak and maritime meadow plant communities tending to occur on islands without resident deer or geese (Isaac-Renton et al. 2010;Martin et al. 2011;Arcese et al. 2014). ...
... Intensive efforts to introduce Canada geese (Branta canadensis) to the region similarly led to exponential increases in population size and the establishment of resident populations on islets with few terrestrial predators (Best and Arcese 2009;Isaac-Renton et al. 2010). As a consequence, selective herbivory by deer (Gonzales and Arcese 2008;Arcese et al. 2014) and the fertilization and transportation of exotic seeds by geese (Best 2008;Best and Arcese 2009) has resulted in most intact examples of Garry oak and maritime meadow plant communities tending to occur on islands without resident deer or geese (Isaac-Renton et al. 2010;Martin et al. 2011;Arcese et al. 2014). The invasion history of non-native plants to the region is also relatively recent and well-known, coinciding with the rapid expansion of European settlement after 1864 (MacDougall et al. 2004;Bennett 2013;, and the likely introduction of non-native earthworms thereafter (Addison 2009). ...
Article
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Non-native earthworms can drive ecosystem change, simplify native plant communities, and promote invasion by non-native plants, but we know little about their pathways into island archipelagos, which currently support about 40% of the worlds threatened species. We studied links among non-native earthworms, human settlement, deer, and plant communities on 26 islands in the San Juan and Southern Gulf Island archipelagos of the Georgia Basin Ecoregion of western North America. We evaluated the (1) invasion pathways and occurrence of non-native earthworms on islands, (2) influence of non-native earthworms on herbaceous and woody plant cover, and (3) potential for synergistic interactions among deer, earthworms and non-native plants. Human settlement was a pre-condition to detecting non-native earthworms on islands. Non-native earthworm abundance was related positively to the cover of non-native herbaceous and woody plants, effects which may be exacerbated by high deer density. Our findings suggest that the absence of non-native earthworms on many small islands makes their protection crucial to the conservation of intact examples of native ecosystems, including critically endangered Garry oak and maritime meadows in Canada.
... Although widely regarded as native in the San Juan and Gulf Islands, Canada geese were actually infrequent winter visitors prior to the 1960s, when races from the Midwest were interbred to establish "a breeding population that would allow a harvestable excess" (Smith 2000, Isaac-Renton et al. 2010). The first record of these introduced, non-migratory birds nesting in the San Juans was in 1986 (Lewis and Sharpe, 1987). ...
... The small islands appear to be favored as nesting sites by the geese, perhaps due to the absence of many ground-based predators and other disturbances found on the larger islands. The impacts of these geese to the vegetation of many of the islands were profound ( Figure 5), a pattern that has been observed in the Gulf Islands as well (Isaac-Renton et al. 2010). Large areas dominated by native species were often heavily grazed, and extensive numbers of plants were ripped out of the ground or smothered in droppings, especially where multiple nests were present. ...
Technical Report
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This report presents the results of floristic surveys of small islands carried out in 2017-2018 in the San Juan Islands National Monument in Washington
... Recently, Arcese and Rodewald (2019) compared insular plant communities with and without nonnative earthworms of the genus Lumbricus to suggest that their introduction to endangered maritime meadow habitats of the Pacific Northwest of North America reduced the diversity and abundance of native herbaceous and woody plant species, independent of the occurrence of deer (Cervidae) or geese (Branta canadensis), each of which causes rapid trophic downgrading in the absence of native predators or human hunting (Best and Arcese 2009;Isaac-Renton et al. 2010;Martin et al. 2011;Arcese et al. 2014). Here, we test whether earthworms act as direct agents of change in maritime meadows by enhancing conditions favorable to the exotic plants species that co-evolved with earthworms (Bohlen et al. 2004). ...
Article
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Non-native earthworms can alter ecosystems by modifying soil structure, depredating seeds and seedlings, and consuming soil organic matter, yet the initial responses of plant communities to earthworm invasions remain poorly understood. We assessed the effect of non-native earthworms on seedling survival during germination and after establishment using six native and six non-native plant species grown from seed in single- and multi-species experimental mesocosms. We examined the extent to which earthworms (1) influenced seedling survival, (2) selectively depredated native versus non-native plants, (3) impacted establishment based on seed size and/or root morphology, and (4) shaped community assembly. The effect of earthworms on seedling survival varied temporally and among species but inconsistently with respect to species origin. Differences in seed/seedling survival translated to changes in community assembly. Earthworms tended to reduce species abundance, richness, evenness, and diversity in multi-species mesocosms and led to the divergence of communities by treatment. In general, species with large seeds and fibrous roots dominated communities with earthworms present, whereas species with small seeds and taproots only persisted in multi-species mesocosms without earthworms. Our findings suggest that earthworms act as ecological filters in the early stages of invasion to shape community composition based on plant morphological traits.
... Although most of the common waterfowl species are considered predominantly plant eaters, dabbling ducks can go through dietary shifts both temporally and spatially depending on food availability and metabolic requirements at certain times of year and for different life phases (e.g.Tidwell et al., 2013).Demand for protein during reproduction is relatively fixed but not tied to any specific food. Macrophyte cover had a negative association with Canada goose density and there was no association with grass (ungrazed rank or grazed pasture) despite this species being almost entirely herbivorous (e.g.Best & Arcese, 2009;Isaac- Renton, Bennett, Best, & Arcese, 2010), although these geese also commonly consume crops (D. Klee, Fish & Game NZ, personal observation). ...
Article
Ponds are increasingly being constructed to enhance amenity values in human-dominated lowland landscapes, but little is known of design features that influence macroinvertebrates which can provide important food resources for fish and waterbirds. We quantified pond characteristics, sampled benthic and water-column macroinvertebrates in winter, and related abundance, biomass and community composition to characteristics of 34 ponds on the lower Waikato River floodplain, northern New Zealand. Compositionally different macroinvertebrate communities occupied water-column and benthic habitats. Landscape setting had a significant effect on benthic composition in terms of biomass, while the independent effects of frequency of pond drying or flooding, respectively, influenced benthic (abundance, biomass) or water-column (abundance) communities. Distance-based linear models highlighted physicochemical conditions, water depth regimes and riparian vegetation composition as key variables accounting for dissimilarity in total macroinvertebrate biomass between ponds. Our results indicate that managing hydrological stability of ponds in terms of permanence and flooding influences the composition and biomass of invertebrate communities. Within this hydrological template, design of appropriate depth regimes and riparian conditions can enhance accrual of macroinvertebrate biomass, thereby increasing food resources for fish and waterbirds.
... Although most of the common waterfowl species are considered predominantly plant eaters, dabbling ducks can go through dietary shifts both temporally and spatially depending on food availability and metabolic requirements at certain times of year and for different life phases (e.g.Tidwell et al., 2013).Demand for protein during reproduction is relatively fixed but not tied to any specific food. Macrophyte cover had a negative association with Canada goose density and there was no association with grass (ungrazed rank or grazed pasture) despite this species being almost entirely herbivorous (e.g.Best & Arcese, 2009;Isaac- Renton, Bennett, Best, & Arcese, 2010), although these geese also commonly consume crops (D. Klee, Fish & Game NZ, personal observation). ...
Article
• Ponds can provide important refuges for aquatic biota on developed floodplains and are increasingly being constructed in an effort to enhance native biodiversity and ecosystem services in degraded landscapes. This study examined 34 constructed ponds to investigate the influence of design features on community composition, native biodiversity, and the biomass or abundance of common fish and waterbirds on the lower Waikato River floodplain, northern New Zealand. • Inundation frequency appeared to be a key factor affecting biomass of the native shortfin eel Anguilla australis and three invasive fish species (common carp [Cyprinus carpio ], brown bullhead [Ameiurus nebulosus ], and goldfish [Carassius auratus ]), suggesting that colonisation occurred during flooding by adjacent waterbodies. Linear models indicated that shortfin eel abundance and total eel biomass were positively associated with the biomass of potential fish prey, the area occupied by islands and cover by trees in the riparian zone. • Native waterbird species richness was strongly related to water area, edge length (including islands) and area:perimeter ratio, with little increase in richness for ponds >1 ha in area, perimeters longer than 800 m, and ratios over 20. The protected grey teal (Anas gracilis ), and the recreationally hunted species phenotypically assigned as mallard (Anas platyrhynchos + hybrids) and grey duck (Anas superciliosa + hybrids) appeared most strongly influenced, respectively, by perimeter length, water depth, and biomass of potential macroinvertebrate food supplies, suggesting variable effects of pond design attributes among waterfowl species. • Overall, these results indicate that constructed ponds can be designed to promote native waterbird diversity, enhance eel fishery and waterfowl gamebird services, and also limit the proliferation of some non‐native invasive fish species in degraded floodplain landscapes. A hierarchy of constructed pond design attributes was identified, involving landscape position and connectivity, pond morphology and complexity, and riparian maturity and buffering, which were associated with direct (habitat) and indirect (food supply, physicochemistry) effects on biodiversity and provisioning services.
... 3 A connection has been found between non-migratory Canada Geese and the spread of exotic plant life in meadow ecosystems of Vancouver Island (Isaac-Renton et al. 2010 ...
Technical Report
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The purpose of this report is to summarize the status of resident goose pop- ulations in the City of Vancouver and present management options to miti- gate impacts from over-abundant geese in parks and other areas. Resident urban Canada Geese were introduced into Metro Vancouver in the 1970s and have thrived in the urban environment. Vancouver and many other municipalities in North America are seeking ways to manage their Canada Geese populations, navigating complex political decisions trying to under- stand goose behaviors and human behaviors related to geese.
Article
Waterbirds are often used as indicators of ecosystem function across broad spatial and temporal scales. Resolving which species are declining and the ecological characteristics they have in common can offer insights into ecosystem changes and their underlying mechanisms. Using 20 years of citizen science data collected by the British Columbia Coastal Waterbird Survey, we examine species-specific trends in abundance of 50 species in the Salish Sea and 37 species along the outer Pacific Ocean coast that we considered to form the core wintering coastal bird community of British Columbia, Canada. Further, we explore whether ecological commonalities increase the likelihood of a species undergoing declines by testing the hypotheses that waterbird abundance trends are influenced by dietary specialization and migration distance to breeding grounds. Results suggest that most populations are stable (i.e., temporal trends are not significant) in both the Salish Sea (36 of 50 spp.) and Pacific coast (32 of 37 spp.) regions. Twelve species displayed significant decline trends in the Salish Sea, whereas two had significant increases. Along the Pacific coast, only three species displayed significant decline trends, and two significant increases. This result is corroborated by guild-specific trends indicating that waterbirds occupying the Salish Sea are faring significantly worse than those residing along the outer coastal regions, almost irrespective of dietary specialization or migration distance. Our results provide evidence that differential environmental pressures between the inner and outer coastal regions may be causing overall loss of wintering waterbirds within, or abundance shifts away from, the Salish Sea. Potential mechanisms responsible for these observed patterns are discussed, including environmental (e.g., climate) and human-induced (e.g., nutrient and chemical pollution) pressures. Collaborative, inter-disciplinary research priorities to help understand these mechanisms are suggested
Preprint
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We used 116-years of floral and faunal records from Mandarte Island, British Columbia, Canada, to estimate the indirect effects of humans on plant communities via their effects on the population size of a surface-nesting, colonial seabird, the Glaucous-winged gull ( Larus glaucescens ). Comparing current to historical records revealed 18 extirpations of native plant species (32% of species historically present), 31 exotic species introductions, and one case of exotic introduction followed by extirpation. Contemporary surveys indicated that native species cover declined dramatically from 1986 to 2006, coincident with the extirpation of ‘old-growth’ conifers. Because vegetation change co-occurred with an increasing gull population locally and regionally, we also tested predictions from the hypothesis that the presence and activities of seabirds help to explain those changes. Specifically, we predicted we would observe high nutrient loading and exotic plant species richness and cover on nearby islands with versus without gull colonies as a consequence of competitive dominance in species adapted to high soil nitrogen and trampling. As predicted, we found that native plant species cover and richness were lower, and exotic species cover and richness higher, on islands with versus without gull colonies. In addition, we found that soil carbon and nitrogen on islands with nesting gulls were positively related to soil depth and exotic species richness and cover across plots and islands. Our results suggest that gulls have the potential to drive rapid change in insular plant communities by increasing nutrients and disturbing vegetation. Because human activities have contributed to long-term change in gull populations, our results further suggest compelling, indirect links between human management decisions and plant community composition on islands of the Georgia Basin.
Preprint
Full-text available
We used 116-years of floral and faunal records from Mandarte Island, British Columbia, Canada, to estimate the indirect effects of humans on plant communities via their effects on the population size of a surface-nesting, colonial seabird, the Glaucous-winged gull ( Larus glaucescens ). Comparing current to historical records revealed 18 extirpations of native plant species (32% of species historically present), 31 exotic species introductions, and one case of exotic introduction followed by extirpation. Contemporary surveys indicated that native species cover declined dramatically from 1986 to 2006, coincident with the extirpation of ‘old-growth’ conifers. Because vegetation change co-occurred with an increasing gull population locally and regionally, we also tested predictions from the hypothesis that the presence and activities of seabirds help to explain those changes. Specifically, we predicted we would observe high nutrient loading and exotic plant species richness and cover on nearby islands with versus without gull colonies as a consequence of competitive dominance in species adapted to high soil nitrogen and trampling. As predicted, we found that native plant species cover and richness were lower, and exotic species cover and richness higher, on islands with versus without gull colonies. In addition, we found that soil carbon and nitrogen on islands with nesting gulls were positively related to soil depth and exotic species richness and cover across plots and islands. Our results suggest that gulls have the potential to drive rapid change in insular plant communities by increasing nutrients and disturbing vegetation. Because human activities have contributed to long-term change in gull populations, our results further suggest compelling, indirect links between human management decisions and plant community composition on islands of the Georgia Basin.
Article
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Flocks of free-ranging Canada Geese (Branta canadensis) often are considered nuisances when they graze on lawns because they litter the sites with fecal material, and their grazing often is perceived to be detrimental to the turf. I tested whether goose grazing had changed the composition of grass species at 20 sites in Connecticut where geese were considered nuisances. At these sites Kentucky bluegrass (Poa pratensis) was less prevalent in areas grazed heavily by geese than in areas of the same lawn that received light grazing. At six sites where up to several hundred geese wait daily for food handouts, 46% of the ground was devoid of vegetation except for a moss. I examined the palatability of different grass species to Canada Geese by giving captive birds the opportunity to feed in plots of five cool-season turf-grass species. The birds spent more time feeding in plots of Kentucky bluegrass and less time feeding in plots of tall fescue (Festuca Araneidae cv. K-31) that would have been expected if the geese were grazing among plots at random. Time spent grazing in plots of colonial bent grass (Agrostis tenuis cv. Highland), perennial ryegrass (Lolium peatland), and red fescue (F. rubra) did not differ from the expected. Feeding preferences for grass species were negatively correlated with the ash content of the leaves and with the amount of force required to sever a specific leaf mass. Captive Canada Geese would not feed on common periwinkle (Inca minor), Japanese pachydermum (pachydermum terminals), or English ivy (headnotes helix). These results suggest that Canada Goose numbers can be reduced at sites where they are foraging on turf if lawns are replaced by an unpalatable ground cover, or, to a lesser extent, with a tough-leaf grass species such as tall fescue.
Article
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Garry oak (Quercus garryana) savannas are among the most endangered ecosystems in Canada. In coastal British Columbia the vast majority of Garry oak savannas have been developed for residential, agricultural, or industrial use. Much of the remaining habitat is threatened by conifer encroachment or invasion by exotics. In this study tree-ring analysis was used to reconstruct stand composition and structure of an oak savanna at Rocky Point, on southern Vancouver Island, British Columbia. At the time of European settlement, the site was largely open prairie, with a few scattered oak trees. Establishment of Garry oak at Rocky Point began shortly after settlement of the area by Europeans—probably due to the cessation of frequent burning by indigenous peoples. Since the 1950s, however, Douglas-fir and grand fir have been encroaching on the Garry oak savanna. The causes of this invasion are unclear, but the invasion signals a clear transition in the stand structure and composition. Oak seedlings are abundant, but few saplings exist—suggesting that oak is not regenerating at Rocky Point. Mechanical removal of conifers would help maintain the Garry oak community over the near future, but given the nearly complete lack of oak recruitment, some restoration of ecosystem processes—most likely fire—will be required to maintain a functioning Garry oak ecosystem. This management strategy will be severely constrained by changes in stand structure resulting from decades of fire exclusion, the potential for weedy exotics to invade the post-fire communities, and the presence of an ammunition depot nearby.
Article
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Acknowledgments Weespecially want to thank M. R. Conover and the Jack H. Berryman Institute, Utah State University, Logan, Utah, for reviews and support in producing this manual. The Wildlife Society-WildlifeDamage,Management,Working Group’s Urban/Suburban Canada Goose Task Force Committee also provided review and support for this manuscript. The manual was also reviewed,by Wildlife Society council members,Dan Decker, James Miller, and Robert Warren, who provided helpful comments. The Wildlife Society also provided financial support for this project. Wealso thank S. Beckerman, H. Blokpoel, E. C. Cleary, R. A. Dolbeer, M. E. Eckstrom, G. Haas, R. S. Lutz, P. J. Rodrick, K. Solberg, W E. Wheeler, P. C. Whitford, members of the Mississippi Flyway Council Technical Section Giant Canada Goose Committee, especially G.Zenner and J. Bergquist. This publication should be cited as Smith, A. E., S. R. Craven, and P. D. Curtis. 1999. Managing,Canada geese in urban environments. Jack Berryman Institute Publication 16, and Cornell University Cooperative Extension, Ithaca, N.Y. Additional copies of this publication may,be purchased from Cornell University, Media and Technology Services Resource
Article
Canada Geese (Branta canadensis) were introduced in the Lower Fraser Valley (LFV) in the late 1960s and early 1970s to provide a breeding population for recreational use. These birds originated from Minnesota, Ontario, Saskatchewan, Alberta, and south-central British Columbia and included birds of the subspecies B. c. moffitti and hybrids of B. c. interior and B. c. maxima. Increased urbanization in the LFV brought local hunting closures to some areas, and harvest rates did not restrict the growth of the population, which increased to over 12 600 individuals in fall 1995. An overabundance of geese in the urban setting created problems with geese and humans competing for the same habitats. High densities of geese cause excessive noise, territorial threats, contamination of public areas, and damage to agricultural fields. Nesting and moulting flocks also present problems when they occur at inappropriate locations. Present management prescriptions include egg addling - to limit recruitment; flock relocation - to relieve pressures of too many geese in urban areas and to expose the geese to hunting pressure by moving them into huntable areas; and modification of hunting seasons and bag limits - to increase hunter harvest. In the LFV, over 8200 eggs have been addled within urban areas since 1988, over 12 000 geese have been relocated from urban settings to huntable areas, bag limits have been increased to five birds a day, and a triple-split hunting season was introduced in part of the valley. Bandreturn data indicated that the majority of geese harvested in the LFV originated there, and that relocated geese showed a strong fidelity to their original capture site. Present management practices are, for the most part, accepted by the public and allow monitoring of control activity through reporting requirements of the permit. Management objectives are to maintain appropriate numbers of geese for viewing and hunting, while minimizing nuisance complaints. To achieve these goals, other means may need to be applied, such as liberalization of local firearm restrictions, changes to traditional habitat management practices, and culling of flocks. Canada Goose management in the LFV requires a long-term commitment among multiple levels of governments and property managers to determine and maintain acceptable Canada Goose population levels.
Article
Net above-ground primary production of ungrazed marsh vegetation on the southern shore of Hudson Bay and dominated by Puccinellia phryganodes and Carex subspathacea was c100 g m-2yr-1 in 1979 and 50-60 g m-2y-1 in 1980. Grazing by lesser snow geese Chen caerulescens caerulescens significantly increased net above-ground primary production (NAPP). In 1979 NAPP of a grazed sward containing both Puccinellia and Carex was c135 g m-2yr-1. Corresponding figures in 1980 were 99 g m-2yr-1 for grazed Puccinellia and 92 g m-2yr-1 for grazed Carex. Productivity of grazed and ungrazed sites was similar early in the growing season. After mid-July productivity continued at a similar rate on grazed sites, but declined on ungrazed sites. Geese consumed c80% of the net above-ground primary production of the grazed marsh. Grazed plants of both species had significantly higher total N concentrations than did ungrazed plants, and maintained high N levels until late in the growing season. Grazed and ungrazed plots did not differ significantly with respect to standing crops of dead above-ground biomass or below-ground biomass. Grazing may stimulate primary production by accelerating cycling in the marsh ecosystem.-from Authors
Article
1 Arrowgrass (Triglochin palustris) is a preferred forage species of geese in the Yukon-Kuskokwim Delta (south-western Alaska) where it is found primarily on slough levees in coastal areas. Geese may affect nutrient availability, interspecific light competition, and salinity. These variables were manipulated in order to identify interactive effects of interspecific competition and abiotic factors on arrowgrass size, biomass allocation and distribution, which are likely to be significant in relation to the effects of herbivory on arrowgrass abundance and distribution. 2 Arrowgrass individuals were transplanted from two slough levee communities to the same two communities and to the adjacent slough margin and wet Carex meadow communities. Geese were excluded and nutrient availability, light competition and salinity levels were manipulated. 3 When light levels were not manipulated, fertilization had a negative effect on plant biomass and allocation to bulbs. Under decreased competition for light, plant biomass of fertilized plants was not significantly different from that of control plants. Fertilization appears to have a negative effect on arrowgrass as a result of increased competition for light. 4 Plants in the slough margin habitat were smallest, had the lowest allocation to leaves and stolons, and the lowest N concentrations and total N mass. Results from the fertilization treatment suggest plants in this community are limited primarily by physical factors. 5 Plants in the Carex wet meadow had higher allocation to leaves than in other communities under unfertilized conditions, but decreased allocation to leaves under fertilization. Plants in this community appear light- and nutrient-limited under unfertilized conditions, and primarily light-limited under fertilization. 6 Our results suggest that the presence of geese may control arrowgrass distribution because (a) faeces deposition has a negative effect on arrowgrass, (b) this negative effect is ameliorated by consumption of neighbours, and (c) the combination of high light competition and highly selective foraging for arrowgrass limit expansion of arrowgrass into the Carex meadow community. These explanations can now be tested.