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Increased pollinator activity in urban gardens with more native flora


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Declining pollinator abundance has become a major global environmental concern. Almost 90% of flowering plants rely on animal pollinators for reproduction, and negative effects of pollinator declines on crop production have been shown. Urbanization is at least partially responsible for pollinator declines, and public programs have been developed to encourage pollinator-friendly gardens. Here, in an observational study, we investigate the relationship between pollinator activity and the proportion of native species in unmanipulated private gardens in an urban area. Pollinator activity in each of ten gardens was recorded at nine times throughout the growing season. Pollinator frequency differed among gardens, and visitation was positively associated with percent area planted with native species, after correcting for effects of time of year, plant density and total garden area. The effect of proportion native plant area on pollinator activity differed among pollinator guilds, and was particularly strong for bumble bees and large bees. The observation of heightened pollinator activity with increasing native area in this correlational study suggests that cultivating native plant species should be encouraged in urban gardens. We discuss that, although such observational studies have the advantage of realism, they cannot determine underlying causal factors driving the observed correlation.
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Fukase Simons: Increased pollinator activity in urban gardens with more native flora
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APPLIED ECOLOGY AND ENVIRONMENTAL RESEARCH 14(1): 297-310. ISSN 1589 1623 (Print) ISSN 1785 0037 (Online)
2016, ALÖKI Kft., Budapest, Hungary
1Institute of Environmental Science, Carleton University, Ottawa, ON, K1S 5B6, Canada
2Department of Biology, Carleton University, Ottawa, ON, K1S 5B6, Canada
(phone: +1-613-859-1135; fax: +1-613-520-3539)
*Corresponding author
(Received 7th Jun 2015; accepted 19th Dec 2015)
Abstract. Declining pollinator abundance has become a major global environmental concern. Almost
90% of flowering plants rely on animal pollinators for reproduction, and negative effects of pollinator
declines on crop production have been shown. Urbanization is at least partially responsible for pollinator
declines, and public programs have been developed to encourage pollinator-friendly gardens. Here, in an
observational study, we investigate the relationship between pollinator activity and the proportion of
native species in unmanipulated private gardens in an urban area. Pollinator activity in each of ten
gardens was recorded at nine times throughout the growing season. Pollinator frequency differed among
gardens, and visitation was positively associated with percent area planted with native species, after
correcting for effects of time of year, plant density and total garden area. The effect of proportion native
plant area on pollinator activity differed among pollinator guilds, and was particularly strong for bumble
bees and large bees. The observation of heightened pollinator activity with increasing native area in this
correlational study suggests that cultivating native plant species should be encouraged in urban gardens.
We discuss that, although such observational studies have the advantage of realism, they cannot
determine underlying causal factors driving the observed correlation.
Keywords: exotic plant species; native flora; pollinator decline; urbanization
The decline of domesticated and wild insect pollinators has become a global
environmental concern (Potts et al., 2010; Stokstad, 2006; Thomann et al., 2013), and a
high priority in conservation efforts (Gallai et al., 2009; Withgott, 1999). For example,
several Bombus species are in serious decline across North America (Cameron et al.,
2011; Colla et al., 2012), Ireland (Fitzpatrick et al., 2007), and declines are occurring in
parallel in Britain and the Netherlands (Biesmeijer et al., 2006). Habitat lost to human
activity including urbanization is considered to be a major cause of pollinator decline
(Connor et al., 2002; Matteson and Langellotto, 2011; Vanbergen et al., 2013),
prompting a movement for pollinator friendly gardens, often encouraging the cultivation
of native flowering plant species. However, the effectiveness of the relative
“nativeness” of gardens in promoting pollinator activity is still unresolved. Here, in an
observational study, we investigate whether the proportion of native plants in urban
gardens affects pollinator activity, and thus whether the inclusion of native flora should
be considered in the development of programs to promote pollinator conservation.
Both managed and wild insect pollinators, such as honey bees (Apis sp.) and
bumblebees (Bombus sp.), play ecologically and economically crucial roles in both
natural and human-altered environments (Ashman et al., 2004; Gallai et al., 2009; Klein
et al., 2007; Potts et al., 2010). Close to 90% of all flowering plants are pollinated by
Fukase Simons: Increased pollinator activity in urban gardens with more native flora
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2016, ALÖKI Kft., Budapest, Hungary
animals (Ollerton et al., 2011), and 35% of global food production depends, at least in
part, on animal pollinators (Klein et al., 2007). The demand for agricultural products
requiring animal pollinators has increased with population increases in recent decades
(Aizen and Harder, 2009; Calderone, 2012), and the estimated annual value of
pollination services provided by insect pollinators in the United States reached $15.1
billion in 2009 (Calderone, 2012). Negative effects of pollinator decline on global crop
production and reproduction of wild flowers has been documented (Aizen and
Feinsinger, 1994; Biesmeijer et al., 2006; Gallai et al., 2009).
Much research aims to investigate and explain the causes of pollinator decline. There
are several possible causes, and there is growing consensus that declines are
multifactorial (Bryden et al., 2013; Vanbergen et al., 2013) and include habitat loss and
fragmentation through intensification of land use (Aizen and Feinsinger, 1994; Connor
et al., 2002); competition with invasive pollinator species (Thomson, 2004); diseases
such as Varroa destructor mite infection (Finley et al., 1996) and the Israeli acute
paralysis virus (Cox-Foster et al., 2007); exposure to pesticides (Brittain et al., 2010),
and reduced floral diversity as a result of invasive plant species (Dietzsch et al., 2011;
Koutika et al., 2011; Simons, 2003).
Potential pollinator habitat is limited by available green space in urban areas.
However, the use of private gardens is increasingly being recognized for its potential
contribution to pollinator conservation through the provision of habitat with a high
diversity of flowering plants (Comba et al., 1999; Goddard et al., 2010; Matteson and
Langellotto, 2011). The effectiveness of urban gardens in pollinator conservation is
expected to depend on the composition of the garden (McFrederick and LeBuhn, 2006).
Specifically, the selection of particular plant species can account for much of the
activity of insect pollinators such as bumblebees (Goulson et al., 2008; Thomson,
2004). Conservation programs provide information on how to create a “pollinator
friendly” garden, often encouraging the cultivation of native plant species rather than
non-native ornamentals and invasive species (Mysliwy, 2014). However, knowledge of
the effectiveness of planting native species in attracting diverse and abundant insect
pollinators is still needed. Existing studies (Corbet et al., 2001; Frankie et al., 2005;
Tuell et al., 2008) were designed specifically to examine pollinator attraction using
prescribed flower choices, and no study to date examines pollinator attraction and
degree of garden nativeness in unmanipulated gardens.
In this study we ask whether the degree of nativeness influences pollinator attraction
in intact gardens by monitoring pollinator foraging activity in ten urban gardens with
different compositions of flowerbed area, plant density and the proportion of area
planted with native flora. We address several questions; two basic to pollination, and
two more focused on effects of nativeness: 1) whether pollinator abundance changes
through the growing season and/or in response to ambient temperature; 2) whether the
effect of time of year on pollinator activity differs among pollinator type or “guild”; 3)
whether the proportion of garden area planted with native species (as well as plant
density and total garden area) affects pollinator activity; 4) because pollinators may
differ in their degree of host range specialization, whether the effect of nativeness on
visitation frequency differs among pollinator guilds.
Fukase Simons: Increased pollinator activity in urban gardens with more native flora
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APPLIED ECOLOGY AND ENVIRONMENTAL RESEARCH 14(1): 297-310. ISSN 1589 1623 (Print) ISSN 1785 0037 (Online)
2016, ALÖKI Kft., Budapest, Hungary
Material and methods
Measures of garden variables
The ten gardens for the study were selected with the assistance of the Backyard
Habitat Program established by the Canadian Wildlife Federation. The gardens were
located in two main municipal districts separated by a distance of 12km within the
greater Ottawa, ON region: three in Centretown and seven in Lincoln Fields. Gardens
within each area were selected on the basis of the feasibility of sampling every area
within a two-day window for dates throughout the season. Two properties had distinct
back and front gardens, and these were considered separate based on differences in
floral composition. In each garden, floral area, plant density, and the area planted with
native species were measured in mid-May (Table 1). Floral area was measured as the
total area of flowerbeds. Some gardens had two to several flowerbeds, whereas a few
had a single large flowerbed. Plant were identified to species, and density was estimated
as the count of individual stems divided by the flowerbed area. In gardens that were
large and densely planted, a randomized 1 m2-quadrat sampling technique was applied.
A “proportion native area” value was calculated for each garden as the proportion of
total flowerbed area sown with native species. These proportions (p) were transformed
as arcsine (p^0.5) to improve normality. To reduce sampling error in estimates of
pollinator activity in large gardens, two plots were established for the six gardens >
50m2, and means of measurement values across plots were used in analyses. To avoid
random selection of anomalous plot areas, plot location was selected subjectively, based
on representative flora and density of flowering plants.
Table 1. Characteristics of the ten selected study gardens in Ottawa, ON. Total area
represents the cultivated area within the garden.
Total area
flower area
native area
145. 81
LF= Lincoln Fields, CT= Centretown
Measures of pollinator frequency
Pollinator activity was observed in each garden plot on nine dates spanning July 3rd
to September 13th. Because pollinator activity is influenced by temperature,
observations were conducted only if the temperature was between 15°C and 30°C, and
were postponed in cases of severe rainfall. Observations of gardens in Lincoln Fields
and Centretown were conducted on consecutive days due to time constraints.
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2016, ALÖKI Kft., Budapest, Hungary
Observations were made between 8:30 and 17:30, and the order in which gardens were
visited within each area was randomized for each day. Summary weather conditions
(temperature and sun index) were recorded for each observation. At each plot, two 10-
minute observations were conducted, and the mean value of the two observations was
used to calculate pollinator frequency (pollinators per minute).
All flower visitors initially present and all new visitors observed foraging within the
plot were monitored, although not all flower-visiting insects are effective pollinators
(Schemske et al., 1978). Potential pollinators were photographed rather than collected
for identification during observation, and were categorized into nine guilds: bumblebee
(Bombus sp.), honey bee (Apis sp.), small bee (halictid and colletid bees), large bee
(megachilid, and andrenid bees), small fly (syrphid flies), large fly (calliphorid and
bombyliid flies), wasp (vespids), butterfly (lepidopterans), and other invertebrates (e.g.
coleopterans), and counted. Events were scored as pollinator activity only if physical
contact was made with the flower; those pollinators merely traversing the plot were
ignored. This sampling method may overestimate true pollinator activity, but it does
not lead to bias across plots because all observations were made by a single researcher
(Hennig and Ghazoul, 2012).
Statistical analyses
Second-degree polynomial (quadratic) regression was first performed to assess
seasonal change in frequency of overall pollinator visitation. The residuals from this
quadratic fit were then used to ask whether there is also an effect of temperature on
pollinator activity independent of time of year. To account for overall changes in
pollinator activity through the season, “residual pollinator activity” from this quadratic
fit was used as the response variable in analyses as noted below. To ask whether the
change in pollinator activity through the season differs among pollinator guilds, we used
a mixed-model ANOVA where pollinator guild is a fixed effect, and day and the
interaction are random effects.
A preliminary one-way ANOVA was conducted to test for variation in the frequency
of total pollinator visitation among gardens (where the response variable is residual
pollinator activity, above), followed by a post-hoc Tukey test. To address the main
question of the effect of native plants on pollinator activity, multiple regression was
used to simultaneously examine the effect of proportion native, plant density and flower
area on residual pollinator activity to account for possible covariation among predictor
variables. Finally, ANCOVA was used to ask whether the effect of proportion native
flora differs among pollinator guilds, where proportion native, plant density, total area
are continuous, and pollinator guild is categorical. All analyses were performed using
SPSS 18.0 or JMP 10.0.
A total of 1699 pollinators were recorded over the observation period. Although it
explains less than 10% of the variance, quadratic regression shows a highly significant
change in pollinator abundance through time (R2=0.087, F141=6.70, P= 0.002).
Therefore, to correct for the effect of time of year, these residuals were used as
pollinator frequency values in the following analyses. (Linear regression would not be
an appropriate analysis of pollinator abundance vs. time of year, because pollinator
abundance is lowest at both ends of the season.) Using the residuals from this quadratic
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fit in a linear regression (there is no apparent nonlinear effect of temperature) shows no
independent effect of temperature on pollinator activity after correcting for time of year
(R2=0.015, F142=2.21, P=0.14).
Much variation was observed among pollinator guilds in average frequencies of
visitation (Figure 1), with small bees showing highest frequencies, followed by
bumblebees, with lowest frequencies observed for butterflies. Furthermore, the relative
abundance of pollinator guilds differed across dates (Figure 2): bumblebee and
honeybee abundance increased mid to late season while small bees were common in
early summer and persisted throughout the summer. Large bees and dipteran pollinators
appeared early in the observation period although their frequencies were generally
lower than that of bumblebees, honeybees and small bees. This pattern is confirmed by
the significant interaction between pollinator guild and time of year (Table 2).
Table 2. Mixed-effects ANOVA results for effect of pollinator guild and time of year on total
pollinator frequency. Guild is a fixed, whereas Day and the interaction terms are random
effects. Because different pollinator guilds may appear at different times during the season,
Day was treated as a categorical effect.
Mean square
Pollinator guild
Pollinator guild*Day
Figure 1. The total abundance of each of the nine pollinator guilds observed in urban garden
plots in Ottawa, ON. Values are from counts conducted for two, ten-minute periods for each
plot on nine occasions from the beginning of July through mid September.
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Bu erfly
Figure 2. The abundance of each of the nine defined pollinator guilds across the nine
observation dates. Values represent total counts conducted for two, ten-minute periods for each
plot from the beginning of July through mid September.
A one-way ANOVA shows that there was a highly significant difference in the
frequency of (residual) pollinator visitation among gardens (F9,134; P<0.001). A post-hoc
Tukey test further reveals that differences in the average frequency of pollinator
visitation was largely attributable to differences between two subsets; a group of four
(6, 7, 8, 9) and two (3 & 4) gardens (Table 3).
Table 3. Post-Hoc Tukey HSD test from one-way ANOVA for differences among gardens in
pollinator activity. For the Tukey HSD column, values not sharing a letter are significantly
Pollinator activity
Garden #
proportion native area
Tukey HSD
a b
a b c
a b c d
a b c d
b c d
c d
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Multiple regression (Table 4) including proportion native area, plant density and total
area (R2=0.251) confirmed a highly significant independent effect of proportion native
plant area on residual pollinator activity. The independent effect of plant density was
also significant, with no evidence for an effect of total area. There are no qualitative
differences in results if pollination activity is used as the response variable instead of
the residuals from the quadratic regression of pollinator activity on time of year.
Table 4. Multiple regression of proportion native area, plant density, and total planted area
on pollinator activity. Proportion native area was arcsine-squareroot transformed, and
pollinator activity was corrected for time of year (see text).
SE Estimate
Proportion native area
Plant density
Total area
Although the slopes of the relationships between activity and proportion native area
have positive values for all pollinator guilds (Figure 3), analysis of covariance
(ANCOVA) shows that the effect of proportion native area on activity differs among
pollinator guilds (Table 5): a significant interaction between pollinator guild and the
degree of garden nativeness exists (Table 6).
0 0.1 0.2 0.3 0.4 0.5 0.6
Propor onna vearea(arcsinp0.5transformed)
Bu erfly
Log10pollinatorac vity
Figure 3. Relationship between observed activity for each of the pollinator guilds and the
proportion of urban garden area planted with native plants. The category “All pollinators” is
the total abundance at each garden for all plotted pollinator guilds. Pollinator activity is
plotted on a log10 scale to allow visualization of low-abundance pollinators, and is based on
counts conducted for two, ten-minute periods for each plot on nine occasions during the
growing season
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Table 5. ANCOVA results including interaction between pollinator guild and proportion
native area in gardens. Model R2=0.224; F19d,1276=19.35; P<0.001. For individual
interaction effect sizes, see Table 6.
Proportion native
Plant density
Total area
Pollinator guild
Pollinator guild*
proportion native
Table 6. Coefficients for each pollinator guild x proportion native plant interaction effect
from the ANCOVA model (Table 5). Note that all slope values for pollinator guild vs.
proportion native area are positive; coefficients represent difference from mean effect.
Interaction term (all
X Proportion native)
Small bee
Large bee
Small fly
Large fly
Programs meant to combat declining pollinator abundance by
encouraging the planting of native species in urban gardens are widespread. However,
empirical evidence for the success of this approach is equivocal (Bergerot et al., 2010;
Hanley et al., 2014; Matteson and Langellotto, 2011). A complicating factor in
assessing the efficacy of native gardens is that the effect of native flora may differ
among plant species, and relative frequencies of pollinators may change through time.
We thus took the approach of sampling several pollinator guilds, and at several times
throughout the growing season. As expected, the overall frequency of pollinator
visitation changed through time, presumably both reflecting pollinator phenology and
changes in ambient temperature that affect the activities of insects (Bergman et al.,
1996); the interaction effect between pollinator guild and time of year suggests that
pollinator guilds have asynchronous life cycles (Ginsberg, 1983), perhaps timed to
coincide with the availability of resources that they depend on (Tuell et al., 2008).
The present results suggest that gardens with higher proportion native flora exhibit
elevated pollinator activity, and that the various pollinator guilds contribute differently
to this overall effect. In agreement with previous work (Smith et al., 2006), we found
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no relationship between total garden area and pollinator abundance, although we found
an effect of plant density on pollinator activity. However, the main result showing an
effect of native flora on pollinator activity persists after controlling for both plant
density and total cultivated area.
Because pollinator activity is corrected for time of year, and because proportion
native area is arcsine-squareroot transformed prior to analysis, the biological
interpretation of the coefficients from the multiple regression is not straightforward.
However, back-transforming the regression predictor equation suggests that the effect of
proportion native area is strong: the expectation is for one additional pollinator per
minute for an increase of about 20% in the area cultivated with native plants.
An observational study of the effects of native flora has the advantage that the
species and range of native flora are known a priori to reflect realistic choices of
gardeners. However, like all observational studies, causation cannot be drawn from the
observed relationships. For example, no attempt is made to manipulate gardens to
represent a random sample of native and non-native species. Thus, the relationship
between proportion native area and pollinator activity is more cautiously interpreted as
valid for the particular flora cultivated by gardeners in the study. Neither could we
control the range in the predictor variable; proportion native area. Although a
manipulation experiment could theoretically include a range from 0 to 100%, the
present study included gardens composed of from 3% to 53% native area (Table 1).
Furthermore, we cannot control for the effect of individual gardeners. It is conceivable
that the most skilled gardeners plant a higher proportion native specieseven after
correcting for density and total garden areawhich somehow results in higher pollinator
An observational study does not experimentally control the choices of native and
non-native flora with respect to their timing of flowering. Because peak abundance of
the various pollinator guilds does not coincide, results may be explained by the
influence of particular plant species (Goulson et al., 2008; Hanley et al., 2014), perhaps
because of their flowering phenology and the resulting seasonal availability of floral
rewards. It is possible that gardens with greater proportion native area provided
resources more continuously than gardens dominated by non-native species. For
example, Asclepias incarnata (swamp milkweed) has been shown to be a native flower
highly attractive to North American bumblebees (Tuell et al., 2008), and its presence
likely influenced pollinator visitation to the gardens where it was abundant in this study
(see Appendix). Also, because plant phenology interacts with environmental variation
(Hughes and Simons, 2014), patterns of visitation are expected to differ among years.
Many native flowers adapted to local conditions bloom in the late summer in eastern
North America (Tuell et al., 2008), whereas many non-native plants had finished
flowering by the end of the observation period. This difference may help explain the
effect of native area on pollinator activity, specifically through increases in the
frequency of bumblebees and large bees. Some species of large univoltine solitary bees
(andrenids and megachilids) appear either early or late in the summer (Ginsberg, 1983),
and late species are known to specialize on late-blooming Solidago spp. (Ginsberg,
1983). Small beeswhich did not show a strong preference for native flowersappeared
early in the season when the abundance of native flowers was low in comparison to later
in the summer. Flower morphology may also contribute to the effect of native flora on
pollinator activity. For example, syrphid flies were frequently found foraging on native
umbelliferous flowers, which are characterized by shallow corollas that allow access to
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both pollen and nectar (Colley and Luna, 2000), and there were few common non-native
umbelliferous flowers in the gardens. Much debate surrounds the effect of native flora
on lepidopterans; (Bergerot et al., 2010; Burghardt et al., 2009; Matteson and
Langellotto, 2011; Tallamy and Shropshire, 2009); however the low abundance of
butterflies observed in this study precludes any inferences here.
Pollinator diversity observed here is not meant to be representative of the pollinator
composition in the Ottawa area. First, bee species may vary in abundance and
richness among years (Dupont et al., 2009; McFrederick and LeBuhn, 2006). Second,
the landscape matrix surrounding gardens (Hennig and Ghazoul, 2012) and the
proximity of sites of reproduction (nests/hives etc.) to gardens (Greenleaf et al., 2007)
may influence pollinator activity on a more local scale. It should also be noted that
our response variable is pollinator activity, and not pollination success. Any useful
extrapolation from this study must thus assume that pollinator activity is positively
associated with pollination.
A commonly cited mechanism underlying the benefits of native flora is the
coevolutionary history of plants and their pollinators: exotic flowers may be either less
accessible and/or attractive to native insect pollinators (Comba et al., 1999; Corbet et
al., 2001). It has been pointed out that, in the context of coevolution, a comparison of
native and exotic plant species does not satisfactorily address mechanisms underlying
pollinator attraction, and a more relevant consideration is the shared biogeographic
distribution of plants and their pollinators (Hanley et al., 2014). This insight, however,
is complicated by the fact that shared biogeography at the species scale does not
necessarily imply an expectation of coevolution, because intraspecific genetic
population differentiation in plant life history traits including flowering phenology is
common (Wagner and Simons, 2009), and plant traits involved in pollination are
expected to evolve in response to uncertainty in pollinator availability (Burd et al.,
2009; Simons, 2011; Thomann et al., 2013).
In conclusion, this study has shown a positive relationship between pollinator
activity and proportion of a garden planted with native flowers. Observational studies
such as this can provide insight into effects that occur over realistic ranges of
independent variables (here, cultivation decisions taken freely by real gardeners), but
trade-off this realism for the ability to ascertain underlying causes of the relationship.
Even if, as suggested by our data, the increase in pollinator activity is dependent on the
particular flora chosen and effects differ among pollinator guilds, programs that
encourage the cultivation of native flora are expected to succeed in increasing pollinator
activity in general, assuming that the gardeners’ choices of plant species in this study
are a representative sample of choices in the general gardening public. Although the
mechanisms underlying the relationship between degree of nativeness and pollinator
activity must still be worked out, this study demonstrates that this relationship exists.
Acknowledgements. The authors thank K. Henein and P. Hennesey for comment, and the participating
homeowners in Ottawa for property access. This research was supported by a Natural Sciences and
Engineering Research Council (of Canada) Discovery Grant to AMS.
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Appendix 1. List of Ontario native flowers cultivated in the study gardens.
Common name
Scientific name
Observed pollinator
Largeflower bellwort
Uvularia grandiflora
Marsh violet
Viola palustris
Virginia bluebells
Mertensia virginica
Aquilegia canadensis
Dutchman's breeches
Dicentra cucullaria
Wild bleeding heart
Dicentra eximia
Celandine poppy
Stylophorum diphyllum
White trillium
Trillium grandiflorum
Late spring
Canada anemone
Anemone canadensis
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Early summer
Foxglove beard-tongue
Penstemon digitalis
Hairy beard-tongue
Penstemon hirsutus
Wild geranium
Geranium maculatum
Small bees
Pale corydalis
Corydalis sempervirens
Campanula rotundifolia
Small bees
Mid summer
Monarda didyma
Bumblebees, small
and large bees
Achillea millefolium
Black-eyed Susan
Rudbeckia hirta
Large flies, others
Silphium perfoliatum
Giant hyssop
Agastache foeniculum
Tradescantia ohioensis
Bumblebees, small
Purple coneflower
Echinacea purpurea
Bumblebees, large
flies, others
Lanceleaf tickseed
Coreopsis lanceolata
Chelone glabra
Small and large bees,
Swamp wilkweed
Asclepias incarnata
Bumblebees, honey
bees, small and large
bees, wasps,
butterflies, others
Butterfly weed
Asclepias tuberosa
Blue vervain
Verbena hastata
Tall meadow-rue
Thalictrum pubescens
Daisy fleabane
Erigeron annuus
Eupatorium perfoliatum
Wild mint
Mentha arvensis
Oenothera biennis
Pearly everlasting
Anaphalis margaritacea
False sunflower
Heliopsis helianthoides
Joe pye weed
Eupatorium purpureum
Bumblebees, small
Late summer
New England aster
Aster novae-angliae
Bumblebees, small
White wood aster
Eurybia divaricata
Smooth blue aster
Symphyotrichum laeve
Bumblebees, small
bees, small flies
Obedient plant
Physostegia virginiana
Bumblebees, large
bees, small bees
Canada goldenrod
Solidago canadensis
Bumblebees, small
Zigzag goldenrod
Solidago flexicaulis
Gray goldenrod
Solidago nemoralis
Stiff goldenrod
Solidago rigida
Observed pollinator type is identified based on casual observation, thus not conclusive.
... The continuity of plants such as Asteraceae, Lamiaceae, Brassicaceae that bloom every spring and summer, or the fruit trees in the parks in our neighborhoods, or the tomatoes, peppers and sage that we plant in our small gardens depends on the presence and health of the bees that live with us in our cities. Without bees, we may have to say goodbye to many plants in urbans (42,43). ...
... (64). In a study carried out in urban gardens, it was observed that areas with a high density of native plant species have more bee biodiversity compared to non-native plants (43). Furthermore, plants that produce high amounts of nectar and pollen in parks and gardens also serve as a source of support for pollinators (38,65). ...
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Insects are critically important for the terrestrial ecosystems. Nevertheless, worldwide insect populations and species diversity are progressively dwindling. Among insects, bees (Hymenoptera: Apoidea) have a special place due to their contribution to pollination service which 87% of the flowering plants (angiosperms) are dependent. Bees also pollinate many flowering plants in urban ecosystems. Urban inhabitants are living with many native bees without knowing their existence on their balcony, in the backyard or in the city gardens and parks. So, the actual species diversity of bees in cities is quite wider than it has been thought. Despite their significance, the presence of bees in urban areas is under serious threats so the bee biodiversity is declining due to urbanization pressures.
... Though we considered including two papers that were just under this 50% threshold (Lowenstein et al., 2015(Lowenstein et al., , 2019, these papers were ultimately excluded due to low taxonomic resolution in combination with only utilizing sight identification, thus making it difficult to evaluate identifications. Coarse groupings of bees as "small" and "large" bees (e.g., Fukase and Simons, 2016) precluded functional trait assignments to garden bees from some studies. It is possible that a lack of available taxonomists (Drew, 2011) might have limited taxonomic resolution in these and other urban garden bee studies. ...
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Urban garden spaces are potentially important habitats for bee conservation. Gardens can host diverse flora, which provide floral resources across foraging seasons for bee species. Recent reviews have focused on the impacts of cityscapes on urban bee assemblages in different green spaces. Urban gardens are distinct from other urban green spaces, and bee communities in urban spaces have been an increasing topic of study over the past few decades. We reviewed 28 urban garden bee studies spanning five decades and 14 countries to compile an original metadataset of bee species' functional traits to understand the conservation value of gardens, identify gaps in bee sampling efforts, and summarize the calls to action included by their authors. Studies of urban garden bees have documented between 674 (conservative count, excluding morphospecies) and 830 (liberal count, including morphospecies) bee species. Urban garden bee communities were taxonomically and functionally diverse, although bee species that were non-eusocial, ground-nesting, generalist foragers, and native were most common in garden habitats. The proportion of parasitic bee species and specialist foragers found in urban gardens was comparable to proportions for global bee taxa. This suggests that gardens contain the hosts and forage needed to support bees with specialized life history requirements, and thus represent high quality habitat for a subset of bee communities. Garden bee research was strongly biased toward the northern hemisphere, which signifies a large gap in our understanding of garden bee communities in other regions. The variety of, and non-standard sampling methods in garden bee research makes it difficult to directly compare results between studies. In addition, both intentional low taxonomic resolution and a lack of collaboration with taxonomists constrains our understanding of bee diversity. Our analyses highlight both successes of past urban garden bee studies, and areas of opportunity for future research as we move into a sixth decade of garden bee research.
... The growth of cities and urban sprawl has led to the expansion of "novel ecosystems" where actions like wildlife-friendly gardening are becoming available to a larger number of people (Klaus & Kiehl, 2021). Native plant gardening, a component of wildlife-friendly gardening, can support species biodiversity in urbanized spaces (Berthon et al., 2021;Burghardt et al., 2009;Fukase, 2016;Lerman & Warren, 2011). Many native insect species can only survive with plants that they co-evolved with and native plants host more diverse larval populations for native bird diets, so creating a network of habitat in urban areas can support native species survival (Burghardt et al., 2009). ...
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Motivating people to take environmentally friendly action, especially collective actions that promote greater social engagement, is important for addressing environmental issues like biodiversity loss. We conducted an online workshop-based field experiment to target social-psychological perceptions to motivate people to plant native plants and encourage others to do the same. To shift these perceptions, we added 13 microinterventions to half the workshops, including normative messaging, public commitment-making, and providing feedback on the impact of reaching out to others. We used a voucher system to track real-world behavior by partnering with native plant nurseries. Compared to an information-only control workshop, our intervention workshops initially increased certain social-psychological perceptions related to encouraging others to plant native plants. However, they did not change behaviors, or many perceptions, compared to control workshops. Additional exploratory analyses revealed differing patterns of behavioral perceptions 2 months after the workshops. Further research is needed that implements experimental methods and real-world measures of conservation behavior to evaluate the impacts of theory-based outreach tactics on collective actions. Keywords: efficacy, field experiment, social diffusion, social norm, workshop intervention
... In fact, the presence of exotic species consistently causes negative effects on butterfly communities and decreases in caterpillar survival (reviewed by Yoon and Read 2016). Given the general importance of native plants to butterflies and moths (Simonson et al. 2001;Collinge et al. 2003;Burghardt et al. 2009;Kurylo et al. 2020) bees (McIntyre andHostetler 2001;Pardee and Philpott 2014;Threlfall et al. 2015Threlfall et al. , 2017Egerer et al. 2020), and other insects (Smith et al. 2015;Fukase and Simons 2016;Tallamy et al. 2020) it is possible that the relatively strong effects of plant species richness in our study (it was associated with increased butterfly species richness, abundance, and proportion of host-specialist species) were magnified because most of the plant species were native rather than exotic. While gardens that contain primarily native plants do not represent the majority of current gardens, they may represent best practices for butterfly conservation while also having positive effects on other taxa. ...
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Public and private flower gardens could be valuable for slowing pollinator decline in urbanized areas, as they can potentially provide crucial foraging and reproductive resources in fragmented landscapes. We conducted surveys of adult butterflies at 26 gardens that contained a majority of native species; we then evaluated how the impervious surface percentage (IS%) surrounding each site and the gardens’ local characteristics (garden area, plant species richness, and planting density) influenced butterfly communities. Butterfly diversity and abundance were strongly influenced by interactions between IS% and local characteristics. IS% interacted significantly with plant species richness to affect butterfly species richness (p = 0.027) and also interacted significantly with both garden area (p < 0.001) and planting density (p = 0.001) to affect butterfly abundance. In each of these interactions, increasing IS% had a negative effect on butterfly abundance, but that effect was mitigated by increases in the interacting factor. In all cases, the strength of this mitigation was greater in more urban gardens, i.e. those with higher IS%. For example, while larger gardens always had higher butterfly abundances, this difference was greatest when comparing large urban gardens with small urban gardens. Garden area is also critical; in addition to the interaction with IS%, garden area significantly affected butterfly species richness (p = 0.037). As gardens increased in size, so did butterfly species richness regardless of IS%. Our results show that gardens can positively affect urban butterfly diversity and abundance. Urban conservation efforts should focus on establishing new gardens/habitat patches, as well as increasing the size of currently established gardens.
... Our finding of a positive association between occupancy probability and floral cover within a relatively low range (0-22 % floral cover) underscores the importance of small pollinator habitat plantings to achieve conservation goals (Hall et al., 2017). Planting a garden or pocket prairie in an otherwise flowerless landscape can have large conservation gains by increasing the occurrence probability of Bombus communities, including rare species (Fukase and Simons, 2016). Contrary to our prediction, we did not find a communitywide correlation between impervious surface and bumble bee occurrence, with a negative correlation only detected for a single species. ...
... Given the long history of coevolution between native plants and their insects, the widespread use of non-native plants across urban and non-urban landscapes has likely contributed to global insect declines through the loss of suitable food and habitat [60]. Although pollinators are often attracted to various non-native ornamental plants in urban landscapes [56,61], research increasingly demonstrates that overall pollinator abundance and diversity are greatest in landscapes with native plants [55,59,62]. Pardee and Philpott [56], for example, found that for city backyard gardens, both bee richness and abundance are higher in gardens that contain more native plants and thus have more floral abundance, taller vegetation, more cover, and more potential nesting sites. ...
Full-text available
Pollinators are responsible for the reproduction of many plant and crop species and provide important diversity for food webs and cultural value. Despite the critical ecosystem services provided by pollinators, rapid pollinator declines are occurring in response to anthropogenic activities that cause the loss of suitable habitat. There is an opportunity for urban green space to support pollination ecosystem services locally and across the landscape. However, there is a lack of practical but evidence-based guidance on how urban green space can be designed effectively to provide floral resources and other habitat needs to a diverse assemblage of pollinators. We examine the existing pollinator research in this paper to address the following questions specific to insect pollinators in temperate urban settings: (1) Which pollinators can be the focus of efforts to increase pollinator ecosystem services in cities? (2) Which plants and what arrangements of plants are most attractive and supportive to urban pollinators? (3) What do urban pollinators need beyond floral resources? (4) How can the surrounding landscape inform where to prioritize new habitat creation within cities? Using these questions as a framework, we provide specific and informed management and planning recommendations that optimize pollinator ecosystem value in urban settings.
... Native plants may better support arthropods (compared to non-native plants) because they are locally adapted to regional environmental conditions and because native perennials may provide year-round provision of resources to arthropods [28]. Notably, native plants in urban ecosystems may benefit the abundance, richness, and activity of pollinators [29], bee abundance [30], parasitoid longevity [31], and local animal biodiversity more generally [32]. Yet, there may be nuanced relationships between native plants and arthropods depending on arthropod taxon, the response variable examined, foraging preferences of arthropods, time since native plant establishment, and plant traits. ...
Full-text available
(1) Urbanization threatens biodiversity, yet urban native plants support native biodiversity, contributing to conservation and ecosystem services. Within urban agroecosystems, where non-native plants are abundant, native plants may boost the abundance and richness of beneficial arthropods. Nevertheless, current information focuses on pollinators, with little attention being paid to other beneficials, like natural enemies. (2) We examined how the species richness of native plants, garden management, and landscape composition influence the abundance and species richness of all, native, and non-native bees, ladybeetles, ants, and ground-foraging spiders in urban agroecosystems (i.e., urban community gardens) in California. (3) We found that native plants (~10% of species, but only ~2.5% of plant cover) had little influence on arthropods, with negative effects only on non-native spider richness, likely due to the low plant cover provided by native plants. Garden size boosted native and non-native bee abundance and richness and non-native spider richness; floral abundance boosted non-native spider abundance and native and non-native spider richness; and mulch cover and tree and shrub abundance boosted non-native spider richness. Natural habitat cover promoted non-native bee and native ant abundance, but fewer native ladybeetle species were observed. (4) While native plant richness may not strongly influence the abundance and richness of beneficial arthropods, other garden management features could be manipulated to promote the conservation of native organisms or ecosystem services provided by native and non-native organisms within urban agroecosystems.
... Being one of the most important drivers of biodiversity richness and pollinator conservation, native flora help in building a sustainable environment by conserving water and providing habitat opportunities for all life forms -fungi and microorganisms in the soil, to birds and small mammals on land (Beckwith et al., 2022). Native flora is more biologically and ecologically resourceful; maintains certain plant-animal relationships with a positive relationship between pollinator activity and the proportion of area in gardens planted with native flora (Fukase, 2016 spaces. ...
Globally, urbanisation trends suggest that cities are indirectly linked to climate change, given the shift in land use from vegetated/green spaces to hard-built surfaces creating the UHI (Urban Heat Island) effect. These densely populated areas, in turn, also increase pressure on the natural resources required to maintain these built environments. Ahmedabad, one such fast-growing city, has jumped from 80 sq km to 237 sq km in a built-up area between 1976 and 2017, along with a 48% fall in its green cover. Located in an arid bio-geographical zone with a harsh climate, Ahmedabad has been facing freshwater scarcity across different land-use types, while green spaces and green cover are necessary. This combination of factors requires wise decisions in planning, design, and building to achieve the dual aim of sustainable resource use and biodiversity conservation. Within this context, the thesis assesses green spaces, designed and natural, in the newly developing parts of south-west Ahmedabad to propose sustainable planting and greening recommendations in times of severe resource depletion. Human health and well-being, climate change resilience, resource management of water and soil, as well as maintaining critical ecosystem services are some of the fundamental reasons why the design of green space needs to transform from being places centred around human use and recreation to also being inclusive of resource & biodiversity conservation. How green spaces are designed is influenced by how people use the space, the plot owner’s choice, perceived aesthetic appeal, and a landscape designer’s approach toward a planting palette. A shift in their approach towards ecological planting and sustainability, by landscaping with native flora and agro-ecological planting of local and indigenous flora that have human use, is water-wise, would increase pollinator activity and biodiversity. This shift would be one way of reducing the city’s ecological footprint and making cities with such dense habitation be better equipped to deal with climate change events. This shift in perception of how and what to plant, will make cities function as catalysts of a sustainable development program, and is the central goal of this thesis, as part of a larger district-wide project. With this vision in mind, this research to compare flora across designed and undesigned landscapes, was carried out in the newly developing parts of south-west Ahmedabad, both the urban and the peri-urban (Zone 1: Shela - South Bopal; Zone 2: Mumatpura; Zone 3: Prahladnagar - Makarba). These three areas are interconnected by a series of landscapes with different user-groups, socio-economic classes, and land-use patterns that are changing because of infrastructure development (Development Plan by AUDA 2021). The thesis is structured into 4 data chapters (3,4,5,6) and a final chapter on Recommendations. In Chapter 3, green cover (designed and natural/undesigned) in the study area is mapped to assess land use change over 12 years. The percentage change in built-form versus green cover (open or woody green spaces, including public gardens and parks) was estimated. This is done by laying a grid of 100 x 100 m on the satellite imagery of the study area for 2010 and 2022 and then colour-coded different categories of land use. The results show that the percentage of green cover has changed from 29.5% to 18.9% in Zone 1, 33.6% to 25.7% in Zone 2, and 7.1% to 3.5% in Zone 3. In Chapter 4, public green spaces, such as community gardens and vacant plots1 in their vicinity were studied to collect data on native floral diversity in these spaces. The surveys were done based on the different stages along the Introduced–Naturalised–Invasive continuum, and plants were assessed based on these three categories in relation to Native, for better evaluation. The surveys were carried out using strip transects that give uniform coverage to a given area. Five gardens in the study area were surveyed using landscaped pathways as transect paths, while 11 vacant plots were surveyed using zig-zag transects and edge surveys. Chapter 4 presents the results of these surveys, where a total of 208 plant species were documented in 11 vacant plots, and 72 species were documented in the five gardens that were assessed. Of the 208 species, 182 were native species (109 being perennial) versus 30 of 72 species were native plants in the public gardens. A point to be noted in regards to the gardens, is that at least 10 of these species are repeated across all five parks and of the 30 native plants, only 13 are from deciduous origins, and only two are local to Ahmedabad district’s ecology, the rest are from evergreen ecologies of India. In Chapter 5, the sustainability of designed green spaces has been assessed using social methods. Semi-structured interviews were conducted with six nursery owners in the three zones. The answers of these interviews show that there is intense resource investment, monetarily, time-wise and of natural resources such as soil and water, in sourcing and maintaining the demand for ornamental non-native species. The knowledge of local native flora was limited to those used in religious rituals or as a source of food or medicine. There was also a disconnect in local knowledge of plants, as most nursery owners were not from Gujarat. Moreover, the current market trend that drives nursery stocks has an exclusionary approach towards gardening, and the concept of wild or native flora in private lands is not yet embraced; and hence nurseries do not yet promote native flora. Finally, to be able to influence greening palettes in urban design, we carried out an online interview with ten Developers and Architects, all based in Ahmedabad. Our aim was to assess their readiness to carry out flora and fauna surveys on lands before developing them and to include already present native flora (a catalogue of local native flora that have minimum energy and water needs was presented to them) in future projects. The results show that the knowledge gaps in flora were similar to nursery owners, with some participants identifying plants such as Bougainvillaea glabra and Tecoma stans as native; and two participants suggested that Ahmedabad fell in the western ghats biogeographic zone, with evergreen forest type. Yet, this group of stakeholders were interested in concepts of biodiversity richness, pollination success, wildlife conservation and exhibited the will to include natives in future floral palette. They stated that they were limited by knowledge and access to native species. In Chapter 7, we weave together all our results and discuss pertinent points as recommendations that need to be applied at different scales of urban design. The urgent need to redress the exclusionary trend towards planting natives in designed green spaces is highlighted. As a note of thanks and farewell, we also present some species complexes that were observed during our study. These are symbols of positive relationships between different species of plants, soil and their pollinators, that have been formed in extremely harsh environments, over large time periods. We recommend them as examples of design clusters that can be used while planning green spaces
... Thus, for these groups, the origin of the plants where they were found was not important, as several studies have shown that exotic flowers are not necessarily less attractive to urban pollinators (Frankie et al., 2019;Garbuzov et al., 2015;Garbuzov & Ratnieks, 2014;Hinners & Hjelmroos-Koski, 2009;Majewska et al., 2018;Martins et al., 2017;Matteson & Langellotto, 2011;Wenzel et al., 2020). On the other hand, some highly specialized bee species may be at a disadvantage and become scarce in cities due to the prevalence of exotic flowers, such as those used in landscaping and those from ruderal invasions (Cane, 2005;Cane et al., 2006;Chrobock et al., 2013;Frankie et al., 2005;Fukase & Simons, 2016;Jain et al., 2016;McFrederick & Le Buhn, 2006;Rocha-Filho et al., 2018;Wenzel et al., 2020). Our results corroborate this scenario because the total bee richness and the abundance of specialist bees increased with the increase in the proportion of native plants present in the urban areas in this study (i.e. ...
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Resumo As abelhas são importantes polinizadores que têm sido impactados negativamente por mudanças ambientais antropogênicas, tais como a urbanização. Além disso, o desenvolvimento urbano pode reduzir e degradar o habitat natural das abelhas aumentando a proporção de superfícies impermeáveis, diminuindo as áreas verdes e aumentando o número de plantas ornamentais exóticas. Entretanto, as cidades podem oferecer refúgio para as abelhas porque elas proporcionam um ambiente com uma grande variedade de recursos para alimentação e nidificação. O objetivo de nosso estudo foi avaliar a riqueza e a abundância de abelhas, seus respectivos grupos funcionais e composição comunitária ao longo de um gradiente de urbanização em 21 localidades distribuídas entre 6 cidades brasileiras de médio porte (com populações entre 80.000 e 170.000 habitantes). Também avaliamos o efeito da riqueza, número de plantas e proporção de plantas nativas. Coletamos um total de 132 espécies de abelhas. A riqueza total de abelhas diminuiu com o aumento da cobertura impermeável e aumentou com a heterogeneidade da paisagem, o que também teve um efeito positivo sobre a riqueza de abelhas que nidificam acima do solo e abelhas generalistas. Em relação aos dados de abundância, as abelhas solitárias e as abelhas que nidificam no solo foram positivamente influenciadas pelo aumento da cobertura de gramíneas. O número total de plantas nativas e exóticas coletadas influenciou positivamente a abundância total de abelhas, bem como a abundância de abelhas eusociais, que nidificam acima do solo e no solo, e generalistas. A proporção de plantas nativas influenciou positivamente a riqueza total e a abundância de abelhas especializadas. Os nossos resultados indicam que as áreas urbanas de médio porte podem albergar uma grande diversidade de espécies de abelhas, mas as espécies que nidificam no solo e as espécies especialistas podem ser mais sensíveis à urbanização e à diminuição da oferta de recursos florais.
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The effects of weather on the flight and flower-visiting activity of bumblebees and butterflies were studied at a subarctic-alpine site in northern Swedish Lapland. The study focused on the insects' role as potential pollinators and the effect of bumblebee flight and foraging activity on plant reproductive success. The activity rates of both bumblebees and butterflies were significantly correlated with ambient air temperature and solar radiation, and as a consequence, both bumblebees and butterflies exhibited a regular diurnal activity pattern. The butterflies' activity was more constrained by low temperature and solar radiation then the bumblebees' activity, and small worker bees were more affected by the weather than the larger queens. Only 1% of the butterflies observed were visiting flowers, as compared to 69% of the bumblebees. Thus, butterflies seem to be less important pollinators for the alpine plant community than bumblebees. Short-term micrometeorological impact on the reproduction of two bumblebee-pollinated plant species, Bartsia alpina and Diapensia lapponica, was also studied. In both species, reproductive success, measured as seed production, was significantly reduced during a spell of cold weather in comparison to a warmer period.
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On the basis of detailed floristic survey the level of invasion in various EUNIS habitat types identified in NW Poland was assessed. In a data set of 2131 floristic lists the mean number and mean proportion of native species, archaeophytes and neophytes was calculated for each of 25 habitat types. Relationships between this three groups of species were analysed using Pearson correlation. A total of 840 vascular plant species, including 77 archaeophytes and 114 neophytes were recorded. The most invaded habitats were: arable land, fallows and field margins, trampled areas, gardens and parks, lines of trees, anthropogenic tall-forb stands (they contained on average 20-67% alien plants). Most of mean numbers and mean percentage numbers of both alien plants groups in particular habitat types were higher compared to the results obtained from phytosociological databases, therefore the level of invasion assessed on the basis of phytosociological data can be underestimated.
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Semelparity and iteroparity are considered to be distinct and alternative life-history strategies, where semelparity is characterized by a single, fatal reproductive episode, and iteroparity by repeated reproduction throughout life. However, semelparous organisms do not reproduce instantaneously; typically reproduction occurs over an extended time period. If variation in reproductive allocation exists within such a prolonged reproductive episode, semelparity may be considered iteroparity over a shorter time scale.This continuity hypothesis predicts that "semelparous" organisms with relatively low probability of survival after age at first reproduction will exhibit more extreme semelparity than those with high probability of adult survival. This contrasts with the conception of semelparity as a distinct reproductive strategy expressing a discrete, single, bout of reproduction, where reproductive phenotype is expected to be relatively invariant. Here, we manipulate expected season length--and thus expected adult survival--to ask whether Lobelia inflata, a classic "semelparous" plant, exhibits plasticity along a semelparous-iteroparous continuum. Groups of replicated genotypes were manipulated to initiate reproduction at different points in the growing season in each of three years. In lab and field populations alike, the norm of reaction in parity across a season was as predicted by the continuity hypothesis: as individuals bolted later, they showed shorter time to, and smaller size at first reproduction, and multiplied their reproductive organs through branching, thus producing offspring more simultaneously. This work demonstrates that reproductive effort occurs along a semelparous-iteroparous continuum within a "semelparous" organism, and that variation in parity occurs within populations as a result of phenotypic plasticity.
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Background and AimsAlthough urban gardens provide opportunities for pollinators in an otherwise inhospitable environment, most garden plants are not native to the recipient biogeographical region and their value to local pollinators is disputed. This study tested the hypothesis that bumblebees foraging in English urban gardens preferentially visited sympatric Palaearctic-range plants over species originating outside their native range.Methods Twenty-seven surveys of flower availability and bumblebee visitation (Bombus spp.) were conducted over a 3-month summer period. Plants were categorized according to whether they were native British, Palaearctic or non-Palaearctic in origin. A phylogeny of the 119 plant species recorded was constructed and the relationship between floral abundance and the frequency of pollinator visits investigated by means of phylogenetically independent contrasts. Differentiation in utilization of plant species by the five bumblebee species encountered was investigated using niche overlap analyses.Key ResultsThere was conflicting evidence for preferential use of native-range Palaearctic plant species by bumblebees depending on which plants were included in the analysis. Evidence was also found for niche partitioning between species based on respective preferences for native and non-native biogeographical range plants. Two bumblebees (Bombus terrestris and B. pratorum) concentrated their foraging activity on non-Palaearctic plants, while two others (B. hortorum and B. pascourum) preferred Palaearctic species.Conclusions The long-running debate about the value of native and non-native garden plants to pollinators probably stems from a failure to properly consider biogeographical overlap between plant and pollinator ranges. Gardeners can encourage pollinators without consideration of plant origin or bias towards 'local' biogeographical species. However, dietary specialist bumblebees seem to prefer plants sympatric with their own biogeographical range and, in addition to the cultivation of these species in gardens, provision of native non-horticultural ('weed') species may also be important for pollinator conservation.
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Current bee population declines and colony failures are well documented yet poorly understood and no single factor has been identified as a leading cause. The evidence is equivocal and puzzling: for instance, many pathogens and parasites can be found in both failing and surviving colonies and field pesticide exposure is typically sublethal. Here, we investigate how these results can be due to sublethal stress impairing colony function. We mathematically modelled stress on individual bees which impairs colony function and found how positive density dependence can cause multiple dynamic outcomes: some colonies fail while others thrive. We then exposed bumblebee colonies to sublethal levels of a neonicotinoid pesticide. The dynamics of colony failure, which we observed, were most accurately described by our model. We argue that our model can explain the enigmatic aspects of bee colony failures, highlighting an important role for sublethal stress in colony declines.
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Bumble bees are an important group of wild pollinators in North America and considerable concern has been expressed over declines in their populations. However, before causes for declines can be assessed, it is essential that the geographical and chronological patterns of decline be discovered. Hitherto a lack of assessment of historical data has hindered our efforts to determine which species are most at risk. Here, the status of 21 North American bumble bee species (Hymenoptera: Apidae) occurring in the eastern nearctic biogeographic region is assessed using a specimen-level database from compiled museum and survey records dating back to the late nineteenth century from various institutional collections. Using a combination of measures, bumble bee declines were assessed over their entire native ranges. We report here that half of the selected fauna is in varying levels of decline (especially Bombus ashtoni, B. fervidus, and B. variabilis), with the remaining species exhibiting stable or increasing trends (e.g., B. bimaculatus, B. impatiens, and B. rufocinctus). Suggestions for prioritizing conservation efforts for this important group of pollinators are given.
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Fecundity characteristics, phenology, and behavior of insect flower-visitors were studied for 7 early flowering woodland herbs: Claytonia virginica, Dentaria laciniata, Dicentra canadensis, Dicentra cucullaria, Erythronium albidum, Isopyrum biternatum, and Sanguinaria canadensis. Sanguinaria canadensis is facultatively autogamous, the Dicentras are obligate outcrossers, and the remainder are self-compatible, at least within a stem. All are insect pollinated except sometimes S. canadensis. The numbers of ovules per flower and flowers per stem tended to be inversely correlated, and large-seeded species (S. canadensis, E. albidum, I. biternatum) had lower numbers of potential seeds per stem than did small-seeded species. Flowering of all species typically occurred during the first prolonged period of weather suitable for pollinator activity and ceased by the time the canopy closed. Annual differences in flowering times were associated with differences in average temperatures (i.e., early blooming in a warm, early spring), but cumulative degree-hours or degree-days of air or soil temperatures were not well correlated with flowering times. Other constraints on flowering phenology are discussed, including the predictability of suitable conditions, a proposed "fail-safe" mechanism that may assure flowering before canopy closure even if temperatures are abnomally low, and the importance of nontemperature factors in defining suitable conditions. Flowering time was not very finely tuned to the temperature regime and pollinator activity; flowers blooming during the flowering peak often had low seed production and the fertilization rate of most species was low. Evidence that seed production may have been pollinator limited for several species was obtained by comparing the success of hand pollination and of natural pollination, rarity of certain specialized pollinators, and estimates of the abortion rates of fertilized ovules. We suggest that flowering in early spring is a high-risk option in terms of insect-mediated sexual reproduction. Certain flower-visiting insects favored D. laciniata out of proportion to its abundance, but no effect on seed set of other species was detectable. Honeybees were abundant and active flower visitors with the potential for disrupting ecological/evolutionary relationships between native insects and flowers.
Thousands of honey bee colonies died in a region-wide epidemic in the northeastern United States during the winter and spring of 1995-96. In an effort to assess the tremendous colony losses, Pennsylvania bee-keepers were asked to provide information on their colony losses and treatments they applied. In all, 252 Pennsylvania beekeepers provided information on 6,054 colonies, or about 22% of the colonies in our state. The average colony mortality was 53%. Colony losses were even higher (71.6%) among beekeepers who did not treat colonies for mites or disease. This is similar to the 85% mortality that we saw in feral colonies in central Pennsylvania. Apistan, Terramycin extender patties, and Fumidil-B all significantly decreased colony mortality. Tracheal mite treatments, including menthol and grease (vegetable shortening) patties, did not reduce colony losses. Many beekeepers applied tracheal mite treatments at the wrong time of year, which most likely lead to ineffective tracheal mite control. Overall, we conclude that aggressive treatment for honey bee mites and other diseases significantly increases colony survival.
In an abandoned field near Ithaca, New York, bee species partitioned the available flower resources, primarily by foraging at different times of the season and by visiting different flower species. Honey bees Apis mellifera concentrated on different flower species than did native bees. Spring-flying native bees included many species adapted to forage on native flower species that bloom before the tree canopy closes. Apis mellifera outcompeted native bees at large clusters of attractive flower species such as apple and the crucifer Barbarea vulgaris. Halictus ligatus was the most common early summer halictine. This species concentrated its foraging on different flowers than did other early summer bees, probably because of differences in innate flower preferences. A profuse bloom of native goldenrods Solidago in late summer was the seasonal peak of resource abundance. Native bees must complete their seasonal cycles and enter diapause before the killing frost. They therefore foraged on early-blooming goldenrod species such as S. juncea. In contrast, honey bees overwinter as active bees within their hives. They were most common on later-blooming goldenrods, where they collected much of the nectar required for overwintering. Competition among foraging bees was probably most important in the spring at this site. Partitioning of flower species and differences in seasonal flight times resulted primarily from differences in innate flower preferences, number of generations per season, and means of overwintering.-Author
Biological invasions represent both an increasingly important applied problem and a tool for gaining insight into the structure of ecological communities. Although competitive interactions between invasive and native species are considered among the most important mechanisms driving invasion dynamics, such interactions are in general poorly understood. The European honey bee (Apis mellifera) is a widespread and economically important invader long suspected to competitively suppress many native bee species. Yet the extent to which this introduced species alters native communities remains controversial, reflecting ongoing debate over the importance of resource competition in regulating pollinator populations. I experimentally tested the effects of competition with Apis on colony foraging behavior and reproductive success of a native eusocial bee, Bombus occidentalis Greene, in coastal California. B. occidentalis colonies located near experimentally introduced Apis hives had lower mean rates of forager return and a lower ratio of foraging trips for pollen relative to nectar. Both male and female reproductive success of B. occidentalis were also reduced with greater proximity to introduced Apis hives. Reproductive success correlated significantly with measures of colony foraging behavior, most strongly with the relative allocation of foraging effort to pollen collection. This pattern suggests that B. occidentalis colonies exposed to competition with Apis experienced increased nectar scarcity and responded by reallocating foragers from pollen to nectar collection, resulting in lowered rates of larval production. These results provide evidence that Apis competitively suppresses a native social bee known to be an important pollinator, with the potential for cascading effects on native plant communities. This work also contributes to a greater understanding of the role competitive interactions play in pollinator communities, particularly for social bees.