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Assessing declines of North American bumble bees (Bombus spp.) using museum specimens

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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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ORIGINAL PAPER
Assessing declines of North American bumble bees
(Bombus spp.) using museum specimens
Sheila R. Colla Fawziah Gadallah Leif Richardson
David Wagner Lawrence Gall
Received: 28 January 2012 / Accepted: 4 October 2012 / Published online: 11 October 2012
ÓSpringer Science+Business Media Dordrecht 2012
Abstract 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 chro-
nological 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.
Keywords Pollinator decline Bumble bees Bombus Grid cell Museum data
Insect collections
S. R. Colla (&)
Department of Biology, York University, 4700 Keele Street, Toronto, ON M3J 1P3, Canada
e-mail: scolla@yorku.ca
F. Gadallah
University of Ottawa, 550 Cumberland St, Ottawa, ON K1N 6N5, Canada
L. Richardson
Dartmouth College Life Sciences Center, 78 College Street, Hanover, NH 03755, USA
D. Wagner
Department of Ecology and Evolutionary Biology,
University of Connecticut, Storrs, CT 06269-3043, USA
L. Gall
Entomology Division, Peabody Museum of Natural History,
Yale University, New Haven, CT 06511, USA
123
Biodivers Conserv (2012) 21:3585–3595
DOI 10.1007/s10531-012-0383-2
Introduction
In recent years, the loss of pollinators and the services they provide has become a sig-
nificant conservation issue because of potentially enormous ecological and economic
impacts (Aizen et al. 2009; Beismeijer et al. 2006; Berenbaum et al. 2007). Bumble bees
(Bombus spp.) are the primary pollinators of many wild and cultivated plants, especially in
temperate latitudes (Berenbaum et al. 2007; Kearns and Inouye 1997). These eusocial
insects accumulate pollen and nectar resources throughout the spring and summer and
produce reproductive individuals (queens and males) primarily towards the end of the
growing season (Heinrich 2004; Laverty and Harder 1988). Colony fitness is thus directly
related to resource accumulation over a relatively long period of activity (e.g. Morandin
et al. 2005; Otti and Schmid-Hempel 2008). Because of their haplodiploid sex determi-
nation genetics, at low effective population sizes bumble bees may produce high pro-
portions of sterile males, leading to local extirpation (Whitehorn 2009; Zayed and Packer
2005).
Bumble bee declines have been noted worldwide, including in North America, Britain,
continental Europe and China (Cameron et al. 2011; Williams and Osborne 2009; Williams
et al. 2009). However, while some Bombus species have declined, others have remained
common throughout their native ranges, have expanded into new regions (e.g. Sheffield
et al. 2003) or have been successfully introduced (e.g. Ruz 2002) indicating differential
vulnerability among species. In North America, changes in bumble bee distribution and
abundances have been studied only in a few historically common species (Cameron et al.
2011; Colla and Packer 2008; Thorp 2005), or at small spatial scales (Colla and Packer
2008; Giles and Ascher 2006; Grixti et al. 2009). Regional studies have found evidence of
decline for some species over recent decades (Cameron et al. 2011; Colla and Packer 2008;
Grixti et al. 2009; Thorp 2005). In particular, historically common members of the sub-
genus Bombus sensu stricto (B. affinis,B. terricola,B. franklini and B. occidentalis) and
their social parasite (B. ashtoni) have been found to have declined wherever good baseline
data exist (Cameron et al. 2011; Colla and Packer 2008; Grixti et al. 2009; Thorp 2005;
Wagner and Van Driesche 2010). Less is known about other species, as declines may have
been more subtle; having occurred in more difficult to identify or already rare species.
Additionally, given the scarcity of good historical comparative data, most declines have
been inferred primarily using relative abundance data from a subset of species, which even
with other corroborating sources may be a difficult source for assessment given the biases
in many historic collections (Pyke and Ehrlich 2009). Also, yearly fluctuations in bee
numbers make comparisons of modern short-term studies to long-term historical data
collections problematic (Roubik and Wolda 2001).
In Europe, where bumble bees have been the subject of study for over two centuries,
historical data have allowed detailed analyses of the whole bumble bee fauna. In Britain
and Ireland, the conservation statuses for up to 17 bumble bee species has been assessed for
a portion of their native ranges based upon 50 950 km grid cells using multiple long-term
datasets from taxonomists, museums and naturalist groups (e.g. Williams 2005; Fitzpatrick
et al. 2007). The extent of decline for the UK fauna and ecological characteristics shared
among declining species has been determined by comparing grid cell occupancy between
historical and recent time periods with appropriate ecological data (Williams 1982,1989).
Historically uncommon species show the greatest declines (Fitzpatrick et al. 2007). In
North America, we have yet to assess the status of the majority of bumble bee species,
including several historically less common ones. Here we use a dataset compiled from
museum collections to assess the conservation status of the entire eastern North America
3586 Biodivers Conserv (2012) 21:3585–3595
123
bumble bee fauna throughout their native ranges. We measure decline using changes in
grid cell occupancy and look at changes in relative abundance over four time periods.
Using these decline measures, we assess the conservation status of each species and make
practical recommendations.
Methods
Dataset
Electronic records for North American occurrences of all species of Bombus were
assembled from the holdings and survey data of the American Natural History Museum;
the Canadian National Collection of Insects, Arachnids and Nematodes; the Peabody
Museum of Natural History, University of Connecticut; Steve Javorek (Agriculture Can-
ada); the Packer Collection at York University, University of Guelph Insect Collection; the
Royal Ontario Museum; Sam Droege (USGS); Kevin Matteson (Fordham University);
Rachel Winfree (Rutgers University); Zadock Thompson Natural History Collection of the
University of Vermont; Middlebury College; Vermont Forests, Parks, and Recreation
Entomology Laboratory; Vermont State Colleges Entomology Collections; Canadian
Museum of Nature; University of Massachusetts; University of New Hampshire; New
York State Museum; Connecticut Agricultural Experiment Station and various private
collections. Incomplete records (e.g. those lacking determinations or essential label data)
were removed. Records without georeference were either removed or assigned coordinates
based on the site description on the label. ArcGIS (ESRI 2010) was used to check
georeference accuracy against associated label data. Records with erroneous coordinates
were either corrected or removed from the dataset. Publicly available records of Bombus
occurrence from the Illinois Natural History Survey were also included (http://
www.inhs.illinois.edu/). These data underwent the same quality checks as data from
other sources; in addition, specimens that were not recorded as having been identified by a
mellitologist were removed from this dataset.
Measuring decline
After data cleaning, a total of 69,600 North American Bombus records were accumulated,
encompassing the period from 1864 to 2009. For these analyses we used 44,797 records,
considering only species which occur primarily in the eastern Nearctic bumble bee bio-
geographic region (Williams 1996). To test whether there have been changes in distri-
bution or abundance of these species, data were divided into four time periods: pre-1931,
1931–1960, 1961–1990, and 1991–2009. These divisions were chosen based on the tem-
poral spread of samples (Fig. 1), to consider time periods before agricultural intensification
(pre-1931), before large-scale urbanization (pre-1961) and to look for possible long term
declines in Bombus s.s. before the documented rapid declines in the mid-1990s (COSEWIC
2010; Evans 2008; Thorp 2005). We consider here 21 species occurring primarily in the
eastern region of North America, but our analyses cover the entire North American ranges
for these species permitting us to assess whether declines occur throughout their ranges
(Fig. 2). Due to insufficient data, we do not treat species found in subarctic eastern Canada.
For comparison with previous European bumble bee studies (Fitzpatrick et al. 2007;
Williams 2005), the continent was divided into 50 950 km grid cells, and the presence of
our target species in each cell for each time period was determined. Historic range size was
Biodivers Conserv (2012) 21:3585–3595 3587
123
estimated to be the total number of grid cells occupied over all time periods also following
Fitzpatrick et al. (2007) and Williams (2005).
In order to determine the current status of species in their former ranges, we considered
only grid cells from prior time periods which contained a record (for any of the 21 species)
in the 1991–2009 time period and therefore were known to have been sampled for bumble
bees. This ‘persistence’ value was calculated as the proportion of re-sampled historical
cells (1864–1990; determined by the presence of at least one Bombus specimen in the
database) occupied by the species in the most recent time period (1991–2009). Only re-
sampled squares were considered because fewer grid cells were sampled in the most recent
time period than historically.
The relative abundance of each species for each time period was calculated by dividing
the number of specimens for that species by the total number of eastern specimens in the
given time period. A logistic regression treating, for each species, the proportion of
individuals in a given period as the dependent variable and time period as a continuous
explanatory variable was performed using SAS, Proc Genmod.
Using these measures, the conservation status of each species was assessed using cri-
teria modified from the IUCN red list (IUCN 2001). IUCN red list criteria assess declines
in the previous 10 years, or three generations, whichever is the longer. However, as rel-
atively few cells were sampled within the last 10 years in comparison to the historical data
available, we determine conservation status based on changes between our historical (all
pre-1991) and recent time (1991–2009) periods instead of ten years.
Results
North American bumble bee species vary substantially in their patterns of abundance over
the past century. The most declining species was the cuckoo bumble bee B. variabilis
which showed severe decline in both measures, being completely absent from all samples
from our dataset in the most recent time period (1991–2009). Other species which persisted
in less than 50 % of their re-sampled range include B. affinis,B. ashtoni,B. auricomus,B.
borealis,B. fernaldae,B. fraternus,B. insularis,B. pensylvanicus, and B. sandersoni
Fig. 1 Eastern Nearctic Bombus specimens collected by decade from our combined and cleaned dataset
3588 Biodivers Conserv (2012) 21:3585–3595
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(Table 1). Species which remained persistent through the majority of their resampled range
include B. citrinus,B. griseocollis,B. impatiens,B. ternarius, and B. bimaculatus
(Table 1). There was a significant positive relationship between pre-1990 range size (i.e.
grid cell occupancy) and persistence (Fig. 3,R
2
=0.34, p\0.05).
Our analyses considering relative abundance of our species over time found significant
declines in B. ashtoni,B. fervidus,B. fraternus and B. variabilis (p\0.05; Table 2).
Conversely, B. bimaculatus,B. impatiens, and B. rufocinctus were found to have signifi-
cantly increased (p\0.05; Table 2). Using our occupancy and relative abundance decline
measures, we assessed one species as Critically Endangered, six species as Endangered and
four as Vulnerable (Table 3).
Discussion
Biological collections and their associated taxonomic databases and utility in systematic
research have recently proved to be an invaluable resource to address conservation issues
(Pyke and Ehrlich 2009). Using these resources, this study is the first to assess the status of
eastern nearctic bumble bee species throughout their native ranges. However, long-term
studies of bumble bee declines face problems of biased sampling and inconsistent sampling
effort among time periods (Kosior et al. 2008; Williams and Osborne 2009). Additionally,
historical collections tend to be geographically and temporally biased (Pyke and Ehrlich
2009). To reduce the effects of these biases, our dataset combined specimen data from
numerous collections and surveys in Canada and the United States. Our spatial analyses
used only grid cells which were sampled both historically and in the most recent time
period, taking into account the smaller geographical spread of recent surveys, and we used
coarse time periods to decrease temporal biases from collection-effort fluctuations. By
using coarse grid cells instead of smaller cells (e.g. Maes et al. 2012), we were able to
reduce the effect of imprecise locality information for most historical records.
Fig. 2 Distribution of Bombus records for species which occur in the eastern Nearctic biogeographic region
Biodivers Conserv (2012) 21:3585–3595 3589
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Table 1 Estimated historical range size as the number of grid cells occupied over all time periods and
persistence measured as the proportion re-sampled historical cells (1864–1990) occupied of recent time
period sampled cells (1991–2009) for each species
Species Estimated range size: 50 950 km cells occupied (area km
2
) Persistence
B. affinis 163 (407,500 km
2
) 0.265
B. ashtoni* 209 (522,500 km
2
) 0.347
B. auricomus 147 (367,500 km
2
) 0.492
B. bimaculatus 235 (587,500 km
2
) 0.837
B. borealis 242 (605,000 km
2
) 0.271
B. citrinus* 177 (442,500 km
2
) 0.759
B. fernaldae* 141 (352,500 km
2
) 0.233
B. fervidus 561 (1,402,500 km
2
) 0.552
B. fraternus 59 (147,500 km
2
) 0.273
B. frigidus 154 (385,000 km
2
) 0.571
B. griseocollis 357 (892,500 km
2
) 0.722
B. impatiens 426 (1,065,000 km
2
) 0.841
B. insularis* 251 (627,500 km
2
) 0.444
B. pensylvanicus 322 (805,000 km
2
) 0.344
B. perplexus 254 (635,000 km
2
) 0.631
B. rufocinctus 283 (707,500 km
2
) 0.667
B. sandersoni 122 (305,000 km
2
) 0.268
B. ternarius 309 (772,500 km
2
) 0.722
B. terricola 323 (807,500 km
2
) 0.518
B. vagans 386 (965,000 km
2
) 0.631
B. variabilis* 35 (87,500 km
2
)0
Total (all N.A. species) 2,171
* Denotes cuckoo species
Fig. 3 Scatterplot showing the relationship between the ranges of 21 North American bumble bees (x-axis)
as the total number of 50 950 km grid cells occupied and persistence (y-axis) as the proportion of
historical (pre-1991) range recently occupied (1991–2009)
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Table 2 Number of records and relative abundance of eastern Bombus species by time period and results of logistic regression on relative abundance. Statistically significant
values in bold
Species Total records \1931 1931–1960 1961–1990 1991–2009 Slope (sign indicates
direction of change)
V
2
pvalue
B. affinis 1,563 355 303 812 93 -0.2779 0.5281 0.4674
B. ashtoni*941 311 280 267 83 20.5166 13.7488 0.0002
B. auricomus 493 140 79 192 82 -0.2538 1.7924 0.1806
B. bimaculatus 2952 202 149 863 1,738 0.7152 16.0216 0.0001
B. borealis 1,067 303 501 125 138 -0.5508 2.9737 0.0846
B. citrinus* 1,202 222 217 178 585 0.1750 0.4106 0.5217
B. fernaldae* 474 77 277 97 23 -0.5064 1.0955 0.2952
B. fervidus 3,937 1,346 1,213 736 642 20.4905 24.0150 <0.0001
B. fraternus 145 41 64 30 10 20.5740 4.2850 0.0385
B. frigidus 1,830 134 1,282 217 197 -0.4056 0.4080 0.5230
B. griseocollis 2,870 398 337 606 1,529 0.3807 3.2199 0.0727
B. impatiens 9,111 1,141 851 2,709 4,410 0.3984 6.7176 0.0095
B. insularis* 1,025 159 361 470 35 -0.3099 0.5251 0.4687
B. pensylvanicus 2,024 435 525 912 152 -0.3001 0.9360 0.3333
B. perplexus 1,735 354 288 597 496 -0.0258 0.0842 0.7717
B. rufocinctus 2,671 215 503 745 1,208 0.3426 15.7771 0.0001
B. sandersoni 472 91 131 219 31 -0.2832 0.6915 0.4056
B. ternarius 2,502 461 300 621 1,120 0.1978 1.1654 0.2803
B. terricola 3,724 963 456 1,632 673 -0.1723 0.4516 0.5016
B. vagans 3,965 659 376 1,518 1,412 0.1705 0.8186 0.3656
B. variabilis*94 76 11 7 0 21.7406 39.2118 <0.0001
Total (eastern species) 44,797 8,083 8,504 13,553 14,657 0.0417 1.1907 0.2752
Total (all species) 69,600 12,375 16,093 21,386 19,746 -0.0682 1.2002 0.2733
* Indicates the species is a social parasite (cuckoo) and bold face indicates significant change in relative abundance over time
Biodivers Conserv (2012) 21:3585–3595 3591
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These methods do have limitations, however. Re-sampled grid cells were selected
because at least one bumble bee was collected in the most recent time period. We cannot
assume all species were searched for equally, meaning a species may be absent due to
actual absence or from lack of collection. Additionally, areas not sampled due to difficulty
of access or distance from roads and urban centers are not well represented in sampled grid
cells. Thus, the true historic range or presence in recent time periods for each species may
be underrepresented. Finally, because we consider only resampled grid cells, our method
does not allow us to assess whether there is spatial autocorrelation among the grid cells lost
in species declines. Anecdotal information suggests, however, that for many of these
species, there is geographic patterning to their decline (e.g. persistence in some areas of
their historic range and extirpation in others).
By combining data from numerous museums and collections from both Canada and the
USA, we determined that 11 of the 21 native eastern Bombus species have likely suffered
substantial population declines (i.e. [50 %). These declines were striking given the
increase in the number of Bombus specimens collected (i.e. sampling effort) in the most
recent time period (Fig. 1). Eight of the 21 species show signs of stability or increases in
relative abundance. Consistent with European studies (Fitzpatrick et al. 2007; Williams
1982,1989,2005), we found that species with smaller historically occupied ranges had
lower persistence (Fig. 3). In particular, B. variabilis, the eastern species with the lowest
number of historically occupied grid cells, is assessed as most at risk, and the extensive
extent of decline had not been noted previously.
Using this dataset, alternative methods of analyses, and considering each species’ entire
North American range, our results contribute additional information about the status of
wild bumble bees gained from recent Bombus decline studies (Cameron et al. 2011; Colla
and Packer 2008; Grixti et al. 2009). Our study recommends 11 species (B. affinis,
B. ashtoni,B. auricomus,B. borealis,B. fernaldae,B. fervidus,B. fraternus,B. insularis,
B. pensylvanicus,B. sandersoni and B. variabilis) for immediate conservation attention.
Table 3 Assessment of declining eastern North American bumble bee species using modified IUCN red list
criteria
Species Rank Rationale
B. affinis EN RD [70 %
B. ashtoni* VU RD [50 %
B. auricomus VU RD [50 %
B. borealis EN RD [70 %
B. fernaldae* EN RD [70 %
B. fervidus EN ID [70 %
B. fraternus EN RD [70 %
B. insularis* VU RD [50 %
B. pensylvanicus VU RD [50 %
B. sandersoni EN RD [70 %
B. variabilis* CR RD [90 %
The rationale ‘‘RD’’ (range decline) provides the decline in occupancy in re-sampled historical range
between 1960–90 and 1991–2009 time periods, ‘‘ID’’ (index decline) provides the decline in index of
relative abundance over all time periods
CR critically endangered, EN endangered, VU vulnerable
* Denotes cuckoo species
3592 Biodivers Conserv (2012) 21:3585–3595
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It should be noted that using a more conservative approach to deal with Type I error due to
performing multiple statistical tests affects the interpretation of extent of decline for B.
fraternus only (Table 3).
While B. affinis has previously been described as suffering large-scale declines
throughout its North American range (Cameron et al. 2011; Colla and Packer 2008;
COSEWIC 2010) and B. ashtoni,B. borealis,B. pensylvanicus and B. variabilis have been
found to be in decline at the regional level (Cameron et al. 2011; Colla and Packer 2008;
Grixti et al. 2009), the declines of the remaining species have been thus far unnoticed. This
is likely due to the fact that they have been uncommon historically, are more difficult to
identify, and may be more specialized to certain habitat types.
The declines estimated here are less marked than previously reported in B. affinis (e.g.
87 % decline, Cameron et al. 2011), likely due to the coarse time scale we used. This
suggests that the decline of B. affinis occurred most precipitously within our most recent
time period (1991–2009) as has been previously hypothesized (Thorp and Shepherd 2005).
Additionally, previous studies considering only a portion of the range for B. terricola found
this species to be among the most sharply declining species (Cameron et al. 2011; Colla
and Packer 2008). However, our analyses take into consideration the full native range of
this widespread species and rank it as of lower conservation concern. In fact, B. terricola
remains common in portions of its Canadian and Northeastern US range with suitable
habitat (Colla and Dumesh 2010; L. Richardson, Pers. Obs.).
Future work determining the cause of differential vulnerability among species is
required to further understand Bombus conservation issues. Williams et al. (2009) found
species with narrow climatic niche, species locally at the edge of their climatic niche and
species with late queen emergences to be more vulnerable to extrinsic threats. Here, we
show select species with shared ecological traits to be most at risk. Similar to previous
findings (e.g. Dupont et al. 2011; Goulson et al. 2005; Williams et al. 2009), we find the
late-emerging, long-tongued species to be in decline (B. auricomus,B. fervidus,B. pen-
sylvanicus). Additionally, as is commonly found amongst other taxa (e.g. Stefanescu et al.
2011), species restricted to certain habitat types were also found to be more at risk
(B. borealis,B. fraternus,B. sandersoni). Lastly, the majority of the eastern cuckoo
bumble bees (B. ashtoni,B. insularis,B. fernaldae,B. variabilis) were also found to be
among the most at-risk. The cuckoo species must be sensitive to changes in the abundance
of host species and, depending upon their degree of host specificity, have likely always
been rarer than their hosts. Further work to better assess the intrinsic patterns of vulner-
ability of these species in addition to assessments of threats is urgently needed to conserve
our bumble bee fauna as a whole. Until then, habitat protection of current populations of
species found to be at higher risk should be implemented.
Acknowledgments This work would not have been possible without the use of valuable insect specimens
from many well-curated collections and recent surveys. We thank additional data providers J. Ascher,
Caroline Scully, Mike Arduser, Steve Javorek and Kevin Matteson. We would like to thank Michael
Otterstatter for help with statistical analyses and Paul Williams, Ignasi Bartomeus, Sarina Jepsen and
reviewers for valuable comments. We thank NSERC-CANPOLIN and NSERC CGS to Fawziah Gadallah
and Sheila Colla respectively for providing funding for this work. Data capture at the American Museum of
Natural History (AMNH), the University of Connecticut, Rutgers University, and Cornell University was
supported by NSF DBI Grant (0956388, P. I. John S. Ascher), with additional support at AMNH from
Robert G. Goelet and at University of Connecticut and the Peabody Museum of Natural History by a state
wildlife Grant (09DEP10012AA, P.I. DLW). This is contribution No. 58 from the Canadian Pollination
Initiative (NSERC-CANPOLIN).
Biodivers Conserv (2012) 21:3585–3595 3593
123
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... Widespread phenomena of urbanization are driving deep changes on landscape features, their temperature and pollutants, creating conditions that impact biodiversity (Foley et al. 2005;Weng et al. 2007;Wenzel et al. 2020). Plants and animals can respond to these environmental variations by shifting their distribution (Colla et al. 2012), phenology (Huchler et al. 2020), and/or shaping some morphological traits considered "functional", i.e. relevant for their ecology, fitness and behavior (Alberti et al. 2017;Eggenberger et al. 2019;Nooten and Rehan 2020). In bees, trait variation due to environmental alteration could affect the efficiency of the pollination ecosystem service they provide though impacting the way they interact with plants (Buchholz and Egerer 2020;Biella et al. 2019a, b). ...
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... Growing evidence, mostly from western Europe and North America, shows widespread but heterogeneous declines in the abundance and diversity of many wild pollinators (Potts et al. 2010;Ollerton et al. 2011;Colla et al. 2012;Zattara and Aizen 2021). Abundance changes (declines and increases) can vary geographically within species (Aizen and Harder 2009b;Thomson 2016) and among congeneric species and taxonomic families (Biesmeijer et al. 2006;Cameron et al. 2011;Richardson et al. 2019;Zattara and Aizen 2021). ...
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... Indeed, studies evaluating the potential effects of these two stressors have revealed both synergistic and additive negative effects on lethal (Linguadoca et al., 2021;Tosi et al., 2017) and sublethal endpoint measurements (Schmehl et al., 2014;Stuligross and Williams, 2020), respectively. Given that similar negative effects are observed on key reproductive physiological parameters in bumble bees, such as on the glands required for digestion and brood care (i.e., hypopharyngeal glands (HPGs)) or sperm traits (i.e., spermatozoa counts and viability), this may provide an additional plausible mechanistic explanation for recent population declines (Bommarco et al., 2012;Colla et al., 2012). However, the impact of any given stressor can vary depending upon the level, e.g. in ants additive effects of virus and pesticide were observed at the level of individuals and castes, while co-exposure with both stressors elicited antagonistic effects on colony size (Schläppi et al., 2021). ...
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... This implies that increasing aridity vis-à-vis temperature are probably going to have detrimental effects on wild bees, albeit their thermophilic character. The individualistic response to climate change has also been observed in other insect groups [64,139], in high-altitude occurring species [63] and in other regions as well [39,138,[140][141][142], pointing that even phylogenetically close and ecologically similar species might differ in their vulnerability against climate stressors [143]. It might as well be that abrupt future temperature rise will exceed the species' thermal tolerance and/or the species' ability to track their niche, especially in areas with low environmental heterogeneity, such as low altitude Aegean islands. ...
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Biogeographic regions are widely regarded as real entities, or at least as useful summaries of the complex patterns of spatial concordance among species. The problem is that, whereas some parts of the transition zones between regions may be strong and abrupt, other parts of the same zones may be weak or broad, so that the corresponding parts of border lines drawn on maps, although convenient, are arbitrary constructs. One approach to investigating transition zones ascribes values to the area units themselves, by quantifying the spatial turnover among species within the surrounding neighbourhoods of areas on maps. Using data for bumble bee distributions world-wide, I show that quantitative measures of neighbourhood turnover can discover many of the transition zones that are found by classification techniques when applied to the same data. But unlike classification techniques, turnover measures, particularly when used in combination, can show how a transition zone varies along its length, not only in its strength (the proportion of species contributing to the zone) but also in its breadth (the degree of spatial overlap or the degree of coincidence among species replacements across it). For bumble bees at least, these transition zones are also negatively associated with areas that have a combination of both high species richness and high species nestedness.
Conference Paper
... compelling but untested hypothesis for the cause of decline in the United States (10) entails the spread of a putatively introduced pathogen , Nosema bombi, which is an obligate intracellular microsporidian parasite found commonly in bumble bees throughout Europe (13–16 ...
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We provide keys for identifying the 26 bumble bee species (Bombus and Psithyrus) found in Canada east of Manitoba, and information on their ecology and distribution. The keys are designed for field use and rely primarily on colour patterns rather than on microscopic features.
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