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Habitat filtering influences plant-pollinator interactions in prairie ecosystems
Journal: Botany
Manuscript ID cjb-2018-0134.R2
Manuscript Type: Article
Date Submitted by the Author: 12-Dec-2018
Complete List of Authors: Bizecki Robson, Diana; The Manitoba Museum, Hamel, Cary; Nature Conservancy of CanadaNeufeld, Rebekah; Nature Conservancy of CanadaBleho, Barbara; Golder Associates Ltd.
Keyword: Functional groups, Habitat filtering, Moisture gradient, Plant-insect interactions, Pollination
Is the invited manuscript for consideration in a Special
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Habitat filtering influences plant-pollinator interactions in prairie ecosystems
Diana Bizecki Robson, Manitoba Museum, 190 Rupert Avenue, Winnipeg, MB, R3B 0N2
Cary Hamel, Nature Conservancy of Canada, 200-611 Corydon Ave., Winnipeg, MB, Canada, R3L 0P3
Rebekah Neufeld, Nature Conservancy of Canada, 207-1570 18th Street, Brandon, MB, Canada R7A 5C5
Barbara I. Bleho, Golder Associates Ltd., 100 – 6925 Century Avenue, Mississauga, Ontario, Canada
L5N 7K2
Corresponding Author:
Diana Bizecki Robson
Manitoba Museum
190 Rupert Avenue
Winnipeg, MB
R3B 0N2
Phone: 204-988-0653
Fax: 204-942-3679
E-mail: [email protected]
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Abstract
The xeric hypothesis is that bees are more abundant pollinators in dry, temperate biomes than
anthophilous flies, and the habitat filtering hypothesis is that differences in the proportions will impact
plant community composition because different pollinators favour different floral traits. However, few
studies have examined the predictive value of these hypotheses. In particular, differences in plant-
pollinator compositions within biomes, such as the Prairie Ecozone, have not been compared. We
documented plant-pollinator interactions and plant abundance in three Canadian prairie types. Flower
visits in moist tall grass prairie were mainly by flies in the Syrphidae, while visits in the drier fescue and
mixed grass prairie were mainly by long-tongued bees in the Apidae. Short-tongued bee visits were not
significantly different between the prairie types. Insect visits to tubular, zygomorphic, violet/blue- and
white-flowered plants were higher in drier fescue and mixed grass prairie than in moister tall grass prairie.
Further, proportions of plants with these features were lower in the tall grass prairie. Thus differences in
the proportion of flies and long-tongued bees, likely affected by habitat conditions including moisture
levels, appear to be influencing the types of plants that dominate each prairie type providing some support
for these hypotheses.
Key words: Functional groups, habitat filtering, moisture gradient, plant-insect interactions, pollination,
prairie
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Introduction
The xeric hypothesis (Michener 1979) is that bees are more common pollinators in temperate, xeric
habitats than in cool arctic or humid tropical habitats. This is based on the observation that deserts have
more bees than prairies, which have more bees than tundra (Michener 1979). Both temperature and
humidity are thought to be responsible for these differences (Michener 1979; Arroyo et al. 1982; Devoto
et al. 2009). High levels of moisture can flood subterranean bee nests and humid conditions can result in
fungal infection of stored food and immature bees (Michener 1979). Further, bees have higher energy
demands than flies and butterflies due to nest provisioning, which can make living in marginal habitats
difficult for these organisms (Arroyo et al. 1982; Kearns 1992). As a result, anthophilous flies are more
common pollinators than bees in cooler areas such as arctic (Mosquin and Martin 1967; Kevan 1972;
Elberling and Olesen 1999) and alpine (Kearns 1992; Gray et al. 2018; Lefebvre et al. 2018) habitats, and
in moister habitats (Devoto et al. 2005, 2009; Robson 2008). However, not all bees are restricted to xeric
habitats: short-tongued bees in the Halictidae and Andrenidae of North America are more common in
temperate, mesic conditions than in xeric ones (Michener 1979). Unfortunately, studies documenting
variation in abundance of bee families across geographic space remain uncommon likely due to the large
sampling effort required (McCall and Primack 1992; Pellissier et al. 2012; Villalobos and Vamosi 2018).
Flies are less studied than bees but are being increasingly recognized as important pollinators (Larson et
al. 2001; Ssymank et al. 2008; Woodcock et al. 2014). Although flies as a group are typically more
common in moist habitats than dry ones (Kearns 1992; Devoto et al. 2005, 2009; Pellissier et al. 2010),
they are highly diverse and breed in a wide range of conditions (Marshall 2012). Studies to date have not
been able to make any generalized statements regarding why some anthophilous fly families are more
abundant in certain ecosystems than others (Kearns 1992; Devoto et al. 2005, 2009). This may be
because unlike bees, flies are often omnivorous, relying on other animals, or bacteria for food in their
larval stage and on plant resources when adults. Anthophilous bee flies (Bombyliidae) and parasitic flies
(Tachinidae), for example, have larvae that parasitize other insects while soldier flies (Stratiomyidae) are
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mostly aquatic filter feeders as larvae (Larson et al. 2001; Marshall 2012). Additional data on geographic
variation in fly families is needed to help us understand which environmental variables are affecting their
distribution and abundance.
Sargent and Ackerly (2008) hypothesized that the composition of a pollinator community could act as a
habitat filter if the absence of certain pollinators prevents the establishment of some plant species.
Pellissier et al. (2012) state that “changes in the proportion of pollinator functional groups are likely to
trigger changes in the structure and composition of natural plant communities” because different
pollinators favour different floral traits. One trait that influences whether a plant is predominantly bee or
fly visited, is flower type (e.g. open or tubular) (McCall and Primack 1992; Fontaine et al. 2006). This
may be because insects without specialized mouthparts, like most flies, are actually constrained to visit a
smaller subset of species than longer-tongued insects like bees and butterflies (Fontaine et al. 2006;
Vamosi et al. 2014). Symmetry (i.e. actinomorphic or zygomorphic), and colour are other variables that
have been investigated for their impact on pollinators (McCall and Primack 1992; Hegland and Totland
2005; Fontaine et al. 2006; Stang et al. 2006; Woodcock et al. 2014; Inouye et al. 2015; Gray et al. 2018;
Kemp et al. 2018). Bees tend to excel at accessing nectar in complex flowers such as zygomorphic ones
while flies prefer open, actinomorphic ones (McCall and Primack 1992; Fontaine et al. 2006; Pellissier et
al. 2012). Regarding colour, white and yellow flowers are more common in fly-dominated habitats such
as the arctic (Kevan 1972) likely because anthophilous flies are sensitive to UV and yellow light (Lunau
and Maier 1995; Woodcock et al. 2014). Interestingly, longer-tongued bomblyiid flies are an exception to
this general observation; they sometimes prefer bluish flowers that are often tubular (Woodcock et al.
2014). Insects of course do not respond to any of these floral variables in isolation; they experience them
all at approximately the same time so teasing out which factor is most important is difficult and likely
varies somewhat from species to species.
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Although pollination studies in northern prairies have been conducted recently (Robson 2008; Evans
2013; Semmler 2016; Robson et al. 2017; Olynyk 2018), only a few compare the pollinator community or
floral characteristics of different prairie types to each other (Chamberlain et al. 2014; Vamosi et al. 2014).
The purpose of this paper is to evaluate the predictive value of the xeric (Michener 1979) and the habitat
filtering (Sargent and Ackerly 2008) hypotheses within a biome by: (1) documenting if and how the
composition of the insect-pollinated plant and pollinator communities differs among three prairie types in
Canada—tall grass, mixed grass and fescue—using Canonical Correspondence Analysis (CCA),
Sorenson similarity and Mantel tests, and (2) testing several hypotheses regarding any compositional
differences. The specific hypotheses we tested using Generalized Linear Mixed Models (GLMMs) were
that: (1) the proportion of fly, and long- and short-tongued bee floral visits would differ among the three
prairie types, (2) the proportion of fly visits would be higher and that of long- and short-tongued bees
lower, at sites with higher moisture (measured via humidity and annual precipitation), (3) the proportion
of pollinator visits to plants with tubular, zygomorphic and violet/blue-coloured flowers would be lower
at the prairie type with more flies, and (4) the proportion of plants with tubular, zygomorphic and
violet/blue-coloured flowers would be lower at the prairie type with more flies. To do this, plant-
pollinator network data and insect visitation rates from previous studies in the tall grass (Robson 2008,
2010, 2013) and fescue (Robson et al. 2017) prairie were re-analysed and compared to new data collected
at a fifth prairie site consisting of mixed grass prairie.
Methods
Study Sites
Insect floral visitors were observed and sampled at five study sites in three Ecoregions of the Prairie
Ecozone (Ecological Stratification Working Group 1995) in southern Manitoba (Figure 1). There were
three tall grass prairie sites (i.e. Birds Hill Provincial Park (BHPP), Living Prairie Museum (LPM), and
Tall Grass Prairie Preserve (TGPP) (Table 1), one mixed grass prairie site at Yellow Quill Prairie
Preserve (YQPP), and one fescue prairie site at the Fescue Prairie Preserves (FPP). The tall grass prairie
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sites were dominated by tall and mid-grass species such as Andropogon gerardii Vitman, Schizachyrium
scoparium (Michx.) Nash and Sporobolus heterolepis (A. Gray) A. Gray, and were considered either dry,
mesic or wet depending on drainage patterns and landscape position (i.e. upland or lowland). The YQPP
was on sandy, well-drained land and was dominated by mid- and short grasses including Bouteloua
gracilis (Kunth) Lag. ex Griffiths, Koeleria macrantha (Ledeb.) Schult., Hesperostipa spartea (Trin.)
Barkworth and Elymus spp. The fescue prairie site occurred on well-drained, raised flatland dominated
by Festuca halli (Vasey) Piper, K. macrantha, Elymus trachycaulus (Link) Gould & Shin. and H. spartea.
Three sites had well-drained Dark Brown Chernozemic soils with lots of sand or gravel: FPP, YQPP and
BHPP. The presence of the BHPP site on an esker (Holland et al. 1996) made it the driest of the three tall
grass prairie sites. In contrast, the TGPP had Gleysolic soils due to prolonged water saturation and LPM
had Vertisolic soils due to the high clay content. In addition to different soils, the tall grass prairie sites
also had slightly different elevations and annual precipitation but the same number of frost free days. The
FPP had the shortest growing season and the highest elevation, and the YQPP was the driest.
LPM is the smallest preserve, only 0.12-km2, as it is an urban park, within and owned by, the City of
Winnipeg. A few of the plots at LPM were on land that had been burned the spring of the first survey
year (2004). The FPP included plots in the Cleland and Elk Glen properties owned by the Nature
Conservancy of Canada (NCC), as well as some provincial government land. Due to the close proximity
of these properties to each other (<3 km), they are considered as one site in this analysis although they
have also been analyzed separately to calculate insect generalization scores (Robson et al. 2017).
Together the Cleland and Elk Glen properties are 5.8-km2 in size but they are bordered to the north by
Riding Mountain National Park and adjacent to provincially- and privately-owned intact prairie and
boreal forest to the east. The six plots at the Cleland property had been grazed by cattle the year before
the surveys began and those at the Elk Glen property were grazed about eighteen years before. The
YQPP, also owned by NCC, is 8.4-km2 but adjacent to additional intact prairie at the provincial
Assiniboine Corridor Wildlife Management Area and the federal Canadian Forces Base Shilo. All
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pastures where the plots were located at YQPP had been grazed the same years as the surveys were
conducted. The sections of the TGPP surveyed were owned by NCC and Nature Manitoba and were
about 10 km apart. Prescribed fire is utilized as a management tool at TGPP, with a prescribed rotation of
six years at any given site. No fire occurred during the survey period, however. In total the TGPP is 22-
km2 and adjacent to some privately-owned prairie mainly being used for cattle grazing. The largest site
was BHPP at 35-km2 but it is largely surrounded by residential development, aggregate mines and
agricultural cropland, and much of the area in the park is forested.
Vegetation Surveys
Plant-insect visitor datasets from the tall grass (Robson 2008, 2010, 2013) and fescue prairie (Robson et
al. 2017) were compared to a new dataset from the mixed grass prairie (i.e. YQPP). After an initial
reconnaissance of each site, areas with predominantly native prairie were identified. To maximize the
length of time that could be spent surveying insect visitors each day, areas relatively close to access roads
and trails were selected and the plots placed randomly within them. However, the BHPP plots were
selected to contain at least one stem of Symphyotrichum sericeum (Vent.) G.L. Nesom as the research at
this site was conducted in part to document pollinators to this rare Canadian species. We established 24
plots at YQPP, 18 plots at FPP, 16 plots at BHPP, and 6 plots each at LPM and TGPP. At YQPP and FPP
each plot was 4-m2 in size, at BHPP each plot was 2.5-m2 and at LPM and TGPP the plots were 5-m2.
The plots were at least 5-m apart and typically scattered over several kilometers of the preserves to better
capture the biological diversity. Plots were closest together at LPM due to its small size. In total the
plots covered an area ranging from 72-m2 to 150-m2.
In 2014 vegetation surveys at FPP were conducted for four consecutive days, which was repeated four
times in mid-June, -July, -August and -September (16 days total). These surveys were repeated in 2015
on the previously established plots except there were two survey periods in June (early and late), and one
each in July and August (16 days total). In 2016 surveys at YQPP were conducted for two consecutive
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days twice each month of June, July, August and September (16 days total). These surveys were repeated
in 2017 on the previously established plots in May, June, July and August (16 days total). Sampling in
BHPP was conducted for 37 non-consecutive days: 6 days in June (2011), 12 days in July (2010 and
2011), 11 days in August (2008 and 2010), and 8 days in September (2008). Sampling at LPM and TGPP
occurred from June to September on four non-consecutive days each month in both 2004 and 2005 for a
total of 32 days at each site. Surveying at slightly different times each year enabled us to observe a
greater number of plant species than if the plots were visited on the exact same calendar days over two
years. Insect-pollinated plant richness and number of inflorescences in the plots was recorded each
survey day. Due to natural variability in abundance, some plant species were present in more plots than
others and thus were observed for longer periods of time. Nine species had solitary or axillary flowers
which typically gave them a smaller floral display area than the others, which all had multiple flowers per
inflorescence. The floral attributes of each plant (i.e. shape, symmetry and colour) were determined by
examining the species in the field. Plants with open flowers had sepals and/or petals unfused or fused
into tubes <3mm in length while those considered tubular were fused for >3mm in length. Zygomorphic
as opposed to actinomorphic plants had flowers that were bilaterally symmetrical. Colour was as seen by
human eyes. Vouchers of each insect-pollinated plant species surveyed were collected and deposited in
the Manitoba Museum’s (MM) botanical collection. The vouchers were assigned both collection and
catalogue numbers, and were entered into the Museum’s database.
Floral Visitor Surveys
Floral visitor surveys occurred on the same days as the plant surveys for set periods of time (10
minutes/plot at FPP, YQPP and BHPP, and 30 minutes/plot at LPM and TGPP) each sampling day; total
sampling effort at each site ranged from 96 to 128 hours over 2-3 years. The sites were surveyed over at
least two seasons to partially account for inter-annual variation in insect populations, which can be high
(Williams et al. 2001). Surveys were conducted between 09:30 h and 17:00 h when insect foraging
activity is at a maximum (Kevan and Baker 1983) thus nocturnal insects were not observed. The order in
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which the plots were visited was randomized each day. During the survey periods, weather conditions at
the nearest (less than 37 km away) Environment Canada (2018) weather station were recorded, and the
values averaged for each survey year.
All insect visitations to any inflorescences in the plots where the reproductive organs were touched for
more than two seconds were recorded; these visits have been used as a proxy for pollination (Kantsa et al.
2017). Some of the flower-visiting insects (e.g. crabronid wasps in the genus Ectemnius Dahlborn, 1845)
may have been predators of other flower-visiting insects. Regardless of whether the wasps were foraging
for pollen, nectar or prey, they were considered potential pollinators and their visitations recorded.
However, ambush bugs (Phymata spp. Latreille, 1802) and crab spider (Misumena spp. Latreille, 1804)
visits were not recorded as they remain stationary on one inflorescence for a long time and thus were
unlikely to act as pollinators. Henceforth, all insect visitors are referred to as pollinators although some
may have been inefficient relative to other taxa.
The first time an insect taxon was observed a specimen was obtained and given a unique collection
number. When the same (or what appeared to be the same) taxon was observed later, the collection
number was used to link that insect visit to the plant. Although this technique does not allow for
complete identification “on the wing” (resulting in an underestimate of taxa) it does enable evaluation of
insect visitation frequency, which was then used to determine the visitation rate for each plant species.
As the sites had different plant abundances, the mean number of insect visits inflorescence-1 hour-1 was
determined to enable comparison between species. Only days when the plant was in flower were
included as part of the total hour count. Pollinator visits at each site are expressed as a proportion rather
than an absolute number to enable comparison. All insect voucher specimens were identified by
zoologists to the lowest taxonomic level possible using reference specimens at the MM and Wallis
Roughley Entomology Museum in Winnipeg, Manitoba; the specimens were deposited in MM’s zoology
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collection. The vouchers were assigned both collection and catalogue numbers, and were entered into the
Museum’s database.
Data Analysis
The total number of species-species interactions at each site (I) and the connectance (C =100I/ap, where
“a” and “p” represent the number of animal and plant species, respectively), was determined (Jordano
1987). To assess plant and insect compositional differences between sites and determine which
environmental factors were most influential, we used CCA as a method of ordination of the five sites.
CCA is a direct gradient analysis that examines species and environmental data simultaneously (Ter
Braak 1986). The plant data used was the proportion of all plots at each site that contained each species,
while the insect data used was the proportion of all visits at that site by each insect family. Environmental
variables examined were: average temperature, humidity and wind speed during the survey periods,
elevation, and mean annual precipitation and frost free days. Environmental data were log (n+1)
transformed. The Sørensen similarities (where maximum similarity is 0 and 1 is completely dissimilar) in
plant and insect communities between the five sites were calculated. Mantel tests (Monte Carlo
randomization) for association between the environmental variables and both plant and insect matrices
were performed. Geographic locations of all plots were recorded in the field using GPS satellites; the
geographic midpoints between all plots at each site were used for Mantel tests (Monte Carlo
randomization) to determine spatial autocorrelation (McCune and Grace 2002). These tests were carried
out using PC-ORD (McCune and Mefford 1999).
As the numbers of taxa and visitations were significantly skewed and could not be transformed to
approach normality, nonparametric statistics were used. Pearson Chi-square was used to compare
differences in proportions of insect functional groups and families between the sites (Analyze-it Software
Ltd., 1997-2008). We also used GLMMs using the GLIMMIX procedure in SAS v. 9.3 (SAS Institute
2010) to test for differences among prairie types (tall grass vs. mixed grass vs. fescue) in floral attributes
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(shape, symmetry, colour), pollinator visits (by functional group), and flower visits (by floral attribute).
GLMMs are appropriate for hierarchical study designs such as here because they retain subsample units
as replicates, but calculate degrees of freedom separately for each nested spatial scale, resulting in
increased power of detection. These models also allow evaluation of both continuous and categorical
predictor variables.
We first assigned each plant species to the applicable category for each floral attribute and then grouped
count data for number of inflorescence and number of flower visits by each floral attribute. Similarly, we
assigned each pollinator species or genus to the applicable functional group and grouped count data by
pollinator functional group. We then converted count data for number of inflorescence by floral attribute,
number of flower visits by floral attribute, and pollinator visits by functional group to proportional values
to reconcile differences in plot size and visit duration within and among sites. We selected flies (Diptera)
and bees (long-tongued and short-tongued) as our pollinator functional groups in analyses as they were
responsible for 96% of all flower visits and so were of primary interest in our study. We differentiated
long- from short-tongued bees as these groups tend to exploit different flower types, and have different
nesting habitats (Fenster et al. 2004; Sheffield et al. 2014).
To determine if flowering communities differ among prairie types, we evaluated the following floral
attributes as response variables: shape (proportion open flowers), symmetry (proportion actinomorphic
flowers), and colour (proportion violet/blue, white, pink, yellow/orange flowers), totalling six models.
For shape and symmetry, we evaluated only one of two available attribute categories because the two
categories as proportions were perfectly correlated within plots. To determine if pollinator communities
differ among prairie types, we evaluated proportion of total flower visits by flies, long-tongued bees and
short-tongued bees (three models). To determine if floral attributes are visited by pollinators differently
(by proportion of total visits) among prairie types, we evaluated proportion of total pollinator visits to
each floral attribute: shape (proportion of visits to open flowers), symmetry (proportion of visits to
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actinomorphic flowers), and colour (proportion of visits to violet/blue, white, pink, yellow/orange
flowers), totalling six models. Prairie type was the predictor variable in the models, with plot treated as a
random, nesting variable.
We also used GLMMs to evaluate how climate and floral attributes influence pollinator visits. Climate
variables (temperature, humidity, annual precipitation) and floral attributes (shape, colour, symmetry)
were included as predictor variables in each model. We also included an interaction term between
temperature and humidity. Our response variables were proportionate of total flower visits by flies, long-
tongued bees and short-tongued bees (three models). Plot was treated as a random nesting variable.
Results
The environmental differences during the survey periods tended to reflect the average conditions that
occur at these sites over time (Table 2). The most northerly site, FPP, had the lowest temperature and
second lowest humidity reflecting its short growing season, and moderate annual precipitation.
Meanwhile, the YQPP was hot, windy and dry, as is typical in a mixed grass prairie. The three tall grass
prairie sites were the most humid, with TGPP having the highest humidity, and BHPP was the warmest.
The CCA ordinations (Figure 2) showed that humidity and annual precipitation were the variables most
strongly correlated with the first axes for both plants (Humid r = 0.980 and Annu prec r = 0.890) and
insects (Humid r = 0.954 and Annu prec r = 0.827). Temperature was the factor most strongly correlated
with axis 2 for the plants (r = -0.699) but elevation (r = -0.763) for insects. Wind speed and frost free
days were not strongly correlated with either axis. Mantel tests for both plants (r = 0.474, p = 0.138) and
insects (r = 0.064, p = 0.370) indicate that there is a positive (but not significant) association between the
species and environmental matrices. Further, Mantel tests show a positive (but not significant)
association between plants (r = 0.475, p = 0.138) and insects (r = 0.06, p = 0.370) with respect to spatial
differences. Thus sites closer together geographically were no more similar in terms of their vegetation
and insect composition than could be expected by chance, so variables other than distance are influencing
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composition. The sites that had the most similar insect-pollinated plant and insect composition were the
fescue and mixed grass prairie sites: FPP and YQPP (Table 3). The tall grass prairie sites that were most
similar were LPM and TGPP with respect to plants, and LPM and BHPP with respect to insects.
Regarding insects, TGPP was strongly dissimilar to the other sites because there were quite a few insect
taxa (mostly flies) that were present only at that site, or in unusually high quantities.
YQPP had the highest number of plant species surveyed and visited although the percentage of plants
visited was not the highest; LPM had the highest percentage (Table 4). Not all insects could be identified
to species due to varying expertise and lack of good taxonomic keys for some taxa. The percentage of
taxa identified to species ranged from 76% at FPP to only 46% at TGPP; this was because bees (which
are easier to identify) were more abundant at the former site and flies at the latter. The FPP also had the
highest number of insect visitor taxa; 62 more than observed at the site with the lowest number, TGPP.
The connectance of the plant-insect visitor system was highest at TGPP and lowest at LPM. Of the 347
insect taxa observed, only two were seen at all five sites: Bombus ternarius Say, 1837 and B. rufocinctus
Cresson, 18631. Five taxa were observed at four of the sites, 14 at three sites and 32 at two sites. Eight
taxa (three Bombus spp. Latreille, 1802, two Andrena spp. Fabricius, 1775, one Lasioglossum sp. Curtis,
1833 and two Syrphidae) were seen at only one site but made more than 1% of all visits. The five insect
species with the highest mean percentage of flower visits were: Bombus ternarius (18.8%), Toxomerus
marginatus (Say, 1823) (5.9%), Odontomyia pubescens Day, 1882 (3.4%), Andrena asteris Robertson,
1891 (3.3%) and B. griseocollis (De Geer, 1773) (3.1%).
In total, 84 plant species in 24 different families were observed being visited with the majority being in
the Asteraceae (27 species) followed by Fabaceae (13 species) and Rosaceae (12 species). All insect taxa
that were observed visiting the plants, even once, were recorded2. The five plants with the highest total
1 Table S12 Table S2, S3
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number of insect visitor taxa, in decreasing order were: Solidago nemoralis Ait. (88), Solidago rigida L.
(75), Erigeron glabellus Nutt. (60), Solidago canadensis L. (50) and Symphoricarpos occidentalis Hook.
(44). Long-tongued bees were the dominant visitors (i.e. making > 50% of all visits) to 15 plant species.
Two species (Amorpha nana Nutt. and Prunus pumila L.) were only visited by long-tongued bees, three
species were only visited by long-tongued bees and Lepidoptera (Hedysarum boreale Nutt., Lathyrus
venosus Muhl. ex. Willd. and Pediomelum esculentum (Pursh) Rydb.) and Geum triflorum Pursh was only
visited by bees. Short-tongued bees were the dominant visitors to three plant species (Antennaria
parvifolia Nutt., Heuchera richardsonii R.Br. and Hypoxis hirsuta (L.) Coville), and Lepidoptera to two
species (Liatris punctata Hook. var. punctata and Viola pedatifida G. Don). Diptera were the dominant
visitors to 27 plants with four species being visited only by flies: Anticlea elegans (Pursh) Rydb.,
Dasiphora fruticosa (L.) Rydb., Prunella vulgaris L. and Ranunculus rhomboideus Goldie var. multifidus
Nutt. A wasp (i.e. Other Hymenoptera) was the only visitor observed to Amelanchier alnifolia (Nutt.)
Nutt. ex Roemer. The remaining 35 plant species were visited by a variety of functional groups.
Additionally, 23 surveyed plant species were not observed being visited by any insects3.
Although the percentage of long-tongued bee taxa was low at all of the sites (Figure 3), this functional
group was responsible for the majority of the visits at three sites: FPP, YQPP and LPM. However, at
BHPP and TGPP the majority of the visits were by flies. GLMM confirmed that long-tongued bee visits
were significantly different amongst all prairie types with the highest proportion of visits occurring in the
fescue prairie (i.e. FPP) (Table 5). Further, flower visits by flies were significantly higher in tall grass
prairie (i.e. BHPP, LPM and TGPP sites) than in the other two prairie types which had a similar
proportion of visits. Visits by short-tongued bees were not significantly different among the prairie types.
There were some compositional differences with respect to the bee families that were most common at
each site (Figure 4). Apidae was the dominant bee family at all sites. Andrenidae were responsible for
3 Table S4
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more than 20% of all bee visits at the three tall grass prairie sites but less than 10% at the others.
Halictidae were frequently observed at TGPP but not so much at the other sites. No Megachilidae were
observed at either LPM or TGPP.
Fly family composition was also different (Figure 5). Bombyliidae were most common at the two of the
drier sites (i.e. YQPP and BHPP) but infrequently seen at TGPP. Syrphidae were observed at all sites but
most frequently at LPM and TGPP. Flies in the Stratiomyidae and other fly families (e.g. Anthomyiidae
and Scatopsidae) were responsible for about half of all fly visits at TGPP. Muscidae were the most
frequently observed fly family at YQPP and Tachinidae at FPP.
The proportion of visits by different trophic groups of insects (i.e. aquatic, and terrestrial herbivores,
omnivores and saprovores) varied among the sites (Figure 6). The majority of insect visitations at all but
one site were by terrestrial herbivores (mainly bees and butterflies). At TGPP, however, almost half of all
insect visitations were by insects with an aquatic larval stage. Insects that were terrestrial saprovores in
the larval stage were also most commonly seen at TGPP. Insects that were terrestrial omnivores (i.e.
carnivorous in the larval stage and herbivorous in the adult stage) were most frequently seen at BHPP.
The results of GLMM show that the proportion of inflorescences with open versus tubular flowers,
actinomorphic versus zygomorphic flowers and in different colour groups was significantly different (p <
0.05) between some of the prairie types (Tables 6)4. The proportions of insect visits to plants of these
different types differed significantly among prairie types except with respect to visits to open flowers
which were the same at all sites. Tubular flowers were more common and visited more frequently at the
fescue and mixed grass prairie than at the tall grass prairie. Zygomorphic flowers were most common and
visited most frequently at the mixed grass prairie. Plants with violet/blue and white flowers were more
common at mixed grass prairie than at tall grass prairie; fescue prairie had more white flowers but the
4 Table S5
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same number of violet/blue flowers as tall grass prairie. Insect visits to violet/blue and white flowers
were higher at the fescue and mixed prairie than at tall grass prairie. Instead yellow/orange-flowered
plants were more common at tall grass prairie, receiving more visits than at the mixed grass prairie.
Visits by all three functional groups increased with increasing temperature (Table 7). Further, a weak
positive interaction between temperature and humidity (r = 0.075) had a negative influence on fly and bee
visits, indicating that the importance of temperature decreased as humidity increased. High humidity
positively affected both fly and short-tongued bee visits but high annual precipitation only increased fly
visits. Long-tongued bee visits were unaffected by both humidity and annual precipitation. A higher
proportion of actinomorphic flowers increased fly and short-tongued bee visits and decreased long-
tongued bee visits. Fly visits also increased with a higher proportion of open, and white and
yellow/orange flowers while long-tongued bee visits increased with an increasing proportion of all flower
colours.
Only 13 of the 84 plant species surveyed were found at all five sites and only five species were actually
observed being visited at all sites (i.e. Campanula rotundifolia, Dalea purpurea var. purpurea,
Lithospermum canescens (Michx.) Lehm., Solidago rigida, Symphyotrichum ericoides (L.) Nesom)5. An
additional 11 plant species were found at four of the five sites, 14 species at three sites and 13 species at
two sites. Visitation rates to plant species found at most of the sites were sometimes quite different. For
example, open, actinomorphic-flowered plants like Achillea borealis Bong. var. borealis, Comandra
umbellata (L.) Nutt., Solidago rigida, Symphyotrichum laeve (L.) A. Love & D. Love and Zizia aptera
(Gray) Fern. tended to have high visitation rates at the tall grass prairie sites but low rates at the fescue
and mixed grass prairie sites while the opposite was true of zygomorphic and/or tubular plants like
Astragalus agrestis Dougl. ex G. Don., Geum triflorum and Pediomelum esculentum.
5 Table S6
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Discussion
The province of Manitoba, Canada has three distinctive prairie types—fescue, mixed grass and tall
grass—providing an excellent opportunity to assess how plant-pollinator communities in these
ecosystems differ. Our research shows that sites within the same prairie type were not necessarily the
most similar in terms of their plant and insect community composition: the three tall grass prairie sites
were more dissimilar than the fescue and mixed grass prairie sites were to each other. Of the
environmental variables we used in the CCA, humidity and annual precipitation appeared to have the
greatest influence on overall plant and insect composition. At the sites with the highest humidity and
precipitation (i.e. LPM and TGPP), flies were responsible for most of the flower visits and at sites with
the lowest humidity and precipitation (i.e. YQPP and FPP) bees (mostly long-tongued) were responsible
for most of the visits, supporting the xeric hypothesis. However, although bees were more common at the
mixed and fescue prairie, differences in temperature and moisture do not seem to be the reason why. The
GLMM showed that bees were not influenced by annual precipitation. Further, high humidity did not
affect long-tongued bee visits and short-tongued bees responded positively (not negatively as predicted)
to it, similar to flies. Instead, the factor that seemed to affect long-tongued bees the most was the
abundance and type of forage available; the presence of flowers of any colour, but particularly
zygomorphic ones, resulted in more long-tongued bee visits. Other researchers have found that flower
richness and density strongly influence long-tongued bee visitation (Kevan and Baker 1983; Rathcke
1983; Stang et al. 2006; Ebeling et al. 2008) and, as Villalobos and Vamosi (2018) suggest, this could
outweigh the impact of climate on bee visitation in some regions. Another reason this may be is because
Bombus species are noted to be particularly well adapted to foraging under humid conditions (Villalobos
and Vamosi 2018); this genus was the dominant long-tongued bee in our region. With respect to short-
tongued bees, Michener (1979) noted that some families (i.e. Andrenidae and Halictidae) are tolerant of
mesic conditions so our results are not wholly unexpected. Temperature positively affected all three
functional groups likely because most pollinators are not active at cool temperatures due to
thermoregulation limits, and temperatures do not get hot enough in this region to inhibit foraging
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behaviour (Kevan and Baker 1983; McCall and Primack 1992). Although our results roughly parallel
those of Devoto et al. (2005, 2009) and Kearns (1992) who found that flies dominated the wetter portion
of an environmental gradient and bees the drier portion, a clear negative effect of moisture on bees was
not observed within the Prairie Ecozone, although it may explain differences between ecozones where
climatic difference are greater.
The mechanism(s) by which moisture influences pollinator communities is likely via breeding habitat. In
a direct way, moisture can affect the availability of larval habitat for flies. Insects with aquatic larvae or
that preferred saturated soil and/or rotting vegetation to breed in (e.g. Stratiomyidae) were seen most
frequently at the wettest site (i.e. TGPP) but were scarce at drier sites (e.g. BHPP, YQPP). Moisture may
have also affected some long-tongued bee genera (but not the group as a whole). Previous research on
Megachile rotundata (Fabricius 1787) in Manitoba found that pollen ball cells were increasingly infected
with fungus as humidity increased (Pitts-Singer and James 2008). The absence of Megachilidae during
our surveys at LPM and TGPP may have been due to high humidity in 2004 and 2005 as species in this
family have been collected by other scientists at these sites in later years (Semmler 2016).
One non-climatic factor that could have affected some pollinating flies is prey abundance. Bee flies
(Bombyliidae) and parasitic flies (Tachinidae) typically parasitize bees and Lepidoptera (Marshall 2012),
insects that were not seen frequently at LPM and TGPP. Bee flies were most frequently observed at the
two sites where bees were common: BHPP and YQPP. Parasitic flies (Tachinidae), which tend to
parasitize Lepidoptera more than bees, were frequently seen at FPP possibly because this site had the
most floral visits by Lepidoptera, and was in close proximity to intact boreal forests in nearby Riding
Mountain National Park, where forest caterpillars are often abundant (Kuussaari et al. 2007). Syrphids,
whose larvae are usually sapro- or phyto-phages or carnivorous on aphids, were common at all sites; this
may be because syrphid populations are noted to be more stable than those of other terrestrial arthropods
(Owen and Gilbert 1989). Through these data we show that flies cannot be considered one cohesive
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group that responds in the same way to environmental differences, similar to other studies which also
noted a lack of a consistent pattern (Kearns 1992; Lambkin et al. 2011).
Another non-climatic factor that could have affected the plant and pollinator compositions was land
management (Stoner and Joern 2004; Kimoto et al. 2012; Lazaro et al. 2016). Insects that prefer bare
ground for nesting (e.g. Halictidae and Andrenidae) and those that require a blood meal to reproduce (e.g.
Stomoxys calcitrans (L., 1758), may directly benefit from the presence and grazing activities of livestock
(Potts and Willmer 1997; Kimoto et al. 2012; Marshall 2012). The most recently and consistently grazed
site, YQPP, had high populations of Muscidae which may have been breeding in dung or biting the
livestock or wild elk in the area. Grazing by large herbivores also influences plant composition
(Augustine and McNaughton 1998; Olff and Ritchie 1998; Towne et al. 2005; Lazaro et al. 2016). For
example, Cerastium arvense L., a species that needs light for seed germination (Willms et al. 1985, 1988),
was likely more abundant at YQPP due to grazing which removed leaf litter, resulting in a high number of
visits to white flowers at this site. Further, as grazing did not begin at this site until June, later season
plants, such as Solidago spp., were more likely to be eaten or trampled.
The habitat filtering hypothesis (Sargent and Ackerly 2008) predicts that the dominant pollinators will
influence the types of plants in the community by acting “as a sieve that filters which species can
establish and persist” (Pellissier et al. 2012). Thus we expected that sites where bees were the dominant
pollinators would contain more plants they are known to favour in terms of colour, shape and symmetry
than fly-dominated sites. Regarding colour, bee preference for violet/blue flowers and fly preference for
white and yellow flowers has been observed in some studies (McCall and Primack 1992; Lunau and
Maier 1995). Differences in insect preferences appear to have resulted in differences in the dominant
flower colour as the pollinator population shifts from bee- to fly-dominant along a Rocky Mountain
elevation gradient (Gray et al. 2018). In our study, we also found differences in the dominant flower
colour: violet/blue and white-flowered plants were more abundant at the xeric, mixed grass prairie than at
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the mesic tall grass prairie, while yellow/orange-flowered plants were more common at the tall grass
prairie. Further, the plants were not being visited in proportion to their abundances: visits to violet/blue
flowers were almost double at both the fescue prairie than at the tall grass prairie, despite no significant
difference between flower abundance. Tall grass prairie had the most visits to yellow/orange flowers.
Similarly, zygomorphic flowers were more commonly visited at the drier mixed grass prairie than at the
wetter tall grass prairie. Tubular plants whose nectar can simply not be accessed by smaller, shorter-
tongued insects were also more abundant and visited more frequently at the drier sites. Similar patterns
such as these are noted in other studies (Fenster et al. 2004; Fontaine et al. 2006; Gray et al. 2018) and
appear to reflect differences in the abundance of bees.
Additional support for the habitat filtering hypothesis came from comparison of the visitation rates. Few
plant species were found at all the sites, but when they were widespread, they often differed in terms of
how frequently they were visited by the functional groups of insects. The variation in individual plant
visitation rates between the sites appears to partly reflect the abundance of various insect groups at those
sites. For example, Achillea borealis Bong. var. borealis and Comandra umbellata (L.) Nutt.—both
white, actinomorphic plants—were frequently visited at the tall grass prairie sites where flies were
common pollinators but hardly at all at the FPP and YQPP, which were dominated by long-tongued bees.
In contrast, Astragalus agrestis Dougl. ex G. Don—a purple, zygomorphic plant— and Geum triflorum—
a pink, tubular plant—were frequently visited at the sites with more long-tongued bees, and infrequently
visited at the fly-dominated sites. In habitats lacking bees, seed production is lower for tubular (Fontaine
et al. 2006) and zygomorphic, blue-flowered plants (Bell et al. 2005). The implications of our
observations for reproductive output are therefore intriguing: seed production of tubular, zygomorphic
and/or violet/blue-flowered plants may be lower in tall grass prairie than in fescue or mixed grass prairie,
due to the different pollinator compositions. However, the fact that violet/blue flowers were still present
in fairly similar compositions at all sites suggests that reproduction is still adequate to maintain
population sizes. This may be because many plants can compensate for a lack of pollination services by
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selfing when necessary (Bond 1994; Wilcock and Neiland 2002). Alternatively, since these plants are
perennials, they may be remnants from a previous drier climatic cycle (George and Nielsen 2002;
Markham 2014). Short term changes in soil moisture dynamics have little immediate effect on prairie
plant composition; it may take a decade of consistent wet or dry weather to alter the composition of a
plant community (Fay et al. 2000). Further investigation of variation in seed production among common
species at these sites is needed to help clarify what is happening with respect to reproduction.
There were other floral characters that may have been influencing insect visitation that we could not
assess in this study. Flower colours were described as they are seen by human eyes but UV colours and
patterns are known to attract different insects as well (Kevan and Baker 1983; Woodcock et al. 2014;
Kemp et al. 2018). Scent also varies among plant species and bumblebees are attracted to flowers with a
different scent profile than flies (Junker et al. 2012; Larue et al. 2016). The degree to which these factors
may have affected the insect community is unknown but as pollinators “likely experience floral traits as
multimodal sensory information, rather than solely visual or olfactory clues” (Kantsa et al. 2017), the
variables chosen by us are likely still relevant and interrelated.
It is also important to note that the plant species are being filtered by environmental conditions in a direct
way, not just indirectly via the pollinator community (Vinton and Burke 1997). Plants found in only one
prairie type likely have a more narrow tolerance to moisture or soil conditions. For example, Euthamia
graminifolia (L.) Nutt., found only at LPM and TGPP, is common in moist habitats, such as damp prairies
and riparian areas, and possesses characteristics that help it tolerate saturated soil and crowd out
competitors in such conditions (Werner and Platt 1976). In contrast, species with adaptations for dry
environments (e.g. short stature, tap roots, small leaves, etc.) (Knight 1965) such as Astragalus spp. and
Oxytropis spp. were most common at the mixed and fescue prairies.
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In summary, the plant and pollinator communities were different with respect to the dominant species but
the fescue and mixed grass prairie types were more similar to each other compositionally than the tall
grass prairie sites were to each other. The proportion of visits by flies was higher at tall grass than fescue
or mixed grass prairie and the proportion of long-tongued bee visits was highest at fescue followed by
mixed grass prairie. However, short-tongued bee visits were not different between the sites. Further,
tubular, zygomorphic and violet/blue-coloured flowers, and visits to these flowers were lower at the
prairie type with more flies (tall grass prairie) as hypothesized. Thus the habitat filtering hypothesis
(Sargent and Ackerly 2008) that the dominant pollinators influence the plant community has some
support. However, whether temperature and moisture are the most influential variables on pollinator
composition as suggested by Michener’s (1979) xeric hypothesis varies among functional groups.
Temperature affected all three functional groups positively and although the proportion of fly visits was
positively affected by increased humidity and precipitation, short-tongued bees responded positively (not
negatively as expected) to humidity and long-tongued bees were not affected at all. Rather, the presence
of large quantities of flowers, zygomorphic ones in particular, had the greatest impact on long-tongued
bee visits. These mixed results suggest that at least within biomes, floral composition may be a more
important influence on bees than climate. The xeric hypothesis may only be applicable at larger
geographic scales.
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Acknowledgements
Financial support for this research was received from The Manitoba Museum Foundation Inc., Nature
Conservancy of Canada (NCC), World Wildlife Fund Canada, Government of Manitoba, and the Science
and Technology Internship Program and Virtual Museum of Canada, Government of Canada. Special
thanks to Heather Flynn, Robert Wrigley, Sarah Semmler, Reid Miller and Jason Gibbs for preparation
and/or identification of insect specimens. The Government of Manitoba, NCC, the City of Winnipeg and
Nature Manitoba graciously allowed us to conduct research on their lands. Dr. Anne Worley and Sarah
Semmler provided valuable comments on an early draft of this paper and Dr. Rob Currie provided advice
regarding statistical analysis. Lastly, thanks to Manitoba Museum and Nature Conservancy of Canada
staff and volunteers for their assistance, especially Dana Krueger for her work on the supplementary
materials.
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31 [accessed 2 May 2018].
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Table 1. Location, environmental* and land management information on the five prairie study sites.
Site Prairie
type
Ecoregion Surficial geology Soil order Elevation
(m)
Annual
precip.
(mean mm)
Frost free
days
(mean #)
Land management
during survey years
Fescue Prairie
Preserves
Fescue Boreal
Transition
Glacial till,
sand/silt
Chernozemic 610 510 96-105 Ungrazed † and
unburned
Yellow Quill
Prairie Preserve
Mixed
grass
Aspen
Parkland
Glaciolacustrine,
sand/silt
Chernozemic 409 474 106-115 Grazed and
unburned
Birds Hill
Provincial Park
Dry tall
grass
Lake Manitoba
Plain
Glaciofluvial,
gravel/silt/sand
Chernozemic 265 550 116-125 Ungrazed and
unburned
Living Prairie
Museum
Mesic
tall
grass
Lake Manitoba
Plain
Glaciofluvial,
clay/silt/sand
Vertisolic 239 521 116-125 Ungrazed; two
2004 plots on
burned prairie
Tall Grass Prairie
Preserve
Wet tall
grass
Lake Manitoba
Plain
Glacial till,
gravel/clay/silt/
sand
Gleysolic 301 581 116-125 Ungrazed and
unburned
* Ecological Stratification Working Group 1995, Environment Canada 2018
† Six of the plots had been grazed the year before the study began (2013).
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Table 2. Survey information and mean weather conditions during the survey periods at each study site.
Site Survey
year
Survey
months
Total area
surveyed (m2)
Sampling
effort (hrs)
Weather station
(km & direction
from study site)*
Temperature
(mean ºC±SE)
Humidity
(mean ±SE)
Wind speed (mean
km hr-1 ±SE)
Fescue Prairie
Preserves
2014 Jun-Sep 72 48 Rossburn 4 North
(~21 km S)
16.4±1.6 56.5±3.1 10.8±0.9
2015 Jun-Sep 72 48 18.9±0.6 49.6±1.8 9.2±0.8
Yellow Quill
Prairie Preserve
2016 Jun-Sep 96 64 Brandon (~27 km
NW)
20.7±0.8 58.9±1.7 16.7±1.6
2017 May-Aug 96 64 23.6±1.0 47.3±4.4 19.9±2.3
Birds Hill
Provincial Park
2008 Aug-Sep 100 35 Winnipeg The
Forks (~22 km S)
19.9±0.6 54.1±1.8 22.8±1.5
2010 Jul-Aug 100 32 24.8±0.6 64.6±1.6 15.8±0.8
2011 Jun-Jul 100 32 23.1±0.4 51.2±1.7 20.7±1.1
Living Prairie
Museum
2004 Jun-Sep 150 48 Winnipeg
Richardson Int’l
Airport (~4 km E)
20.9±0.9 58.5±3.8 17.9±2.1
2005 Jun-Sep 150 48 19.6±1.1 57.9±2.8 16.9±1.9
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Tall Grass Prairie
Preserve
2004 Jun-Sep 150 48 Emerson (~36 km
NE)
17.3±1.2 61.1±3.4 13.6±1.5
2005 Jun-Sep 150 48 20.1±1.2 68.3±4.9 15.3±1.6
* Environment Canada
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Table 3. Sørenson similarity of the plant and insect communities between five study sites.
Sørensen Similarity
Plant Community Insect Community
Prairie Type Site FPP YQPP BHPP LPM TGPP FPP YQPP BHPP LPM TGPP
Fescue prairie Fescue Prairie
Preserves (FPP) 0 0.307 0.349 0.355 0.450 0 0.201 0.442 0.340 0.777
Mixed grass
prairie
Yellow Quill Prairie
Preserves (YQPP) 0.307 0 0.411 0.456 0.526 0.201 0 0.436 0.332 0.760
Tall grass prairie Birds Hill Provincial
Park (BHPP) 0.349 0.411 0 0.380 0.416 0.442 0.436 0 0.322 0.603
Living Prairie Museum
(LPM) 0.355 0.456 0.380 0 0.316 0.340 0.332 0.322 0 0.625
Tall Grass Prairie
Preserve (TGPP) 0.450 0.526 0.416 0.316 0 0.777 0.760 0.603 0.625 0
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Table 4. Number of plant and insect taxa surveyed at each site and connectance data.
Plant taxa Insect taxa
Site Surveyed
(#)
Visited
(# = p)
Visited
(%)
Identified to
species
[# (%)]
Identified
to genus
[# (%)]
Identified to
family/order
[# (%)]
Total
(# = a)
Interactions
(I)
Connectance*
Fescue Prairie Preserves 58 47 81 83 (76) 20 (18) 7 (6) 110 469 9.07
Yellow Quill Prairie Preserve 63 49 78 65 (66) 17 (17) 16 (17) 98 377 7.85
Birds Hill Provincial Park 35 26 74 62 (68) 16 (18) 13 (14) 91 204 8.62
Living Prairie Museum 39 32 82 37 (55) 20 (30) 10 (15) 67 159 7.42
Tall Grass Prairie Preserve 46 33 72 27 (46) 23 (40) 8 (14) 58 204 10.66
* Connectance (C = 100I / ap)
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Table 5. Differences among prairie type in proportional representation of insect visits by three functional
groups.
Functional Group Comparison Estimate SE P
Lower
CL
Upper
CL
Long-tongued Bees Fescue vs. Tall grass 20.037 2.096 0.0002 14.650 25.424
Mixed grass vs. Tall grass 13.593 1.845 0.0007 8.851 18.335
Fescue vs. Mixed grass 6.424 2.153 0.0306 0.890 11.957
Short-tongued Bees Fescue vs. Tall grass -2.172 1.376 0.1753 -5.711 1.366
Mixed grass vs. Tall grass 0.303 1.216 0.8131 -2.823 3.429
Fescue vs. Mixed grass -2.472 1.435 0.1455 -6.161 1.217
Diptera Fescue vs. Tall grass -13.991 2.055 0.0010 -19.273 -8.708
Mixed grass vs. Tall grass -13.876 1.805 0.0006 -18.517 -9.234
Fescue vs. Mixed grass -0.115 2.135 0.9590 -5.603 5.372
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Table 6. Mean proportion of inflorescences and insect visits to plants according to flower shape, symmetry and colour at three prairie types.
Prairie type Flower shape (%)* Flower symmetry (%) Flower colour (%)
Open Tubular Actinomorphic Zygomorphic Violet/blue White Pink Yellow/orange
Fescue prairie Inflorescences 54.6 a 45.4 a 80.5 a 19.5 a 34.1 ab 29.4 a 11.1 a 25.5 a
Mixed grass prairie 68.0 b 31.7 b 72.9 b 26.8 b 36.7 a 38.6 b 6.2 b 18.1 b
Tall grass prairie† 70.4 b 25.2 c 77.4 ab 18.2 a 30.6 b 19.2 c 12.4 a 33.4 c
Fescue prairie Insect visits 41.6 31.1 a 56.4 a 16.2 a 30.9 a 15.9 a 3.9 n/a 21.9 ab
Mixed grass prairie 45.5 19.7 b 43.8 b 21.5 b 26.5 a 16.0 a 4.3 n/a 18.5 b
Tall grass prairie† 48.7 12.6 c 53.0 ab 8.2 c 15.9 b 10.1 b 7.5 n/a 27.7 a
* Lowercase letters indicate statistically significant (p < 0.05) differences in proportions among sites using Generalized Linear Mixed Models.
† Includes data from Birds Hill Provincial Park, Living Prairie Museum and Tall Grass Prairie Preserve sites.
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Table 7. Results of the impact of environmental conditions and proportion of flower types on insect visits by three functional groups.
Pollinator Visits Predictor Variable Estimate SE P Lower CL Upper CL
Diptera Temperature 1.6626 0.28 <.0001 1.1135 2.2118
Humidity 0.6267 0.1347 <.0001 0.3626 0.8908
Temperature*Humidity -0.03063 0.006563 <.0001 -0.0435 -0.01776
Annual precipitation 0.2022 0.03343 <.0001 0.1367 0.2678
Proportion open 0.1108 0.02259 <.0001 0.06649 0.1551
Proportion actinomorphic 0.09434 0.03487 0.0069 0.02597 0.1627
Proportion blue 0.09435 0.05918 0.111 -0.0217 0.2104
Proportion white 0.1424 0.0669 0.0334 0.0112 0.2736
Proportion pink -0.03823 0.07064 0.5884 -0.1768 0.1003
Proportion yellow 0.1711 0.06867 0.0128 0.03645 0.3058
Long-tongued Bees Temperature 0.6812 0.2853 0.017 0.1218 1.2406
Humidity 0.02665 0.1369 0.8457 -0.2418 0.2951
Temperature*Humidity -0.01564 0.006687 0.0194 -0.02875 -0.00253
Annual precipitation -0.02629 0.03446 0.4456 -0.09385 0.04128
Proportion open -0.03895 0.02307 0.0915 -0.08419 0.006289
Proportion actinomorphic -0.1464 0.03565 <.0001 -0.2163 -0.0765
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Proportion blue 0.3244 0.06081 <.0001 0.2051 0.4437
Proportion white 0.233 0.06849 0.0007 0.0987 0.3673
Proportion pink 0.3129 0.07258 <.0001 0.1706 0.4553
Proportion yellow 0.3003 0.07067 <.0001 0.1617 0.4389
Short-tongued Bees Temperature 0.5638 0.1935 0.0036 0.1844 0.9432
Humidity 0.2039 0.09287 0.0283 0.02172 0.386
Temperature*Humidity -0.01087 0.004535 0.0166 -0.01976 -0.00198
Annual precipitation 0.007411 0.02314 0.7488 -0.03797 0.05279
Proportion open -0.00685 0.01563 0.6615 -0.03749 0.0238
Proportion actinomorphic 0.05784 0.02414 0.0166 0.01051 0.1052
Proportion blue 0.05862 0.04098 0.1527 -0.02174 0.139
Proportion white 0.04196 0.04631 0.3649 -0.04885 0.1328
Proportion pink 0.07119 0.04891 0.1457 -0.02473 0.1671
Proportion yellow 0.06395 0.04761 0.1794 -0.02943 0.1573
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Figure 1. Location of the five study sites in southern Manitoba. Copyright © 2001-2018, the Government
of Manitoba. All rights reserved. The Government of Manitoba is the owner of copyright in all
information contained in this Manitoba Land Initiative, unless otherwise noted.
Figure 2. Canonical Correspondence Analysis (CCA) joint plots of (a) plant, and (b) insect family
proportions sampled at five sites (triangles): Birds Hill Provincial Park (BHPP), Fescue Prairie Preserves
(FPP), Living Prairie Museum (LPM), Tall Grass Prairie Preserve (TGPP), and Yellow Quill Prairie
Preserve (YQPP). Lines represent environmental variables: average annual precipitation (Ann prec),
elevation (Elevatio), average humidity during survey days (Humid) and average temperature during
survey days (Temp).
Figure 3. Percentage of (a) all taxa, and (b) all visitations in six functional groups of insects at the five
study sites. Pearsons χ2 (20, N = 500) = (a) 31.83 (p = 0.04), and (b) 144.75 (p<0.0001) indicated these
proportions were significantly different.
Figure 4. Percentage of all bee visitations by long- (Apidae, Megachilidae) and short-tongued bees
(Andrenidae, Colletidae and Halictidae) at the five study sites. Pearsons χ2 (20, N = 500) = 116.35,
p<0.0001 indicated these proportions were significantly different.
Figure 5. Percentage of all visitations by the five dominant families of flies (Diptera) and all other
families, at the five study sites. Pearsons χ2 (20, N = 500) = 324.24, p<0.0001 indicated these proportions
were significantly different.
Figure 6. Percentage of all visitations by different trophic groups at the five study sites. Aquatic species
may have been herbivores, omnivores or carnivores on microorganisms. Pearsons χ2 (12, N = 500) =
186.58, p<0.0001 indicated these proportions were significantly different.
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a)
b)
Tall grass prairie
Fescue prairie
Mixed grass prairie
Tall grass prairie
Mixed grass prairie
Fescue prairie
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10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
Long-tongued bees Short-tongued beesOther Hymenoptera DipteraLepidoptera Coleoptera/Hemiptera
Inse
ct ta
xa (%
)a)
Fescue Prairie Preserves
Yellow Quill Prairie Preserve
Birds Hill Provincial Park
Living Prairie Museum
Tall Grass Prairie Preserve
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
Inse
ct v
isita
tions
(%)
Sites
b)
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DraftFescue Prairie
PreservesYellow Quill
Prairie Preserve
Birds Hill Provincial
Park
Living Prairie Museum
Tall Grass Prairie
Preserve
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
Halictidae
Colletidae
Andrenidae
Megachilidae
ApidaeMea
n in
sect
vis
itatio
ns (%
)
Sites
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DraftFescue Prairie
Preserves
Yellow Quill Prairie
Preserve
Birds Hill Provincial
Park
Living Prairie Museum
Tall Grass Prairie
Preserve
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
Other
Tachinidae
Syrphidae
Stratiomyiidae
Muscidae
Bombyliidae
Mea
n in
sect
vis
itatio
ns (%
)
Sites
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DraftFescue Prairie
Preserves
Yellow Quill Prairie
Preserve
Birds Hill Provincial
Park
Living Prairie
Museum
Tall Grass Prairie
Preserve
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
Terrestrial saprovore
Terrestrial omnivore
Terrestrial herbivore
AquaticMea
n in
sect
vis
itatio
ns (%
)
Sites
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