2
Abstract
Many invasive plants are equipped with a suite of mechanisms that can allow them to
out-compete native plants. Recently, there has been growing recognition that invasive plants may
affect native plants by luring away shared pollinators. The goal of this research was to explore
the properties that make invasive plants strong competitors for pollination services from insects
and birds. I addressed the question: How commonly do invasive plants offer more attractive
nectar and pollen rewards than their native counterparts? I examined quality and quantity of
floral rewards from native-invader pairs by measuring nectar production rate, nectar sugar
concentration, pollen grain volume, total amount of pollen, and pollen protein content. I found
that for the response variables of amount of pollen and nectar sugar concentration, most of the
invasive plants in this study were more rewarding than their native counterparts. To assess
whether invasive plants offering more rewarding nectar and pollen than natives is a general
phenomenon will require the study of a broader selection of species pairs.
Key Words: pollinators, mating system, competition, preference, nectar, pollen
3
Introduction
Invasive species are a leading component of environmental change and a second leading
cause of biodiversity loss worldwide (Houlahan and Findlay 2004). Invasives by definition are
species establishing in the wild beyond their natural distribution ranges following transportation
of whole plants or propagules by humans or human related activities (Mooney and Hobbs 2000)
and are typically associated with negative impacts on the environment, human activities, or
human health (Lee 2002). Invasive plants infest agricultural fields and reduce crop yields and
cattle forage (Pimentel et al. 2000), restrict recreational access to infested areas, increase soil
erosion, and form dense stands that dry and become fire hazards (Erickson and White 2007).
Many invaders are equipped with a suite of mechanisms that help them out-compete native
plants. These include resistance to native herbivores and disease as well dominating nutrients and
space (Ledger and Forister 2005). There is also growing recognition that invasive plants can
affect native plants by luring away shared pollinators (reviewed in Bjerknes et al. 2007);
however, the mechanisms driving such changes in pollinator behavior are not well explored.
Several studies suggest that certain invasive plants may be more superior competitors than
similar native plants because they offer more attractive floral rewards for pollinators. Chittka and
Schürkens (2001) found that the invasive forb Impatiens glandulifera (Balsaminaceae) offered
significantly more nectar than the native and morphologically similar Stachys palustris
(Lamiaceae), a trait which correlated with reduced pollinator visitation and subsequent seed set
of co-occurring S. palustris. Similarly, Brown et al. (2002) observed that the highly invasive
Lythrum salicaria (Lythraceae) has larger and more numerous flowers, and greater nectar and
pollen rewards, than native Lythrum alatum, making the invasive a putatively more attractive
plant than its native congener. Furthermore, a study of invasive Linaria vulgaris (Plantaginaceae)
4
demonstrated that it may be especially attractive to pollinators because its nectar has a higher
sugar concentration than that of many native plants (Irwin, unpublished data). While these
individual studies suggest that increased floral rewards may be a general pattern among invasive
species compared to natives, a broad comparative investigation of pollen and nectar in native and
invasive plants has yet to be done.
The purpose of this project was to explore nectar and pollen characteristics that make
invasive plants strong competitors for the pollination services of insects and birds. I addressed
the question: How commonly do invasive species offer more rewarding nectar and pollen than
their native counterparts? I examined floral rewards (nectar and pollen quantity and quality) from
several native and invasive plants. Plants were organized into invasive-native species pairs based
on phylogenetic relatedness or similar morphology. Specifically, I quantified nectar and pollen
rewards by measuring: number of flowers per plant, nectar production rate per flower, nectar
sugar content, pollen grain size, pollen production per flower, and collected pollen samples for
future nitrogen content analysis (to measure protein content).
I predicted that floral rewards would correlate with the mating system of plants because
plants can control their mating opportunities by manipulating the behavior of pollinators (Barrett
2003). Plants that are self-incompatible and thus obligate outcrossers require pollination from a
genetically different individual of the same species and are dependent on wind, water, or insects
to spread pollen. I hypothesized that successful invasive species that are self-incompatible (Table
1) and rely on pollinators for reproduction would show increased floral rewards compared to
native plants with similar mating and pollination systems. In contrast, plants that are selfcompatible and that can self-pollinate do not require a means to transport pollen. Therefore I
5
predicted that there would not necessarily be a strong difference between invasive plants that are
self-compatible (Table 1) and similar self-compatible native species.
Methods
Location. This study was conducted in June through August 2010 at the Rocky Mountain
Biological Laboratory (RMBL) in Gothic, Gunnison County, Colorado, USA. The RMBL is
located roughly 13 kilometers from the town of Crested Butte, CO which is commonly
recognized as the wildflower capital of the world, so there were a wide variety of flowering
plants to select from. Samples were collected from 19 locations in Gunnison County including
the Gothic town site, Slate River Road, Washington Gulch, and Cement Creek (Table 2). I
sampled from sites where both the native and invasive plants occurred together as often as
possible.
Taxon sampling. I selected four invasive species from the Colorado Noxious Weeds List
(Colorado Department of Agriculture) that are common to the Gunnison basin (Taylor 1999).
They represented four different families: Boraginaceae, Brassicaceae, Convolvulaceae, and
Plantaginaceae. I paired each invasive species with a native plant that often co-occurred with it
and also shared similar flower size, color, shape, and flowering phenology. I selected invasive
and native plants that were con-familial whenever possible. One invasive species, Linaria
vulgaris, was part of a triplet with two native species since two familial matches existed (Table
3). Two of the invasive species that I selected are known to be self-incompatible and two are
self-compatible. Many of the native plants that were used in this study have not been studied
6
extensively and thus their mating system is unknown (Table 1). For each study species, I aimed
to collect samples from at least three sites, ten plants per site (Table 2).
Mean floral display. I counted the number of open flowers on 10 plants at three different
sites for a total of 30 plants for each species. This was done within about one week of peak
flowering time for each species. For Linaria vulgaris, I counted the number of flowers per ramet
because ramets are connected by underground rhizomes and the boundary of an individual plant
was unfeasible to determine. Mean floral display size was used to estimate the nectar and pollen
rewards available to pollinators on a per-plant basis (see below).
Nectar production rate. For the same plants that I counted the number of open flowers, I
used a microcapillary tube to drain the standing crop of nectar from all of the flowers. Often, the
standing crop of nectar in flowers was already zero if pollinators had already visited the plant
that day. I then covered the entire plant with a mesh bag to exclude pollinators and returned
approximately 24 hours later to measure the volume of nectar that had been produced in three
flowers on each plant using a microcapillary tube and calipers. This was done for a total of
approximately 90 flowers per species (3 flowers x 10 plants x 3 sites). I always measured nectar
between 0900 and 1100 to control for potential variation in nectar production rate that can occur
throughout the day. I did not take nectar measurements on mornings after it had rained because
water pooling in flowers changes nectar measurements. I also did not measure nectar production
rate or nectar sugar concentration (described subsequently) in Barbarea vulgaris or Draba
because the flowers were too small to measure nectar given the field analytical techniques I used.
Nectar sugar concentration. I used the nectar that was collected for nectar production rate
measurements and a light refractometer to measure the refractive index in the field. The
7
refractive index of nectar is used as a measure of sucrose equivalents (Kerns and Inouye 1993).
For nectar production rates that were too small, I could not get concentration estimates. Nectar
was usually pooled from all of the flowers on a plant in order to get enough volume to get a
reading of sugar concentration. This was done for about 30 plants per species (10 plants x 3
sites).
Amount of sugar in nectar. The raw amount of sugar, in contrast to the percent sugar, is an
important to consider because it is liberated from the natural variation that comes from
differences in temperature and humidity. Percent sugar changes as volume of nectar changes and
nectar volume is sensitive to rain and evaporation. The amount of sugar (mg sugar/flower and
mg nectar/plant) is a combination of the mean nectar production rate for a plant and the nectar
sugar concentration. When these measurements are simply multiplied together, there is an
increasingly large error at high percentages (Bolten et al. 1979). The percent sugar that was
measured with a refractometer was converted to mg sugar/100 mL nectar using a conversion
table that accounts for this error (Kerns and Inouye 1993). The mg sugar/100 mL nectar was
multiplied by the nectar production rate volume measurements (μl nectar/flower and total μl
nectar/plant) to calculate the raw amount of sugar produced in 24 hours (mg sugar/flower and mg
sugar/plant).
Pollen quantity. For 10 plants at the same sites used for nectar measurements, I removed all
of the anthers from three buds per plant that were near full maturity. Buds were used instead of
open flowers so that anthers were fully developed but not yet dehiscing. I dried the anthers in
mircocentrifuge tubes for at least two weeks. I added 1500 uL of ethanol to each sample and
sonicated it for five minutes to release any remaining pollen from the anthers. Once the pollen
grains were in suspension using a vortex, I counted the number of pollen grains in a five uL
8
subsample with a hemocytometer under a dissecting microscope (Kerns and Inouye 1993). I
counted four subsamples for the pollen collected from each flower and used the average to
calculate the number of pollen grains for that flower. I attempted to get pollen counts for 90
flowers per species (3 flowers x 10 plants x 3 sites) but actually only counted a total of 582
flowers.
Pollen volume. Pollen samples came from several different individual plants from at least 3
different locations. To measure average pollen grain volume for each species, I followed
methods used in several other studies (da Silveria 1999; O’Rourke and Buchmann 1991;
Roulston et al. 2000). I stained pollen samples on slides with basic fuchsin dye (Kearns and
Inouye 1993). I measured the polar and equatorial aspects of pollen grains with an ocular
micrometer under a compound microscope at 400x magnification. This was done for
approximately 10 haphazardly encountered noncollapsed pollen grains per slide for a total of 10
slides (Figure 1). For this study, I measured pollen size for a total of 669 pollen grains. The
volume of each pollen grain was estimated using volumetric formulas for spheres (1/6πp3) and
ellipsoids (1/6πe2p) where p = polar axis and e = equatorial axis. For roughly spherical pollen
grains, I set p equal to the average of the two axis measurements that I took. Because all
measurements were taken using the same microscope, units are consistent across the entire study.
However, the measurements could not be assigned meaningful units such as micrometers
because I was unable to calibrate the microscope. Therefore the results are just reported in terms
of units3.
Pollen nitrogen/protein. I pooled pollen from several plants within a site in order to collect
enough pollen for quantification. Nitrogen content was to be measured using combustion at the
Dartmouth College Analytical Laboratory. Pollen protein could be calculated from the nitrogen
9
content estimates. I could not do this analysis for Barbarea vulgaris and Draba due to difficulty
collecting an adequate amount of pollen from such very small flowers. This analysis was not
able to be completed by the conclusion of this project but the results are expected to be
incorporated into a future publication.
Data analysis. The independent variable in this study is the status of the plant (native or
invasive) and the dependent variables are the measures of nectar and pollen quality and quantity,
specifically nectar production rate, nectar concentration, amount of sugar in nectar, pollen
volume and counts, and pollen protein content. I tested the null hypothesis that there was no
difference between the invasive and native species pairs. Nectar production rate per flower was
normalized using square root transformation. All other nectar variables were normalized using
log (x+1) transformation. I used t-tests for each native-invader pair for all of the nectar response
variables. I did not apply the sequential Bonferroni correction to significance levels because this
method can inflate the Type II error rate (Moran, 2003; Gotelli and Ellison, 2004). I instead
follow the guidelines provided by Moran (2003) and Gotelli and Ellison (2004) and report
unadjusted significance values. I combined data across sites and use plant as the unit of
replication in all analyses. Statistical analyses were performed in JMP.
Results
Nectar production rate per flower. Only the invasive Linaria Vulgaris showed a higher
nectar production rate than native plants. Linaria vulgaris had 35% higher nectar production rate
per flower than native Penstemon strictus (t58 = -2.606, P = 0.0116) and was nearly tenfold
higher than native Mimulus guttatus (t56= -9.747, P<0.0001). Conversely, the invasive
10
Cynoglossum officinale and Convolvulus arvensis had significantly lower nectar production rates
per flower than their corresponding native plants (respectively, t67= 3.118, P=0.0013 and t 28=
6.166 P = <0.0001; Figure 2).
Nectar production rate per plant. When examined at the plant level (multiplying nectar
production per flower by number of flowers per plant), Linaria vulgaris had a nectar production
rate that was 75 times greater than Mimulus guttatus (t39 = -12.036, P<0.0001), but there was not
a significant difference between Linaria vulgaris and Penstemon strictus in whole-plant nectar
production rate (t54= -1.235, P=0.111). Conversely, the invasives Cynoglossum officinale and
Convolvulus arvensis had a lower nectar production rate per plant than their corresponding
natives (respectively, t56= -2.476, P=0.0082 and t21= 3.803, P = 0.0005; Figure 3).
Nectar sugar concentration. For the four complete pairs on which I had data, all of the
invasive species had a significantly higher mean sugar concentration then their corresponding
native species (Linaria-Penstemon: t30= -7.157, P<0.0001; Linaria-Mimulus: t=-10.999,
P<0.0001; Cynoglossum-Mertensia: t38= -2.373, P=0.0114; Convolvulus-Geranium: t9= -2.457,
P=0.0182; Figure 4).
Nectar sugar per flower. For the mean amount of sugar produced per flower, Linaria
vulgaris was 32 times greater than Mimulus guttatus and five times greater than Penstemon
strictus (respectively, t28= -4.459, P<0.0001 and t32= -3.962, P=0.0002). However, the invasives
Convolvulus arvensis and Cynoglossum officinale were not significantly different from their
corresponding natives on a per-flower basis (t9=1.841, P=0.095 and t39=0.234, P=0.592; Figure
5).
11
Nectar sugar per plant. On a per-plant basis (multiplying nectar sugar by number of flowers
open), invasive Linaria vulgaris was 280 times greater than Mimulus guttatus and three times
greater than Penstemon strictus (respectively, t18=-4.057, P=0.0004 and t 30=-2.972, P=0.0029)
and invasive Cynoglossum officinale was 5 times greater than its native counterpart (t30= -5.200,
P<0.0001). However, the amount of sugar produced per plant by invasive Convolvulus arvensis
was 6 times less than its native counterpart (t9=2.541, P=0.0158; Figure 6).
Pollen Volume. The size of a pollen grain was significantly greater for natives Draba,
Penstemon strictus, Mimulus guttatus, and Geranium richardsonii as compared to their
comparable invasive plants (respectively, t198= 2.081, P < .0194; t178=18.449, P < 0.0001; t172=
32.272, P < 0.0001; and t168= -16.581, P < 0.0001). The only invasive plant that had a
significantly greater pollen grain volume than its paired native was Cynoglossum officinale (t117=
-8.148, P < 0.0001; Figure 7).
Amount of pollen per flower. In four out of five pairs, invasive plants produced
significantly more pollen per flower compared to native plants. The invasives Barbarea vulgaris
and Convolvulus arvensis had significantly higher pollen counts than their native counterparts
(respectively, t26= -4.354, P < 0.0001and t21= -7.247, P < 0.0001). Likewise, invasive Linaria
vulgaris had significantly higher pollen per flower than both of its analogous natives Penstemon
strictus (t30= -3.888, P < 0.000) and Mimulus guttatus (t13= -5.233, P < 0.0001). Only invasive
Cynoglossum officinale had significantly less pollen than its native counterpart (t25= -7.247, P <
0.0001; Figure 8).
12
Discussion
I predicted that floral rewards would correlate with mating system of invasive plants.
Although the data generally matched my predicted pattern, I am limited in my ability to assess
the generality of the pattern because of the small sample size. Invasive Linaria vulgaris, which is
self-incompatible, was more rewarding than its native counterparts for all response variables.
Cynoglossum officinale, a self-compatible invasive, had greater nectar concentration and raw
amount of sugar but a lower nectar production rate and amount of pollen than its corresponding
native plant. Thus these two pairs do support that dependency on pollinators for reproduction
corresponds to more evidence for increased floral rewards. However, because I was unable to
attain nectar data for the other invasives, Convolvulus arvensis (self-incompatible) and Barbarea
vulgaris (self-compatible), it is not reasonable to make any further general conclusions about the
role of mating systems. Additionally, a goal of this study was to use a broad sample of species to
assess the generality of the pattern so it would have been advantageous to also consider more
invasive-native pairs such as self-incompatible Matraicaria perforata-Erigeron speciosus and
Cirsium arvense-native Cirsium, and self-compatible Tragopogon dubius-Agroseris glauca and
Erodicum cicutarium-Geranium richardsonii. However due to time constraints and difficulty in
attaining nectar and pollen samples from species with composite flowers, I had fewer study
species than what was ideal and I am not able to suggest that this pattern would hold true for all
invasive plants.
Overall, I found that invasives are usually more rewarding than natives in many, but not all,
estimates of nectar and pollen quality and quantity. With regards to pollen rewards, my
hypothesis that invasive species would have a greater amount of pollen per flower than natives
was supported by the fact that in four out of five pairs, invasives had higher pollen counts than
13
their counterparts. Although other studies have considered pollen protein content as a driving
factor in pollinator plant preference (Roulston et al. 2000, Roulston and Cane 2000), pollen
quantity has not been previously explored. These results warrant further investigation about the
role that total pollen output has on pollinator preference and how increased pollen output could
be an advantageous trait in invasive plants. It is important to also note that higher pollen
production in invasive plants may not only be important as a pollinator reward but also directly
in invasive plant reproduction.
Measurements of nectar production rate were not in agreement with my hypothesis that
invasive species should have a higher nectar production rate than their native counterparts. In
fact, L. vulgaris was the only invasive to show a higher nectar production rate than for natives on
a per-flower and per-plant basis. Conversely, the data for nectar sugar concentration supported
my hypothesis in that invasives had more highly concentrated nectar than their native
counterparts. However, it is important that these two variables not be viewed in isolation, since
nectar volume and concentration alone are not necessarily a good indication of nectar rewards.
The quantity of nectar in a plant that is sheltered from pollinators still fluctuates through time as
volume is supplied by condensation from humid air or precipitation or as it is lost by
evaporation. Rates of secretion also show high variation between flowers and between plants and
are highly dependent on microclimate (Corbet 2003). Especially for open flowers that contain
small amounts of nectar as is characteristic of insect pollination, concentration also often
fluctuates rapidly. Corbet (2003) advocates that studies of sugar content be based on
measurements of concentration as well as volume in individual flowers. My calculation of the
raw amount of sugar is comprised of both nectar production rate and sugar concentration and
therefore is an important nectar response variable to consider. With respect to my hypothesis, I
14
found that in two out of four pairs, invasives had more sugar per flower than natives. In three out
of four pairs, invasives also produced more sugar on a per-plant basis. A possible reason for why
I did not find a consistent pattern for nectar response variables across invasive species is that if
nectar is costly to produce, some plants may invest more in growth than in nectar production.
Some of the results of this study coincide with several species-level studies that also found
invasive plants to have greater nectar rewards for pollinators than their native counterparts
(Chittka and Schürkens 2001, Brown et al 2002). It also confirmed previous observations that L.
vulgaris has particularly highly concentrated nectar. R. E. Irwin (personal communication)
speculated that the high sugar concentration might be why pollinators have a strong preference
for L. vulgaris (unpublished data). The data from this study suggests that L. vulgaris is also
highly rewarding in terms of most other aspects of floral rewards including nectar production
rate and amount of pollen per flower. It would be interesting in future studies to consider which
component or combination of components of floral rewards drives the observed pollinator
preference for L. vulgaris. Additionally, while sugar content of nectar is usually of primary
interest because sugars provide the energy that fuels activity or provisions the larvae (Corbet
2003), it is certainly not the only characteristic of nectar that pollinators find valuable. Studies
have shown that some nectivores prefer nectar with high amino acid content (Blüthgen and
Fiedler 2004, Schütz and Erhardt 2005). Likewise pollen is known to have attractive
components besides protein, including vitamins, growth regulators, and lipids (Herbert et al.
1980). Future studies might also explore these other attractive components as they relate to
pollinator preference and invasive plants.
The suggestion that invasive plants are more rewarding for pollinators than native plants
leads to several interesting questions about the nature of floral rewards. For example, do
15
increased floral rewards develop as a successful invasive plant adapts to its new range, or are
they an inherent trait of the invasive plant and therefore facilitate its invasion? This question
could be illuminated by comparing floral rewards for invasive plants in their native range and in
their invasive range. Understanding the plasticity of floral rewards of invasives and how they
compare to native plants could provide novel predictive insight into which introduced plants are
most likely to become invasive in their new range.
Another interesting concept to consider is how increased nectar and pollen rewards in
invasives might affect the reproductive success of the surrounding native plant community.
Most studies have found that invasive plants compete with native plants for pollinator visits but
the reproductive output in the native species was not necessarily reduced (reviewed by Bjerknes
et al. 2007). Conversely, one study found that a native species suffered significantly reduced
seed set in the presence of a similar aggressive invasive plant that shares pollinators (Brown et
al. 2002). Future studies might look into how nectar and pollen rewards relate to studies that
examine pollination and subsequent seed set, like those of Brown et al. (2002).
Lastly, my results introduce some noteworthy management implications for invasive plants.
Because some invasive species in this study appear to be more rewarding than natives, then
eradication could mean eliminating a high quality food source for pollinators, like bumble bees.
For example, although L. vulgaris is not the only plant from which bumble bees collect nectar
and pollen, it is often their preferred food source wherever it exists (R. E. Irwin, unpublished
data). Therefore, if all L. vulgaris was eliminated from an area where it is dominant now, it is
reasonable to hypothesize that the bumble bee population might decrease in the following years
due to reduced food supply for larvae.
16
Conclusions
A comparison of native-invader pairs found that invasive plants often have higher quality
nectar rewards and higher quantities of pollen than similar native plants. The narrow sample size
limits my ability to assess how universal this phenomenon is. Future study should focus on
broadening the array of invasive and native plants sampled and also considering other
components of nectar and pollen rewards.
Acknowledgements
It is a pleasure to thank those who made this thesis possible. I am grateful to Dr. Randy
Mitchell, Dr. Rebecca Irwin and Dr. Jessamyn Manson for their valuable guidance and
assistance, to all of those who commented on this manuscript, and to NSF for its generous
financial support. In addition, I thank the public and private landowners who allowed me to work
on their property.
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Tables
Table 1. Reported Mating Systems of Focal Taxa
Taxa
Mating System
Invasive Plants
Barbarea vulgaris
self-compatible (van Leur et al. 2006)
Cynoglossum officinale
self-compatible (Klinkhamer and Dejong 1987)
Linaria vulgaris
self-incompatible (Stout et al. 2000)
Convolvulus arvensis
self-incompatible (Westwood et al. 1997)
Native Plants
Draba (most species)
self-compatible (Mulligan and Findlay 1970)
Geranium Richardsonii
mixed (Williams et al. 2000)
Mimulus guttatus
mixed (Ritland 1989)
Penstemon strictus
unknown
21
Table 2. Summary of Research Sites
Site Description
CB-Mt CB bike path between 1st and 2nd service stations heading south
Empty lot next to 308 Horseshoe, Pitchfork, Mt. Crested Butte
Cement creek along side of road, 1/6 mile past pioneer guest cabins
RMBL, beaver ponds across creek from dining hall
RMBL, behind Ruby Lounge
RMBL, below beanpod cabin
RMBL, dining hall above volley ball court
RMBL, north of path behind Johnson cabin
RMBL, northeast corner of research meadow
RMBL, parking lot near maintenance shop
RMBL, research meadow along trail uphill at 1st right
RMBL, research meadow near entrance
RMBL, stream behind billy barr's house
RMBL, west of Teocallii/Pumpkin cabin
RMBL, west side of main road above Gothic hill
Road to deer creek, beneath willow
Slate River Rd. between cattle corral and Nicholson Lake
Slate River Rd. past Nicholson Lake
Slate River Rd., 1/4 mile down road across from cattle corral
Taxa Sampled
Barbarea vulgaris
Barbarea vulgaris
Penstemon strictus
Linaria vulgaris
Penstemon strictus
Mimulus guttatus
Convovulus arvensis
Mertensia ciliata
Linaria Vulgaris
Geranium richardsonii
Geranium richardsonii
Barbarea vulgaris
Draba
Barbarea vulgaris
Draba
Draba
Mertensia ciliata
Mertensia ciliata
Cynoglossum officinale
Cynoglossum officinale
Penstemon Strictus
Linaria Vulgaris
Cynoglossum officinale
Penstemon Strictus
Linaria Vulgaris
Note: GPS coordinates for all sampling sites are deposited at the RMBL in the spatial data
archive.
22
Table 3. Invasive and Native Pairings
Invasive
Linaria vulgaris (Plantaginaceae, formerly
Scrophulariaceae)
Penstemon strictus
(Plantaginaceae,
formerly
Scrophulariaceae)
Native
Mimulus guttatus
(Scrophulariaceae)
Convolvulus arvensis (Convolvulaceae)
Geranium richardsonii (Geraniaceae)
Cynoglossum officinale (Boraginaceae)
Mertensia ciliata (Boraginaceae)
Barbarea vulgaris (Brassicaceae)
Draba (Brassicaceae). Unable to identify
conclusively to species level. Likely to be Draba
aurea or Draba spectabilis.
23
Figures
Figure 1. Method for measuring size of an ellipsoid shaped pollen grain.
24
Figures 2-8. Results shown for each response variable. Charts include: actual data points, box
plots showing minimum, Q1, median, Q3, and max for each population for a species; the species
mean (purple bar); heavy black dividing lines between invasive-native pairs; t-test results for
pairs.
Figure 2. Nectar production rate per flower.
25
Figure 3. Nectar production rate per plant.
26
Figure 4. Nectar sugar concentration.
27
Figure 5. Nectar sugar amount per flower.
28
Figure 6. Nectar amount of sugar per plant.
29
Figure 7. Pollen grain volume. Population origin was not recorded for pollen volume
measurements, thus box plots and points show data combined across all sites.
30
Figure 8. Amount of pollen per flower