Predators in Higher Trophic Levels Affect Selection on Floral Traits by
Altering Plant-Pollinator Interactions
by
Amanda Decker Benoit
A Thesis
Presented to
The University of Guelph
In partial fulfillment of requirements
for the degree of
Master of Science
in
Integrative Biology
Guelph, Ontario, Canada
© Amanda Decker Benoit, August 2016
PREDATORS IN HIGHER TROPHIC LEVELS AFFECT SELECTION ON
FLORAL TRAITS BY ALTERING PLANT-POLLINATOR INTERACTIONS
ABSTRACT
Amanda Decker Benoit
Advisor:
University of Guelph, 2016
Professor Christina M. Caruso
Interactions between plants and pollinators generate selection on floral traits. These
interactions, and the resulting selection, may be affected by predators in higher trophic levels that
consume and alter the behavior of pollinators. The effect of predators on selection on floral traits
should depend on the predator’s hunting-mode. I examined the effects of active-pursuit predators
(dragonflies) and sit-and-wait predators (ambush bugs) on selection on floral traits of the
bumblebee-pollinated wildflower Lobelia siphilitica. Contrary to my predictions, I found that
while selection did differ between dragonfly treatments, it was not caused by decreased plantpollinator interaction strength. Consistent with my predictions, I found that ambush bugs prefer
plants with larger daily displays, and significantly decrease the strength of selection on daily
display size. My results suggest that predators in higher trophic levels may be an
underappreciated cause of selection on plant traits.
iii
ACKNOWLEDGEMENTS
I would like to sincerely thank all who inspired, guided, and encouraged me throughout
the process of completing this thesis. I especially appreciate the thoughtful mentorship of my
advisor, Dr. Christina Caruso. Her expertise, passion, and practicality made this work possible. I
am also indebted to my committee members, Dr. Shannon McCauley and Dr. Brian Husband. I
would like to thank them for their time, enthusiasm, and helpful suggestions. This work would
not have been possible without the countless hours of field and laboratory work performed by
Katie Brown, our work study students: Emily Williams, Aaron Hudson, Ann Lee, Cassia Dal
Bello, Heather Van Den Diepstraten, and Lauren Paulson, and our wonderful volunteers,
Stephanie Otto and Tracy Yuen. I also greatly appreciate the hard work and dedication of the
Phytotron staff, as well as the staff and researchers at Koffler Scientific Reserve. Especially,
Stephan Schneider for being a phenomenal manager. I would also like to thank Dave Punzalan
for providing information about ambush bug identification. Last, but certainly not least, I would
like to thank members of the Caruso-Maherali lab, and Daniel Siksay, for thought provoking
discussion, and helpful feedback.
iv
TABLE OF CONTENTS
Abstract……………………………………………………………………………………………ii
Acknowledgements……………………………………………………………………………....iii
Table of Contents…………………………………………………………………………………iv
List of Figures………………………………………………………………………………….v-vi
List of Tables………………………………………………………………………………..vii-viii
Introduction …………………………………………………………………………………….1-6
Methods………………………………………………………………………………………..6-19
Study Species…………………………………………………………………………....6-7
Germination and Growth………………………………………………………………..7-8
Experimental Design……………………………………………..…………..…..……8-11
Trait Data Collection…………………………………………………………………11-13
Fitness Data Collection……………………………………………………………….13-14
Statistical Analysis…………………………………………………………………...14-19
Results………………………………………………………………………………………..19-22
Discussion…………………………………………………………………………………….22-27
Literature Cited……………………………………………………………………………….28-34
v
LIST OF FIGURES
Figure 1. Map of field site locations at the Koffler Scientific Reserve. Field sites 1 and 2
are less than 100m from ponds. Fields sites 3 and 4 are greater than 500m
from ponds. Field sites 1 and 3 were found to have high dragonfly
abundance, and field sites 2 and 4 were found to have low dragonfly
abundance…………………………………………….35
Figure 2. Floral traits of Lobelia siphilitica: a) corolla length was measured from the
base of the bracts to the corolla tube opening, b) width of the corolla tube
opening was measured from the bottom of the corolla tube to the top at the
point where the corolla tube opening begins, c) banner petal size was
measured as the total area of the top and bottom petals outlined in white, d)
banner petal shape was measured as the ratio of length to width of the bottom
three banner petals, which serve as a landing platform, e) the nectar guide
stripes were measured as the white stripes (arrow 1) and the purple
background (arrow 2), nectar guide contrast was measured by subtracting the
modal gray value of purple background from the modal gray value of the
white stripes, and f) the area of the corolla underside was measured as the
area outlined in white, nectar guide size was measured by dividing the area of
the underside of the corolla tube (f) by the area of the nectar guide stripes (e.
arrow 1)……………………………………………………………………...36
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Figure 3. Relationship between daily display size and fruits per plant for Lobelia
siphilitica in low- and high-dragonfly abundance treatments. Fruit per plant is
a female fitness measure, and was relativized for each treatment by dividing
each plant’s fitness by the mean plant fitness within the treatment. Daily
display size was standardized by subtracting the treatment mean daily display
from each plant’s daily display size, and then dividing by the treatment
standard deviation, resulting in a mean of zero and a variance of one…….37
Figure 4. Relationship between daily display size, and fruits per plant (a, b) and seeds
per plant (c, d) for Lobelia siphilitica in ambush bug absent and ambush bug
present treatments. Fruit per plant, and seeds per plant are female fitness
measures. They were relativized for each treatment by dividing each plant’s
fitness by the mean plant fitness within the treatment. Daily display size was
standardized by subtracting the treatment mean daily display from each
plant’s daily display size, and then dividing by the treatment standard
deviation, resulting in a mean of zero and a variance of one.……………..38
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LIST OF TABLES
Table 1. Split-plot analysis of variance model of the effect of dragonfly treatment (low
abundance vs. high abundance), pollination treatment (hand vs. open), and
the interaction between the dragonfly and pollination treatments on three
female fitness components of Lobelia siphilitica
plants………………………...………………………………………………39
Table 2. Mean values for three female fitness components ± 1 SE (N) for hand- and
open-pollinated Lobelia siphilitica plants.………………….………………40
Table 3. Mean values for three female fitness components ± 1 SE (N) for open-pollinated
Lobelia siphilitica plants in low- and high-dragonfly abundance treatments.41
Table 4. Linear selection gradients ± 1 SE estimated via three female fitness components
for 10 floral and inflorescence traits of open-pollinated Lobelia siphilitica
plants in the low- and high-dragonfly abundance treatments. F compares the
strength of selection between the low- and high-dragonfly abundance
treatments. …………………………………………………………………..42
Table 5. Linear selection differentials ± 1 SE estimated via three female fitness
components for 10 floral and inflorescence traits of open-pollinated Lobelia
siphilitica plants in the low- and high-dragonfly abundance treatments. F
compares the strength of selection between the low- and high-dragonfly
abundance treatments..................................................................................…43
viii
Table 6. Linear selection gradients ± 1 SE estimated via three female fitness components
for 10 floral and inflorescence traits of hand- and open-pollinated Lobelia
siphilitica plants. F compares the strength of selection between the hand- and
open-pollination treatments.………………………………………….……..44
Table 7. Linear selection differentials ± 1 SE estimated via three female fitness
components for 10 floral and inflorescence traits of hand- and openpollinated Lobelia siphilitica plants. F compares the strength of selection
between the hand- and open-pollination treatments..……………………….45
Table 8. Mean trait values ± 1 SE (N) for open-pollinated Lobelia siphilitica plants with
ambush bugs present and absent. t compares mean trait values between plants
with ambush bugs present and absent..……………………………………..46
Table 9. Linear selection gradients ± 1 SE estimated via three female fitness components
for 10 floral and inflorescence traits of open-pollinated Lobelia siphilitica
plants with ambush bugs present and absent. F compares the strength of
selection between plants with ambush bugs present and absent..………….47
Table 10. Linear selection differentials ± 1 SE estimated via three female fitness
components for 10 floral and inflorescence traits of open-pollinated Lobelia
siphilitica plants with ambush bugs present and absent. F compares the
strength of selection between plants with ambush bugs present and absent.48
INTRODUCTION:
Mutualists and antagonists that interact with plants can generate selection on plant traits
(Benkman 2013; Vanhoenacker et al. 2013). Examples include pollinators selecting on floral
traits such as spur length (Johnson and Steiner 1997) and herbivores selecting on plant defense
traits such as secondary metabolites (Coley and Barone 1996). However, plants and their
mutualists and antagonists are embedded in larger food-webs, and therefore, predators in higher
trophic levels may affect the interactions between plants and their mutualists and antagonists –
and the resulting selection. Predators in higher trophic levels can alter plant-animal interactions
by consuming, and altering the behavior of plant mutualists and antagonists. Predators of plant
mutualists and antagonists have been shown to alter plant fitness, plant productivity, and plant
community composition (Hairston et al. 1960; Paine 1980; Pace et al. 1999; Preisser et al. 2005;
Knight et al. 2006; Romero et al. 2011). Less is known about how predators of plant mutualists
and antagonists affect selection on plant traits. But logic suggests, if predators in higher trophic
levels alter plant-animal interactions, and plant-animal interactions generate selection, then
predators in higher trophic levels should affect the strength of selection on plant traits (Estes
2013).
The effects of predators in higher trophic levels on mutualist- and antagonist-mediated
selection on plant traits is important to consider for two reasons. First, if predators indirectly
affect the strength of selection, then fluctuation in predator abundance may explain observed
variation in the strength of mutualist- and antagonist-mediated selection on plant traits
(Thompson 2005; Siepielski et al. 2009, 2013; Bartkowska and Johnston 2015; but see
Morrissey and Hadfield 2012). Second, understanding the evolutionary consequences of topdown effects of predators on plants may allow us to better predict the consequences of trophic-
1
downgrading – the expatriation and extinction of top predators (Estes 2011). By understanding
how predators in higher trophic levels affect plant ecology and evolution, we can make betterinformed conservation and management decisions for areas affected by trophic-downgrading.
My goal is to determine whether predators of pollinators alter the strength of pollinatormediated selection on plant traits. Predators, however, are not all equal; they can have different
hunting-modes that range from sit-and-wait to active-pursuit hunting. Therefore, I do not expect
all pollinator predators to affect the strength of selection on plant traits in the same way.
Predators with different hunting modes should affect selection via different mechanisms.
Some pollinator predators, such as dragonflies (Odonata e.g. Aeshnidae: Anax junius),
birds (Aves e.g. Hirundinidae: Hirundo pyrrhonota), and bee-wolves (Sphecidae e.g. Philanthus
bicinctus), have an active-pursuit hunting mode (Dukas 2005; Meehan et al. 2005; Knight et al.
2005). Active-pursuit predators should increase the strength of pollinator-mediated selection by
weakening interactions between plants and pollinators. Active-pursuit predators are highly
mobile, conspicuous, and have high prey capture rates (Pianka 1966; Schoener 1971; Huey and
Pianka 1981). Because of their mobility and high prey capture rates, these predators should
decrease the abundance of pollinators through consumptive effects (Hairston et al. 1960).
Furthermore, because these predators are conspicuous, they are expected to alter pollinator
behavior (Schmitz et al. 1997; Preisser et al. 2005; Romero et al. 2011). For example, in
response to predator cues, prey (i.e. pollinators) may forage less (spending more time hidden and
inactive), leave the patch in search of a new safer patch, or forage on different resources
(Schmitz et al. 1997; Preisser et al. 2005; Romero et al. 2011; Belgrad and Griffen 2016). As a
result, active-pursuit predators are expected to decrease the number of foraging pollinators in a
patch (Lima 1998; Guariento et al. 2014; Keiser et al. 2015). Therefore, active-pursuit predators
2
should decrease plant-pollinator interaction strength (Preisser et al. 2005; Knight et al. 2006;
Trussell et al. 2006; Romero et al. 2011). Interaction strength can have multiple definitions
(Menge et al. 2004; Wootton and Emmerson 2005), but here, I use the same definition as
Benkman (2013): interaction strength is the proportion of a population that is either directly
benefited or harmed by an interaction. This definition takes into account both the individual per
capital effect of the interaction as well as population size (Menge et al. 2004; Benkman 2013).
Weaker plant-pollinator interactions, i.e. when a smaller proportion of the plant population
benefits from interactions with pollinators, should decrease plant fitness. As mean plant fitness
decreases, and the variance in fitness either stays constant or increases, the opportunity for
selection will increase (Benkman 2013). Opportunity for selection is the maximum strength of
selection possible in a population (Crow 1958; Arnold and Wade 1984; Benkman 2013). The
opportunity for selection increases as mean fitness decreases because the opportunity for
selection is the variance in fitness over mean absolute fitness squared (Benkman 2013).
Therefore, by weakening plant-pollinator interactions, active-pursuit predators should increase
the opportunity for strong pollinator-mediated selection.
In contrast, other predators, such as crab spiders (Thomisidae) and ambush bugs
(Phymatinae), have a sit-and-wait hunting mode (Balduf 1939; Morse 1979). Sit-and-wait
predators differ from active-pursuit predators in that they are primarily stationary, cryptic, and
have low prey capture rates (Pianka 1966; Schoener 1971; Huey and Pianka 1981). Another key
difference is that sit-and-wait predators hunt at the plant level, while active-pursuit predators
hunt at the patch level. This means that while active-pursuit predators should be randomly
distributed in relation to plant phenotypes, sit-and-wait predators are expected to be more
common on plants with phenotypes that increase prey (pollinator) attraction (Greco and Kevan
3
1994, 1995; Morse 1999; Bhaskara et al. 2009; Hanna and Eason 2012; Defrize et al. 2014). In
contrast to active-pursuit predators, it is unlikely that behavioral changes caused by sit-and-wait
predators will decrease the abundance of pollinators in a patch. This is because pollinators can
use cues to detect and avoid specific flowers or plants that harbor cryptic sit-and-wait predators
(Abbott 2006; Dawson and Chittka 2012; Dawson and Chittka 2014), and pollinators can avoid
these predators by moving to a neighboring plant (sit-and-wait predators do not pursue their
prey). Instead of decreasing pollinator abundance, it is more likely that behavioral changes
caused by sit-and-wait predators will cause decreased pollinator visitation to some plants and
increased visitation to neighboring plants, such that overall visitation at the patch level remains
unchanged. Again in contrast to active-pursuit predators, sit-and-wait predators are unlikely to
greatly reduce the abundance of pollinators through consumptive effects, because sit-and-wait
predators have low prey capture rates (Morse 1979; Mason 1986; Elliott and Elliott 1991, 1994).
Given that sit-and-wait predators are not expected to decrease pollinator abundance through
either behavioral or consumptive effects, it is unlikely that sit-and-wait predators will decrease
the strength of plant-pollinator interactions. In contrast, sit-and-wait predators are expected to
affect the strength of pollinator-mediated selection by increasing or decreasing the frequency of
pollinator visits to certain plant phenotypes. Sit-and-wait predators should be more common on
plants with traits that attract prey (Greco and Kevan 1994, 1995; Morse 1999; Bhaskara et al.
2009; Hanna and Eason 2012; Defrize et al. 2014) causing predation risk to be unevenly
distributed across plant phenotypes. In response, pollinators should visit safer plants more
frequently, and riskier plant less frequently. Plants with traits preferred by sit-and-wait predators
should receive fewer pollinator visits and should decrease in fitness relative to plants with traits
not preferred by predators. This should weaken the strength of selection on traits preferred by sit-
4
and-wait predators. For example, when predators preferentially hunt on plants with large nectar
rewards, pollinators shift to foraging on less rewarding flowers (Jones 2010; Jones and Dornhaus
2011). Such a shift should increase the fitness of the less rewarding flowers relative to more
rewarding flowers, causing weaker selection on rewarding traits.
To determine if and by which mechanism predators affect pollinator-mediated selection
on plant traits, I examined the effects of active-pursuit predators and sit-and-wait predators on
selection on inflorescence and floral traits of the bumblebee-pollinated wildflower Lobelia
siphilitica. The active-pursuit predators I studied were a community of dragonflies (Odonata),
including several species of darners (Aeshnidae), and other large-bodied dragonfly species found
in southern Ontario. Because not all species of dragonflies are activity pursuit predators, this
study focused only on families that are (Aeshnidae and Corduliidae), excluding families which
have a perch and pursue hunting mode (Libellulidae). Active-pursuit dragonflies fly
continuously, circling above fields, often in groups, intercepting and consuming other flying
insects (Olberg et al. 2005; Jones et al. 2008; personal observation). They are fast, agile fliers
and have high prey-capture success rates (Olberg 2000; Combes et al. 2013). I predict that
dragonflies will decrease the number of foraging pollinators in a patch. This will weaken plantpollinator interactions, causing plant reproduction to become pollen limited (Ashman et al.
2004), resulting in decreased plant fitness. Decreased plant fitness will cause the strength of
pollinator-mediated selection to increase (Benkman 2013). The sit-and-wait predators I studied
were ambush bugs (Phymatinae: Phymata pennsylvanica americana) (Balduf 1939, 1941).
Ambush bugs perch on flowers or leaves, and use their raptorial claws to capture and hold
insects that come within reach (Balduf 1939; Mason 1986). They select a plant to hunt on, and if
prey visitation is low they fly to a different plant. After settling on a plant, ambush bugs can
5
remain on the plant as long as prey availability is high (Balduf 1939, 1941; Greco and Kevan
1994, 1995). Because ambush bugs choose hunting sites based on prey availability, and
pollinators are their main prey (Balduf 1939), they should be more common on plants with traits
that attract pollinators. By increasing predation risk on plants with preferred traits, ambush bugs
should decrease pollinator visitation to those plants, and increase pollinator visitation to plants
with non-preferred traits. Thereby, ambush bugs should weaken selection on their preferred plant
traits.
I measured phenotypic selection via female fitness on ten traits of L. siphilitica that were
(1) open-pollinated with low and high dragonfly abundance, (2) open-pollinated with ambush
bugs absent and present, and (3) hand-pollinated with supplemental pollen. I used these data to
answer the following questions: (1) Do dragonflies increase the strength of selection on floral
and inflorescence traits by decreasing the strength of plant-pollinator interactions?; and (2) Do
ambush bugs affect the strength of selection on floral traits by changing predation risk across
plant phenotypes?
METHODS:
Study species
Lobelia siphilitica (Campanulaceae) is a short-lived, perennial wildflower native to
wetlands and woodlands in eastern North America (Johnston 1991a and references therein). It
produces three-centimeter-long, tubular flowers, ranging from blue to purple, on a single
racemose inflorescence, and occasionally on axillary branches (Parachnowitsch and Caruso
2008; Caruso et al. 2010). In Ontario, L. siphilitica flowers from August to early October, and
6
fruits dehisce from mid-September to November (Johnston 1991a). Lobelia siphilitica is
gynodioceous; plants can be female or hermaphroditic, but in northern populations females are
rare (<10%) (Caruso and Case 2007). Hermaphroditic L. siphilitica plants are self-compatible;
however, they are protandrous with non-overlapping male and female phases. Therefore, selfing
in this species must be pollinator mediated (Johnston 1991b). Lobelia siphilitica is primarily
pollinated by two species of bumblebee, Bombus vagans and B. impatiens (Flanagan et al. 2011).
Germination and Growth
The L. siphilitica plants used in this experiment were germinated from seeds collected in
2012 from 49 open-pollinated plants at the T.J. Dolan Natural Area, Stratford, Ontario, Canada
(43.370N, 80.997W). Two methods were used to break seed dormancy. In the first method, seeds
were placed on moist filter paper in a Petri dish, wrapped in Parafilm, and stratified at 4˚C for
eight weeks (Johnston 1992). In the second method, seeds were treated with a 16:1:1 solution of
distilled water, 70% ethanol, and bleach, with four drops of Triton X-100 detergent added per
two liters of solution (Dudle et al. 2001). The seeds were then rinsed with distilled water and
stored at 4˚C overnight. Cold stratification began on January 27, 2015 and the bleach treatment
was performed on February 18, 2015.
After breaking dormancy, approximately 90 seeds per family were planted into plug trays
filled with Sunshine Mix (Sun Gro Horticulture Canada Ltd., Vancouver, British Columbia,
Canada). The trays were placed in standing water on benches at the University of Guelph
Phytotron, Guelph, Ontario, Canada. Five to seven weeks after sowing, 1-50 seedlings from 37
families with adequate germination were transplanted (N = 1397 plants) from plug trays into 635
7
mL3 Deepots (Stuewe and Sons Inc., Tangent, Oregon, USA) filled with Sunshine mix (Sun Gro
Horticulture Canada Ltd., Vancouver, British Columbia, Canada). Each plant was randomly
assigned a position on the greenhouse bench. Plants were watered as needed, exposed to
supplementary light for 16 hours per day, and fertilized with eight prills of Nutricote total 13-1313 with micronutrients (Plant Products, Brampton, Ontario, Canada).
Experimental design
The week of June 8, 2015, plants which had bud primordia, or were ≥ 8 cm tall were
assigned to be experimental plants (N= 300), pollen donors (N=80), or extras (N=118). Plants
which were already flowering, or that had multiple racemes, were assigned to be pollen donors.
All other plants were randomly assigned to be experimental plants, pollen donors, or extras. The
experimental plants, pollen donors, and extras were then transported to Koffler Scientific
Reserve (KSR), King City, Ontario (44.03N, 79.52W) where the field experiment was
conducted. The experimental plants were transplanted into 8.7 liter pots filled with Sunshine
Mix, and remained potted for the duration of the experiment. Pollen donors and extras remained
in Deepots and were housed in the field station’s greenhouse.
From the old fields of the reserve, I selected four fields similar in size, exposure (full
sun), and plant community composition (typical old field communities including Solidago spp.,
Daucus carota, Asclepias syriaca, and Vicia cracca). Within each selected field, I constructed a
10m × 10m fence that was 2.4m tall. Within each fenced site, I placed 75 randomly assigned,
potted L. siphilitica plants. They were arranged in arrays, with half a meter between each plant
and its nearest neighbor. The plants were placed into the field sites on June 18 and 19, 2015, and
8
remained in the field for eight weeks. For the duration of the experiment, vegetation within the
fence was trimmed to below the rims of the pots, and flowers from plants other than L. siphilitica
were removed. All plants were sexed upon flowering. Female plants (N = 6) were replaced with
randomly selected hermaphrodites from the extras group for the first three weeks of the
experiment. Plants discovered to be female after three weeks were excluded from analysis (N =
4). Three additional plants were replaced with extras because they were found to be Lobelia
cardinalis × siphilitica hybrids.
Dragonfly abundance was not manipulated; instead, the locations of the field sites
utilized natural variation of dragonfly abundance. I selected two sites adjacent to ponds (<100m),
where I predicted dragonfly abundance would be high, and two fields greater than 500m from
ponds, where I predicted dragonfly abundance would be low. Dragonfly abundance was
measured on 31 days over the course of the eight-week experiment, approximately every second
day as weather allowed. Dragonfly abundance was measured by conducting point counts, as
done in other studies (e.g. Knight et al. 2005a). During these point counts, an observer stood at
each corner of each field site for five minutes looking continuously in one direction and counting
the number of dragonflies that entered their field of view. Each dragonfly was classified as small,
medium, or large. Because small and medium dragonflies were rarely observed, and are unlikely
to prey upon bumblebees, my analysis is restricted to the abundance of large dragonflies.
However, the inclusion of small or medium dragonflies does not change the classification (low
vs. high dragonfly abundance) of the field sites (data not shown).
Contrary to my expectations, dragonfly abundance was not related to pond proximity. Of
the two field sites adjacent to ponds (Fig. 1), site one had high dragonfly abundance (mean ( 1
SE) dragonflies/20 minutes = 23.8 (8.53); Range = 0-214), and site two had low dragonfly
9
abundance (mean (1 SE) = 2.5 (0.51); Range = 0-11). Of the two field sites greater than 100m
from ponds (Fig. 1), site three had high dragonfly abundance (mean (1 SE) = 10.6 (3.04); Range
= 0-68), and site four had low dragonfly abundance (mean (1 SE) = 2.6 (0.60); Range = 0-11).
Consequently, site two (<100m from a pond) was reclassified as a low-dragonfly site, and site
three (>500m from a pond) was reclassified as a high-dragonfly site (Fig. 1).
In all four field sites, I allowed naturally occurring ambush bugs from the surrounding
fields to colonize the potted L. siphilitica plants. The first adult ambush bug was observed on
July 21. After July 21 the plants were inspected every second day for ambush bugs. From July
21 to 28 plants were classified as having ambush bugs either present to absent. From July 28 to
August 12 the number of ambush bugs on each plant was counted. If ambush bugs were found on
a plant for one or more days, then for analysis the plant was placed in the ambush bug present
category; if ambush bugs were never observed on a plant, then the plant was placed in the
ambush bug absent category. Of the 195 open pollinated plants, 127 were in the ambush bugs
present category and 68 were in the ambush bugs absent category. During the experiment,
ambush bugs were observed both attempting to prey upon and successfully consuming
bumblebees and other floral visitors, including butterflies and syphid flies (personal
observation).
The majority of plants in the ambush bug present category hosted ambush bugs on two or
more of the days that plants were examined. Of the plants in the ambush bug present category,
48 plants had ambush bugs present on only one of the nine days they were examined, 33 plants
had ambush bugs present on two of the nine examination days, 17 plants had ambush bugs on 3
of the 9 days, 15 plants had ambush bugs present on 4 of the 9 days, and 15 plants had ambush
bugs present on 5 or more of the examination days. The number of ambush bugs on a plant at a
10
given time ranged between zero and nine. The mean number of ambush bugs observed on each
plant in the ambush bug present category was 1.6. Other studies have demonstrated that the mean
residence time for ambush bugs on a plant is 5.2 days with a range of 1-14 days (Yong 2005).
The ratio of plants with and without ambush bugs was similar for field sites two, three,
and four. However, field site one had fewer ambush bugs than the others. In field site one,
ambush bugs were present on 13 plants and absent on 37 plants. In field site two, ambush bugs
were present on 40 plants and absent on 9 plants. In field site three, ambush bugs were present on
41 plants and absent on 8 plants. In field site four, ambush bugs were present on 35 plants and
absent on 14 plants.
To determine if selection on floral traits of L. siphilitica was pollinator mediated, 25
plants at each field site were randomly assigned to be hand-pollinated. To hand pollinate plants,
pollen was collected from at least five haphazardly-chosen pollen donors, and applied with a
paintbrush to the stigmas of plants in the hand-pollination treatment (Caruso et al. 2010;
Parachnowitsch and Caruso 2008). Plants in this treatment received supplemental pollen every
second day (as in Parachnowitsch and Caruso 2008; Rivkin et al. 2015). Comparing selection
between open- and hand-pollinated plants determines whether selection is the result of
interactions with pollinators (Ashman et al. 2004).
Trait data collection
To measure floral morphology, I scanned one of the first five flowers from each plant.
Using one of the first five flowers controls for within plant variation in floral traits along the
raceme (Diggle 1995). Each flower was open for a minimum of two days, to ensure that it was
11
fully expanded, before collection for scanning. Collected flowers were stored on ice and
transported to the University of Guelph, where they were scanned using WinRhizo© (Regent
Instruments Inc., QC, Canada).
Using these scanned images and ImageJ (Abramoff et al. 2004), I measured four floral
morphology traits. Corolla length was measured as the distance from the tube opening to where
the bracts attached (Fig. 2a). Width of the tube opening was measured as the length from the
base of the tube to the top of the tube at the tube opening (Fig. 2b). Banner petal size was
measured as the total area of the top and bottom banner petals i.e. the petals visible to an
approaching pollinator (Fig. 2c). Banner petal shape was measured as the ratio of length to width
of the three lower banner petals, which bumblebees use as a landing platform (Fig. 2d).
I measured four floral color traits. The first two traits, nectar guide contrast and size, were
measured by analyzing scanned images in ImageJ. These measurements quantify variation in the
white and purple striping on the underside of the corolla tube. Nectar guide contrast was
calculated by subtracting the modal gray value of the purple background from the modal gray
value of the white stripes (Fig. 2e). Nectar guide size was determined by dividing the area of the
underside of the corolla tube (Fig. 2f) by the area of the nectar guide stripes (Fig 2e). The second
two traits, chroma and brightness, were measured using an Ocean Optics USB2000
spectrophotometer (Dunedin, FL) with a deuterium-tungsten halogen light source (Caruso et al.
2010). I measured the proportion of light reflected from L. siphilitica petals at wavelengths
between 400 and 800 nm at a 0.3-nm interval, standardized against black and white reference
standards (Caruso et al. 2010). Principal component analysis was used to reduce the spectra into
two of the three fundamental components of color: brightness and chroma, the first and third
principal components respectively (Grill and Rush 2000). Brightness is the amount of light
12
reflected: bright flowers are light purple, while less bright flowers are dark purple. Chroma is the
amount of gray mixed with hue: flowers with high chroma are vibrant purple, while flowers with
low chroma are faded purple.
Lastly, I measured two inflorescence traits: inflorescence height and daily display.
Inflorescence height was measured after fruits matured as the distance from the soil to the top of
the raceme. Daily display size was measured as the mean number of flowers open on a plant each
day. Flowers were counted daily at the start of the experiment, and every fourth day after July
13, 2015.
Fitness data collection
On August 17 and 18, 2015, the plants were transported to the University of Guelph
Bovey Greenhouse, where fruits were allowed to mature. Before dehiscence, between 6 and 29
fruit per plant were collected (mean (1 SE) = 15.4 (0.21); N = 257 plants and 3965 fruit). To
ensure that I collected fruits from along the length of the raceme, that were representative of the
pollination environment for the full timeframe of the experiment, I marked the raceme at the topand bottom-most open flower approximately every 10th day for the duration of the experiment.
From each marked section of the raceme, I collected three randomly selected fruit. Three
additional fruit were collected from fruiting axillary branches, when present.
I measured three female fitness components: fruit per plant, seeds per fruit, and seeds per
plant. To measure fruit per plant, I counted the total number of fruit produced by each plant
during the experiment (fruit produced after the end of the experiment were not counted). To
measure seeds per fruit, I separated the seeds from the fruit capsule, and weighed them using a
13
microbalance (Sartorius LA120S, Sartorius Corporation, Bohemia, NY). I then found the
average mass of 50 seeds from a random sample of 62 fruit. To estimate the total number of
seeds per fruit, I divided the mass of all the seeds in the fruit by the mean mass of 50 seeds, and
multiplied by 50 (Johnston 1991b). To measure seeds per plant, I multiplied the mean number of
seeds per fruit by the number of fruits per plant. Because, cleaning and weighing seeds was time
consuming, seeds per fruit and seeds per plant data were measured for a randomly selected subsample of 254 plants.
Statistical analysis
Q1: Do dragonflies increase the strength of selection on floral and inflorescence traits by
decreasing the strength of plant-pollinator interactions?
I predicted that dragonflies would decrease interactions between plants and pollinators,
causing plant reproduction to become pollen limited. To determine if the magnitude of pollen
limitation was greater in high dragonfly abundance sites than in low dragonfly abundance sites, I
used a split-plot analysis of variance (ANOVA) model. Dragonfly treatment (low vs. high) was
included as the between-plot fixed factor. Pollination treatment (hand vs. open) was included as
the within-plot fixed factor. The model also included dragonfly × pollination treatment as a fixed
factor, and field site nested within dragonfly treatment as a random factor. Separate models were
run for each of three fitness measures: fruit per plant, seeds per fruit, and seeds per plant. If the
pollination term is significant, and plants in the hand-pollinated treatment have higher fitness
than plants in the open-pollinated treatment, I can conclude that plant reproduction is pollen
limited. If the dragonfly term is significant, and plant fitness is lower in the high-dragonfly
14
abundance treatment, I can conclude that dragonflies decrease plant fitness. If the dragonfly ×
pollination term is significant, and plant fitness is lowest in the high-dragonfly abundance/openpollination treatment combination, I can conclude that dragonflies increase the magnitude of
pollen limitation by weakening plant-pollinator interactions.
To test whether dragonflies strengthened selection on floral and inflorescence traits of L.
siphilitica, I calculated directional selection gradients and differentials for open-pollinated plants
in the low- and high-dragonfly abundance treatments. Selection differentials estimate both direct
and indirect selection on a trait, whereas selection gradients estimate only direct selection (Lande
and Arnold 1983). To calculate directional selection gradients, I used multiple regression. The
model included relativized fitness as the dependent variable. Fitness was relativized for each
treatment by dividing the individual fitness value by the treatment mean fitness value (Lande and
Arnold 1983). The ten standardized traits were included in the model as continuous independent
variables. Traits were standardized for each treatment by subtracting the treatment mean from the
individual value, and then dividing by the treatment standard deviation, resulting in a mean of
zero and a variance of one (Sokal and Rohlf 1995). Data from the same treatment but different
field sites was pooled. Field site was included in the model as a fixed factor to control for
differences in fitness or traits caused by field site. Ambush bug presence/absence was also
included as a fixed factor to control for any effect of ambush bugs. The same model was used to
calculate directional selection differentials, but only one trait was included per model. Separate
models were run for three fitness measures: fruit per plant, seeds per fruit, and seeds per plant.
After I had estimated selection on floral and inflorescence traits of plants in the low- and
high-dragonfly abundance treatments, I tested whether selection gradients and differentials for
open-pollinated plants differed between dragonfly treatments using a split-plot analysis of
15
covariance (ANCOVA). Dragonfly treatment (low vs. high) was included as the between-plot
fixed factor. Ambush bug treatment (absent vs. present) was included as the within-plot fixed
factor. The 10 standardized traits were included as continuous independent variables. The
interactions between dragonfly treatment and each trait were also included in the model. Field
site nested within dragonfly treatment was included as a random factor. Relativized fitness was
included as the dependent variable. To determine if the strength of linear selection differentials
differed between the low- and high-dragonfly abundance treatments, I used the same model, but
with only one trait included per model. Separate models were run for each of three fitness
measures: fruit per plant, seeds per fruit, and seeds per plant. If the dragonfly × trait term is
significant, then I can conclude that selection on that trait differs between the low- and highdragonfly abundance treatments.
In comparing selection between low- and high-dragonfly abundance treatments, I
assumed that dragonflies were affecting pollinator-mediated selection on floral traits. However,
not all selection on floral traits is pollinator-mediated. To test whether selection of floral traits of
L. siphilitica was pollinator-mediated, I calculated directional selection gradients and
differentials for plants in the hand- and open-pollination treatments. To calculate linear selection
gradients, I used multiple regression. The model included relativized fitness as the dependent
variable, the ten standardized traits as continuous independent variables, and field site and
ambush bug presence/absence as fixed factors. The same model was used to calculate directional
selection differentials, but only one trait was included per model. Separate models were run for
three fitness measures: fruit per plant, seeds per fruit, and seeds per plant. Fitness was relativized
and traits were standardized for each treatment.
After I had estimated selection on floral and inflorescence traits of plants in the hand- and
16
open-pollinated treatments, I tested whether selection gradients and differentials differed
between pollination treatments using analysis of covariance (ANCOVA). The model included
relativized fitness as the dependent variable, the ten standardized traits as continuous
independent variables, and pollination treatment (hand vs. open), ambush bug (absent vs.
present), and field site as fixed factors. The model included interactions between pollination
treatment and each trait. To determine if the strength of linear selection differentials differed
between the hand and open-pollination treatments, I used the same model, but with only one trait
included per model. If the pollination treatment × trait term is significant, then I can conclude
that selection on that trait is pollinator-mediated.
Q2: Do ambush bugs affect the strength of selection on floral traits by changing predation risk
across plant phenotypes?
To determine if ambush bugs have preferences for plant traits, I used two approaches.
First, I used an independent samples t-tests to compare trait means between plants with ambush
bugs absent and present. The t-tests included ambush bugs (absent vs. present) as the
independent variable, and the ten floral and inflorescence traits as the dependent variables. If a ttest is significant, then I can conclude that ambush bugs have a trait preference. Second, I used a
generalized linear model with a Poisson distribution and a log link function. The model included
the maximum number of ambush bugs found on a plant as the dependent variable, field site as
the predictor factor, and the ten traits as continuous independent variables. If a trait term is
significant, then I can conclude that ambush bugs have a preference for that trait. Because the
results of this model were consistent with the results of the t-tests, only the results of the t-tests
are included here.
17
To test whether ambush bug preferences for traits of L. siphilitica affect selection on
those traits, I calculated directional selection gradients and differentials for plants with ambush
bugs absent and present (Lande and Arnold 1983). To calculate selection gradients, I used
multiple regression. The model included relativized fitness as a dependent variable, the ten
standardized traits as continuous independent variables, and field site as a fixed factor. Fitness
was relativized, and traits were standardized separately for each treatment. The same model was
used to calculate selection differentials, but with only one trait included per model. Separate
models were run for each of three fitness measures: fruit per plant, seeds per fruit, and seeds per
plant.
After I had estimated selection on floral and inflorescence traits of plants in ambush bug
absent and present treatments, I tested whether selection gradients and differentials differed
between the treatments using analysis of covariance (ANCOVA). To test if gradients differed,
the ANCOVA model included relativized fitness as the dependent variable, the ten standardized
traits as continuous independent variables, and ambush bug (absent vs. present) and field site as
fixed factors. The model included interactions between ambush bug treatment and all traits. The
same model was used to determine if selection differentials differed, but only one trait was
included per model. Separate models were run for each of three measures of fitness: fruit per
plant, seeds per fruit, and seeds per plant. If the ambush bug × trait term is significant, I can
conclude that ambush bugs alter the strength of selection on that trait.
I tested assumptions of normality and homogeneity of variance for all models. To test for
normality, I compared unstandardized residuals to a normal distribution using a KolmogorovSmirnov test. A significant test would indicate that the assumption of normality was violated. To
test for homogeneity of variance, I estimated the Spearman correlation between the predicted
18
values and the residuals. A significant correlation would indicate that the assumption of
homogeneity of variance was violated.
All analysis was done with IBM SPSS Statistics for Windows, Version 23.0 (Armonk,
NY: IBM Corp.). Twelve plants were excluded from analysis because they died during the
experiment, were female, or were missing trait data.
RESULTS:
Overall, I found significant selection on both inflorescence traits and all eight floral traits
of L. siphilitica in at least one treatment. I consistently found significant selection for larger daily
display size, taller inflorescences, wider corolla tube openings, larger banner petals,
longer/narrower banner petal shape, increased nectar guide contrast, and lower chroma (Tables 4,
5, 6, 7, 9, and 10). For three traits, I found that the direction of selection was more variable: I
found significant selection for both shorter and longer corolla tubes (Tables 6 and 7), larger and
smaller nectar guides (Tables 6 and 9), and brighter and less bright flowers (Tables 4, 6, and 9).
Generally, I detected more significant selection via fruit per plant and seeds per plant than by
seeds per fruit.
Q1: Do dragonflies increase the strength of selection on floral and inflorescence traits by
decreasing the strength of plant-pollinator interactions?
In the split-plot analysis of variance model, which tested whether the treatments affected
plant fitness, no terms were significant (Table 1). The pollination term was not significant for
19
any of the three female fitness components of L. siphilitica (Table 1). The magnitudes of the
fitness differences between hand- and open pollinated plants were very small. There were
~0.01% fewer fruits per plant, ~0.08% more seeds per fruit, and ~0.06% more seeds per plant in
the hand-pollinated treatment than in open-pollinated treatment (Table 2). The dragonfly term
was not significant for any of the three female fitness components of L. siphilitica (Table 1). The
magnitudes of fitness differences between the low- and high-dragonfly abundance treatments
were small. There were ~0.09% more fruit per plant, ~0.08% fewer seeds per fruit, and ~0.03%
more seeds per plant in the high-dragonfly abundance treatment than in the low-dragonfly
abundance treatment (Table 3). Furthermore, the dragonfly × pollination term was not significant
for any of the three female fitness components (Table 1).
Although dragonflies did not affect mean plant fitness, selection on two floral traits, daily
display size and petal brightness, differed between the low- and high-dragonfly abundance
treatments. Direct selection via fruit per plant on daily display size was ~53% stronger in the
high-dragonfly abundance treatment than in the low-dragonfly treatment (Table 4). The selection
differential estimated via fruit per plant for daily display size was ~46% stronger in the highdragonfly abundance treatment than in the low-dragonfly abundance treatment (Fig. 3 and Table
5). Direct selection via fruit per plant was for brighter flowers in the low-dragonfly abundance
treatment, but the strength of selection decreased by ~332% resulting in selection for less bright
flowers in the high-dragonfly abundance treatment (Table 4). For all other traits, there was no
significant difference in the strength or direction of selection between the low- and highdragonfly abundance treatments (Tables 4 and 5).
The strength of selection on inflorescence and floral traits of L. siphilitica in the handand open-pollinated treatments differed for one trait. When selection was calculated using seeds
20
per plant, hand-pollinated plants experienced direct selection for larger nectar guides, while
open-pollinated plants experienced direct selection for smaller nectar guides that was ~15%
weaker (Table 6). For all other traits, selection was consistent between the hand- and openpollination treatments (Tables 6 and 7).
Q2: Do ambush bugs affect the strength of selection on floral traits by changing predation risk
across plant phenotypes?
One of ten trait means differed significantly between plants with ambush bugs present
and absent. Plants with ambush bugs present had a mean daily display 16% larger than plants
without ambush bugs (Table 8). Means of the other nine traits did not differ between plants with
and without ambush bugs (Table 8).
The strength of selection differed between plants with and without ambush bugs for three
traits, daily display size, banner petal shape, and banner petal size. The selection gradient for
daily display size estimated via fruit per plant was ~47% weaker when ambush bugs were
present than when they were absent, and when estimated via seeds per plant it was ~57% weaker
when ambush bugs were present than when they were absent (Table 9). The selection differential
for daily display size estimated via fruit per plant was ~41% weaker when ambush bugs were
present, compared to when they were absent (Fig. 4 and Table 10). The selection differentials
estimated via fruit per plant showed selection for larger banner petals when ambush bugs were
absent, but for smaller banner petals when ambush bugs were present (Table 10). The selection
differentials estimated via seeds per plant showed selection for longer/narrower banner petals
21
when ambush bugs were absent, but for shorter/wider banner petals when ambush bugs were
present (Table 10).
DISCUSSION:
My results support the hypothesis that ambush bugs decrease the strength of selection on
the plant traits they prefer, by increasing predation risk and shifting pollinator visitation away
from plants with preferred traits. Ambush bugs were more abundant on plants with larger daily
displays than plants with smaller daily displays (Table 8). Furthermore, when ambush bugs were
present, selection for larger daily display size was significantly weaker than when ambush bugs
were absent (Tables 9 and 10). From these results, I can infer that the bumblebee pollinators of L.
siphilitica shifted from visiting plants with large daily displays to visiting plants with smaller
daily displays to avoid predation risk (Jones 2010; Jones and Dornhaus 2011; Wang et al. 2013),
and that this shift weakened selection on daily display size. These results are consistent with
previous studies, which have found that bumblebees can recognize and avoid cryptic sit-and-wait
predators (Abbott 2006; Ings and Chittka 2009; Wang et al. 2013) and that the presence of sitand-wait predators on flowers decreases pollinator visitation frequency and duration (Dukas and
Morse 2003, 2005; Suttle 2003; Robertson and Maguire 2005; Reader et al. 2006; GoncalvesSouza et al. 2008; Higginson et al. 2010; Romero et al. 2011).
In contrast, my hypothesis that dragonflies increase the strength of pollinator-mediated
selection by decreasing plant-pollinator interactions (Benkman 2013) was not supported. While I
did find stronger selection on daily display size in the high-dragonfly abundance treatment, and a
shift from selection for brighter to less bright petals (Table 4 and 5), plants did not have lower
22
mean fitness in the high-dragonfly abundance treatment (Tables 1 and 3). Furthermore there is
not consistent evidence that dragonfly abundance affected bumblebee abundance (Appendix A).
My results showing stronger selection on daily display size, and a directional shift from selection
for brighter to less bright flowers in the high-dragonfly abundance treatment, have two possible
explanations. First, it is possible that dragonflies did affect selection, but through a mechanism
other than altered interaction strength. For example, it is possible that dragonflies altered
bumblebee behavior and preferences without decreasing their abundance. Second, it is possible
that a factor other than dragonfly abundance caused the differences in selection between the lowand high-dragonfly abundance treatments. To determine which of these two possibilities cause
selection to differ between the dragonfly treatments, a follow-up study could be done which
manipulates dragonfly abundance. Then, if selection differs between the dragonfly treatments, I
could conclude that dragonflies are affecting selection through a mechanism other than
interaction strength.
While I found no evidence that active-pursuit predators affect interaction strength or
selection via interaction strength, previous studies have found that active-pursuit predators,
including dragonflies, decrease mean plant fitness by decreasing interactions between plants and
pollinators (Knight et al. 2005a; Dukas 2005; Meehan et al. 2005). Differences in the strength of
the top-down effects of active-pursuit predators may be due to differences in the body size of
prey (Romero et al. 2011). In a system reliant on small-bodied pollinators, including sweat bees
(Agapostemon spp.) and flies (Syrphidae and Bombyliidae), Knight et al. (2005a) found that
dragonflies exerted a strong top-down effect, which significantly decreased plant fitness. In
contrast, in a system reliant on large-bodied pollinators, bumblebees, I found that dragonflies had
a weak top-down effect and did not affect plant fitness. Dragonfly prey-capture success
23
decreases as prey size increases (Combes et al. 2013). Thus, while dragonflies do consume
bumblebees (Beatty 1951; Wright 1944; Warren 1915), they may not pose as great a risk to
bumblebees as they do to smaller-bodied pollinators (Rodriguez-Girones and Bosch 2012). If the
strength of the top-down effects of predators depends on prey body size, then predators should
have a stronger top-down effect on plants pollinated by small-bodied pollinators, such as sweat
bees, and a weaker top-down effect on plants pollinated by large-bodied pollinators, such as
bumblebees.
My results suggest that sit-and-wait predators, by altering predation risk across plant
phenotypes, have a stronger top-down effect on selection on plant traits than active-pursuit
predators. This suggests first, that predators, particularly sit-and-wait predators, have the ability
to affect the evolution of plant traits. And second, that interaction strength, contrary to popular
belief, may not be the most important mechanism mediating the strength of selection. These
findings may be generalizable beyond my study system. Many sit-and-wait predators use direct
and indirect cues to choose hunting sites with high prey availability (Morse 1988, 1993, 1999,
2000; Greco and Kevan 1994, 1995; Heiling et al. 2004; Bhaskara et al. 2009; Hanna and Eason
2012; Defrize et al. 2014). Because sit-and-wait predators use cues to select hunting sites, they
should not be randomly distributed across plant phenotypes. Rather, sit-and-wait predators
should be more common on plants with traits that attract pollinators. Sit-and-wait predators,
despite low prey capture rates (Morse 1979), can have a significant effect on pollinator visitation
frequency and duration (Dukas 2001; Suttle 2003; Dukas and Morse 2003, 2005; Robertson and
Maguire 2005; Abbott 2006; Knight et al. 2006; Llandres et al. 2013; Dawson and Chittka
2014). Therefore, by altering predation risk across plant phenotypes, sit-and-wait predators may
be common, yet underappreciated, agents of selection on plant traits.
24
In addition to finding that predators of pollinators can have top-down effects on selection
on floral and inflorescence traits, I also found that the majority of selection on floral and
inflorescence traits was not pollinator-mediated. By comparing the strength of selection on traits
of hand- and open-pollinated plants, I found that while there was significant selection on the
majority of traits, selection was pollinator-mediated for only one trait (Tables 6 and 7). These
results, along with results from other studies, demonstrate that the absence of pollinator-mediated
selection does not imply that there is no selection on floral traits. They also demonstrate the
inverse: that finding significant selection on floral and inflorescence traits does not imply that
selection is pollinator-mediated (Parachnowitsch and Caruso 2008; Caruso et al. 2010).
While ambush bugs preferred plants with larger daily displays, and thereby decreased the
strength of selection on daily display size, selection on two other traits, banner petal shape and
banner petal size, also differed between plants with ambush bugs present and absent (Tables 9
and 10). However, there is no evidence that ambush bugs had preferences for these traits (Table
8). Given the lack of predator preference, one possible explanation is that banner petal shape and
size have a functional role in pollination that is affected by ambush bugs. Banner petal shape
measures the ratio of length to width of the bottom three banner petals that serve as a landing
platform for visiting bumblebees. This landing platform has a vital functional role; without these
three petals, successful bumblebee visitation is reduced from ~90% to ~27% (Owen and
Bradshaw 2011). Previous studies have found that pollinators can have strong preferences for,
and exert selection on petal shape (Møller 1995; Galen and Cuba 2001; Gomez et al. 2008). If
banner petal shape and size affects the efficiency of visiting bumblebees (Owen and Bradshaw
2011) or their risk of being attacked, then selection on banner petal size and shape may have
shifted in response to ambush bug presence. Shorter/wider banner petals may allow bumblebees
25
to find and enter the corolla tube opening more quickly. This could decrease the opportunity for
a hidden ambush bugs to attack visiting bumblebees, thereby allowing plants with shorter/wider
banner petals to receive more successful visits. Similarly, if ambush bugs hide behind banner
petals to conceal themselves from approaching pollinators, and smaller banner petals offer less
opportunity to for concealment, then pollinators may be safer visiting plants with smaller banner
petals. If banner petal shape and size do have functional roles which affect the safety of visiting
pollinators, then this could explain why there was selection for shorter/wider and smaller banner
petals when ambush bugs were present.
One limitation of my study is that I did not manipulate predators. Because of this, I
cannot rule out the possibility that other agents of section influenced my results (Wade and
Kalisz 1990). I elected to utilize a natural gradient of dragonfly abundance because building
dragonfly enclosures or exclosures at such a large scale was infeasible. In contrast to dragonfly
abundance, ambush bug presence could have been manipulated, but I elected to allow ambush
bugs to choose plants freely in order to determine if they had trait preferences. While this
decision limits my ability to make causal claims, if I had manipulated ambush bug presence, I
would not have observed their preference for plants with larger daily displays, and I would not
have been able to test my hypothesis that sit-and-wait predator preferences affect the strength of
selection on plant traits. A follow-up study which manipulates ambush bug presence/absence
would be informative. If ambush bugs were randomly assigned to plants, then they should not
affect the strength of selection.
My results suggest that predators in higher trophic levels may be an underappreciated
cause of selection on plant traits. Given that since Hairston, Smith, and Slobodkin published their
classic paper (1960), evidence has mounted that predators produce cascading ecological effects,
26
it is not surprising that predators can also have cascading evolutionary effects (Estes 2013). The
cascading evolutionary effects of predators in higher trophic levels have received little attention,
because most studies of selection have been restricted to one or two trophic levels (e.g. Benkman
2013). However, my results demonstrate the importance of incorporating additional higher
trophic levels when studying selection on plant traits. My results also suggest that anthropogenic
trophic downgrading (Estes 2011) in addition to damaging ecological consequences, may have
unforeseen cascading evolutionary consequences.
27
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34
Figure 1. Map of field site locations at the Koffler Scientific Reserve. Field sites 1 and 2 are less
than 100m from ponds. Fields sites 3 and 4 are greater than 500m from ponds. Field sites
1 and 3 were found to have high dragonfly abundance, and field sites 2 and 4 were found
to have low dragonfly abundance.
35
Figure 2. Floral traits of Lobelia siphilitica: a) corolla length was measured from the base of the
bracts to the corolla tube opening, b) width of the corolla tube opening was measured from the
bottom of the corolla tube to the top at the point where the corolla tube opening begins, c) banner
petal size was measured as the total area of the top and bottom petals outlined in white, d) banner
petal shape was measured as the ratio of length to width of the bottom three banner petals, which
serve as a landing platform, e) the nectar guide stripes were measured as the white stripes (arrow
1) and the purple background (arrow 2), nectar guide contrast was measured by subtracting the
modal gray value of purple background from the modal gray value of the white stripes, and f) the
area of the corolla underside was measured as the area outlined in white, nectar guide size was
measured by dividing the area of the underside of the corolla tube (f) by the area of the nectar
guide stripes (e. arrow 1)
36
Figure 3. Relationship between daily display size and fruits per plant for Lobelia siphilitica in
low- and high-dragonfly abundance treatments. Fruit per plant is a female fitness measure, and
was relativized for each treatment by dividing each plant’s fitness by the mean plant fitness
within the treatment. Daily display size was standardized by subtracting the treatment mean daily
display from each plant’s daily display size, and then dividing by the treatment standard
deviation, resulting in a mean of zero and a variance of one.
37
Figure 4. Relationship between daily display size, and fruits per plant (a, b) and seeds per plant
(c, d) for Lobelia siphilitica in ambush bug absent and ambush bug present treatments. Fruit per
plant, and seeds per plant are female fitness measures. They were relativized for each treatment
by dividing each plant’s fitness by the mean plant fitness within the treatment. Daily display size
was standardized by subtracting the treatment mean daily display from each plant’s daily display
size, and then dividing by the treatment standard deviation, resulting in a mean of zero and a
variance of one.
38
Table 1. Split-plot analysis of variance model of the effect of dragonfly treatment (low
abundance vs. high abundance), pollination treatment (hand vs. open), and the interaction
between the dragonfly and pollination treatments on three female fitness components of Lobelia
siphilitica plants.
Fitness
measure
Fruit per plant
Seeds per fruit
Seeds per plant
Source
Fdf
P
Dragonfly
Pollination
Dragonfly × Pollination
Dragonfly
Pollination
Dragonfly × Pollination
Dragonfly
Pollination
Dragonfly × Pollination
0.7011,3.4
0.2341,44.5
0.1471,44.5
0.2721,3.6
3.2111,82.4
0.0751,82.4
0.0521,3.6
1.6631,81.7
0.4191,81.7
0.458
0.631
0.703
0.633
0.077
0.785
0.833
0.201
0.519
39
Table 2. Mean values for three female fitness components ± 1 SE (N) for hand- and openpollinated Lobelia siphilitica plants.
Fitness measure
Hand-pollinated
Open-pollinated
Fruit per plant
Seeds per fruit
Seeds per plant
88.73 ± 3.84 (93)
633.23 ± 18.55 (86)
56496.71 ± 2811.04 (86)
89.54 ± 2.72 (195)
581.15 ± 14.86 (168)
53079.06 ± 2200.91 (168)
40
Table 3. Mean values for three female fitness components ± 1 SE (N) for open-pollinated
Lobelia siphilitica plants in low- and high-dragonfly abundance treatments.
Fitness
measure
Fruit per plant
Seeds per fruit
Seeds per plant
Low-dragonfly
abundance
85.041 ± 7.843 (97)
605.803 ± 47.441 (84)
52,274.34 ± 8821.595 (84)
41
High-dragonfly
abundance
93.908 ± 7.843 (98)
556.493 ± 47.441 (84)
53,883.777 ± 8821.595 (84)
Table 4. Linear selection gradients ± 1 SE estimated via three female fitness components for 10
floral and inflorescence traits of open-pollinated Lobelia siphilitica plants in the low- and highdragonfly abundance treatments. F compares the strength of selection between the low- and
high-dragonfly abundance treatments.
Fitness
measure
Fruit per
plant
Trait
Banner petal shape
Banner petal size
Brightness
Chroma
Corolla length
Daily display
Inflorescence height
Nectar guide contrast
Nectar guide size
Width of tube opening
Low-dragonfly
abundance
-0.072 ± 0.038
-0.014 ± 0.053
0.044 ± 0.039
-0.074 ± 0.037*
-0.028 ± 0.039
0.140 ± 0.037**
0.059 ± 0.037
-0.029 ± 0.037
-0.014 ± 0.039
0.034 ± 0.047
N = 97
High-dragonfly
abundance
0.001 ± 0.038
-0.003 ± 0.044
-0.102 ± 0.039*
-0.106 ± 0.039**
-0.055 ± 0.043
0.298 ± 0.042**
0.007 ± 0.042
0.061 ± 0.044
0.029 ± 0.040
0.028 ± 0.045
N = 98
Fdf
2.2031,170NS
0.0021,172NS
7.5951,172**
0.1301,172NS
0.1991,171NS
10.2251,171**
0.9441,171NS
2.8521,168NS
0.6231,171NS
0.0191,171NS
Seeds per
fruit
Banner petal shape
Banner petal size
Brightness
Chroma
Corolla length
Daily display
Inflorescence height
Nectar guide contrast
Nectar guide size
Width of tube opening
0.055 ± 0.036
0.022 ± 0.057
-0.079 ± 0.037*
0.049 ± 0.035
0.023 ± 0.038
0.013 ± 0.035
0.057 ± 0.036
0.040 ± 0.035
-0.070 ± 0.038
0.057 ± 0.047
N = 84
0.026 ± 0.041
0.025 ± 0.047
-0.040 ± 0.042
-0.017 ± 0.042
0.033 ± 0.046
-0.016 ± 0.045
0.052 ± 0.044
0.022 ± 0.050
0.008 ± 0.044
-0.009 ± 0.050
N = 84
0.4861,143NS
0.0041,153NS
0.3811,144NS
1.2941,144NS
0.0571,144NS
0.3521,143NS
0.0501,143NS
0.0661,145NS
1.1241,143NS
0.7791,143NS
Seeds per
plant
Banner petal shape
Banner petal size
Brightness
Chroma
Corolla length
Daily display
Inflorescence height
Nectar guide contrast
Nectar guide size
Width of tube opening
-0.043 ± 0.051
-0.031 ± 0.080
-0.039 ± 0.053
-0.007 ± 0.049
-0.010 ± 0.054
0.163 ± 0.049**
0.084 ± 0.051
0.027 ± 0.050
-0.062 ± 0.054
0.162 ± 0.067*
N = 84
0.034 ± 0.056
0.001 ± 0.064
-0.154 ± 0.057**
-0.101 ± 0.057
-0.030 ± 0.062
0.296 ± 0.060 **
0.094 ± 0.060
0.149 ± 0.067*
0.026 ± 0.059
-0.019 ± 0.067
N = 84
1.0921,143NS
0.0501,143NS
2.4031,144NS
1.3471,144NS
0.0461,143NS
2.6231,143NS
0.0011,143NS
2.8781,145NS
0.8801,143NS
3.2371,143NS
* P < 0.05, ** P ≤ 0.01
42
Table 5. Linear selection differentials ± 1 SE estimated via three female fitness components for
10 floral and inflorescence traits of open-pollinated Lobelia siphilitica plants in the low- and
high-dragonfly abundance treatments. F compares the strength of selection between the low- and
high-dragonfly abundance treatments.
Fitness
measure
Fruit per
plant
Trait
Banner petal shape
Banner petal size
Brightness
Chroma
Corolla length
Daily display
Inflorescence height
Nectar guide contrast
Nectar guide size
Width of tube opening
Low-dragonfly
abundance
-0.037 ± 0.037
0.028 ± 0.039
0.035 ± 0.038
-0.063 ± 0.038
0.000 ± 0.038
0.142 ± 0.036**
0.095 ± 0.036**
-0.032 ± 0.037
-0.007 ± 0.038
0.050 ± 0.038
N = 97
High-dragonfly
abundance
0.006 ± 0.047
0.028 ± 0.047
-0.016 ± 0.046
-0.062 ± 0.047
-0.015 ± 0.046
0.261 ± 0.039 **
0.089 ± 0.046
0.011 ± 0.049
0.049 ± 0.046
0.032 ± 0.047
N = 98
Fdf
0.6481,188NS
0.0241,190NS
0.7801,189NS
0.0401,190NS
0.0891,188NS
6.9591,189**
0.1831,188NS
0.6091,190NS
0.9401,188NS
0.1471,190NS
Seeds per
fruit
Banner petal shape
Banner petal size
Brightness
Chroma
Corolla length
Daily display
Inflorescence height
Nectar guide contrast
Nectar guide size
Width of tube opening
0.032 ± 0.034
0.057 ± 0.035
-0.011 ± 0.034
0.038 ± 0.034
0.052 ± 0.034
0.030 ± 0.034
0.056 ± 0.033
0.044 ± 0.033
-0.048 ± 0.35
0.073 ± 0.033*
N = 84
0.004 ± 0.038
0.038 ± 0.038
-0.042 ± 0.037
-0.024 ± 0.038
0.049 ± 0.037
-0.009 ± 0.038
0.050 ± 0.037
0.004 ± 0.040
0.017 ± 0.038
0.018 ± 0.038
N = 84
0.3481,162NS
0.3051,162NS
0.4631,162NS
1.3121,163NS
0.0011,161NS
0.6861,161NS
0.1201,161NS
0.6091,163NS
1.1481,161NS
1.1311,162NS
Seeds per
plant
Banner petal shape
Banner petal size
Brightness
Chroma
Corolla length
Daily display
Inflorescence height
Nectar guide contrast
Nectar guide size
Width of tube opening
-0.035 ± 0.051
0.106 ± 0.052*
0.016 ± 0.052
-0.012 ± 0.052
0.048 ± 0.051
0.178 ± 0.048**
0.134 ± 0.049**
0.019 ± 0.051
-0.036 ± 0.053
0.158 ± 0.049**
N = 84
0.024 ± 0.062
0.039 ± 0.063
-0.048 ± 0.062
-0.043 ± 0.063
0.004 ± 0.062
0.266 ± 0.055**
0.161 ± 0.060**
0.074 ± 0.065
0.023 ± 0.063
0.028 ± 0.062
N = 84
0.6061,161NS
1.0061,16NS
0.6151,161NS
0.0861,162NS
0.3771,161NS
1.3581,161NS
0.0011,161NS
0.7071,163NS
0.2661,161NS
2.4501,162NS
* P < 0.05, ** P < 0.01
43
Table 6. Linear selection gradients ± 1 SE estimated via three female fitness components for 10
floral and inflorescence traits of hand- and open-pollinated Lobelia siphilitica plants. F compares
the strength of selection between the hand- and open-pollination treatments.
Fitness
measure
Fruit per
plant
Seeds per
fruit
Seeds per
plant
Trait
Hand-pollinated
Open-pollinated
Fdf
Banner petal shape
Banner petal size
Brightness
Chroma
Corolla length
Daily display
Inflorescence height
Nectar guide contrast
Nectar guide size
Width of tube
opening
0.003 ± 0.038
0.061 ± 0.047
0.013 ± 0.038
-0.112 ± 0.038**
-0.116 ± 0.042**
0.255 ± 0.043**
0.009 ± 0.042
-0.024 ± 0.036
0.087 ± 0.037*
0.043 ± 0.045
-0.018 ± 0.027
0.009 ± 0.034
-0.032 ± 0.028
-0.082 ± 0.027**
-0.044 ± 0.029
0.206 ± 0.028**
0.038 ± 0.028
0.010 ± 0.029
0.002 ± 0.028
0.032 ± 0.033
0.1331,262NS
0.4801,262NS
0.5931,262NS
0.0821, 262NS
1.256 1,262NS
1.071 1,262NS
0.313 1,262NS
0.5561,262NS
3.5781,262NS
0.018 1,262NS
N = 95
N = 198
-0.026 ± 0.033
-0.018 ± 0.040
-0.046 ± 0.033
-0.009 ± 0.032
0.021 ± 0.035
0.000 ± 0.036
0.013 ± 0.037
-0.011 ± 0.031
-0.012 ± 0.032
0.087 ± 0.038*
0.040 ± 0.026
0.028 ± 0.034
-0.053 ± 0.027*
0.015 ± 0.026
0.024 ± 0.028
-0.002 ± 0.027
0.057 ± 0.027*
0.024 ± 0.028
-0.043 ± 0.027
0.028 ± 0.033
N = 86
N = 168
0.025 ± 0.046
0.067 ± 0.057
-0.027 ± 0.047
-0.145 ± 0.045**
-0.102 ± 0.049*
0.256 ± 0.052**
0.013 ± 0.052
-0.019 ± 0.044
0.084 ± 0.046
0.089 ± 0.054
0.017 ± 0.037
0.016 ± 0.048
-0.100 ± 0.038**
-0.048 ± 0.037
-0.027 ± 0.040
0.218 ± 0.039**
0.093 ± 0.039*
0.062 ± 0.039
-0.044 ± 0.039
N = 86
N = 168
Banner petal shape
Banner petal size
Brightness
Chroma
Corolla length
Daily display
Inflorescence height
Nectar guide contrast
Nectar guide size
Width of tube
opening
Banner petal shape
Banner petal size
Brightness
Chroma
Corolla length
Daily display
Inflorescence height
Nectar guide contrast
Nectar guide size
Width of tube
opening
* P < 0.05, ** P < 0.01
44
0.077 ± 0.046
2.4131, 228NS
0.8371, 228NS
0.0011, 228NS
0.1561, 228NS
0.0031, 228NS
1.1 x 10-51, 228NS
0.8031, 228NS
0.6401, 228NS
0.5671, 228NS
1.1331, 228NS
1.17 x 10-41,228NS
0.2481,228NS
0.6291,228NS
1.5541,228NS
0.7371,228NS
0.1241,228NS
1.2841,228NS
1.5791,228NS
4.3881,228*
5.8 x 10-51,228NS
Table 7. Linear selection differentials ± 1 SE estimated via three female fitness components for
10 floral and inflorescence traits of hand- and open-pollinated Lobelia siphilitica plants. F
compares the strength of selection between the hand- and open-pollination treatments.
Fitness
measure
Fruit per
plant
Trait
Banner petal shape
Banner petal size
Brightness
Chroma
Corolla length
Daily display
Inflorescence height
Nectar guide contrast
Nectar guide size
Width of tube opening
Handpollinated
-0.078 ± 0.044
0.099 ± 0.044*
0.036 ± 0.048
-0.108 ± 0.044*
-0.016 ± 0.046
0.234 ± 0.041**
0.044 ± 0.046
-0.005 ± 0.045
0.058 ± 0.044
0.059 ± 0.045
N = 95
Open-pollinated
Fdf
-0.012 ± 0.030
0.027 ± 0.031
0.007 ± 0.030
-0.062 ± 0.030*
-0.006 ± 0.030
0.201 ± 0.027**
0.093 ± 0.029**
-0.010 ± 0.031
0.015 ± 0.030
0.040 ± 0.031
N = 198
1.4911,280NS
0.9111,280NS
0.3471,280NS
0.1191,280NS
0.0121,280NS
0.3141,280NS
0.5541,280NS
0.0101,280NS
0.7011,280NS
4.72 x 10-41,280NS
Seeds per
fruit
Banner petal shape
Banner petal size
Brightness
Chroma
Corolla length
Daily display
Inflorescence height
Nectar guide contrast
Nectar guide size
Width of tube opening
-0.017 ± 0.030
0.044 ± 0.029
-0.039 ± 0.031
-0.002 ± 0.031
0.037 ± 0.030
0.014 ± 0.032
0.038 ± 0.030
-0.009 ± 0.029
-0.013 ± 0.030
0.081 ± 0.028**
N = 86
0.016 ± 0.025
0.048 ± 0.026
-0.026 ± 0.025
0.008 ± 0.025
0.050 ± 0.025*
0.011 ± 0.025
0.052 ± 0.025*
0.026 ± 0.025
-0.025 ± 0.026
0.048 ± 0.026
N = 168
0.7911, 246NS
0.0041, 246NS
0.2911, 246NS
0.0601, 246NS
0.0641, 246NS
0.0301, 246NS
0.2731, 246NS
0.7521, 246NS
0.1701, 246NS
0.5121, 246NS
Seeds per
plant
Banner petal shape
Banner petal size
Brightness
Chroma
Corolla length
Daily display
Inflorescence height
Nectar guide contrast
Nectar guide size
Width of tube opening
-0.064 ± 0.053
0.127 ± 0.050*
-0.002 ± 0.056
-0.135 ± 0.051*
-0.007 ± 0.052
0.239 ± 0.049**
0.075 ± 0.053
-0.008 ± 0.052
0.054 ± 0.052
0.104 ± 0.051*
N = 86
0.000 ± 0.040
0.070 ± 0.041
-0.018 ± 0.040
-0.028 ± 0.041
0.027 ± 0.040
0.223 ± 0.037**
0.149 ± 0.038**
0.044 ± 0.0± 0.040
-0.012 ± 0.041
0.097 ± 0.041*
N = 168
1.0291,246NS
0.3631,246NS
0.0011,246NS
1.6841,246NS
0.2471,246NS
0.0911,246NS
1.5811,246NS
0.6361,246NS
1.2101,246NS
0.0511,246NS
* P < 0.05, P <0.0
45
Table 8. Mean trait values ± 1 SE (N) for open-pollinated Lobelia siphilitica plants with ambush bugs present and absent. t compares
mean trait values between plants with ambush bugs present and absent.
Traits
Banner petal shape
Banner petal size
Brightness
Chroma
Corolla length
Daily display
Inflorescence height
Nectar guide contrast
Nectar guide size
Width of tube opening
* P < 0.05
Ambush bugs
Absent
1.402 ± 0.019 (68)
1.510 ± 0.044 (68)
-0.124 ± 0.108 (68)
0.189 ± 0.121 (68)
1.345 ± 0.010 (68)
15.327 ± 0.689 (68)
46.087 ± 1.321 (68)
23.265 ± 1.986(68)
6.977 ± 0.752 (68)
0.587 ± 0.008 (68)
Ambush bugs
Present
1.416 ± 0.016 (127)
1.532 ± 0.031 (127)
0.044 ± 0.090 (127)
-0.022 ± 0.086 (127)
1.336 ± 0.010 (127)
17.801 ± 0.622 (127)
47.104 ± 0.823 (127)
22.126 ± 1.350 (127)
6.014 ± 0.206 (127)
0.588 ± 0.006 (127)
46
tdf
P
0.510193
0.414193
1.147193
-1.438 193
-0.615193
2.502193*
0.686193
-0.485193
-1.23577.186
0.109193
0.610
0.680
0.253
0.152
0.539
0.013*
0.493
0.628
0.221
0.913
Table 9. Linear selection gradients ± 1 SE estimated via three female fitness components for 10
floral and inflorescence traits of open-pollinated Lobelia siphilitica plants with ambush bugs
present and absent. F compares the strength of selection between plants with ambush bugs
present and absent.
Ambush bugs
Fitness
measure
Fruit per
plant
Trait
Absent
Present
F1, 143
Banner petal shape
Banner petal size
Brightness
Chroma
Corolla length
Daily display
Inflorescence height
Nectar guide contrast
Nectar guide size
Width of tube opening
0.039 (0.046)
0.006 (0.056)
-0.018 (0.050)
-0.108 (0.050)*
0.000 (0.051)
0.308 (0.053)**
-0.031 (0.052)
0.058 (0.056)
0.025 (0.050)
0.034 (0.052)
N = 68
-0.061 (0.034)
-0.020 (0.045)
-0.047 (0.034)
-0.079 (0.033)*
-0.053 (0.037)
0.163 (0.033)**
0.070 (0.034)*
-0.023 (0.036)
-0.036 (0.036)
0.054 (0.043)
N = 127
2.466
0.332
0.339
0.139
0.630
7.848**
2.944
0.187
0.408
0.017
Seeds per
fruit
Banner petal shape
Banner petal size
Brightness
Chroma
Corolla length
Daily display
Inflorescence height
Nectar guide contrast
Nectar guide size
Width of tube opening
0.113 (0.049)*
0.018 (0.058)
-0.092 (0.056)
0.019 (0.053)
0.048 (0.056)
-0.016 (0.057)
0.057 (0.056)
0.024 (0.059)
-0.015 (0.053)
0.018 (0.056)
N = 109
0.009 (0.031)
0.066 (0.044)
-0.034 (0.031)
0.016 (0.030)
-0.008 (0.035)
0.015 (0.031)
0.066 (0.031)*
0.001 (0.033)
-0.070 (0.034)*
0.023 (0.041)
N = 109
4.292*
0.811
1.088
0.112
1.428
0.695
2.61 x 10-4
0.281
0.324
0.023
Seeds per
Plant
Banner petal shape
Banner petal size
Brightness
Chroma
Corolla length
Daily display
Inflorescence height
Nectar guide contrast
Nectar guide size
Width of tube opening
0.124 (0.061)*
0.018 (0.073)
-0.149 (0.07)*
-0.084 (0.067)
0.035 (0.07)
0.377 (0.072)**
0.028 (0.071)
0.127 (0.074)
-0.011 (0.067)
0.013 (0.071)
N = 68
-0.041 (0.046)
0.019 (0.066)
-0.09 (0.047)
-0.043 (0.045)
-0.068 (0.052)
0.164 (0.046)**
0.138 (0.046)**
0.002 (0.050)
-0.078 (0.051)
0.112 (0.062)
N = 109
4.953*
0.002
5.85
0.155
1.902
5.414*
1.588
2.121
0.554
1.198
* P < 0.05, ** P < 0.01
47
Table 10. Linear selection differentials ± 1 SE estimated via three female fitness components for
10 floral and inflorescence traits of open-pollinated Lobelia siphilitica plants with ambush bugs
present and absent. F compares the strength of selection between plants with ambush bugs
present and absent.
Ambush bugs
Fitness
measure
Fruit per
plant
Trait
Absent
Present
F1, 161
Banner petal shape
Banner petal size
Brightness
Chroma
Corolla length
Daily display
Inflorescence height
Nectar guide contrast
Nectar guide size
Width of tube opening
0.051 (0.054)
0.079 (0.055)
0.074 (0.054)
-0.029 (0.055)
0.017 (0.055)
0.280 (0.045)**
0.070 (0.054)
0.024 (0.056)
0.021 (0.057)
0.080 (0.055)
N = 68
-0.044 (0.036)
-0.010 (0.038)
-0.025 (0.036)
-0.078 (0.036)*
-0.013 (0.036)
0.166 (0.033)**
0.110 (0.035)**
-0.020 (0.037)
-0.016 (0.037)
0.026 (0.038)
N = 127
0.850
4.615*
0.039
1.545
0.497
5.962*
0.027
0.155
0.863
1.876
Seeds per
fruit
Banner petal shape
Banner petal size
Brightness
Chroma
Corolla length
Daily display
Inflorescence height
Nectar guide contrast
Nectar guide size
Width of tube opening
0.087 (0.045)
0.033 (0.046)
-0.075 (0.046)
0.005 (0.046)
0.047 (0.046)
-0.003 (0.048)
0.027 (0.046)
-0.017 (0.048)
0.010 (0.048)
0.024 (0.047)
N = 59
-0.007 (0.030)
0.065 (0.030)*
-0.003 (0.029)
0.01 (0.030)
0.042 (0.029)
0.029 (0.029)
0.069 (0.028)*
0.035 (0.030)
-0.043 (0.03)
0.054 (0.03)
N = 109
3.512
1.008
2.318
0.006
0.127
0.998
0.738
0.583
0.146
0.365
Seeds per
plant
Banner petal shape
Banner petal size
Brightness
Chroma
Corolla length
Daily display
Inflorescence height
Nectar guide contrast
Nectar guide size
Width of tube opening
0.138 (0.072)
0.109 (0.073)
0.025 (0.075)
0.023 (0.073)
0.042 (0.075)
0.337 (0.061)**
0.091 (0.072)
0.060 (0.076)
0.002 (0.077)
0.112 (0.074)
N = 59
-0.054 (0.049)
0.055 (0.051)
-0.033 (0.049)
-0.057 (0.049)
0.014 (0.049)
0.187 (0.045)**
0.187 (0.045)**
0.027 (0.050)
-0.018 (0.051)
0.092 (0.050)
N = 109
5.168*
0.247
0.321
0.864
0.200
2.582
1.525
0.109
0.031
0.015
* P < 0.05, ** P < 0.01
48
Appendix A
Bumblebee Abundance
Bumblebee abundance at each field site was determined during the dragonfly point
counts. During each point count in addition to counting the number of dragonflies, the observers
noted the number of bumblebees that entered their field of view. While bumblebees were not
identified to species level, the majority of bumblebees counted were actively foraging on Lobelia
siphilitica flowers (personal observation). Sites one through three had similar bumblebee
abundance, mean (1 SE) 8.4 (1.79); range: 0-51, 9.5 (1.75); range: 1-40, and 7.8 (1.07); range: 226) bumblebees per 20 minutes of observation, respectively. Site four however, consistently had
higher bumblebee abundance, (mean bumblebees/20 minutes of observation (1 SE) 17.8 (1.52);
range:10-41). Field site 4 was far from ponds and had low dragonfly abundance (Fig. 1).
49