Download I expect that pollinator visitation rates will be positively correlated

Stacy Endriss
PhD student, Graduate Degree Program in Ecology
Title: Impact of herbivory on pollination ecology of Verbascum thapsus
Questions: An important question within pollination ecology is how herbivory influences plant-pollinator
interactions. Although plants are commonly being simultaneously attacked by many different types of
herbivores, understanding of how differences in herbivory influences plant-pollinator interactions is still
frustratingly unclear. I therefore propose to specifically address the following questions: (1) how do
differences in herbivory influence plant traits as well as pollinator visitation, and (2) do differences in
herbivory differentially influence introduced versus native populations.
Study System: Verbascum thapsus (common mullein) is an ideal system for investigating pollinator
ecology. An introduced class C noxious weed within Colorado, V. thapsus is a typically biennial forb that
produces a showy inflorescence that can grow up to 2m tall and display more than a hundred small,
yellow flowers over the course of a season. Although flowering is indeterminate, with blooms as early as
June and as late as first frost, individual flowers are only open for less than 24 hours. V. thapsus
experiences delayed self-pollination in the absence of an outcrossing event, though little is known about
how V. thapsus interacts with pollinator communities within either the introduced or native range.
Hypothesis: Pollinator identity and visitation rates will be influenced by population size and density,
distance to the closest flowering neighbor, plant size, level of herbivory, as well as reproductive metrics
such as number of open flowers, the height of the inflorescence, and nectar quantity and quality.
Methods:
Field surveys of V. thapsus populations: During Summer 2013 undergraduate interns and I will survey
eight V. thapsus populations, each of which has been selected based on differences in the composition of
the surrounding plant community. Within each population we will tag up to 50 second-year rosettes.
Every two weeks, we will count the number of open flowers on each plant. As floral resources can
influence pollinator visitation rates, we will also estimate population size and density. We will categorize
the size of the population based on one of four different categories: < 20 plants, 21-100 plants, 101-500
plants, or > 500 plants. To estimate population density, we will walk a 50m transect through the densest
part of the population and count every plant that is within 1m of the path. For each plant we will also
measure: flower diameter, distance to the closest flowering neighbor, plant size (rosette diameter and
length of the longest leaf), level of herbivory (estimates of percent damage on both young and old leaves),
as well as pollination metrics (i.e. pollinator identity and visitation rate). In addition, for a subset of 5
plants within each population we will determine nectar quality (using microcapillary tubes) for up to 5
open flowers per plant. We will also measure nectar quality by then placing these nectar samples on a
refractometer to determine sugar composition. During one early-, mid-, and late- season sampling we will
subsample nectar from at least 2 populations to measure levels of defensive chemistry using HPLC.
For each sampling period we will also note which 5 plants have the lowest measured levels of herbivory
as well as which 5 plants have the highest measured levels of herbivory. For each of these plants, we will
mark up to 5 open flowers by tying a thin piece of wire around the base of the flower (different colors will
signify different sampling periods). Once fruits begin to ripen, we will harvest fruit capsules to estimate
seed set and seed quality for each of the different treatments. As plants with taller inflorescences have
been shown to receive a greater and more diverse pollen load than plants with shorter inflorescences, we
will also record the height of each capsule before removing it from the plant. We will estimate average
seed mass from each of the capsules by weighing 33 seeds from each capsule. By then dividing the total
mass of all seeds within the capsule by the average seed weight we will in turn estimate the total number
of seeds within the capsule.
To determine how herbivory influences pollinator visitation rates we will also sample early, mid-, and
late-season using standardized methods for netting at flowers. We will only sample pollinators between
6AM and 10AM on days with no precipitation and low levels of wind. We will perform six 10-minute
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sample periods and record and then collect all individuals that touch the reproductive parts of the flower.
All collected specimen will be preserved for later identification.
Expected Outcomes:
I expect that pollinator visitation rates will be positively correlated with population size and density,
distance to the closest flowering neighbor, plant size, number of open flowers, the height of the
inflorescence, as well as nectar quantity and quality. In addition, I predict that pollinator visitation will be
negatively correlated with levels of herbivory, as plants with high levels of herbivory will experience
elevated levels of defensive chemistry within their nectar (which will in turn deter pollinators). I also
predict that seed quality and quantity from capsules taken from plants that experienced low levels of
herbivory will be significantly greater than that of capsules taken from plants that experienced high levels
of herbivory.