skip navigation

TEACHING ALL VOLUMES SUBMIT WORK SEARCH TIEE
VOLUME 2: Table of Contents TEACHING ISSUES AND EXPERIMENTS IN ECOLOGY
EXPERIMENTS


Introduction:

       Why are flowers so pretty? It is of little benefit to a wild plant to be admired. Why have plants put so much energy into the structure of flowers and production of nectar and other rewards? Of course, the flower’s purpose is to result in sexual reproduction. Berenbaum (1995) states that “Sexual reproduction is just as important for plants as it is for animals when it comes to generating genetic variation, but plants have a singular disadvantage compared to animals when it comes to sex: they can't just get up and find themselves a mate. " Plants must rely on pollen vectors, from wind to insects to birds, to transport their pollen to another individual. Those visitors must be attracted to the same species repeatedly to bring about pollination. Visitors must cause pollen transfer for flowers to be successful. Usually this means that the visitor must be attracted, collect pollen accidentally by brushing floral parts, or purposefully collect pollen to take back to a nest, and then visit another flower of the same species and brush up against the stigma, effecting pollination. Flowers can attract pollinators by providing ample nectar of the right composition, and by advertising this nectar by deep shape and recognizable floral patterns, by providing excess pollen as food, or by providing shelter or a place to raise (and feed) young - or by at least looking as if they do (Faegri and van der Pijl 1971). We human observers have tastes that are somewhat similar to those of the birds and bees when it comes to floral attractiveness (although we vary from carrion beetles and flies, as we do NOT consider the smell of rotting meat attractive). Our perception of color differs from that of non-vertebrate pollinators. Bees don’t see colors at the red end of what we consider the visible light spectrum, but they do see colors of ultraviolet. Many flowers have ultraviolet markings that act as nectar guides that cue insects on where to find floral rewards (Barth 1991, Buchmann and Nabhan 1996, Proctor et al 1996).

       In some cases, there are many species of plants, or many flowers of the same species, open at the same time, resulting in a shortage of possible pollinators and competition among the plants for visits (Mosquin 1971, Waser 1983, Caruso 2000). This may result in differences in flowering time to reduce competition for pollinators (Frankie 1975, Anderson and Schelfhout 1980) or in changes in floral structure (Waser 1983, Fishman and Wyatt 1999, Medel et al 2003). In other cases, pollinators must compete with each other, as the floral rewards are in short supply (Pleasants 1981, Pyke 1982, Thomson 2004).

       Natural selection has favored those flowering plants that are most attractive to pollinators, and those pollinators best able to get floral rewards. Millions of years of coevolution between flowering plants and their pollinators, with each participating species population acting as a selective agent on the other, have resulted in overwhelming biodiversity of both insects and flowering plants (Stebbins 1983). “The shapes and colors of the flowers, their scent, their location on the stalks, the season and daily schedule of their pollen and nectar offerings, as well as other qualities we admire but seldom understand, are adjusted precisely to attract particular species of insects; and those specialists in turn, whether beetles, butterflies, bees, or some other group, are genetically adapted to respond to certain kinds of flowers” (Wilson 1999). The mutualistic relationships that develop between a flowering plant , which benefits by cross-fertilization and its most effective pollinator, which benefits from an enhanced food source, are also influenced by other species populations. There may be herbivores and nectar thieves that visit flowers to acquire rewards, but do not disperse pollen (Irwin and Brody 1999, Maloof and Inouye 2000). We see the results of reciprocal selective pressures, with adaptations of the flower to restrict nectar acquisition to a visitor with the ability to move its pollen effectively, and adaptations of the insect to acquire nectar in long, curved tubes. Other organisms that interact with both flower and pollinator may complicate the system, resulting in apparent or real maladaptations (Thompson et al 2002).

       The most frequent visitor to a flowering plant is not necessarily the one that is the most effective pollinator. Natural selection will result in the flower adapting to the pollinator that is most likely to bring about effective pollen transfer for fertilization. Floral color, shape, placement, timing, and reward will be selected, with the most attractive flower getting the most effective visits, producing more viable seed, and leaving more offspring in future gene pools. The most likely pollinator can often be deduced from floral characteristics, with flowers that attract the same type of pollinator converging in morphology into what we call pollination syndromes (Table 1, Key 2) but it usually takes careful observation to sort out specific pollination relationships. Most flowers attract several different kinds of pollinators, with several different attractants which may not be consistent with the “syndrome”.

<top>

______________________________________________________________


Materials and Methods:


Study Site(s).

       Outdoors, anywhere there are plants in flower - ranging from spring beauties in the lawn, to flower beds on campus, to insect pollinated trees like crabapple or buckeye.


Overview of Data Collection and Analysis Methods.

Example Assignment - Natural History Observations.

       The class might be divided into three groups, one to estimate visitation rates, one to determine the phenology (seasonal and/or daily timing of flowering) of the flowering species (Ohio buckeye in this case), and one to collect visitors and determine how much and what kind of pollen is carried. Alternatively, any one part could be used. Once you have these kinds of data about the most likely flowering plants and visitors for the time of your class, you may want to provide students with these data and go straight to the student-generated questions. Many of the techniques are described in Kearns and Inouye (1993).

____________________________________________


Group 1 - Visitation Rates

Equipment Needed:

Procedure:

  1. Each person in the group should find a branch (buckeye) or a plot (spring beauties) with receptive flowers (pollen is being shed, and/or stigmatic surface is visible and sticky). Mark the branch or plot with marking tape (buckeye) or flags (spring beauties).
  2. Carefully examine the flowers. Use the key to pollination syndromes (sets of traits of flowers thought to attract and/or accommodate pollen vectors, and sets of traits of animals that allow them to exploit flowers with those traits) to predict visitors by floral morphology.
  3. Count the number of individual OPEN flowers on the branch to be observed, or in the marked plot. Count only the flowers you will observe - and record visits on only the flowers selected or in the plot.
  4. Observe the marked branch or plot for three observation periods of exactly 10 minutes, counting the number of visits by each category of visitor (categories include honeybees, bumblebees, small bees, flies, butterflies, beetles, and birds).
  5. Note the ambient air temperature in oC.
  6. Return to the same branch or plot and make three ten-minute observations at additional assigned time (later in the day, or earlier on another day - assigned times will cover two hour blocks - 0800 to 1000, 1000 to 1200, 1200 to 1400, 1400 to 1600, and 1600 to 1800 hr),
  7. Note the temperature in oC
  8. Calculate the average number of visits per category of visitor per flower per observation period.,
  9. Is the most frequent visitor the one you expected from the pollination syndrome key? If not, why might there be a difference in what the flower appears to attract and what actually visits most frequently?
  10. What were each of the visitors doing at the flower? Does that activity promote pollen transfer?
  11. Turn in your observations to your instructor so that they may be compiled. Pick up the compiled observations from all groups,
  12. Estimate the number of visits that can be expected per flower at each observation time.



Report Form: Visitation Rates - Group 1, In-class Observation

Time of Day ______________ Temperature ___________ (oC)

Number of flowers observed_____________ Number of minutes observed _______________

Categoryobservation #1observation #2 observation #3average
Honeybees




Bumblebees




Small bees




Flies




Butterflies




Beetles




Other




Total





Report Form: Visitation Rates - Group 1, Additional Assigned Observation

Time of Day ______________ Temperature ___________ (oC)

Number of flowers observed_____________ Number of minutes observed _______________

Categoryobservation #1observation #2 observation #3average
Honeybees




Bumblebees




Small bees




Flies




Butterflies




Beetles




Other




Total






Key 1 - Major Insect Visitors to Flowers (based on Central Illinois)

1 Wings not visible, or hard wing covers concealing wings.... 2
2 No wings, narrow area between thorax and abdomen.... Ants
2' Hard wing covers conceal flight wings, form line down middle of back, chewing mouthparts.... Beetles
1' Wings visible.... 3
3 One set of filamentous wings, eyes large and obvious (careful - some Syrphid flies mimic bees).... Flies
3' Two sets of wings.... 4
4 Both sets of wings often colorful, covered with scales.... 5
5 Antennae with knob-like ends, wings usually folded when at rest.... Butterflies
5' Antennae with feathered ends, no knob, wings often open at rest.... Moths
4' Wings membranous, usually clear.... Bees and
Wasps, 6
6 Thorax and abdomen joined by narrow “waist,” abdomen often pointed.... Wasps
6' “Waist” not as marked, body usually hairy.... Bees, 7
7 Pollen carried on “belly”.... Megachilid
leafcutting bee
7' Pollen carried mainly on leg.... 8
8 Usually small (~5-10mm), black or metallic green, short tongued.... Halictids,
“sweat bees”
Andrenids
8' Long tongue, usually over 12 mm.... 9
9 Spur on hind leg, abdomen often appears striped.... Anthophorid
digger bees
9' No spur, body robust, usually over 20mm, yellow and black, eyes not hairy.... Bumblebees
Bombus
9" No spur, golden brown color, 12-15mm, hairy eyeballs(!).... Honeybees
Apis mellifera


Key 2 - Dichotomous key to pollination syndromes

1 Flowers small, inconspicuous and usually green or dull in color, petals reduced or absent.... Wind
1' Flowers conspicuous, usually with white or colored petals.... 2
2 Flowers regular in shape, radially symmetrical.... 3
3 Flowers purple-brown or greenish in color, often with strong odor of rotting fruit or meat, little floral depth.... 4
4 Flowers purple-brown, sometimes with a “light window”.... Flies
4' Odor day or night, dull color.... Beetles
3' Flowers with little odor, or sweet odor.... 5
5 Flowers with deep corolla tube.... 6
6 Flowers red, open in day, little or no odor, no nectar guide, nectar plentiful.... Hummingbirds
6' Flowers not pure red, usually sweet odor.... 7
7 Flowers yellow, blue, or purple, corolla tube not narrow, but sometimes needing forced opening, often with nectar guides.... Long tongued bees
7' Flowers red, purple or white, corolla tube or spur narrow, usually lack nectar guide.... 8
8 Flowers purple or pink, diurnal, upright, with landing area.... Butterflies
8' Flowers white or pale, pendant, open or producing odor at night.... Moths
(in some
areas, bats)
5' Flowers more dish-shaped, reward accessible, yellow, or with abundant pollen.... Bees, Flies,
small moths
2' Flowers irregular in shape, bilaterally symmetrical.... 9
9 Flowers red, little or no odor.... Hummingbirds
9' Flowers with odor, usually with nectar guides.... Bees


Table 1. Pollination Syndromes: Characteristics of flowers and the animal vectors that shape them.
Vector Characteristics of Flower Characteristics of Vector
Wind * Inconspicuous, green or dull in color, petals reduced or absent, abundant and in canopy * Abiotic
Beetles, flies * Dull colors, dark red, strong, spicy odor, or odor of rotting flesh, flat shape
* May have light window (flies)
* Good sense of smell
* Some lay eggs in rotting flesh
Bees * Often blue or yellow, with landing platform
* Often have markings that act as nectar guides, sometimes in UV spectrum
* Reduced numbers of floral parts
* Often irregular in shape
* May have deep tube or spur for nectar
* Good sense of vision, smell
* Often have body hairs
* Can perceive depth, “count” petals
* Do not see true red - see UV
Moths * Open at dusk or night, emit sweet odor at night
* Often dull or white
* Long corolla, no landing platform
* Most active at night
* Strong sense of smell
* Have long proboscis for nectar acquisition
Butterflies * Open in day, emit some odor in day
* Landing platform
* Long corolla tube, narrow
* May be blue, purple, red, yellow
* May have nectar guide
* Active in day
* Have long, thin proboscis for nectar acquisition
* Can see red
* Alight on blossoms
Hummingbirds * Red, large flowers with deep nectar tube and abundant nectar
* Little or no fragrance
* Open in day
* No landing platform
* No nectar guide
* Vision much like human - see red
* Long bill and tongue, large body
* Little sense of smell
* Intelligent - remember and return to flowers with abundant reward
* Active in day
* Approach flower and hover

____________________________________________


Group 2 - Phenology

Equipment Needed:

Procedure:

  1. Carefully examine flowers. Determine if some are shedding pollen, with stigmatic surfaces not receptive (functionally male), or if pollen is all shed, but stigmatic surfaces are open and sticky (functionally female), or if both stamens and pistils are functional, or if both are finished. Note presence of a scent or nectar. Note color patterns, and presence of nectar guides.
  2. Mark at least 15 individual flowers with numbered small tags, with colored floss, or with colored toothpicks. Note the phase or phenological state of each. The group should decide on criteria to describe each phase (see examples for spring beauty and partridge pea).
  3. If population size estimates are required, randomly select at least 10 one-meter squared plots and count number of individual plants of the target species that are in flower.
  4. Return to collect data on phenological phase of marked flowers and population in flower at four additional assigned times or days. Observations of class members should continue for at least 5 days, or at 4 times of day on one day if the flowers are short-lived.
  5. Turn in observations to your instructor so that they may be compiled. Make sure to pick up compiled data.
  6. Determine the sequence of each phase of floral function, the average longevity of each phase, and how long the flower is open for visitation.


Phase Key must be designed or modified depending upon what species of flowering plant is the target.

For Spring Beauty: Key to Phases: (from closed bud to flower finished)



Date and Time of Observations:
Observation Date Time
1

2

3

4

5



Phase at Each Observation
Flower # Phase A Phase B Phase C Phase D Phase E
1




2




3




4




5




6




7




8




9




10




11




12




13




14




15




____________________________________________


Group 3 - Visitors

Equipment Needed to Collect Insects:

Equipment Needed to Collect Pollen:

Procedure:

  1. Individuals with allergic reactions to bee stings should not participate in this portion.
  2. Keep the basic fuchsin gelatin out of the sun, in a small ice chest. If phenol was added, do not touch the gel.
  3. In an area away from where the visitation observations are underway, collect as many visitors to flowers as you can with an insect net.
  4. Use ethyl acetate to kill or stun insects.
  5. Use a dissecting needle to cut a 0.5 x 0.5 cm cube of glycerin fuchsin jelly. Holding the cube of jelly on the dissecting needle, wipe pollen from the body of the visitor. Make separate slides for jelly wiped on mouthparts, abdomen, and legs of the insect (or other body parts as observed). Carefully remove all visible pollen from the body section. If the specimen is a bee with a pollen load, crush the load so that pollen grains are identifiable.
  6. Place the cube on a glass slide, put on a coverslip, and gently melt over a candle or lighter, or place on dark surface in the sun.
  7. Label the slide with a Sharpee - date, time, type of visitor, body part.
  8. Make a reference slide from the flowering species for comparison. Wipe a cube of the jelly on an anther that is shedding pollen. Label it with date, time, species, and phase of flower.
  9. Examine your slides under a microscope at 100X. Count the number of pollen grains of the target species (buckeye or spring beauty) AND the number of non-target species pollen grains. Identify non-target species if possible. Start at one corner of the coverslip and systematically move the slide back and forth so that the field of view covers the slide the same way that an eraser would erase a blackboard.
  10. If pollen grains are too numerous for a complete count, sample the slide by counting 10 fields of view. Estimate how many fields of view there are (this will differ depending on the magnification you are using). Then, multiply the average number of pollen grains of both target and non-target species counted in your 10 fields by the number of fields to estimate the number of pollen grains on the slide.
  11. Calculate the proportion of target species pollen in the pollen load or jelly wipe that is from each part of the body. If non-target pollen is identified, calculate the proportion of each species pollen on the slide.


Group 3, Visitor - Target Flower Species:

Visitor Species # Pollen
Legs - Target
# Pollen
Legs - Nontarget
# Pollen
Body - Target
# Pollen
Body - Nontarget
# Pollen
Mouth - Target
# Pollen
Mouth -Nontarget
1





2





3





4





5





6





7





8





9





10





11





12





13





14





15





16





____________________________________________

Additional Instructions for All 3 Groups:

       Turn in the report form from your section of the investigation. Pick up the compiled data from all groups and answer the following questions. After examining the data, develop five questions the data raise and testable hypotheses for each of those questions.

       In addition, you may be asked to choose one of those hypotheses to to design and carry out an experiment to test. If so, use the Format for Written Research Report below to write your lab report.

____________________________________________

Format for Research Report - should include the following sections:

* TITLE

* INTRODUCTION

* METHODS AND MATERIALS

* RESULTS

* DISCUSSION

* LITERATURE CITED

IN ADDITION: PROOFREAD AND USE SPELL CHECK!!!!!

____________________________________________

Preparation of Basic Fuchsin Gelatin and Pollen Slides.

       A jelly containing stain to make a semi-permanent microscope slide of pollen, as suggested by Beattie, A. J. 1971. Pan-Pacific Entomologist 47: 82.

Use the following ingredients:


Procedure to Make the Fuchsin Gelatin:

  1. add the gelatin to the distilled water in a beaker and heat until the gelatin dissolves,
  2. add the glycerin,
  3. add phenol, if desired,
  4. add basic fuchsin crystals a few at a time until the solution is the color desired. Too light will not stain the pollen, but too dark may mask details of the pollen,
  5. filter the solution through glass wool or cheesecloth,
  6. pour into sterile containers such as petri plates that can be covered. If phenol is not used, refrigerate the plates and slides. They will keep about a month without refrigeration.


Preparation of Pollen Slides

  1. keep the prepared slides, and the unused jelly, out of the sun, and cool enough not to melt,
  2. with a dissecting needle, cut a small cube of the jelly out of the petri plate,
  3. brush the cube of jelly against an anther containing pollen, or on the insect body part,
  4. place the cube containing the pollen sample on a glass slide,
  5. place a coverslip on top of the cube of jelly,
  6. gently heat the slide over a candle flame until the jelly melts. Do not overheat, or scorch the slide. If it is warm and sunny, the jelly may be melted by placing the slide on a dark surface in the sun instead of using the candle flame. This will make a semi-permanent, stained specimen,
  7. using a permanent marker, label the glass slide (date, species sample was collected on).
<top>

______________________________________________________________


Questions for Further Thought and Discussion:


  1. Describe the phenology of the target species. Are there color changes in floral display? If so, can you think of any advantage to the flower for the change? To the pollinator?

  2. Draw a graph to display the average visitation rate by all categories of visitors over daily time. Are flowers more likely to be visited at certain times of day?

  3. Draw another graph to display the average visitation rate by all categories of visitor at different temperatures. How does this graph compare to the time of day graph?

  4. Calculate the probability of visit to each flower by any possible pollinator. You will need data from the phenology group to know how long the flowers are open for visitation, as well as average visitation rates, or rates for specific time periods.

  5. Which visitors carry the most pollen? Is that pollen likely to be transferred to another flower of the same species? If more than one species of pollen is present, is it in the same place on the visitor’s body?

  6. Which visitors are likely to be the most effective pollinators? Why? Which visitors are probably not pollinators?

  7. What attributes of the plant population (e.g. density, patch size, identity of neighbors) might raise or lower the predicted visitation rate?


*** Note: Answers to many of these questions and numerous other comments by the contributing author can be found in the "NOTES TO FACULTY" page.

<top>

______________________________________________________________


References and Links:



<top>

______________________________________________________________


Tools for Assessment of Student Learning Outcomes:

Guidelines for Assessment:

       Each student's grade will be based on 15% from the answers to pre-lab questions, 15% for group data collected, 10% for answers to questions for further thought, and 10% for questions and testable hypotheses generated. The other 50% will be from the individual research-style papers submitted (Ten points for each section - Introduction, Methods and Materials, Results, Discussion, and Literature Cited). A practical exam will test knowledge of flower parts and species of flowering plants and insect visitors.



Formal Report Scoring Sheet:

Section Possible
Points
Points
Earned
Comments
Introduction
    Background
    Justification
    Literature used
    Hypothesis clear
10

Methods - repeatable, clear 10

Results
    Tables and/or figures properly drawn & labeled
    DATA PRESENTED ONCE
    Pertinent results presented
8

    Narrative - describes trends 1

    No explanation (only in discussion) 1

Discussion
    Results compared to hypothesis 3

    Explanations, alternative explanations 2

    Literature used correctly 2

    Demonstrated understanding of coevolution 2

    Strong conclusion 1

Literature Cited
    At least 3 primary sources 6

    Proper format 4

TOTAL 50

<top>

______________________________________________________________


Tools for Formative Evaluation of this Experiment:



An extensive discussion on Evaluation appears in the Teaching section of this site.

<top>