Authors: Ally Ruttan and Christopher J. Lortie Shrubs are a convenient and common functional group used to explore facilitation in desert ecosystems. In the southwestern United States, the dominant shrub Larrea tridentata has been documented for its positive effects on other plant species (Huenneke et al. 2002; Titus et al. 2002). It interacts positively with the surrounding community through stress amelioration, water retention, and soil nutrient concentration, resulting in an increase in flowering plants within its understory (Yeaton 1978; Drezner 2006). This interaction is ecologically relevant to pollinators because it provides an island of concentrated floral resources for them to exploit. Using L. tridentata as a model, we will test the capacity for these shrubs to act as magnets for the pollination of understory annual plants as well as contrast the direct and indirect effects they have on desert pollinator communities and annual plant reproductive traits. Given the global declines of pollinators and the many important ecosystem services they provide (Kremen et al. 2007), understanding the role of keystone plant species in their conservation is critical. In order to do this, we will first conduct a survey to determine the strength of the magnet effect of L. tridentata and a wind-pollinated shrub that is not in flower (Ambrosia Dumosa) on annual plant pollination, pollen deposition, seed production and plant life history traits. Following this, the effects of L. tridentata and understory annual plants on the facilitation of desert pollinators will be examined. Video and in-situ observation will be used to compare pollinator visitation rates between three treatment groups (+shrub/+annuals, +shrub/-annuals, -shrub/+annuals) using three different methods (natural flower variation, controlled flower variation using burlap exclosures, controlled flower variation using a phytometer). This will allow for the separation of the direct and indirect pathways of this facilitation (described in Fig. 1). This research program is an important step forward in determining if dominant plants can be used as a tool for maintaining and restoring declining desert pollinator communities, as well as managing the effects of desertification in light of climate change.

1. PROGRESS REPORT – MARCH 2015
Ally Ruttan, MSc Candidate
Decoupling the direct and indirect pathways of
basal plant facilitation: A test of the double
magnet hypothesis for pollinators.

2. Positive interactions

3. Positive interactions

4. Magnet hypothesis

5. Magnet Hypothesis
Magnet
(shrub) + + -
Target
(annuals) + - +
Net
Outcome

6. Magnet
(shrub) + + -
Target
(annuals) + - +
Net
Outcome
Magnet Hypothesis

7. Magnet Hypothesis
Magnet
(shrub) + + -
Target
(annuals) + - +
Net
Outcome + O O

8. + –
+
+ +
+
1
2
3
4 5
6
1. Facilitation of annuals by shrub indirectly provides resources for pollinators
2. Positive, bidirectional relationship between shrub and insect (pollination of shrub, food resource for pollinators)
3. Positive, indirect, bidirectional relationship between shrub and annuals for the attraction of pollinators
4. Shrub facilitates annuals directly through amelioration stress and access to resources
5. Bidirectional competition between shrub and annuals for resources (including pollinators)
6. Positive, bidirectional relationship between annuals and pollinators (pollination of annuals, food resource for pollinators)

9. A
B
1
1
2
2
Double magnet hypothesis

10. Chapter 1
A systematic review of the magnet species hypothesis
for plant-pollinator interactions

11. Chapter 1 - Purpose
To determine the extent of study and
methodologies applied to the magnet
species hypothesis.

12. Chapter 1 – Research Questions
What ecosystems are most commonly used to test the magnet
hypothesis?
What is the diversity of pollinators tested via the magnet hypothesis?
What observational approached have been used in testing the magnet
hypothesis (e.g. pan trapping, video, in-situ, etc.)?
Are there specific focal species used as a magnet or many different
ones?
Are there underlying ecological theories explicitly associated with the
magnet hypothesis?

13. Chapter 1 - Methods
(Magnet hypothesis OR magnet species OR pollinator
facilitation OR co-flower) AND pollinat*

14. Chapter 1 - Methods
(Magnet hypothesis OR magnet species OR pollinator
facilitation OR co-flower) AND pollinat*
228

15. Chapter 1 - Methods
Refined for relevance….
44

16. All
visitors
(5)
All insects
(27)
Bees
(2)Target bee
species
(7)
Social bees
(2)
Birds
(1)

17. 0
5
10
15
20
25
30
35
40
45
in situ collection proxy video
Frequency
Observation method

18. Chapter 2
A survey examining the island effect of shrubs on
understory plant pollination and reproductive patterns

22. Study site

23. Chapter 2 - Purpose
Test the capacity for entomophilous and anemophilous
shrubs to act as magnets for the pollination of
understory plants and the shrubs themselves.

24. Chapter 2 - Hypothesis
Shrubs (particularly entomophilous ones) act as
resource islands/magnets that attract pollinators and
increase the reproductive success of understory plants.

25. L. tridentata A. dumosa

26. Chapter 2 - Predictions
1. Annuals under shrubs (especially L. tridentata,
flowering magnet) will do better than in open
microsites (abundance, pollen deposition, seed
production).
2. Lower annual density = increased seed set
(competitor release)
3. Seed set of L. tridentata (but not A. dumosa) will
be positively correlated with annual floral density
(assuming strict pollinator matching to floral
densities, ignoring resource levels).

27. 30 paired shrub/open sites
2 species
3 annuals x 3 species
per site
1 flower per annual
1 seed head per annual

28. Chapter 3
The double magnet hypothesis: separating the direct
and indirect effects of plant facilitation on pollinators
X Y Z

29. Chapter 3 - purpose
Contrast direct/indirect effects of shrubs and annual
plants on desert pollinator communities and map
important shrub traits onto extended community
dynamics

30. Chapter 3 - Hypothesis
The floral resource island created by shrubs and the
beneficiary annual plants will have positive non-
additive effects on pollinator visitation rates.

31. Chapter 3 - Predictions
1. Annual plants under shrubs will have a higher frequency of
pollinator visitations than annual plants in open microsites
2. Shrubs with annuals in their understory will have a higher
frequency of pollinator visitations than shrubs without annuals
due to the concentration of floral resources
3. Sites with both shrubs and annuals will have the highest
frequency of pollinator visitations to both the shrubs and the
annuals

32. InsectVisitation
Annuals
Removed
Annuals
Intact
shrub microsite open Microsite

33. A.
B.
C.
natural variation controlled variation phytometer

34. = shrub only
= annual first,
then shrub
= annual only
= shrub first,
then annual
*bonus experiment 1* – Fluroescent pollen dye

35. Timeline
Chapter 1: Data inputted/writing complete Sept 2015
Chapter 2: Survey mid-March to mid-April
Chapter 3: March: removals
Late March – April (natural/controlled variation)
April – early May (phytometer)

36. Thanks!