The Relative Strength of Abiotic and Biotic Controls on Species Range Limits

161 Pages·2017·5.21 MB·English

University of Colorado, Boulder
CU Scholar
Environmental Studies Graduate Theses &
Environmental Studies
Dissertations
Spring 1-1-2016
The Relative Strength of Abiotic and Biotic
Controls on Species Range Limits
Allison Marie Louthan
University of Colorado at Boulder, [email protected]
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Louthan, Allison Marie, "The Relative Strength of Abiotic and Biotic Controls on Species Range Limits" (2016).Environmental Studies
Graduate Theses & Dissertations. 32.
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THE RELATIVE STRENGTH OF ABIOTIC AND BIOTIC CONTROLS ON SPECIES RANGE
LIMITS
by
ALLISON LOUTHAN
B.A., Grinnell College, 2008

A thesis submitted to the
Faculty of the Graduate School of the
University of Colorado in partial fulfillment
of the requirement for the degree of
Doctor of Philosophy
Environmental Studies Program
2016
This thesis entitled:
The Relative Strength of Abiotic and Biotic Controls on Species Range Limits
written by Allison Marie Louthan
has been approved for the Environmental Studies Program

Daniel Doak

Sharon Collinge

Brett Melbourne

Date

The final copy of this thesis has been examined by the signatories, and we
find that both the content and the form meet acceptable presentation standards
of scholarly work in the above mentioned discipline.
ABSTRACT
Louthan, Allison Marie (Ph.D., Environmental Studies Program)
The Relative Strength of Abiotic and Biotic Controls on Species Range Limits
Thesis directed by Professor Daniel F. Doak

Study of the determinants of species’ geographic distributions has a rich tradition in ecology and
evolution, and understanding these determinants is becoming increasingly important in the face
of climate change. While we know many range limits are set by abiotic stress, species
interactions can also be important drivers of range limits. However, we lack any well-tested
predictive framework for when and where each of these two broad classes of factors will most
commonly set range limits.
A long-standing, but still nearly untested, hypothesis suggests that abiotic stress most
often sets range limits in seemingly stressful areas, such as arctic, high-alpine, or arid systems,
with species interactions having more influence in apparently benign environments, such as the
tropics, low-elevation, or mesic places. In my dissertation, I experimentally tested a fundamental
assumption of this hypothesis: namely, that the relative importance of species interactions and
abiotic stress for population performance varies systematically with abiotic stress. I tested the
relative importance of abiotic stress vs. three species interactions (herbivory, neighbors, and
pollinators) for population dynamics of a model plant species in central Kenya, Hibiscus meyeri,
across a sharp aridity gradient.
I find broad-scale support for Darwin’s hypothesis, with stronger effects of herbivores,
neighbors, and pollinators on population growth rate in mesic areas v. arid areas. Interestingly, I
find universal competitive effects of neighbors (rather than the switch from facilitative to
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competitive with increasing rainfall predicted by recent theoretical and empirical work). This
work suggests that species interactions might be critical drivers of range limits only in
unstressful regions of a species range.
This work also has implications for projecting shifts in species’ distributions. While in
some cases, leaving biotic interactions out of species’ distribution models reduces accuracy, the
vast majority of projections of shifts in distributions with climate change do not include such
interactions. This work suggests that species distribution modelers should include species
interactions in their predictions only in abiotically benign portions of a species range.

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DEDICATION
In memory of Antony Eschwa.

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ACKNOWLEDGEMENTS
I would like to thank the invaluable help I have received from my advisor, Daniel Doak, as well
as from my committee members and the Principal Investigators of UHURU. I am also grateful
for funding from the P.E.O. Scholar Award, The University of Colorado-Boulder, the L’Oréal-
UNESCO Award for Women in Science, the American Philosophical Society (Lewis and Clark
Fund), a Doctoral Dissertation Improvement Grant, NSF DEB-1311394, NSF DEB-0812824 to
D. Doak, the University of Wyoming, the Wyoming NASA Space Grant, and the Bureau of Land
Management.

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CONTENTS

CHAPTER

I. INTRODUCTION .................................................................................... 1

Purpose of the Study ........................................................................... 2

Experimental Design of the Study ...................................................... 3

Arrangement of the Thesis ................................................................. 4

II. WHERE AND WHEN DO SPECIES INTERACTIONS
SET RANGE LIMITS? ................................................................ 6

Abiotic and Biotic Determinants of Species Ranges .................... 7

A Brief History of Range Limit Theory ....................................... 9

Tests of the Forces Governing Range Limits ............................. 10

A Clear Definition of SIASH ..................................................... 12

Possible Mechanisms Determining Species Interaction
Strength across Stress Gradients ........................................... 15

Concluding Remarks and Future Directions .............................. 21

Supporting Details ...................................................................... 24

III. CLIMATIC STRESS MEDIATES THE IMPACTS OF HERBIVORY
ON PLANT POPULATION STRUCTURE AND COMPONENTS
OF INDIVIDUAL FITNESS .................................................................. 30

Introduction ................................................................................ 31

Materials and Methods ............................................................... 34

Results ........................................................................................ 40

Discussion ................................................................................... 47

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IV. MECHANISMS OF PLANT-PLANT INTERACTIONS:
CONCEALMENT FROM HERBIVORES IS MORE IMPORTANT
THAN ABIOTIC-STRESS MEDIATION IN AN
AFRICAN SAVANNAH ....................................................................... 54

Introduction ................................................................................ 55

Materials and Methods ............................................................... 57

Results ........................................................................................ 64

Discussion ................................................................................... 68

V. SPECIES INTERACTIONS MORE STRONGLY AFFECT
POPULATION GROWTH RATE IN UNSTRESSFUL AREAS ......... 73

Introduction ................................................................................ 74

Results and Discussion ............................................................... 76

Materials and Methods ............................................................... 82

VI. CONCLUSION ...................................................................................... 88

Summary of consistent patterns .................................................. 89

Future work ................................................................................. 90

BIBLIOGRAPHY……………………..…………………………………………Error!
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APPENDIX

A. CHAPTER 2 APPENDIX .................................................................... 111

B. CHAPTER 3 APPENDIX .................................................................... 112

C. CHAPTER 4 APPENDIX .................................................................... 121

D. CHAPTER 5 APPENDIX .................................................................... 126

E. PERMISSIONS TO USE PUBLISHED MANUSCRIPTS ................. 139

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TABLES

1. Possible patterns in abiotic and biotic causes of range limits ......................... 8

2. Effect of Herbivore Exclosures on Population Metrics ................................ 47

3. Best-fit Models of Vital Rates for Neighbor Removal Experiment ............. 61

4. Sample Sizes for Size Distributions ........................................................... 112

5. Effect of Herbivores on the Probability of Floral Initiation ....................... 113

6. Statistical Analyses of Size Distributions (Kolmogorov-Smirnov) .......... 113

7. Statistical Analyses of Size Distributions (other tests) ............................... 114

8. Effect of Herbivores on the Number of Fruits ............................................ 115

9. Effect of Herbivores on Densities .............................................................. 120

10. Rates of Insect Herbivory ........................................................................... 121

11. Basal Area and Heights for Small, Medium, and Large Sizes ................... 121

12. Best-Fit Models of Vital Rates for Neighbor Removal Experiment
(AIC weight >=0.08) ............................................................................ 121

13. Effects of Neighbors and Herbivores on Fitness ........................................ 123

14. Effects of all Species Interactions on Vital Rates ....................................... 126

15. Annual Rainfall ........................................................................................... 127

16. Sample Sizes for Estimating Species Interactions Effects
On Population Growth Rate ................................................................. 127

17. Contribution of Different Mechanisms and Vital Rates
to Stronger Effects of Species Interactions in Mesic Areas ................. 128

18. Variance Explained by Random Effects for Vital Rate Functions ............. 134

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