Limits to range expansion in the native annual
legume Chamaecrista fasciculata






A DISSERTATION
SUBMITTED TO THE FACULTY OF THE GRADUATE SCHOOL
OF THE UNIVERSITY OF MINNESOTA
BY





John Louis Stanton-Geddes






IN PARTIAL FULFILLMENT OF THE REQUIREMENTS
FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY



Ruth Shaw and Peter Tiffin




August 2011

© John Stanton-Geddes 2011

i
Acknowledgements
I gratefully acknowledge the advice, guidance, patience and encouragement of my
advisors, Ruth Shaw and Peter Tiffin. There‟s no doubt that without them, this work
would not be of the quality it is and would most likely not exist.

My undergraduate advisor, Jill Miller at Amherst College, was the person who got me
into evolutionary biology research, and she has been a great friend and resource ever
since. Everett Dietrick of Rincon-Vitova Insectaries showed me that biology can be a
lifetime passion and offered a perspective outside of academia how biology can impact
the world.

My friends, colleagues and lab members have all been tremendous resources through this
process, and made this a lot more fun.

My research has been generously supported by many sources, including the Rothman
Fellowship, Carolyn Crosby Fellowship, Alexander and Lydia Anderson Fellowship,
Center for Community Genetics and the University of Minnesota Graduate School. My
advisors yet again were fantastic and provided many years of summer support and a year
of RA support to complete my research.

Finally, I thank my family. My grandparents are inspirations in pursuing your passion.
My parents supported me throughout this process, giving me a place to stay while on the
road and even letting me use their yard as a field site! Most importantly, my wife Hilary
for everything she does. She put up with me during all these endeavors, and came out to
help with field work each year even when she knew it would just be ticks and
mosquitoes. Thanks Hil!

ii
Dedication
All happy families are alike; each unhappy family is unique in its own way
-Tolstoy

All species have restricted ranges; each range edge is unique
in its own way.


This thesis is dedicated to ecologists and evolutionary biologists who did the work and
wrote the questions that motivate me, and biologists of the future who will further this
work. And finally, my daughter Madeleine who arrived just in time for me to finish this
thesis, and will soon be pursuing her own passion (maybe biology!).

iii
Abstract
Species range limits are determined by historical (e.g., range expansion), ecological (e.g.,
biotic interactions) and genetic (e.g, gene flow) processes, but comprehensively
understanding the relative role of these processes in limiting any single species‟ range has
been elusive. This research is timely for understanding species‟ responses to climate
change. The goal of this research was to examine the processes that limit the range of the
native annual legume Chamaecrista fasciculata, by integrating ecological-genetic field
studies and population genetic laboratory studies. In Chapter 1, I investigate the extent to
which C. fasciculata is in demographic range edge equilibrium at its western and
northern range edges, and the effect of biotic interactions at these range edges. I find that
C. fasciculata fitness is reduced to zero when planted beyond the western and northern
range limits, indicating it is in equilibrium with its range. Neighbors increase early-
season survival, but decrease seedpod production. The goal of Chapter 2 was to examine
if the mutualism between C. fasciculata and its associated rhizobia was disrupted beyond
the range edge, potentially limiting range expansion. The results demonstrate that
compatible rhizobia are nearly absent beyond both range edges, which may limit range
expansion. In Chapter 3, I ask how the habitat where C. fasciculata establishes may
change with range shifts. I conclude that habitat type influences C. fasciculata fitness, but
the outcome depends on both the substrate and competitive environments. Finally, in
Chapter 4, I use population genetic methods to gain insight into the history of range
expansion, population structure and gene flow. Population genetics indicate that the edge
populations have reduced genetic diversity compared to the southernmost interior
population, and are highly differentiated from each other. However, there is little
evidence for contemporary gene flow between populations at the scale investigated.
Overall, this work suggests that ecological-genetic or metapopulation dynamics are likely
to be involved in setting the northern and western range limits. Further, it highlights the
value of integrated approaches to studying species‟ range limits.

134

Figure 4-6. Map showing estimated migration rates (M) among populations and
mutation-scaled effective population sizes (θ) from Full MIGRATE-N model. The
thickness of the arrows is proportional to the migration rate, and the size of the circle
representing each population is proportional to θ. Absolute values are given in Table S4-
6.