Energy Flow and Food Web Ecology along a Hydroperiod Gradient

172 Pages·2012·3.91 MB·English

Energy Flow and Food Web Ecology Along a Hydroperiod
Gradient
by
Tiffany A. Schriever
A thesis submitted in conformity with the requirements
for the degree of Doctor of Philosophy
Ecology and Evolutionary Biology
University of Toronto
© Copyright by Tiffany A. Schriever 2012
Energy Flow and Food Web Ecology Along a Hydroperiod
Gradient
Tiffany A. Schriever
Doctor of Philosophy
Ecology and Evolutionary Biology
University of Toronto
2012
Abstract
Identifying the ecological mechanisms that determine food web structure is critical for
understanding the causes and consequences of diversity. The objective of this thesis was to
identify the mechanisms structuring aquatic food webs across environmental gradients from a
multi-level perspective (individual to ecosystem) using integrative methodology and field
experiments to test classic ecological theory. My results demonstrate support for the dynamic
constraints hypothesis, which predicts habitats with greater disturbance should have shorter food
chains, but are not consistent with the ecosystem size hypothesis that predicts larger ecosystems
have longer food chains. Insect and amphibian richness increased with increasing pond size and
hydroperiod, indicating that insertion of new consumers into pond communities was driving
variation in food-chain length. A multivariate analysis testing the influence of physicochemical
variables on food-web characteristics revealed that hydroperiod and pond area had a strong
influence on amphibian and invertebrate assemblages, trophic diversity and 15N range. Food-
chain length did not respond strongly to any one variable, but instead responded weakly to
multiple environmental variables, suggesting interacting influences on food-web structure.
Conversely, the trophic niche of amphibian larvae was not influenced by pond hydroperiod, but
did exhibit ontogenetic diet shifts. Populations of amphibian larvae with broader niche widths
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also had increased individual variation, supporting the niche variation hypothesis. In addition, I
assessed whether species diversity influenced the magnitude of cross-habitat resource flow
between aquatic and terrestrial habitats via emerging aquatic insects, metamorphosing
amphibians, and litter deposition. Deposition into ponds far exceeded carbon exported via insect
and amphibian emergences. We found a negative relationship between resource flux and the
diversity of amphibians and insects, which contradicts the general pattern of positive
biodiversity-ecosystem function relationships. My research strongly suggests environmental
variation is a key process in shaping food-web structure and function and that multiple
methodologies are needed to understand temporal and spatial dynamics of aquatic ecosystems.
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Acknowledgments
My first and biggest thank you goes to Devin Bloom. I can’t possibly express in words how
grateful I am for all his love, support, and pep-talks. I surely would not have kept any sliver of
sanity or had any fun without him around. He has been my greatest supporter, best friend, and
editor for too many years to count. I wish I was half as great of a scientist as he is. Thank you
and I love you!
I owe an enormous thank you to my advisor, Prof. Dudley Williams. He took a chance and gave
me the opportunity to become part of his legacy. Dudley gave me the freedom to develop as an
aquatic biologist through his guidance and financial support through the years. I am grateful for
the quick turnaround and thorough edits of manuscripts and positive feedback. I owe him a big
thank you because he is responsible for my newfound love of invertebrates. I am saddened that I
am one of the last members of the Williams’ lab, but humbled to have been part of it.
I am thankful for my PhD advisory committee, Rudy Boonstra, Bob Murphy, and Marc Cadotte.
I appreciate all their helpful comments and discussion throughout the different stages of my
program.
A big muddy, pond-smelling hug for all those who labored in the field with me: Jennifer
Arblaster, Katherine Bannar-Martin, Devin Bloom, Shauna Bloom, Kristen Brochu, Monica
Candelaria, Kirsten Comberford, Mark Conboy, Jake Cowper Szamosi, Klara Jaspers-Fayer,
Maria Modanu, Stephen Pynn, David Stitt, Caroline Tucker, and Tristan Willis. I am thankful for
Fiona Munro who walked around the ponds numerous times with a heavy Trimble GPS unit and
gave me my first introduction to GIS. I loved working in the ponds all day with such wonderful
company! Caroline, I’m sorry for the nights lacking sleep and bug bites. Kirsten thanks for
pointing out all the spiders even though you were not thrilled to see them – everywhere! Kat,
Maria, Kristen, Pynn and Devin, you are all workhorses with strong backs and big smiles. Thank
you again and again!
I could not have processed as many samples, sorted as many invertebrates, or dried and ground
as many amphibians as I did without the amazing help of: Sarah Booth, Siao Ryan Yang, Ruby
Sambi, Gaayathiri Jegatheeswaran, Maryam Mahmood, Gagan Gill, David Stitt, Katie Keogh,
Kristen Beck, and Christen Dschankilic. Some of these students stuck with me for multiple years
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and even after graduating! I wish you all the best of luck in your academic and life endeavors.
Thank you so much for all your countless hours of unpaid work. Sarah and Jenn you are
amazing, intelligent, and creative biologists. I can’t wait to see what you bring to the field. Thank
you for letting me practice playing the role of advisor on you!
I want to extend a special thanks to Marc Cadotte and his lab members, Caroline, Lanna, Nick,
Kelly, and Robin, for welcoming me into their joyful lab. I enjoyed the meetings, get-togethers,
birthday cakes and the candy that was always available. Marc you have been kind, extremely
helpful, and a very important mentor to me.
I thank Brian Crother, my master’s supervisor, who continually provides spirited support and
encouragement. Nate Lovejoy, thank you for writing a reference letter for my new post-doctoral
position. I am very grateful for the Toronto Zoo staff and Malcolm Campbell for providing an
enjoyable and enlightening way to spend my teaching hours.
My time at UTSC was made tremendously better by the friendship of Emily, Maria, Kristen, and
Megan. My lab mates: Catherine, Kevin, Julie, Sarah, Jessica, Oksana, and Judith provided
guidance, support, and good times. I will miss the walks in the valley, lunch on the patio, and
your smiles.
I have enjoyed my time in Toronto, although I have a love-hate relationship with Toronto.
Toronto has been a fun city to have called home for five years, but I will be happy to leave the
traffic behind. I love the friends I’ve met, of whom I will miss greatly. My newly acquired
family of Newfoundlanders: Dave, Steve, Kirsten, and Danielle (yes, you are guilty by
association) were always there to provide awesome food, wine, laughs, and encouragement.
Karen and Hernán I am blessed to have you as friends. Until we all meet again!
Lastly, I am very grateful for a family that has provided support and encouragement through the
many tiring years of graduate work. Mom and Dad, South Dakota will always be my home
regardless of where I live. I am grateful for the Blooms as they have always supported me,
pushed me to be confident, and provided entertainment. My heart is continuously filled with love
from my family and for that I thank you all for giving me the strength to pursue my dreams.
If I forgot someone, please forgive me. So many people have helped in some way and I am
sincerely grateful to all of you.
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Chapter Acknowledgments
This thesis is comprised of four papers that are either submitted, in review, or in preparation for
publication in peer-reviewed scientific journals (Chapters 2 – 5). Co-author Marc Cadotte
provided statistical expertise, editorial comments, and consultation on research topic (Chapter 4).
Chapter 1 provides the framework for this dissertation research and Chapter 6 summarizes the
major findings and importance of this work in the field of Ecology.
Schriever, T.A. and Williams, D. D. Accepted. Ontogenetic and individual diet variation in
amphibian larvae across an environmental gradient. Freshwater Biology
Schriever, T.A. and Williams, D. D. In review. Duration matters, size does not in pond food
webs: a study of the dynamic constraints and ecosystem size hypotheses. Oecologia
Schriever, T.A., Cadotte, M. W. and Williams, D. D. In preparation. How hydroperiod and
species richness affect the balance of resource flows across aquatic-terrestrial habitats.
Ecosystems
Schriever, T. A. In preparation. Food webs in relation to environmental and community
variation: a multivariate approach.
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Table of Contents
Contents
Acknowledgments.......................................................................................................................... iv
Chapter Acknowledgments ............................................................................................................ vi
Table of Contents .......................................................................................................................... vii
List of Tables ................................................................................................................................. xi
List of Figures ............................................................................................................................... xii
List of Appendices ....................................................................................................................... xvi
1 Introduction .................................................................................................................................1
1.1 Freshwater hydroperiod gradient .........................................................................................2
1.2 Food web ecology ................................................................................................................3
1.3 Use of stable isotopes in ecology .........................................................................................4
1.4 Aquatic-terrestrial linkages ..................................................................................................4
1.5 Thesis objectives ..................................................................................................................6
2 Ontogenetic and individual diet variation in amphibian larvae across an environmental
gradient ........................................................................................................................................9
2.1 Abstract ..............................................................................................................................10
2.2 Introduction ........................................................................................................................10
2.3 Methods..............................................................................................................................12
2.3.1 Study area...............................................................................................................12
2.3.2 Field collecting and laboratory methods ................................................................13
2.3.3 Gut content analysis (GCA) ...................................................................................13
2.3.4 Stable isotopes analysis (SIA) ...............................................................................14
2.4 Data analysis ......................................................................................................................15
2.4.1 Quantifying diet variation and niche width ...........................................................15
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2.4.2 Statistical analyses .................................................................................................18
2.5 Results ................................................................................................................................19
2.5.1 Niche variation across hydroperiod gradient .........................................................19
2.5.2 Diet and trophic niche ............................................................................................20
2.5.3 Trophic position .....................................................................................................21
2.5.4 Ontogenetic diet shifts ...........................................................................................22
2.6 Discussion ..........................................................................................................................23
2.6.1 Ontogenetic diet shifts ...........................................................................................27
2.6.2 Conclusions ............................................................................................................28
2.7 Acknowledgements ............................................................................................................29
2.8 Figures................................................................................................................................30
3 Duration matters, size does not in pond food webs: A study of the dynamic constraints and
ecosystem size hypotheses ........................................................................................................36
3.1 Abstract ..............................................................................................................................37
3.2 Introduction ........................................................................................................................37
3.3 Methods..............................................................................................................................40
3.3.1 Study sites ..............................................................................................................40
3.3.2 Field collection of pond communities....................................................................40
3.3.3 Characterization of ecosystem size and environmental variation ..........................41
3.3.4 Insect community composition ..............................................................................42
3.3.5 Sample preparation and stable isotope analysis .....................................................42
3.3.6 Food-chain length and trophic position calculation ...............................................43
3.3.7 Data analysis ..........................................................................................................44
3.4 Results ................................................................................................................................45
3.4.1 Food-chain length and environmental variation ....................................................45
3.4.2 Proximate mechanisms underlying changes in FCL ..............................................45
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3.5 Discussion ..........................................................................................................................47
3.5.1 Ultimate determinants of FCL ...............................................................................47
3.5.2 Proximal mechanisms underlying FCL variability ................................................49
3.6 Acknowledgments..............................................................................................................51
3.7 Figures................................................................................................................................52
4 Reciprocal resource flows in temporary ponds: the importance of community composition
and hydroperiod ........................................................................................................................59
4.1 Abstract ..............................................................................................................................60
4.2 Introduction ........................................................................................................................60
4.3 Methods..............................................................................................................................63
4.3.1 Study sites and physical habitat .............................................................................63
4.3.2 Field and laboratory methodology .........................................................................63
4.3.3 Statistical analyses .................................................................................................65
4.4 Results ................................................................................................................................66
4.4.1 Timing of resource flows and environmental influences .......................................66
4.4.2 Magnitude of fluxes ...............................................................................................66
4.4.3 Diversity-ecosystem function response .................................................................67
4.4.4 Influence of pond area and hydroperiod ................................................................68
4.5 Discussion ..........................................................................................................................69
4.5.1 Diversity-ecosystem function relationship ............................................................71
4.5.2 Conclusions ............................................................................................................72
4.6 Acknowledgements ............................................................................................................73
4.7 Tables .................................................................................................................................74
4.8 Figures................................................................................................................................76
5 Food webs in relation to environmental and species assemblage variation: A multivariate
approach ....................................................................................................................................82
5.1 Abstract ..............................................................................................................................83
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5.2 Introduction ........................................................................................................................83
5.3 Methods..............................................................................................................................86
5.3.1 Study sites ..............................................................................................................86
5.3.2 Field data collection ...............................................................................................87
5.3.3 Laboratory and stable isotope analysis ..................................................................88
5.3.4 Statistical analyses .................................................................................................90
5.4 Results ................................................................................................................................91
5.4.1 Physical-chemical pond environment ....................................................................91
5.4.2 Insect and amphibian community-environment relationships ...............................91
5.4.3 Food web-environment relationships .....................................................................94
5.5 Discussion ..........................................................................................................................96
5.5.1 Summary ................................................................................................................98
5.6 Acknowledgments..............................................................................................................98
5.7 Tables ...............................................................................................................................100
5.8 Figures..............................................................................................................................102
6 Thesis summary ......................................................................................................................107
6.1 Individual, population and ontogenetic trophic niches of amphibian larvae ..................107
6.2 Hydroperiod determines food-chain length .....................................................................108
6.3 Less diverse temporary ponds boast higher subsidy to surrounding terrestrial habitat .108
6.4 Importance of multiple environmental influences on aquatic community and food-web
structure ...........................................................................................................................109
7 References ...............................................................................................................................110
8 Appendices ..............................................................................................................................124

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