GRADUATE STUDENT PROJECT DETAILS |
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JONATHAN
CHASE: Jon was co-advised by Mathew Leibold. His Ph. D. project focused on how size-dependent
interactions affect food web structure and response to ecosystem
productivity. He documented a
curious pattern of snail abundance and plant biomass across a
phosphorous-controlled productivity gradient, characterized by low plants, weak
grazer control, and dominance by Physa at low productivity, high plants, strong grazer control, and
dominance by Helisoma at high
productivity, but a mixture of these different states in different ponds with
intermediate productivity. He also
demonstrated that Physa and
small Helisoma were readily
consumed by predators but that large Helisoma were not. From these results, he postulated that size-dependent predation was
leading to alternative stable states at intermediate productivity, with ponds
in some cases being under strong predator control and others dominated by
predator-resistant grazers. Food
web models with size-structured predation demonstrated, contrary to prior
verbal theory, that size structure alone could not produce alternative stable
states (although the system can stay close to an unstable equilibrium for long
periods in transient dynamics), but that a configuration including an edible
competitor species could generate alternative stable states at intermediate productivity. He then carried out studies in
replicated experimental ponds by varying productivity, predator presence, and
the initial densities of snails, which demonstrated the predicted shift in food
web structure and alternative stable states at intermediate levels. This work is one of the best examples
of alternative stable states yet in ecology. Jon also carried out many side-projects, including studies
of small-scale variation in grassland food chains, and behavior-mediated
effects of tadpoles in rivers. He
also began exploring the interplay of disturbance, productivity, and food web
structure, the niche concept and effects of local and regional processes on
community composition. Jon is now
on the faculty of Washington University, Saint Louis. Publications that
involved work done while a student at U of Chicago include:
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Chase, J. M. 2003. Experimental evidence for alternative stable
equilibria in a benthic pond food web. Ecology Letters 6:733-741.
Chase, J. M. 2003. Strong and weak trophic cascades along a
productivity gradient. Oikos. 101: 187-195.
Chase, J. M. and M. A. Leibold. 2002. Spatial scale dictates the productivity-biodiversity
relationship. Nature 416:427-430.
Chase J.M., M. A. Leibold, A. L. Downing, and J. B. Shurin. 2000.
The effects of productivity, herbivory, and plant species turnover in grassland
food webs. Ecology 81:2485-2497.
Leibold, M.A.., J.M. Chase, J. B. Shurin, and A. L. Downing. 1997.
Species turnover and the regulation of trophic structure. Annual Review of
Ecology and Systematics 28:467-494.
Chase, J. M., M. A Leibold, and E. Simms. 2001. Plant tolerance and resistance in
food webs: Community-level predictions and evolutionary implications.
Evolutionary Ecology 14:289-314.
Chase, J. M., W. G. Wilson, and S. A. Richards, Shane. 2001. Foraging trade-offs and resource patchiness: Theory and experiments
with a freshwater snail community. Ecology Letters 4: 304-312.
Chase, J. M. 1999. To grow or to reproduce? The role of
life-history plasticity in food web dynamics. American Naturalist 154:571-586.
Chase, J. M. 1999. Food web effects of prey size refugia: Variable
interactions and alternative stable equilibria. American Naturalist 154: 559-570.
Chase, J. M. 1998. Central-place forager effects on food web
dynamics and spatial pattern in northern California meadows. Ecology 79:1236-1245.
Chase,
J. M., and M. A. Leibold. 2001. Ecological Niches:
Linking Classical and Contemporary Approaches. University of
Chicago Press.
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JEAN TSAO: Jean has a strong interest in the interface of ecological science and
human health, and developed an ambitious project to examine this issue in a
particularly appropriate system: Lyme Disease. She set out
to experimentally test the hypothesis of Rick Ostfeld and colleagues that the
ecological community comprising the backdrop for the disease could have strong
effects on disease prevalence. To
do this, she worked closely with the laboratories of Durland Fish (Yale
University) and Alan Barbour (University of California, Irvine) to generate an
experimental host "species": white-footed mice vaccinated against Lyme Disease. This experiment was implemented on
large-scale plots (100 x 100 m) and the subsequent dynamics of the disease,
tick populations, and mouse populations were monitored. She then incorporated the results into
a mechanistic model of host-disease ecology and systematically considered
whether key features of the model (mouse density, vaccination treatment,
contributions of mice to tick production) were necessary to produce her
experimental results. She found
that disease levels were depressed as predicted by the host community
hypothesis following mouse vaccination, and that the response was strongly tied
to mouse density. These results
suggest that if it can be effectively deployed (e.g, via digestion with food),
field vaccination of hosts or other methods of manipulating the host community
may be helpful in fighting the disease. Additionally, her results revealed that Lyme Disease levels are strongly
influenced by alternative hosts to mice, in contrast to conventional
wisdom. These responses were
complex, such that if alternative hosts were lost via local or regional
extinction, disease levels are likely to increase. Jean is now on the faculty of Michigan State
University. Publications involving
work done while at U of Chicago include:
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Tsao,
J. I., J. T. Wootton, J. Bunikis, M. G. Luna, D. Fish, and A. G. Barbour. 2004. An ecological approach to preventing human infection: vaccinating wild mouse reservoirs
intervenes in the Lyme disease cycle. Proceedings of the National Academy of
Science 52:18159-18164
Bunikis, J., J. Tsao, C. J. Luke, M. G. Luna, D. Fish, and A. G.
Barbour. 2004. Borrelia
burgdorferi infection in a natural population of Peromyscus leucopus mice: A
longitudinal study in an area where lyme borreliosis is highly endemic. Journal
of Infectious Diseases 189:1515-1523.
Tsao, J., A. Barbour, C. Luke, E. Fikrig, D. Fish. 2001.
Inoculation with OspA causes a decrease in transmission of Borrelia
burgdorferi from infected Peromyscus leucopus to larval Ixodes
scapularis. Vect. Born. Zoon. Dis., 1:65-74.
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AMY
DOWNING: Amy was co-advised by Mathew Leibold. Her Ph. D. investigated the link between biodiversity and
ecosystem function and stability, using experimental pond communities. Prior work on this problem involved
either single trophic level systems (plants) or was restricted to laboratory
settings. Amy's project extended
investigations into the realm of multi-trophic situations characteristic of real
ecosystems. She varied the
diversity of predators, grazers, and macrophytes in a variety of combinations
to explore how overall diversity affected system productivity, respiration,
chemical characteristics, and the resistance and resilience of the system in
response to pulsed perturbations (changes in pH or nutrients, mimicking pulsed
runoff events observed in many areas). She found general patterns of increase in ecosystem function with
diversity. Because she replicated
the species composition of her diversity treatments, Amy's study was also
unique in its ability to tease apart the effects of species identity versus
overall diversity. Her results
indicate that most of the diversity effect is the result of species-specific
contributions and strong synergisms among key combinations of species, and she
was able to identify some of the species causing disproportionately large
effects on the system. Surprisingly, consumer species generally had the strongest effects on
ecosystem processes, although they are more distantly removed from many of them
compared to the plant species she manipulated. Amy is now of the faculty of Ohio Wesleyan University. Publications involving work done while
at U. of Chicago include:
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Downing, A. L., and J. T. Wootton. 2005. Trophic position,
biotic context, and abiotic factors determine species contributions to
ecosystem functioning. Pp. 295-307 In P. de Ruiter, J. Moore, and V. Wolters,
editors. Dynamic Food Webs: Multispecies assemblages, ecosystem development,
and environmental change. Academic Press.
Downing, A. L. 2005. Relative effects of species composition and
richness on pond ecosystem properties in ponds. Ecology 86: 701-715.
Downing, A. L. and M. A. Leibold. 2002. Ecosystem consequences or species richness and
composition in pond food webs. Nature 416:837-841.
Loreau, M., A. Downing, M. Emmerson, A. Gonzalez, J. Hughes, P.
Inchausti, J. Joshi, J. Norberg and O. Sala. 2002. In A new look at the
relationship between diversity and stability. In M. Loreau, S.
Naeem, and P. Inchausti, eds. Biodiversity and Ecosystem Functioning: Synthesis
and Perspectives. Oxford University Press.
Wootton, J. T. and A. Downing. 2002. Understanding the Effects of
Reduced Biodiversity: A Comparison of Two Approaches, In P. Kareiva, and S.
Levin eds. The Importance of
Species: Perspectives on Expendability and Triage. Princeton University Press.
Chase J.M., M. A. Leibold, A. L. Downing, and J. B. Shurin. 2000.
The effects of productivity, herbivory, and plant species turnover in grassland
food webs. Ecology 81:2485-2497.
Leibold, M.A., J.M. Chase, J. B. Shurin, and A. L. Downing. 1997.
Species turnover and the regulation of trophic structure. Annual Review of
Ecology and Systematics. 28: 467-494.
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KEVIN
BRITTON-SIMMONS: Kevin's research focused on the
invasion dynamics and community impacts of an invasive brown seaweed, Sargassum
muticum, in subtidal marine
communities. Sargassum is steadily invading the Puget Sound area of
Washington State and other areas, following its introduction from Japan. Sargassum has a dual mode of dispersal with very local
production of gametophytes and long-distance rafting of attached adults. To investigate invasion dynamics, Kevin
studied invasion fronts of Sargassum and carried out manipulations that demonstrated that invasion speed
was negatively affected by dispersal limitation of Sargassum embryos and by interactions with native species
when they were present in key combinations of functional groups (encrusting
species to preempt recruitment to empty space, canopy species to cast shade on
established recruits). Once
established, Sargassum strongly reduced native kelp species, which indirectly resulted in reduced sea
urchin abundance as their food source was decreased. Finally, he showed that habitat heterogeneity facilitated
coexistence of natives and the invader, with Sargassum performing very poorly on vertical surfaces,
which allowed natives to persist. Kevin is now affiliated with the Friday Harbor Laboratories at the
University of Washington. Publications involving work done while at U. of Chicago include:
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Klinger, T., D. K. Padilla and K. Britton-Simmons. 2005. Two invaders achieve higher densities in marine reserves. Aquatic Conservation. In Press.
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PAMELA GEDDES: Pam
was co-advised by Mathew Leibold. She became interested in the role that external subsidies into systems
play in shaping community structure and stability, and focused on ponds across
a range of forest cover. Her
thesis project emphasized the dissolved organic matter contributed by leaves to
ponds, particularly released humic substances that can stain water brown (e.g.,
tea generated from tea leaves). Experiments in experimental pond mesocosms showed that community
structure shifted with the addition of humic substances and that, contrary to
theory, system stability decreased. Subsequent experiments manipulating the variability of input into the
system unexpectedly failed to increase the system variability, suggesting
strong homeostatic mechanisms within the food web. Finally, small-scale experiments designed to tease apart
mechanisms of action by humic substances by varying zooplankton contact with
enriched water and incident UV light indicated that they had strong short-term
effects on the food web via non trophic pathways, including providing
protection from UV radiation and introducing detrimental defensive compounds
into the water. Pam is currently a
postdoctoral fellow at Loyola University, Chicago.
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DOUG NUTTER: Doug was co-advised with Cathy Pfister. Doug explored several factors affecting the structure of
tidepool communities. One aspect
of his study was to test the hypothesis of community-assembly for multi-trophic
systems put forward by Bob Holt, which predicts steeper species accumulation
curves with area by consumers compared to plant species because prey need to
invade an area before populations of their consumers can be sustained. Using experimental tidepools of
different size, Doug found only a weak trend following this pattern, suggesting
that several fundamental assumptions of the theory (closed systems, lack of feedback
by consumers on prey diversity, lack of strong competitive interactions among
species at a single trophic level) may be violated in real systems. He also experimentally investigated the
nature of disturbances and stress to systems, focusing on the relative
importance of intensity and frequency of events. By varying the delivery rate and mean concentration of
ammonium, a toxic compound when found at high concentrations when delivered
from coastal sources such as runoff from seabird and marine mammal waste, he
found that the intensity of disturbance plays a much stronger role than the
temporal variation of disturbance. Doug now coordinates undergraduate ecology programs at the University of
Arizona.
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LISA (LIS)
NELIS: Lis
is interested in how the invasion of exotic species affects the community
structure through reorganization of the web of interactions among species. Her thesis project is exploring how
synergistic interactions among invaders affect invasion dynamics, focusing on
the invasion of a shrub, Trun (Rosaceae), and European Rabbits on the remote
islands of the Juan Fernandez Archipelago ("Robinson Crusoe Islands") off the
coast of Chile. The invasion of
Trun there appears to be greatly facilitated by the introduction of rabbits,
which may reduce competition with natives via grazing, disperse seeds of the
invader on their fur, and alter nutrient cycling on the landscape through
burrowing and fecal deposition. Lis is conducting large (10 x 10 m) experimental manipulations of
rabbits, trun, and native species to tease apart these invasion dynamics,
combining experimental results with mathematical modeling to explore the rates
of spread of the invading shrub. Her experiments will also shed light on the role that recruitment
limitation and seed bank dynamics play in restoration of native plants
following removal of invasive species.
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MARK
NOVAK: Mark is
very interested in the role of omnivory on food web dynamics and the patterns
of interaction strength throughout communities. He is studying these questions experimentally and with
stable isotope methods in the intertidal zone of New Zealand, which is
relatively unstudied and has a somewhat different structure than well-studied
North American systems. He is
taking advantage of the strong gradient in productivity generated by shifts in
coastal upwelling to determine whether omnivory shifts with productivity as
expected by theory, and how this shift affects community stability. He plans to examine whether patterns of
interaction strength are configured to promote system stability, and to test
experimentally different approaches to estimating interaction strength in the
field (energetics, population dynamics, lab experiments).
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MICHAEL FITZSIMONS: Michael is co-advised by Mike Miller at Argonne National Laboratory. Michael is interested in the determinants of diversity in mycorrhizal fungi and how these communities develop over time. His experiments focus particularly on feedbacks between plant and soil organisms, exploring whether negative or positive feed backs result from prolonged interactions and how that shapes succession. He is combining his small scale experimental results with analyses of soil-plant interactions of the chronosequence of prairie restoration experiments at Fermi National Lab to evaluate how the short-term, small scale patterns play out over larger spatial and temporal scales.
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